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s indicate the direction of the wind, while the "L" symbol denotes the center of the "low". Note the occluded, cold and warm Surface weather analysis.
Extratropical cyclones, sometimes called
mid-latitude cyclones, are a group of cyclones defined as
Synoptic scale meteorology Low pressure area weather systems that occur in the middle latitudes of the Earth having neither
tropical cyclone nor
polar cyclone characteristics, and are connected with Surface weather analysis and horizontal gradients in temperature and dew point otherwise known as "baroclinic zones". Extratropical cyclones are the everyday phenomena which, along with anticyclones, drive the weather over much of the Earth, producing anything from cloudiness and mild
rains to heavy gales and
thunderstorms.
Terminology
Extratropical cyclones encompass a class of storms with many names. Although they are sometimes referred to as "cyclones", this is imprecise;
cyclone applies to numerous types of low pressure areas. The descriptor
extratropical signifies that this type of cyclone generally occurs outside the tropics in the middle
latitudes of Earth. The term
mid-latitude cyclones may be used because of where they form; "post-tropical cyclones" if #Extratropical transition has occurred.ExtraLessonMillUni Weather forecasters and the general public often describe them as "depressions" or "lows".
Extratropical cyclones are classified mainly as baroclinity, because they form along zones of temperature and dewpoint
gradient known as weather fronts. They can become barotropic late in their life cycle, when the distribution of heat around the cyclone becomes fairly uniform with its radius.
Formation
Extratropical cyclones form anywhere within the extratropical regions of the Earth (usually between 30° and 60° latitude from the
equator), either through cyclogenesis or extratropical transition. A study of extratropical cyclones in the
Southern Hemisphere shows that between the 30th and 70th parallels, there are an average of 37 cyclones in existence during any 6-hour period. {{cite web | title = Variability of Southern Hemisphere Extratropical Cyclone Behavior, 1958–97
| author = Ian Simmonds and Kevin Keay
| publisher = http://ams.allenpress.com/ American Meteorology Society (Allenpress Inc)
| date = 2000-02
| url = http://ams.allenpress.com/perlserv/?request=get-document&doi=10.1175%2F1520-0442(2000)013%3C0550:VOSHEC%3E2.0.CO%3B2#I1520-0442-13-3-550-F04
| accessdate = 2006-10-20 --> A separate study in the [Northern Hemisphere suggests that approximately 234 significant extratropical cyclones form each winter. {{cite web
| title = Winter Storms in the Northern Hemisphere (1958–1999)
| author = S.K. Gulev, O. Zolina, and S. Grigoriev
| publisher =
http://www.co2science.org/ CO2 Science
| date = 2001
| url = http://www.co2science.org/scripts/CO2ScienceB2C/articles/V4/N37/C2.jsp
| accessdate = 2006-10-20 -->
Cyclogenesis
Extratropical cyclones form along linear bands of temperature/dewpoint gradient with significant vertical wind shear, and are thus classified as baroclinic cyclones. Initially,
cyclogenesis, or low pressure formation, occurs along
Surface weather analysis near a favorable quadrant of the
Jetstream, usually being the right rear and left front quadrants, where
divergence ensues. This causes air to rush out from the top of the air column which in turn forces
convergence in the low-level wind field and increased upward motion within the column. The increased upward motion causes surface pressures to lower as the upward air motion counteracts gravity, lessening the weight of the atmosphere (atmospheric pressure) in that location, and thus strengthening the
cyclone. As the cyclone strengthens, the cold front sweeps towards the
equator and moves around the back of the cyclone. Meanwhile, its associated
warm front progresses more slowly, as the cooler air ahead of the system is density, and therefore more difficult to dislodge. Later, the cyclones
occlusion as the poleward portion of the cold front overtakes a section of the warm front, forcing a tongue, or trowal, of warm air aloft. Eventually, the cyclone will become barotropically cold and begin to weaken.
A rapidly-falling
atmospheric pressure is possible due to strong upper level forces on the system, and when pressures fall more than 1 millibar (0.029 inHg) per hour, such a cyclone is sometimes referred to as a bomb.{{cite web | title = Bomb cyclones ravage northwestern Atlantic
| author = Jack Williams
| publisher =
http://www.USATODAY.com USA Today
| date = [2005-05-20
| url = http://www.usatoday.com/weather/tg/wnoreast/wbombs.htm
| accessdate = 2006-10-04 -->
{{cite web | title = American Meteorological Society Glossary - Bomb
| publisher =
http://www.allenpress.com Allen Press Inc.
| date = 2000-06
| url = http://amsglossary.allenpress.com/glossary/search?id=bomb1
| accessdate = 2006-10-03 --> The lowest pressure measured from an extratropical cyclone in the United States was 951.7 millibars (28.10 inHg) on [March 1 [ in
Bridgehampton, New York. Between [January 4 and [January 5 [, an extratropical cyclone south of Atlantic Canada deepened to 928 millibars (27.40 inHg), equivalent to a [Saffir-Simpson Hurricane Scale.{{cite web
| title = Flying into a record Nor'easter
| author = JeffMasters
| publisher =
http://www.wunderground.com/blog/JeffMasters/ JeffMasters' Blog on http://www.wunderground.com Wunderground.Com
| date = [2006-02-15
| url = http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=304&tstamp=200602
| accessdate = 2006-10-04 --> In the [Arctic Ocean, the average pressure for cyclones is 988 millibars (29.18 inHg) during the winter, and 1,000 millibars (29.53 inHg) during the summer.{{cite web
| title = A cyclone statistics for the Arctic based on European Centre re-analysis data (Abstract)
| Author = BRÜMMER B.; THIEMANN S.; KIRCHGÄSSNER A.;
| publisher = Springer, Wien, AUTRICHE (1986)
| url = http://cat.inist.fr/?aModele=afficheN&cpsidt=861335
| accessdate = 2006-10-04 -->
Extratropical transition
Tropical cyclones often transform into extratropical cyclones at the end of their tropical existence, usually between 30° and 40° latitude, where there is sufficient
Earth's atmosphere (Meteorology) from upper-level troughs or shortwaves riding the Westerlies for the process of extratropical transition to begin. During extratropical transition, the cyclone begins to tilt back into the colder airmass with height, and the cyclone's primary energy source converts from the release of latent heat of condensation (from thunderstorms near the center) to baroclinic processes. The low pressure system eventually loses its warm core and becomes a cold-core system. During this process, a cyclone in extratropical transition (known in
Canada as the post-tropical stage){{cite web | title = Glossary of Hurricane Terms
| publisher = http://www.atl.ec.gc.ca/weather/hurricane/index_e.html Canadian Hurricane Center
| date = [2003-07-10
| url = http://www.atl.ec.gc.ca/weather/hurricane/hurricanes9.html
| accessdate = 2006-10-04 --> will invariably form or connect with nearby fronts and/or troughs consistent with a baroclinic system. Due to this, the size of the system will usually appear to increase, while the core weakens. However, after transition is complete, the storm may re-strengthen due to baroclinic energy, depending on the environmental conditions surrounding the system. The cyclone will also distort in shape, becoming less symmetric with time.
