A couple people have asked me why it's 21 degrees and sleeting or raining, rather than snowing, where they live. The below image is called a SKEW-T chart -- this is the chart produced from weather balloon data (although this particular chart is generated with weather models instead of straight weather balloon data).

The blue dashed lines are lines of constant temperature. The 0 degree line is freezing, anything to the left of that is below freezing, anything to the right is above. The brown base of the chart represents the surface. The thick red line represents temperature as you climb higher into the atmosphere, and the thick green line represents the dew point.

A large wedge of warm air is sitting just off the surface, with temperatures sitting at or slightly above freezing around 5500 feet. This layer is supposed to warm through the day, causing any snowflakes to melt down to liquid and refreeze when it hits the extremely cold air at the surface. Depending on how much these snowflakes melt, they'll reach the surface as sleet or freezing rain. Surface temperatures are supposed to remain well below freezing through the day, so regardless of whether it's sleet, freezing rain, or snow when it falls, it's going to stick and freeze.

This sounding is from Danville VA.

Here's another one from Knoxville, TN, currently under an ice storm warning for up to a half an inch of solid ice accumulation from freezing rain. It's just below freezing at the surface -- 31 degrees at the Knoxville Airport when I typed this -- but 5000 feet off the ground it's 41 degrees. Anything frozen has a snowball's chance in Knoxville (heh heh heh) when the atmosphere is that warm. Precipitation falls as liquid until it hits the sub-freezing surface, where it freezes on contact.

It's storms like this that make it hard for forecasters to tell you exactly what will happen and when. The tools are there to make it possible, but it's difficult and (surprisingly) nature doesn't always cooperate with forecasts. Just one or two degrees over a 1000 foot layer of the atmosphere means the difference between snow, sleet, or freezing rain.

11:27 AM PT: Clarification: The Danville VA sounding has warmed up beyond the layer shown in the above sounding, to fully allow the sleet/freezing rain to form.

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