What Causes Thundersnow?

[Jane Louise]

WHAT CAUSES THUNDERSNOW?

METEOROLOGIST JEFF HABY

Thundersnow is a thunderstorm than has snow reaching the surface instead of rain. Usually thunder and lightning are more commonly observed with warm season thunderstorms. It is the convective process that creates the favorable charge separation for lightning to occur.

It is fairly rare to have convection within a temperature sounding that can support snow. The reason for this is because the lower troposphere tends to have low dewpoints and temperatures. This dry (low moisture content) and cold lower troposphere creates stability with respect to parcels of air rising from the surface.

Two mechanisms are important to the creation of thundersnow and they are elevated instability and strong dynamic lifting. Each of these mechanisms reinforces the other. The CAPE or instability in the troposphere is a function of the tropospheric temperature profile and the initial temperature and dewpoint of a parcel of air. Surface Based CAPE (SB CAPE) can be zero when at the same time lifting from another pressure level produces positive CAPE. The term for convection starting at a pressure level other than the surface is termed ELEVATED_CONVECTION. It helps to have the parcel initially start with a small dewpoint depression since CAPE is more likely to occur than if the dewpoint depression is large. Keep in mind that environmental dewpoint can increase with height or may not fall that much with height. Even if no elevated CAPE is present, CAPE may still be present when lifting occurs at a slant (known as slantwise convection)

Strong dynamic lifting (UVV) can enhance instability by cooling the middle layers of the troposphere. Cooling the middle layers of the troposphere makes elevated convection more possible. The dynamic lifting is also the "trigger" to cause the air to rise and keep rising from a pressure level that eventually results in elevated convection. Very intense dynamic lifting can often build clouds enough so that the clouds have convective characteristics even if no elevated convective CAPE is present.

A severe thundersnow occurs when the snow is accompanied with hail that is equal or greater than 3/4" in diameter or when the wind is equal or greater than 50 knots. Wind behind a strong cold front can mix down to the surface creating severe winds along with the thundersnow. If the elevated CAPE is high, hail can reach the surface along with the snow. A sounding profile below freezing will limit hail growth but it will not experience melting as it falls to the surface.

The ultimate weather education website: http://www.theweatherprediction.com/

[Thundery wintry showers]

Some more info here:

http://en.wikipedia.org/wiki/Thundersnow

[Jane Louise]

Some more info here:

http://en.wikipedia.org/wiki/Thundersnow

Thanks Tws

A Study in Opposites: Thundersnow

© Keith C. Heidorn

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Nov 1, 2004

As a wet snow falls, I watch the flakes shifting down from a deep-grey sky. Then, to my left, a throaty rumble following a muted flash catches my attention. Lightning and thunder? During the winter season? For most of us, thunder and snow in the same sentence, let alone the same word thundersnow, seems a contradiction, the symbols of summer and winter weather coexisting.

For most North Americans, the combination, known as thundersnow, is a very unusual event. Even where they are most common, thundersnow occurs no more than a few times per year, usually when winter storms with substantial warm and humid air sectors spawn thunderstorms along their fronts, or when air crosses a large lake or rises over a mountain range.

The central American states, mostly those west of the Mississippi River (Missouri, Kansas, Nebraska, Iowa, Colorado, Minnesota, Oklahoma and into northern Texas), define a loose arc in which thundersnows occur a few times each winter. For residents around the Great Lakes, the Great Salt Lake, or in the continent's mountainous terrain, however, thundersnows can be more common, though often many are unreported. Thundersnows have also been reported around the Sea of Japan and the North Sea. Mountaineers scaling Mount Everest, and other high peaks, have reported thundersnows during their ascents as some of their scariest moments.

Observations of thunder/lightning with snowstorms have been found in records and commentaries since the 19th century in the Western world and in China for nearly a millennium. However, only in the past decade have meteorologists delved into the phenomenon of thundersnow, likely due to the uncommonness of the events. In the United States, only 1.3% of cool-season (October-May) thunderstorms reported coincide with snowfall, and only 0.07% of snowstorms have associated thunder. (A five-year study of thundersnows in the central US by University of Missouri meteorologist, Patrick Market, began in 2003.)

Reports of thunder/lightning with snow are also hindered by several factors. Thunderstorms dropping snow to the surface generally produce fewer lightning strokes than rain-dropping thunderstorms. The snowflakes absorb and diffuse the light of lightning and sound of thunder more readily than does falling raindrops. (While thunder during rain is generally heard 6.5-8 km (4-5 miles) away, the range for snow-related thunder is no more than 1.6 km (1 mile). The colder weather in which thundersnow occur may also contribute to the relative rarity of the event as fewer observers are out of doors.

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