On rare occasions, an extratropical cyclone can transit into a tropical cyclone if it reaches an area of ocean with warmer waters and an environment with less vertical wind shear. The peak time of Subtropical cyclone cyclogenesis (the midpoint of this transition) is in the months of September and October, when the difference between the temperature of the air aloft and the
sea surface temperature is the greatest, leading to the greatest potential for instability.{{cite web | title = A Fifty year History of Subtropical Cyclones
| author = David M. Roth
| publisher = http://www.hpc.ncep.noaa.gov/ Hydrometeorological Prediction Center
| date = [2002-02-15
| url = http://www.hpc.ncep.noaa.gov/research/roth/Subpreprint.pdf
| accessdate = 2006-10-04 --> The process known as "tropical transition" involves the usually slow development of an extratropically cold core vortex into a tropical cyclone. {{cite web
| title = Cyclogenesis and Tropical Transition in decaying frontal zones
| author = Michelle L. Stewart, COAPS, Tallahassee, FL; and M. A. Bourassa
| publisher =
http://ams.confex.com/ American Meteorological Society Conference
| date = [2006-04-25
| url = http://ams.confex.com/ams/27Hurricanes/techprogram/paper_108880.htm
| accessdate = 2006-10-24 --> {{cite web
| title = The TT Problem - Forecasting the Tropical Transition of Cyclones
| author = Christopher A. Davis; Lance F. Bosart
| publisher =
http://ams.allenpress.com/ American Meteorological Society Journals Online
| date = 2004-11
| url = http://ams.allenpress.com/archive/1520-0477/85/11/pdf/i1520-0477-85-11-1657.pdf
| format = PDF
| accessdate = 2006-10-24 -->
Structure
Surface pressure/Wind distribution
image of typical extratropical cyclones over the ocean. Note the maximum winds are on the outside of the occlusion.The windfield of an extratropical cyclone constricts with distance in relation to surface level pressure, with the lowest pressure being found near the center, and the highest winds typically just on the cold/poleward side of warm fronts, occlusions, and cold fronts, where the
pressure gradient force is highest.{{cite web | title = WW2010 - Pressure Gradient Force
| publisher =
http://ww2010.atmos.uiuc.edu/(Gh)/abt/home.rxml University of Illinois
| date = [1999-09-02
| url = http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fw/pgf.rxml
| accessdate = 2006-10-11 --> The area north and west of the cold and warm fronts connected to extratropical cyclones is known as the cold sector, while the area south and east of its associated cold and warm fronts is known as the warm sector.
The wind flow around a large cyclone is clockwise and counterclockwise in the northern hemisphere, and clockwise in the southern hemisphere, due to the
Coriolis effect (this manner of rotation is generally referred to as
cyclonic). Near this center, the pressure gradient force (from the pressure at the center of the cyclone compared to the pressure outside the cyclone) and the Coriolis force must be in an approximate balance for the cyclone to avoid collapsing in on itself as a result of the difference in pressure. The central pressure of the cyclone will lower with increasing maturity, while outside of the cyclone, the sea-level pressure is not very low; its typical value is around 1,013 millibars (29.92 inHg), which is the average sea level pressure for Earth. In most extratropical cyclones, the part of the cold front ahead of the cyclone will develop into a warm front, giving the frontal zone (as drawn on
Surface weather analysis) a wave-like shape. Due to their appearance on satellite images, extratropical cyclones can also be referred to as frontal waves early in their life cycle. In the
United States, an old name for such a system is "warm wave".{{cite web | title = The Atmosphere in motion: Pressure & mass
| publisher = http://www.sbs.ohio-state.edu/ Ohio State University
| date = [2006-04-26
| url = http://geog-www.sbs.ohio-state.edu/courses/G520/bmark/Lecture%2013-Chp9,%20Atmosphere%20in%20motion-P&mass.pdf#search=%22average%20surface%20atmosphere%20pressure%20distribution%22
| accessdate = 2006-10-04 -->
Once a cyclone occludes, a
trough
of
warm air
aloft, or "trowal" for short, will be caused by strong southerly winds on its eastern periphery rotating aloft around its northeast, and ultimately northwestern, periphery (also known as the warm conveyor belt), forcing a surface trough to continue into the cold sector on a similar curve to the occluded front. The trowal creates the portion of an occluded cyclone known as its
comma head, due to the comma (punctuation)-like shape of the mid-tropospheric cloudiness that accompanies the feature. It can also be the focus of locally heavy precipitation, with thunderstorms possible if the atmosphere along the trowal is unstable enough for convection.{{cite web | title = What is a TROWAL?
| publisher = St. Louis University
| date = Unknown
| url = http://www.eas.slu.edu/CIPS/Presentations/Conferences/NWA2002/Snow_NWA_02/tsld003.htm
| accessdate = 2006-11-02 -->
Vertical structure
Extratropical cyclones slant back into colder air masses and strengthen with height, sometimes exceeding 30,000 feet (approximately 9 kilometre) in depth.{{cite web | title = Mid-Latitude Cyclones: Vertical Structure
| author = Andrea Lang
| publisher =
http://www.aos.wisc.edu/ University of Wisconsin-Madison Department of Atmospheric and Oceanic Sciences
| date = [2006-04-20
| url = http://www.aos.wisc.edu/~aalopez/aos101/wk14.html
| accessdate = 2006-10-03 -->
Above the surface of the earth, the air temperature near the center of the cyclone is increasingly colder than the surrounding environment. These characteristics are the direct opposite of those found in their tropical cyclones; thus, they are sometimes called "cold-core lows".{{cite web | title = Cyclone Phase Analysis and Forecast: Help Page
| author = Robert Hart
| publisher = http://moe.met.fsu.edu Florida State University Department of Meteorology
| date = [2003-02-18
| url = http://moe.met.fsu.edu/cyclonephase/help.html
| accessdate = 2006-10-03 --> Various charts can be examined to check the characteristics of a cold-core system with height, such as the 700 millibars (20.67 inHg) chart, which is at about 10,000 feet or 3,000 meters in height. [Cyclone phase diagrams are used to tell whether a cyclone is tropical, subtropical, or extratropical. {{cite web
| title = Cyclone phase evolution: Analyses & Forecasts
| author = Robert Hart
| publisher =
http://moe.met.fsu.edu Florida State University Department of Meteorology
| date = [2006-10-04
| url = http://moe.met.fsu.edu/cyclonephase/
| accessdate = 2006-10-03 -->
Cyclone evolution
There are two models of cyclone development and lifecycles in common use - the Norwegian model and the Shapiro-Keyser Model.{{cite web | title = Unified Surface Analysis Manual
| author = David M. Roth
| publisher = http://www.hpc.ncep.noaa.gov/ Hydrometeorological Prediction Center (NOAA)
| date = [2005-12-15
| url = http://www.hpc.ncep.noaa.gov/sfc/UASfcManualVersion1.pdf
| accessdate = 2006-10-11 -->
Norwegian cyclone model
Of the two theories on extratropical cyclone structure and life cycle, the oldest is the Norwegian Cyclone Model, developed during
World War I. In this theory, cyclones develop as they move up and along a frontal boundary, eventually occlusion and reaching a barotropically cold environment.{{cite web | title = The Norwegian Cyclone Model
| author = Shaye Johnson
| publisher =
http://weather.ou.edu/ University of Oklahoma, School of Meteorology
| date = [2001-09-25
| url = http://weather.ou.edu/~metr4424/Files/Norwegian_Cyclone_Model.pdf#search=%22norwegian%20cyclone%20model%22
| accessdate = 2006-10-11 --> It was developed completely from surface-based weather observations, including descriptions of clouds found near frontal boundaries. This theory still retains merit, as it is a good description for extratropical cyclones over continental landmasses.
