Thundersnow

Thundersnow

Thundersnow also known as a winter thunderstorm or a thunder snowstorm is a rare thunderstorm with snow falling as the primary precipitation instead of rain. It commonly falls in regions of strong upward motion within the cold sector of extratropical cyclones between autumn and spring when surface temperatures are most likely to be near or below freezing. Variations exist, such as thundersleet, where the precipitation consists of sleet rather than snow.

Formation

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 in the comma head of an extratropical cyclone 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 kilometers away, while the thunder from thundersnow can usually only be heard within a two to three kilometer radius from the lightning. In the United States, March is their peak month of formation, and on average, only three events are reported per year. For example, on March 27 2008, thundersnow was observed in the area of northern Indiana between South Bend and Gary; forecasters reported over 110 lightning strikes while parts of the Indiana Toll Road experienced whiteout blizzard conditions.

Thundersnow, while 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. It has also been reported around Kanazawa and the Sea of Japan, and even around Mount Everest during expeditions. When such storms happen at ski areas, these mountains are often evacuated for safety.

From lake effect precipitation

This type of 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 25 degrees Celsius or more 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. 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 2.5 km is necessary and an average depth of 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 30 degrees between the ground and 2km in height must be in place, any change in direction greater than 30 degrees through that layer will tear the snow squall apart. A bare minimum fetch of 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 -30C. It is generally accepted that there is no longer any super cooled water vapour 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 have lightning or thunder result.

From synoptic forcing

Synoptic snow storms tend to be large and complex with many possible locations and factors effecting 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 Superstorm of 1993 and White Juan were such cases.

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.

See also

References

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