K. A. Browning and Robert J. Gurney (1999). Global Energy and Water Cycles. Retrieved on 2008-12-26. Within the cold sector north to northwest of a cyclone center, in colder cyclones, small scale, or mesoscale, bands of heavy snow can occur within a cyclone's comma head precipitation pattern with a width of to .
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Similar to the lake effect regime, thundersnow is usually witnessed in terrain in the cold sector of an extratropical cyclone when a shortwave aloft moves into the region. The shortwave will steepen the local lapse rates, allowing for a greater possibility of both heavy snow at elevations where it is near or below freezing, and occasionally thundersnow.National Weather Service Office, Sacramento, California. Alexander Tardy.
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It intensified into Tropical Storm Arlene six hours later. After attaining peak winds of early on April 21, the storm began to revolve counterclockwise around a larger extratropical low. The storm tracked into the cold sector of the cyclone, causing Arlene to lose tropical characteristics around 12:00 UTC on April 21. The post-tropical cyclone moved south and east, before dissipating well west-southwest of the Azores on April 22.
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Animated satellite cloud imagery is a better tool for their early detection and tracking. The low-level convergence caused by the cut-off low can trigger squall lines and rough seas, and the low-level spiral cloud bands caused by the upper level circulation are parallel to the low- level wind direction. This has also been witnessed with upper level lows which occur at higher latitudes. For example, in areas where small-scale snow bands develop within the cold sector of extratropical cyclones.
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Thundersnow formation with an occluded front Thundersnow, also known as a winter thunderstorm or a thundersnowstorm, is an unusualDauna Coulter, NASA Retrieved on 12-20-2012.Christine Dell'Amore, National Geographic News kind of thunderstorm with snow falling as the primary precipitation instead of rain. It typically falls in regions of strong upward motion within the cold sector of an extratropical cyclone. Thermodynamically, it is not different from any other type of thunderstorm, but the top of the cumulonimbus cloud is usually quite low.
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If squall lines form over arid regions, a duststorm known as a haboob can result from the high winds picking up dust in their wake from the desert floor. If outflow boundaries move into areas of the atmosphere which are stable in the low levels, such through the cold sector of extratropical cyclones or a nocturnal boundary layer, they can create a phenomenon known as an undular bore, which shows up on satellite and radar imagery as a series of transverse waves in the cloud field oriented perpendicular to the low-level winds.
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QuikSCAT 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. 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.
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Retrieved on 01-11-2006. Thundersnow can also be located underneath the TROWAL, a trough of warm air aloft which shows up in a surface weather analysis as an inverted trough extending backward into the cold sector from the main cyclone.National Weather Service Office, St. Louis, Missouri. Thundersnow Proximity Soundings. Retrieved on 01-11-2006. In extreme cases, thunderstorms along the cold front are transported towards the center of the low-pressure system and will have their precipitation change to snow or ice, once the cold front becomes a portion of the occluded front. The 1991 Halloween blizzard, Superstorm of 1993, and White Juan are examples of such blizzards featuring thundersnow.
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Once a cyclone occludes an occluded front (a trough of warm air aloft) will be caused by strong southerly winds on its eastern periphery rotating aloft around its northeast, and ultimately northwestern, periphery (also termed the warm conveyor belt), forcing a surface trough to continue into the cold sector on a similar curve to the occluded front. The front 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 front is unstable enough for convection. Banding within the comma head precipitation pattern of an extratropical cyclone can yield significant amounts of rain.
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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.
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