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Thundersnow, also known as a winter thunderstorm or a thunder snowstorm, is an unusual 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 thunderstorms but the top of the cumulonimbus is usually quite low. In addition to snow, graupel or hail falls.
There are usually three forms of thundersnow:
- A normal thunderstorm on the leading edge of a cold front or warm front that can either form in a winter environment or one that runs into cool air and where the precipitation takes the form of snow.
- A heavy synoptic snowstorm that sustains strong vertical mixing which allows for favorable conditions for lightning and thunder to occur.
- A lake effect or ocean effect thunderstorm which is produced by cold air passing over relatively warm water; this effect commonly produces snow squalls over the Great Lakes.
One unique aspect of thundersnow is that the snowfall acts as an acoustic suppressor of the thunder. The thunder from a typical thunderstorm can be heard many miles away, while the thunder from thundersnow can usually only be heard within a two to three mile radius from the lightning. In the United States, March is the peak month of formation, and on average, only 6.3 events are reported per year.
Thundersnow, while relatively rare anywhere, is more common with lake-effect snow in the Great Lakes area of the United States and Canada, the midwestern U.S., and the Great Salt Lake. Thundersnow also occurs in Halifax, Nova Scotia, sometimes several times per winter season. Bozeman, Montana also sees thundersnow more often than average with these storms typically occurring in April or May. The British Isles and other parts of northwestern Europe occasionally report thunder and lightning during sleet or (usually wet) snow showers during winter and spring. It is also common around Kanazawa and the Sea of Japan, and even around Mount Everest. Low pressure events in the eastern Mediterranean that originate from polar origin cause copious thundersnow occurrences during winter storms, especially over the elevated provinces of Israel and Jordan, including Amman and Jerusalem. When such storms happen at ski areas, these mountains are often evacuated for safety.
From lake effect precipitation
Lake effect thundersnow occurs after a cold front or shortwave aloft passes by, which steepens the lapse rates between the lake temperature and the temperatures aloft. A difference in temperature of 45 °F (25 °C) or greater between the lake temperature and the temperature around 5000 feet/1500 meters (the 850 hPa level) usually marks the onset of thundersnow if surface temperatures are expected to be below freezing. However there are several factors affecting its development and other geographical elements. The primary factor is convective depth; this is the vertical depth in the troposphere that a parcel of air will rise from the ground before it reaches the equilibrium (EQL) level and stops rising. A minimum depth of 0.9 mi (1.5 km) is necessary and an average depth of 1.8 mi (3 km) or more is generally accepted as sufficient. Wind shear is also a significant factor, linear snow squall bands produce more thundersnow than clustered bands, thus a directional wind shear with a change of less than 54° between the ground and 1.2 mi (2 km) in height must be in place, any change in direction greater than 54° through that layer will tear the snow squall apart. A bare minimum fetch of 30 mi (50 km) is required in order for air passing over the lake or ocean water to sufficiently saturate with moisture and acquire thermal energy from the water. The last component is the echo top or storm top temperature, which must be at least −22 °F (−30 °C). It is generally accepted that there is no longer any super cooled water vapor present in a cloud at this temperature but rather ice crystals suspended in the air. This allows for the interaction of the ice cloud and graupel pellets within the storm to generate a charge and create lightning or thunder as a result.
From synoptic forcing
Synoptic snow storms tend to be large and complex with many possible locations and factors affecting the development of thundersnow. The best location in a storm to find thundersnow is typically on the northwest side, within what is known as the comma head of a mature extratropical cyclone. 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. 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.
From upslope flow
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.
Thundersnow produces snowfall rates in the range of 2 to 4 inches per hour. Snowfall of this intensity may limit visibilities severely, even during light wind conditions. However, thundersnow is often a part of a severe winter storm or blizzard. Winds of above tropical storm force are frequent with thundersnow. As a result, visibilities in thundersnow are frequently under 1/4 mile. Additionally, such wind creates extreme wind chills and may result in frostbite. Finally, there is a greater likelihood that thundersnow lightning will have a positive polarity which has greater destructive potential than negatively-charged (typical) lightning.
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