Shapiro-Keyser model
A second competing theory for extratropical cyclone development over the oceans is the Shapiro-Keyser model, developed in 1990. {{cite web | title = Determining Midlatitude Cyclone Structure and Evolution from the Upper-Level Flow
| author = David M. Schultz and Heini Werli
| publisher =
http://www.cimms.ou.edu/ Cooperative Institute for Mesoscale Meteorological Studies
| date = [2001-01-05
| url = http://www.cimms.ou.edu/~schultz/papers/marwealog.html
| accessdate = 2006-10-09 --> Its main differences with the Norwegian Cyclone Model are the fracture of the cold front, treating warm-type occlusions and warm fronts as the same, and allowing the cold front to progress through the warm sector [perpendicular to the warm front. This model was based on oceanic cyclones and their frontal structure, as seen in surface observations and in previous projects which used [Fixed-wing aircraft to determine the vertical structure of fronts across the northwest Atlantic.
Warm seclusion
A warm seclusion is the mature phase of the extratropical cyclone lifecycle. This was conceptualized after the ERICA (Meteorology) field experiment of the late 1980s, which produced observations of intense marine cyclones that indicated an anomalously warm low-level thermal structure, secluded (or surrounded) by a bent-back warm front and a coincident chevron-shaped band of intense surface winds. {{cite web | title = Warm seclusion cyclone climatology
| author = Ryan N. Maue
| publisher =
http://ams.confex.com/ American Meteorological Society Conference
| date = [2006-04-25
| url = http://ams.confex.com/ams/27Hurricanes/techprogram/paper_108776.htm
| accessdate = 2006-10-06 --> The Norwegian Cyclone Model, as developed by the Bergen School of Meteorology, largely observed cyclones at the tail end of their lifecycle and used the term occlusion to identify the decaying stages. {{cite web
| title = Wind Throw Damages on Forests - Frequency and Associated Pressure Patterns 1961–1990 and in a Future Climate Scenario
| author = Lars Bärring
| publisher =
http://www.natgeo.lu.se/English/homeines.asp Department of Geography and Ecosystems Analysis, Lund University
| date = [2003-12-11
| url = http://www.natgeo.lu.se/ex-jobb/exj_97.pdf#search=occlusion
| accessdate = 2006-10-30 -->
Warm seclusions may have cloud-free, eye (cyclone)-like features at their center (reminiscent of tropical cyclones), significant pressure falls, hurricane force winds, and moderate to strong convection. The most intense warm seclusions often attain pressures less than 950 millibars (28.05 inHg) with a definitive lower to mid-level warm core structure.Ref A warm seclusion, the result of a baroclinic lifecycle, occurs at latitudes well poleward of the tropics.
As latent heat
flux releases are important for their development and intensification, most warm seclusion events occur over the
oceans; they may impact coastal nations with hurricane force winds and torrential
rain.Ref {{cite web | title = Blizzicanes
| author = Jeff Masters
| publisher = http://www.wunderground.com/blog/JeffMasters/ JeffMasters' Blog on
http://www.wunderground.com Wunderground.Com
| date = [2006-02-14
| url = http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=303&tstamp=200602
| accessdate = 2006-11-01 --> Climatologically, the Northern Hemisphere sees warm seclusions during the cold season months, while the Southern Hemisphere may see a strong cyclone event such as this during all times of the year.
In all tropical basins, except the Northern Indian Ocean, the extratropical transition of a tropical cyclone may result in reintensification into a warm seclusion. For example, Hurricane Maria (2005) of 2005 reintensified into a strong baroclinic system and achieved warm seclusion status at maturity (or lowest pressure).{{cite web | title = Tropical Cyclone Report - Hurricane Maria
| author = Richard J. Pasch; Eric S. Blake
| publisher = http://www.nhc.noaa.gov National Hurricane Center (NOAA)
| date = [2006-02-08
| url = http://www.nhc.noaa.gov/pdf/TCR-AL142005_Maria.pdf
| accessdate = 2006-10-30 -->
Motion
, 2007 radar image of a large Extratropical cyclonic storm system at its peak over the central United States (3 MB)Extratropical cyclones are generally driven, or "steered", by deep westerly winds in a general west to east motion across both the Northern and Southern hemispheres of the Earth. This general motion of atmospheric flow is known as "zonal". {{cite web]
| date = 2000-06
| url = http://amsglossary.allenpress.com/glossary/search?id=zonal-flow1
| accessdate = 2006-10-03 --> Where this general trend is the main steering influence of an extratropical cyclone, it is known as a "[zonal flow".
When the general flow pattern buckles from a zonal pattern to the meridional pattern,{{cite web | title = American Meteorological Society Glossary - Meridional Flow
| publisher =
http://www.allenpress.com Allen Press Inc.
| date = 2000-06
| url = http://amsglossary.allenpress.com/glossary/search?id=meridional-flow1
| accessdate = 2006-10-03 --> a slower movement in a north or southward direction is more likely. [Meridional flow patterns feature strong, amplified troughs and ridges, generally with more northerly and southerly flow.
Changes in direction of this nature are most commonly observed as a result of a cylone's interaction with other low pressure systems, trough (meteorology),
ridge (meteorology), or with anticyclones. A strong and stationary anticyclone can effectively block the path of an extratropical cyclone. Such blocking anticyclone are quite normal, and will generally result in a weakening of the cyclone, the weakening of the anticyclone, a diversion of the cyclone towards the anticyclones periphery, or a combination of all three to some extent depending on the precise conditions. It is also common for an extratropical cyclone to strengthen as the blocking anticyclone or ridge weakens in these circumstances. {{cite web | title = The Interactions between a Midlatitude Blocking Anticyclone and Synoptic-Scale Cyclones That Occurred during the Summer Season
| author = Anthony R. Lupo; Phillip J. Smith
| publisher = http://www.purdue.edu/eas/ Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana
| date = [1997-05-02
| url = http://ams.allenpress.com/perlserv/?request=get-document&doi=10.1175%2F1520-0493(1998)126%3C0502:TIBAMB%3E2.0.CO%3B2#S4
| accessdate = 2006-10-21 -->
Where an extratropical cyclone encounters another extratropical cyclone (or almost any other kind of cyclonic vortex in the atmosphere), the two may combine to become a "Binary cyclone", where the vorticies of the two cyclones rotate around each other (known as the "Fujiwhara effect"). This most often results in a merging of the two low pressure systems into a single extratropical cyclone, or can less commonly result in a mere change of direction of either one or both of the cyclones. {{cite web | title = Theoretical and Applied Climatology - Rotation of mid-latitude binary cyclones: a potential vorticity approach
| author = B. Ziv; P. Alpert
| publisher = Springer Wien (ISSN 0177-798X (Print) 1434-4483 (Online))
| date = [2003-11-20
| url = http://www.springerlink.com/content/nhlkqm3ckujgm106/
| accessdate = 2006-10-21 --> The precise results of such interactions depend on factors such as the size of the two cyclones, their strength, their distance from each other, and the prevailing atmospheric conditions around them.
