Mapping Out The Next 12 Hours

I've posted the maps below to help give you a general picture of what is going to happen over the next 12 hours. The purple and blues represent all snow. The shades of pinks are a mix of snow, sleet and freezing rain. Lastly, the greens are all rain.

I have updated the previous snow accumulation map at the bottom so that you can see additional totals (removing accums from this morning). As you can see, the threat for accumulating snows has now shifted to the north and northeast for the rest of this afternoon and evening. Regardless, be careful if you are going to be on the roads this evening!

Winter Weather Update: Tuesday 5:16AM

A Winter Weather Advisory is in effect Tuesday for nearly the entire Commonwealth of Virginia, including all of the Richmond metro area. Overnight we saw our first rounds of light snow and sleet moving northeast through central Virginia, leading to minimal accumulations. We'll see a more consistent round of snow and sleet developing after sunrise during the morning hours Tuesday as an area of low pressure develops off the coast. We will be in between the developing coastal low and an approaching upper low from the west, bringing us our greatest chance of precipitation during this afternoon and evening hours as these lows begin to phase together to our northeast. I expect to see snow early Tuesday, with a transition to sleet mid-day through the rest of the afternoon, and then a switch back to snow after dark before ending by Midnight in Virginia. Snow and sleet accumulations in central Virginia will range from 1-2", with some spots close to 3".

Sleet Sounding

There is nothing better to look at when trying to determine winter precipitation mode than an atmospheric sounding. You can look at any model you want, and there are many to choose from, but no model will give you an entire look at the moisture and temperature profile like a sounding will. Even the smallest warm layer will result in sleet instead of snow, and completely blow a forecast. Models use mandatory levels and don't "see" everything going on. The above picture is of a skew T diagram, which shows a forecast for the moisture and temperature (and wind) profile for the upcoming winter event. The two black squiggly lines show the dewpoint (on the left) and the temperature (on the right) as you go from the surface (bottom of the chart) to the tropopause (top of the chart, which is above all of the weather). The diagnonal lines in the background of the chart skewed to the right show temperatures, and the key line for winter weather is the 0-degree line. Look at how close the atmospheric temp comes to the zero line (it actually touches it and goes a little to the right of it). That's the warm layer that will cause the snow to melt half way down. The temp line then jumps back to the left of the 0-degree line (going colder than 0), and stays there all the way down to the surface. The flake will melt in the warm layer, and then refreeze in this lower sub-freezing zone into a sleet pellet. Sometimes you'll get only a partial melt and you'll end up with a hybrid snow/sleet pellet. With a warm layer in the mid-levels tomorrow, we'll end up with several hours of sleet.