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Cumulus clouds in fair weather. File:Strong Extratropical Cyclone Over the US Midwest. United States, starting late on October 25 and running through October 27, 2010. Terms like frontal cyclone, frontal depression, frontal low, extratropical low, non-tropical low and hybrid low are often used as well. An upper level jet streak.
DIV areas are regions of divergence aloft, which will lead to surface convergence and aid cyclogenesis. 37 cyclones in existence during any 6-hour period. 234 significant extratropical cyclones form each winter. The divergence causes air to rush out from the top of the air column.
Low-level convergence and upper-level divergence imply upward motion within the column, making cyclones tend to be cloudy. Eventually, the cyclone will become barotropically cold and begin to weaken. 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. 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 process known as “tropical transition” involves the usually slow development of an extratropically cold core vortex into a tropical cyclone. 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.
Dvorak technique is still used if the system begins dissipating without transition. Note the maximum winds are on the outside of the occlusion. The area poleward and west of the cold and warm fronts connected to extratropical cyclones is known as the cold sector, while the area equatorward and east of its associated cold and warm fronts is known as the warm sector. Extratropical cyclones are clockwise spinning in the Southern Hemisphere, just like tropical cyclones.
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. Due to their appearance on satellite images, extratropical cyclones can also be referred to as frontal waves early in their life cycle. It can also be the focus of locally heavy precipitation, with thunderstorms possible if the atmosphere along the trowal is unstable enough for convection. Above the surface of the earth, the air temperature near the center of the cyclone is increasingly colder than the surrounding environment.
Cyclone phase diagrams are used to tell whether a cyclone is tropical, subtropical, or extratropical. There are two models of cyclone development and lifecycles in common use—the Norwegian model and the Shapiro-Keyser Model. 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. A second competing theory for extratropical cyclone development over the oceans is the Shapiro-Keyser model, developed in 1990.
This model was based on oceanic cyclones and their frontal structure, as seen in surface observations and in previous projects which used aircraft to determine the vertical structure of fronts across the northwest Atlantic. A warm seclusion is the mature phase of the extratropical cyclone lifecycle. A warm seclusion, the result of a baroclinic lifecycle, occurs at latitudes well poleward of the tropics. 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.