Effects
General
Extratropical cyclones can bring mild weather with a little rain and surface winds of 15–30 kilometres per hour (10–20
miles per hour), or they can be cold and dangerous with torrential rain and winds exceeding 119 km/h (74 mph),{{cite web | title = Mariners Weather Log, Vol 49, No. 1
| author = Joan Von Ahn; Joe Sienkiewicz; Greggory McFadden;
| publisher = http://www.vos.noaa.gov/ Voluntary Observing Ship Program
| date = 2005-04
| url = http://www.vos.noaa.gov/MWL/april_05/cyclones.shtml
| accessdate = 2006-10-04 --> (sometimes referred to as [European windstorm in Europe). The band of [precipitation (meteorology) that is associated with the [warm front is often extensive. In mature extratropical cyclones, an area known as the '''comma head''' on the northwest periphery of the surface low can be a region of heavy precipitation, frequent [thunderstorms, and [thundersnows. Cyclones tend to move along a predictable path at a moderate rate of progress. During [autumn, winter, and spring, the atmosphere over continents can be cold enough through the depth of the [troposphere to cause snowfall.
Severe weather
Squall lines, or solid bands of strong thunderstorms, can form ahead of cold fronts and Trough (meteorology)#Lee troughs due to the presence of significant atmospheric moisture and strong upper level divergence, leading to hail and high winds. {{cite web | title = WW2010 - Squall Lines
| publisher = http://ww2010.atmos.uiuc.edu/(Gh)/abt/home.rxml University of Illinois
| date = [1999-09-02
| url = http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/svr/type/mline/home.rxml
| accessdate = 2006-10-21 --> When significant directional wind shear exists in the atmosphere ahead of a cold front in the presence of a strong upper level jet stream, tornado formation is possible. {{cite web
| title = Tornadoes: Nature's Most Violent Storms
| publisher = http://www.nssl.noaa.gov/ National Severe Storms Laboratory (NOAA)
| date = [2002-03-13
| url = http://www.nssl.noaa.gov/NWSTornado/
| accessdate = 2006-10-21 --> Although tornadoes can form anywhere on Earth, the greatest number occur in the [Great Plains in the United States, because downsloped winds off the north-south oriented [Rocky Mountains, which can form a dryline, aid their development at any [Fujita scale.
Explosive development of extratropical cyclones can be sudden. The storm known in the UK as the "Great Storm of 1987" deepened to 953 millibars (28.14 inHg) with a highest recorded wind of 220 km/h (137 mph), resulting in the loss of 19 lives, 15 million trees, widespread damage to homes and an estimated economic cost of
pound sterling1.2 billion (
United States dollar2.3 billion). {{cite web | title = The Great Storm of 1987
| publisher =
http://www.metoffice.gov.uk Met Office
| url = http://www.meto.gov.uk/education/secondary/students/1987.html
| accessdate = 2006-10-30 --> {{cite web
| title = What we should fear in 2006
| author = Bill McGuire
| publisher = http://www.channel4.com/news/ Channel 4 News
| date = [2005-12-22
| url = http://www.channel4.com/news/special-reports/special-reports-storypage.jsp?id=1409¶sStartAt=2
| accessdate = 2006-10-30 -->
Although most tropical cyclones that become extratropical quickly dissipate or are absorbed by another weather system, they can still retain winds of hurricane or gale force. In 1954, Hurricane Hazel became extratropical over North Carolina as a strong Category 3 storm. The Columbus Day Storm of 1962, which evolved from the remains of Typhoon Freda, caused heavy damage in Oregon and
Washington, with widespread damage equivalent to at least a Category 3. In 2005, Hurricane Wilma began to lose tropical characteristics while still sporting Category 3-force winds (and became fully extratropical as a Category 1 storm).{{cite web | title = Tropical Cyclone Report - Hurricane Wilma
| author = Richard J. Pasch; Eric S. Blake;, Hugh D. Cobb III; and David P Roberts
| publisher = http://www.nhc.noaa.gov National Hurricane Center (NOAA)
| date = [2006-01-12
| url = http://www.nhc.noaa.gov/pdf/TCR-AL252005_Wilma.pdf
| accessdate = 2006-10-11 -->
Historic storms
A violent storm during the
Crimean War on
November 14 1854 wrecked 30 vessels, and sparked initial investigations into meteorology and forecasting in Europe. In the United States, the
Columbus Day Storm of 1962 led to
Oregon's lowest measured pressure of 965.5 pascal (unit) (28.51 inHg), violent winds, and US$170 million in damage (1964 dollars).George Taylor and Raymond R. Hatton. The 1962 Windstorm. Retrieved on 2006-November 25. A rapidly strengthening storm struck Vancouver Island on
October 11, 1984, and inspired the development of moored buoys off the western coast of
Canada.S. G. P. Skey and M. D. Miles. Advances in Buoy Technology for Wind/Wave Data Collection and Analysis. Retrieved on 2006-
November 25. The Wahine disaster, New Zealand's most infamous maritime disaster, occurred during a major extratropical storm.
See also
References
s indicate the direction of the wind, while the "L" symbol denotes the center of the "low". Note the occluded, cold and warm
Surface weather analysis.
Extratropical cyclones, sometimes called
mid-latitude cyclones, are a group of cyclones defined as
Synoptic scale meteorology Low pressure area weather systems that occur in the middle latitudes of the Earth having neither
tropical cyclone nor
polar cyclone characteristics, and are connected with Surface weather analysis and horizontal gradients in temperature and
dew point otherwise known as "baroclinic zones". Extratropical cyclones are the everyday phenomena which, along with
anticyclones, drive the weather over much of the Earth, producing anything from cloudiness and mild
rains to heavy gales and thunderstorms.
Terminology
Extratropical cyclones encompass a class of storms with many names. Although they are sometimes referred to as "
cyclones", this is imprecise;
cyclone applies to numerous types of low pressure areas. The descriptor
extratropical signifies that this type of cyclone generally occurs outside the tropics in the middle latitudes of Earth. The term
mid-latitude cyclones may be used because of where they form; "post-tropical cyclones" if
#Extratropical transition has occurred.ExtraLessonMillUni Weather forecasters and the general public often describe them as "depressions" or "lows".
