Thundersnow

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Thundersnow is a particularly rare meteorological phenomenon that includes the typical behavior of a thunderstorm, but 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.

Contents 1 Formation 1.1 From Lake Effect Precipitation 1.2 From Synoptic Forcing 1.3 From Upslope Flow 2 See also 3 References 4 External links 4.1 Links to individual events

[edit] Formation

There are usually three forms of thundersnow:

• A normal thunderstorm on the leading edge of a cold front or warm front that either forms in a winter environment or one which runs into cool air and maintains the precipitation as 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 miles 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.^[1]

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., the Great Salt Lake, and 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.

Thundersnow was reported in metro Atlanta during the Great Blizzard of 1993, and in greater New York (and other areas) during the Blizzard of 2006. During a thundersnow event in late March 1997, lightning struck the main ski lift at the Angelfire Resort in Angelfire, New Mexico after closing time. This caused no injuries, but damaged the only lift providing access to and from the operating part of the mountain. Large snow pellets were also reported from this storm. One of the largest Thundersnow events to date is the October 2006 snowstorm (Lake Storm "Aphid") which affected the Buffalo, New York area as well as Fort Erie, Ontario. Thundersnow was reported constantly and during the event's height, cloud to ground lightning strikes were occurring at a rate of 10 strikes per minute or more. On November 21, 2006, Charleston, South Carolina observed thundersnow for the first time in its history.^{[citation needed][dubious — see talk page]}. On the morning of January 21st, 2007 at approximately 6:30am, Sedona, Arizona observed Thundersnow. Areas of Wisconsin including Eau Claire, La Crosse, and Madison observed this during a large winter blizzard mixed with ice and sleet on February 23rd and 24th, 2007, the same storm causing thundersnow to be observed in the area of Dubuque, Iowa.

[edit] From Lake Effect Precipitation

This type of thundersnow occurs after a long days work 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.^[2]

[edit] 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 typically find thundersnow on the northwest side, within what is known as the comma head of a mature extratropical cyclone.^[3] 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.^[4] 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.

[edit] 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. ^[5]

[edit] See also

• Lake effect snow
• Shear
• Snow
• Thunderstorm

[edit] References

[edit] External links

• What causes thundersnow?
• Straight Dope staff report: Why don't snowstorms produce lightning?

[edit] Links to individual events

• Sierra Nevada Mountain Thundersnow: 9-10 November 2000
• Central Victorian, Australia Thundersnow: 14 June 2001
• Boston Thundersnow: 9 December 2005
• Chester County Pennsylvania Thundersnow: 25 January 2006
• Madison, Wisconsin Thundersnow: 16 February 2006
• Aberdeen, Scotland Thundersnow: 3 March 2006
• Buffalo's Thundersnow: 12-15 October 2006
• Columbia, Missouri Thundersnow: 1 December 2006
• Thudersleet is rare weather feat, Lawrence Journal-World, Wednesday, January 24, 1996