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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.

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