Extratropical cyclones are classified mainly as baroclinity, because they form along zones of temperature and dewpoint gradient known as
weather fronts. They can become barotropic late in their life cycle, when the distribution of heat around the cyclone becomes fairly uniform with its radius.
Formation
Extratropical cyclones form anywhere within the extratropical regions of the Earth (usually between 30° and 60° latitude from the equator), either through cyclogenesis or extratropical transition. A study of extratropical cyclones in the
Southern Hemisphere shows that between the 30th and 70th parallels, there are an average of 37 cyclones in existence during any 6-hour period. {{cite web | title = Variability of Southern Hemisphere Extratropical Cyclone Behavior, 1958–97
| author = Ian Simmonds and Kevin Keay
| publisher = http://ams.allenpress.com/ American Meteorology Society (Allenpress Inc)
| date = 2000-02
| url = http://ams.allenpress.com/perlserv/?request=get-document&doi=10.1175%2F1520-0442(2000)013%3C0550:VOSHEC%3E2.0.CO%3B2#I1520-0442-13-3-550-F04
| accessdate = 2006-10-20 --> A separate study in the [Northern Hemisphere suggests that approximately 234 significant extratropical cyclones form each winter. {{cite web
| title = Winter Storms in the Northern Hemisphere (1958–1999)
| author = S.K. Gulev, O. Zolina, and S. Grigoriev
| publisher =
http://www.co2science.org/ CO2 Science
| date = 2001
| url = http://www.co2science.org/scripts/CO2ScienceB2C/articles/V4/N37/C2.jsp
| accessdate = 2006-10-20 -->
Cyclogenesis
Extratropical cyclones form along linear bands of temperature/dewpoint gradient with significant vertical
wind shear, and are thus classified as
baroclinic cyclones. Initially,
cyclogenesis, or low pressure formation, occurs along
Surface weather analysis near a favorable quadrant of the
Jetstream, usually being the right rear and left front quadrants, where divergence ensues. This causes air to rush out from the top of the air column which in turn forces
convergence in the low-level wind field and increased upward motion within the column. The increased upward motion causes surface pressures to lower as the upward air motion counteracts gravity, lessening the weight of the atmosphere (
atmospheric pressure) in that location, and thus strengthening the
cyclone. As the cyclone strengthens, the cold front sweeps towards the equator and moves around the back of the cyclone. Meanwhile, its associated
warm front progresses more slowly, as the cooler air ahead of the system is
density, and therefore more difficult to dislodge. Later, the cyclones
occlusion as the poleward portion of the cold front overtakes a section of the warm front, forcing a tongue, or trowal, of warm air aloft. Eventually, the cyclone will become barotropically cold and begin to weaken.
A rapidly-falling atmospheric pressure is possible due to strong upper level forces on the system, and when pressures fall more than 1 millibar (0.029 inHg) per hour, such a cyclone is sometimes referred to as a bomb.{{cite web | title = Bomb cyclones ravage northwestern Atlantic
| author = Jack Williams
| publisher =
http://www.USATODAY.com USA Today
| date = [2005-05-20
| url = http://www.usatoday.com/weather/tg/wnoreast/wbombs.htm
| accessdate = 2006-10-04 -->
{{cite web | title = American Meteorological Society Glossary - Bomb
| publisher = http://www.allenpress.com Allen Press Inc.
| date = 2000-06
| url = http://amsglossary.allenpress.com/glossary/search?id=bomb1
| accessdate = 2006-10-03 --> The lowest pressure measured from an extratropical cyclone in the United States was 951.7 millibars (28.10 inHg) on [March 1 [ in
Bridgehampton, New York. Between [January 4 and [January 5 [, an extratropical cyclone south of Atlantic Canada deepened to 928 millibars (27.40 inHg), equivalent to a [Saffir-Simpson Hurricane Scale.{{cite web
| title = Flying into a record Nor'easter
| author = JeffMasters
| publisher = http://www.wunderground.com/blog/JeffMasters/ JeffMasters' Blog on
http://www.wunderground.com Wunderground.Com
| date = [2006-02-15
| url = http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=304&tstamp=200602
| accessdate = 2006-10-04 --> In the [Arctic Ocean, the average pressure for cyclones is 988 millibars (29.18 inHg) during the winter, and 1,000 millibars (29.53 inHg) during the summer.{{cite web
| title = A cyclone statistics for the Arctic based on European Centre re-analysis data (Abstract)
| Author = BRÜMMER B.; THIEMANN S.; KIRCHGÄSSNER A.;
| publisher = Springer, Wien, AUTRICHE (1986)
| url = http://cat.inist.fr/?aModele=afficheN&cpsidt=861335
| accessdate = 2006-10-04 -->
Extratropical transition
Tropical cyclones often transform into extratropical cyclones at the end of their tropical existence, usually between 30° and 40° latitude, where there is sufficient
Earth's atmosphere (Meteorology) from upper-level troughs or shortwaves riding the
Westerlies for the process of extratropical transition to begin. During extratropical transition, the cyclone begins to tilt back into the colder airmass with height, and the cyclone's primary energy source converts from the release of latent heat of condensation (from thunderstorms near the center) to baroclinic processes. The low pressure system eventually loses its warm core and becomes a cold-core system. During this process, a cyclone in extratropical transition (known in
Canada as the post-tropical stage){{cite web | title = Glossary of Hurricane Terms
| publisher =
http://www.atl.ec.gc.ca/weather/hurricane/index_e.html Canadian Hurricane Center
| date = [2003-07-10
| url = http://www.atl.ec.gc.ca/weather/hurricane/hurricanes9.html
| accessdate = 2006-10-04 --> will invariably form or connect with nearby fronts and/or troughs consistent with a baroclinic system. Due to this, the size of the system will usually appear to increase, while the core weakens. However, after transition is complete, the storm may re-strengthen due to baroclinic energy, depending on the environmental conditions surrounding the system. The cyclone will also distort in shape, becoming less symmetric with time.
