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Extratropical cyclones, sometimes called mid-latitude cyclones or wave cyclones, are low-pressure areas which, along with the of high-pressure areas, drive the weather over much of the Earth. Extratropical are capable of producing anything from cloudiness and mild rain to severe hail, , , and . These types of cyclones are defined as large scale (synoptic) low pressure that occur in the middle latitudes of the Earth. In contrast with , extratropical cyclones produce rapid changes in temperature and dew point along broad lines, called , about the center of the cyclone.
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.
Atmospheric pressure can fall very rapidly when there are strong upper level forces on the system. When pressures fall more than per hour, the process is called explosive cyclogenesis, and the cyclone can be described as a bomb. These bombs rapidly drop in pressure to below under favorable conditions such as near a natural temperature gradient like the Gulf Stream, or at a preferred quadrant of an upper-level jet streak, where upper level divergence is best. The stronger the upper level divergence over the cyclone, the deeper the cyclone can become. Hurricane-force extratropical cyclones are most likely to form in the northern Atlantic and northern Pacific oceans in the months of December and January. On 14 and 15 December 1986, an extratropical cyclone near Iceland deepened to below , which is a pressure equivalent to a category 5 hurricane. In the Arctic Ocean, the average pressure for cyclones is during the winter, and during the summer.
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 from condensation (from thunderstorms near the center) to Baroclinity processes. The low pressure system eventually loses its warm core and becomes a cold-core low.
The peak time of subtropical cyclogenesis (the midpoint of this transition) in the North Atlantic 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. On rare occasions, an extratropical cyclone can transform into a tropical cyclone if it reaches an area of ocean with warmer waters and an environment with less vertical wind shear. An example of this happening is in the 1991 Perfect Storm. The process known as "tropical transition" involves the usually slow development of an extratropically cold core vortex into a tropical cyclone.
The Joint Typhoon Warning Center uses the extratropical transition (XT) technique to subjectively estimate the intensity of tropical cyclones becoming extratropical based on visible and infrared satellite imagery. Loss of central convection in transitioning tropical cyclones can cause the Dvorak technique to fail; the loss of convection results in unrealistically low estimates using the Dvorak technique. The system combines aspects of the Dvorak technique, used for estimating tropical cyclone intensity, and the Hebert-Poteat technique, used for estimating subtropical cyclone intensity. The technique is applied when a tropical cyclone interacts with a frontal boundary or loses its central convection while maintaining its forward speed or accelerating. The XT scale corresponds to the Dvorak scale and is applied in the same way, except that "XT" is used instead of "T" to indicate that the system is undergoing extratropical transition. Also, the XT technique is only used once extratropical transition begins; the Dvorak technique is still used if the system begins dissipating without transition. Once the cyclone has completed transition and become cold-core low, the technique is no longer used.
The wind flow around an extratropical cyclone is 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 about average. 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 maps) 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".
In the northern hemisphere, 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 into its 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-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.
Warm seclusions may have cloud-free, eye-like features at their center (reminiscent of ), 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. 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 ; they may impact coastal nations with hurricane force and torrential rain. 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) and Hurricane Cristobal (2014) each re-intensified into a strong baroclinic system and achieved warm seclusion status at maturity (or lowest pressure).
When the general flow pattern buckles from a zonal pattern to the meridional pattern, 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 cyclone's interaction with other low pressure systems, troughs, ridges, or with . A strong and stationary anticyclone can effectively block the path of an extratropical cyclone. Such blocking patterns 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 anticyclone's 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.
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 vortices 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. 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.
Explosive development of extratropical cyclones can be sudden. The storm known in Great Britain and Ireland as the "Great Storm of 1987" deepened to with a highest recorded wind of , resulting in the loss of 19 lives, 15 million trees, widespread damage to homes and an estimated economic cost of pound sterling1.2 billion (US$2.3 billion).
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).
In summer, extratropical cyclones are generally weak, but some of the systems can cause significant overland because of torrential rainfall. The July 2016 North China cyclone never brought gale-force sustained winds, but it caused devastating floods in mainland China, resulting in at least 184 deaths and Renminbi33.19 billion (US$4.96 billion) of damage.
An emerging topic is the co-occurrence of wind and precipitation extremes, so-called compound extreme events, induced by extratropical cyclones. Such compound events account for 3–5% of the total number of cyclones.
/ref> extratropical cyclones (so-called atmospheric transients) acts as a mechanism in converting potential energy that is created by pole to equator temperature gradients to eddy kinetic energy. In the process, the pole-equator temperature gradient is reduced (i.e. energy is transported poleward to warm up the higher latitudes).
The existence of such transients are also closely related to the formation of the Icelandic and Aleutian Low — the two most prominent general circulation features in the mid- to sub-polar northern latitudes.Linear Stationary Wave Simulations of the Time-Mean Climatological Flow, Paul J. Valdes, Brian Hoskins, Journal of the Atmospheric Sciences 1989 46:16, 2509–2527 The two lows are formed by both the transport of kinetic energy and the latent heating (the energy released when water phase changed from vapor to liquid during precipitation) from the mid- latitude cyclones.
In the North Atlantic Ocean, the most intense extratropical cyclone was the Braer Storm, which reached a pressure of in early January 1993. Before the Braer Storm, an extratropical cyclone near Greenland in December 1986 reached a minimum pressure of at least . The West German Meteorological Service marked a pressure of , with the possibility of a pressure between , lower than the Braer Storm.
The most intense extratropical cyclone across the North Pacific Ocean occurred in November 2014, when a cyclone partially related to Typhoon Nuri reached a record low pressure of . In October 2021, the most intense Pacific Northwest windstorm occurred off the coast of Oregon, peaking with a pressure of . One of the strongest nor'easters occurred in January 2018, in which a cyclone reached a pressure of .
Extratropical cyclones have been responsible for some of the most damaging floods in European history. The Great storm of 1703 killed over 8,000 people and the North Sea flood of 1953 killed over 2,500 and destroyed 3,000 houses. In 2002, floods in Europe caused by two caused $27.115 billion in damages and 232 fatalities, the most damaging flood in European since at least 1985. In late December 1999, Cyclones Cyclone Lothar and Martin caused 140 deaths combined and over $23 billion in damages in Central Europe, the costliest European windstorms in history.
In October 2012, Hurricane Sandy transitioned into an extratropical cyclone off the coast of the Northeastern United States. The storm killed over 100 people and caused $65 billion in damages, the second costliest tropical cyclone at the time. Other extratropical cyclones have been related to major tornado outbreaks. The tornado outbreaks of April 1965, April 1974 and April 2011 were all large, violent, and deadly tornado outbreaks related to extratropical cyclones. Similarly, winter storms in March 1888, November 1950 and March 1993 were responsible for over 300 deaths each.
In December 1960 a nor'easter caused at least 286 deaths in the Northeastern United States, one of the deadliest nor'easters on record. 62 years later in 2022, a winter storm caused $8.5 billion in damages and 106 deaths across the United States and Canada.
In September 1954, the extratropical remnants of Typhoon Marie caused the Tōya Maru to run aground and capsize in the Tsugaru Strait. 1,159 out of the 1,309 on board were killed, making it one of the deadliest typhoons in Japanese history. In July 2016, a cyclone in Northern China left 184 dead, 130 missing, and caused over $4.96 billion in damages.
For older extratropical storms occurring before the 20th century, new paleotempestological methods can be used to assess their intensity. Cross-referencing environmental and historical records in Western Europe has highlighted the intense storms of 1351-1352, 1469, 1645, 1711 and 1751, which caused severe damage and long-lasting flooding along much of Europe's coastline.
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