On rare occasions, an extratropical cyclone can transit into a tropical cyclone if it reaches an area of ocean with warmer waters and an environment with less vertical wind shear. The peak time of
Subtropical cyclone cyclogenesis (the midpoint of this transition) is in the months of September and October, when the difference between the temperature of the air aloft and the sea surface temperature is the greatest, leading to the greatest potential for instability.{{cite web | title = A Fifty year History of Subtropical Cyclones
| author = David M. Roth
| publisher =
http://www.hpc.ncep.noaa.gov/ Hydrometeorological Prediction Center
| date = [2002-02-15
| url = http://www.hpc.ncep.noaa.gov/research/roth/Subpreprint.pdf
| accessdate = 2006-10-04 --> The process known as "tropical transition" involves the usually slow development of an extratropically cold core vortex into a tropical cyclone. {{cite web
| title = Cyclogenesis and Tropical Transition in decaying frontal zones
| author = Michelle L. Stewart, COAPS, Tallahassee, FL; and M. A. Bourassa
| publisher =
http://ams.confex.com/ American Meteorological Society Conference
| date = [2006-04-25
| url = http://ams.confex.com/ams/27Hurricanes/techprogram/paper_108880.htm
| accessdate = 2006-10-24 --> {{cite web
| title = The TT Problem - Forecasting the Tropical Transition of Cyclones
| author = Christopher A. Davis; Lance F. Bosart
| publisher = http://ams.allenpress.com/ American Meteorological Society Journals Online
| date = 2004-11
| url = http://ams.allenpress.com/archive/1520-0477/85/11/pdf/i1520-0477-85-11-1657.pdf
| format = PDF
| accessdate = 2006-10-24 -->
Structure
Surface pressure/Wind distribution
image of typical extratropical cyclones over the ocean. Note the maximum winds are on the outside of the occlusion.The windfield of an extratropical cyclone constricts with distance in relation to surface level pressure, with the lowest pressure being found near the center, and the highest winds typically just on the cold/poleward side of warm fronts, occlusions, and
cold fronts, where the pressure gradient force is highest.{{cite web | title = WW2010 - Pressure Gradient Force
| publisher =
http://ww2010.atmos.uiuc.edu/(Gh)/abt/home.rxml University of Illinois
| date = [1999-09-02
| url = http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fw/pgf.rxml
| accessdate = 2006-10-11 --> The area north and west of the cold and warm fronts connected to extratropical cyclones is known as the cold sector, while the area south and east of its associated cold and warm fronts is known as the warm sector.
The wind flow around a large cyclone is clockwise and counterclockwise in the northern hemisphere, and clockwise in the southern hemisphere, due to the
Coriolis effect (this manner of rotation is generally referred to as
cyclonic). Near this center, the pressure gradient force (from the pressure at the center of the cyclone compared to the pressure outside the cyclone) and the Coriolis force must be in an approximate balance for the cyclone to avoid collapsing in on itself as a result of the difference in pressure. The central pressure of the cyclone will lower with increasing maturity, while outside of the cyclone, the
sea-level pressure is not very low; its typical value is around 1,013 millibars (29.92 inHg), which is the average sea level pressure for Earth. In most extratropical cyclones, the part of the cold front ahead of the cyclone will develop into a warm front, giving the frontal zone (as drawn on
Surface weather analysis) a wave-like shape. Due to their appearance on satellite images, extratropical cyclones can also be referred to as frontal waves early in their life cycle. In the
United States, an old name for such a system is "warm wave".{{cite web | title = The Atmosphere in motion: Pressure & mass
| publisher =
http://www.sbs.ohio-state.edu/ Ohio State University
| date = [2006-04-26
| url = http://geog-www.sbs.ohio-state.edu/courses/G520/bmark/Lecture%2013-Chp9,%20Atmosphere%20in%20motion-P&mass.pdf#search=%22average%20surface%20atmosphere%20pressure%20distribution%22
| accessdate = 2006-10-04 -->
Once a cyclone occludes, a
trough
of
warm air
aloft, or "trowal" for short, will be caused by strong southerly winds on its eastern periphery rotating aloft around its northeast, and ultimately northwestern, periphery (also known as the warm conveyor belt), forcing a surface trough to continue into the cold sector on a similar curve to the occluded front. The trowal creates the portion of an occluded cyclone known as its
comma head, due to the comma (punctuation)-like shape of the mid-tropospheric cloudiness that accompanies the feature. It can also be the focus of locally heavy precipitation, with thunderstorms possible if the atmosphere along the trowal is unstable enough for convection.{{cite web | title = What is a TROWAL?
| publisher =
St. Louis University
| date = Unknown
| url = http://www.eas.slu.edu/CIPS/Presentations/Conferences/NWA2002/Snow_NWA_02/tsld003.htm
| accessdate = 2006-11-02 -->
Vertical structure
Extratropical cyclones slant back into colder air masses and strengthen with height, sometimes exceeding 30,000 feet (approximately 9 kilometre) in depth.{{cite web | title = Mid-Latitude Cyclones: Vertical Structure
| author = Andrea Lang
| publisher =
http://www.aos.wisc.edu/ University of Wisconsin-Madison Department of Atmospheric and Oceanic Sciences
| date = [2006-04-20
| url = http://www.aos.wisc.edu/~aalopez/aos101/wk14.html
| accessdate = 2006-10-03 -->
Above the surface of the earth, the air temperature near the center of the cyclone is increasingly colder than the surrounding environment. These characteristics are the direct opposite of those found in their
tropical cyclones; thus, they are sometimes called "cold-core lows".{{cite web | title = Cyclone Phase Analysis and Forecast: Help Page
| author = Robert Hart
| publisher = http://moe.met.fsu.edu Florida State University Department of Meteorology
| date = [2003-02-18
| url = http://moe.met.fsu.edu/cyclonephase/help.html
| accessdate = 2006-10-03 --> Various charts can be examined to check the characteristics of a cold-core system with height, such as the 700 millibars (20.67 inHg) chart, which is at about 10,000 feet or 3,000 meters in height. [Cyclone phase diagrams are used to tell whether a cyclone is tropical, subtropical, or extratropical. {{cite web
| title = Cyclone phase evolution: Analyses & Forecasts
| author = Robert Hart
| publisher =
http://moe.met.fsu.edu Florida State University Department of Meteorology
| date = [2006-10-04
| url = http://moe.met.fsu.edu/cyclonephase/
| accessdate = 2006-10-03 -->
Cyclone evolution
There are two models of cyclone development and lifecycles in common use - the Norwegian model and the Shapiro-Keyser Model.{{cite web | title = Unified Surface Analysis Manual
| author = David M. Roth
| publisher = http://www.hpc.ncep.noaa.gov/ Hydrometeorological Prediction Center (NOAA)
| date = [2005-12-15
| url = http://www.hpc.ncep.noaa.gov/sfc/UASfcManualVersion1.pdf
| accessdate = 2006-10-11 -->
Norwegian cyclone model
Of the two theories on extratropical cyclone structure and life cycle, the oldest is the Norwegian Cyclone Model, developed during
World War I. In this theory, cyclones develop as they move up and along a frontal boundary, eventually occlusion and reaching a barotropically cold environment.{{cite web | title = The Norwegian Cyclone Model
| author = Shaye Johnson
| publisher =
http://weather.ou.edu/ University of Oklahoma, School of Meteorology
| date = [2001-09-25
| url = http://weather.ou.edu/~metr4424/Files/Norwegian_Cyclone_Model.pdf#search=%22norwegian%20cyclone%20model%22
| accessdate = 2006-10-11 --> It was developed completely from surface-based weather observations, including descriptions of clouds found near frontal boundaries. This theory still retains merit, as it is a good description for extratropical cyclones over continental landmasses.
Shapiro-Keyser model
A second competing theory for extratropical cyclone development over the oceans is the Shapiro-Keyser model, developed in 1990. {{cite web | title = Determining Midlatitude Cyclone Structure and Evolution from the Upper-Level Flow
| author = David M. Schultz and Heini Werli
| publisher =
http://www.cimms.ou.edu/ Cooperative Institute for Mesoscale Meteorological Studies
| date = [2001-01-05
| url = http://www.cimms.ou.edu/~schultz/papers/marwealog.html
| accessdate = 2006-10-09 --> Its main differences with the Norwegian Cyclone Model are the fracture of the cold front, treating warm-type occlusions and warm fronts as the same, and allowing the cold front to progress through the warm sector [perpendicular to the warm front. This model was based on oceanic cyclones and their frontal structure, as seen in surface observations and in previous projects which used [Fixed-wing aircraft to determine the vertical structure of fronts across the northwest Atlantic.
Warm seclusion
A warm seclusion is the mature phase of the extratropical cyclone lifecycle. This was conceptualized after the ERICA (Meteorology) field experiment of the late 1980s, which produced observations of intense marine cyclones that indicated an anomalously warm low-level thermal structure, secluded (or surrounded) by a bent-back warm front and a coincident chevron-shaped band of intense surface winds. {{cite web | title = Warm seclusion cyclone climatology
| author = Ryan N. Maue
| publisher =
http://ams.confex.com/ American Meteorological Society Conference
| date = [2006-04-25
| url = http://ams.confex.com/ams/27Hurricanes/techprogram/paper_108776.htm
| accessdate = 2006-10-06 --> The Norwegian Cyclone Model, as developed by the Bergen School of Meteorology, largely observed cyclones at the tail end of their lifecycle and used the term occlusion to identify the decaying stages. {{cite web
| title = Wind Throw Damages on Forests - Frequency and Associated Pressure Patterns 1961–1990 and in a Future Climate Scenario
| author = Lars Bärring
| publisher =
http://www.natgeo.lu.se/English/homeines.asp Department of Geography and Ecosystems Analysis, Lund University
| date = [2003-12-11
| url = http://www.natgeo.lu.se/ex-jobb/exj_97.pdf#search=occlusion
| accessdate = 2006-10-30 -->
Warm seclusions may have cloud-free, eye (cyclone)-like features at their center (reminiscent of
tropical cyclones), significant pressure falls, hurricane force winds, and moderate to strong
convection. The most intense warm seclusions often attain pressures less than 950 millibars (28.05 inHg) with a definitive lower to mid-level warm core structure.Ref A warm seclusion, the result of a baroclinic lifecycle, occurs at latitudes well poleward of the tropics.
As latent heat flux releases are important for their development and intensification, most warm seclusion events occur over the oceans; they may impact coastal nations with hurricane force winds and torrential rain.Ref {{cite web | title = Blizzicanes
| author = Jeff Masters
| publisher = http://www.wunderground.com/blog/JeffMasters/ JeffMasters' Blog on
http://www.wunderground.com Wunderground.Com
| date = [2006-02-14
| url = http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=303&tstamp=200602
| accessdate = 2006-11-01 --> Climatologically, the Northern Hemisphere sees warm seclusions during the cold season months, while the Southern Hemisphere may see a strong cyclone event such as this during all times of the year.
In all tropical basins, except the Northern Indian Ocean, the extratropical transition of a tropical cyclone may result in reintensification into a warm seclusion. For example, Hurricane Maria (2005) of 2005 reintensified into a strong baroclinic system and achieved warm seclusion status at maturity (or lowest pressure).{{cite web | title = Tropical Cyclone Report - Hurricane Maria
| author = Richard J. Pasch; Eric S. Blake
| publisher = http://www.nhc.noaa.gov National Hurricane Center (NOAA)
| date = [2006-02-08
| url = http://www.nhc.noaa.gov/pdf/TCR-AL142005_Maria.pdf
| accessdate = 2006-10-30 -->
Motion
,
2007 radar image of a large Extratropical cyclonic storm system at its peak over the central United States (3 MB)Extratropical cyclones are generally driven, or "steered", by deep westerly winds in a general west to east motion across both the Northern and Southern hemispheres of the Earth. This general motion of atmospheric flow is known as "zonal". {{cite web]
| date = 2000-06
| url = http://amsglossary.allenpress.com/glossary/search?id=zonal-flow1
| accessdate = 2006-10-03 --> Where this general trend is the main steering influence of an extratropical cyclone, it is known as a "[zonal flow".
When the general flow pattern buckles from a zonal pattern to the meridional pattern,{{cite web | title = American Meteorological Society Glossary - Meridional Flow
| publisher = http://www.allenpress.com Allen Press Inc.
| date = 2000-06
| url = http://amsglossary.allenpress.com/glossary/search?id=meridional-flow1
| accessdate = 2006-10-03 --> a slower movement in a north or southward direction is more likely. [Meridional flow patterns feature strong, amplified troughs and ridges, generally with more northerly and southerly flow.
Changes in direction of this nature are most commonly observed as a result of a cylone's interaction with other
low pressure systems,
trough (meteorology),
ridge (meteorology), or with
anticyclones. A strong and stationary anticyclone can effectively block the path of an extratropical cyclone. Such blocking anticyclone are quite normal, and will generally result in a weakening of the cyclone, the weakening of the anticyclone, a diversion of the cyclone towards the anticyclones periphery, or a combination of all three to some extent depending on the precise conditions. It is also common for an extratropical cyclone to strengthen as the blocking anticyclone or ridge weakens in these circumstances. {{cite web | title = The Interactions between a Midlatitude Blocking Anticyclone and Synoptic-Scale Cyclones That Occurred during the Summer Season
| author = Anthony R. Lupo; Phillip J. Smith
| publisher =
http://www.purdue.edu/eas/ Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana
| date = [1997-05-02
| url = http://ams.allenpress.com/perlserv/?request=get-document&doi=10.1175%2F1520-0493(1998)126%3C0502:TIBAMB%3E2.0.CO%3B2#S4
| accessdate = 2006-10-21 -->
Where an extratropical cyclone encounters another extratropical cyclone (or almost any other kind of cyclonic
vortex in the atmosphere), the two may combine to become a "Binary cyclone", where the vorticies of the two cyclones rotate around each other (known as the "Fujiwhara effect"). This most often results in a merging of the two low pressure systems into a single extratropical cyclone, or can less commonly result in a mere change of direction of either one or both of the cyclones. {{cite web | title = Theoretical and Applied Climatology - Rotation of mid-latitude binary cyclones: a potential vorticity approach
| author = B. Ziv; P. Alpert
| publisher = Springer Wien (ISSN 0177-798X (Print) 1434-4483 (Online))
| date = [2003-11-20
| url = http://www.springerlink.com/content/nhlkqm3ckujgm106/
| accessdate = 2006-10-21 --> The precise results of such interactions depend on factors such as the size of the two cyclones, their strength, their distance from each other, and the prevailing atmospheric conditions around them.
Effects
General
Extratropical cyclones can bring mild weather with a little rain and surface winds of 15–30
kilometres per hour (10–20 miles per hour), or they can be cold and dangerous with torrential rain and winds exceeding 119 km/h (74 mph),{{cite web | title = Mariners Weather Log, Vol 49, No. 1
| author = Joan Von Ahn; Joe Sienkiewicz; Greggory McFadden;
| publisher =
http://www.vos.noaa.gov/ Voluntary Observing Ship Program
| date = 2005-04
| url = http://www.vos.noaa.gov/MWL/april_05/cyclones.shtml
| accessdate = 2006-10-04 --> (sometimes referred to as [European windstorm in Europe). The band of [precipitation (meteorology) that is associated with the [warm front is often extensive. In mature extratropical cyclones, an area known as the '''comma head''' on the northwest periphery of the surface low can be a region of heavy precipitation, frequent [thunderstorms, and [thundersnows. Cyclones tend to move along a predictable path at a moderate rate of progress. During [autumn, winter, and spring, the atmosphere over continents can be cold enough through the depth of the [troposphere to cause snowfall.
Severe weather
Squall lines, or solid bands of strong thunderstorms, can form ahead of cold fronts and
Trough (meteorology)#Lee troughs due to the presence of significant atmospheric moisture and strong upper level divergence, leading to hail and high winds. {{cite web | title = WW2010 - Squall Lines
| publisher = http://ww2010.atmos.uiuc.edu/(Gh)/abt/home.rxml University of Illinois
| date = [1999-09-02
| url = http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/svr/type/mline/home.rxml
| accessdate = 2006-10-21 --> When significant directional wind shear exists in the atmosphere ahead of a cold front in the presence of a strong upper level jet stream, tornado formation is possible. {{cite web
| title = Tornadoes: Nature's Most Violent Storms
| publisher = http://www.nssl.noaa.gov/ National Severe Storms Laboratory (NOAA)
| date = [2002-03-13
| url = http://www.nssl.noaa.gov/NWSTornado/
| accessdate = 2006-10-21 --> Although tornadoes can form anywhere on Earth, the greatest number occur in the [Great Plains in the United States, because downsloped winds off the north-south oriented [Rocky Mountains, which can form a dryline, aid their development at any [Fujita scale.
Explosive development of extratropical cyclones can be sudden. The storm known in the UK as the "Great Storm of 1987" deepened to 953 millibars (28.14 inHg) with a highest recorded wind of 220 km/h (137 mph), resulting in the loss of 19 lives, 15 million trees, widespread damage to homes and an estimated economic cost of pound sterling1.2 billion (United States dollar2.3 billion). {{cite web | title = The Great Storm of 1987
| publisher = http://www.metoffice.gov.uk Met Office
| url = http://www.meto.gov.uk/education/secondary/students/1987.html
| accessdate = 2006-10-30 --> {{cite web
| title = What we should fear in 2006
| author = Bill McGuire
| publisher =
http://www.channel4.com/news/ Channel 4 News
| date = [2005-12-22
| url = http://www.channel4.com/news/special-reports/special-reports-storypage.jsp?id=1409¶sStartAt=2
| accessdate = 2006-10-30 -->
Although most tropical cyclones that become extratropical quickly dissipate or are absorbed by another weather system, they can still retain winds of hurricane or gale force. In 1954,
Hurricane Hazel became extratropical over
North Carolina as a strong Category 3 storm. The Columbus Day Storm of 1962, which evolved from the remains of Typhoon Freda, caused heavy damage in
Oregon and
Washington, with widespread damage equivalent to at least a Category 3. In 2005, Hurricane Wilma began to lose tropical characteristics while still sporting Category 3-force winds (and became fully extratropical as a Category 1 storm).{{cite web | title = Tropical Cyclone Report - Hurricane Wilma
| author = Richard J. Pasch; Eric S. Blake;, Hugh D. Cobb III; and David P Roberts
| publisher =
http://www.nhc.noaa.gov National Hurricane Center (NOAA)
| date = [2006-01-12
| url = http://www.nhc.noaa.gov/pdf/TCR-AL252005_Wilma.pdf
| accessdate = 2006-10-11 -->
Historic storms
A violent storm during the
Crimean War on
November 14 1854 wrecked 30 vessels, and sparked initial investigations into meteorology and forecasting in Europe. In the United States, the
Columbus Day Storm of 1962 led to Oregon's lowest measured pressure of 965.5
pascal (unit) (28.51 inHg), violent winds, and US$170 million in damage (1964 dollars).George Taylor and Raymond R. Hatton. The 1962 Windstorm. Retrieved on 2006-
November 25. A rapidly strengthening storm struck
Vancouver Island on
October 11, 1984, and inspired the development of moored buoys off the western coast of Canada.S. G. P. Skey and M. D. Miles. Advances in Buoy Technology for Wind/Wave Data Collection and Analysis. Retrieved on 2006-November 25. The
Wahine disaster, New Zealand's most infamous maritime disaster, occurred during a major extratropical storm.
See also
References
Extratropical cyclone - Wikipedia, the free encyclopedia
Extratropical cyclones, sometimes called mid-latitude cyclones, are a group of cyclones defined as synoptic scale low pressure weather systems that occur in the middle latitudes of ...
Glossary of NHC Terms
Extratropical: A term used in advisories and tropical summaries to indicate that a cyclone has lost its "tropical" characteristics. The term implies both poleward displacement of ...
Extratropical storms are major weather makers
06/27/97 - 09:54 AM ET - Click reload often for latest version. Source: The USA TODAY Weather Book by Jack Williams Extratropical storms are major weather makers
HiGEM home page
Extratropical Cyclones in Climate Models. Extratropical cyclones present some of the most dangerous weather hazards in the extratropics. The extreme winds and precipitation ...
extratropical - Wiktionary
Definition from Wiktionary, a free dictionary
Serial clustering of extratropical cyclones - Reading Research ...
Mailier, P. J. and Stephenson, D. B. and Ferro, C. A. T. and Hodges, K. I. (2006) Serial clustering of extratropical cyclones. Monthly Weather Review, 134 (8). pp. 2224-2240.
PROPAGATION CHARACTERISTICS OF EXTRATROPICAL PLANETARY WAVES OBSERVED ...
PROPAGATION CHARACTERISTICS OF EXTRATROPICAL PLANETARY WAVES OBSERVED IN THE ATSR GLOBAL SEA SURFACE TEMPERATURE RECORD Katherine Hill 1, Ian Robinson 1 and Paolo Cipollini 2 1 ...
IngentaConnect A 700 year record of Southern Hemisphere extratropical ...
Authors: Mayewski, Paul A.; Maasch, Kirk A.; White, James W.C.; Steig, Eric J.; Meyerson, Eric; Goodwin, Ian; Morgan, Vin I.; van Ommen, Tas; Curran, Mark A.J.; Souney, Joseph;
Data @ NASA GISS: Atlas of Extratropical Storm Tracks
Atlas of Extratropical Storm Tracks (1961-1998) Extratropical storms directly affect the environment and economy throughout the mid-latitudes, greatly impacting the lives of ...
e-Prints Soton - CLIVAR Exchanges No. 23. Special issue on: Tropical ...
CLIVAR Exchanges No. 23. Special issue on: Tropical-Extratropical Interactions; Villwock, A. and Gould, J. (eds.) (2002) CLIVAR Exchanges No. 23.
