Monday, January 10, 2011

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.

Winter Weather Update: Monday 11:45AM

A Winter Weather Advisory will be in effect for all of the purple highlighted counties in central Virginia from 4 a.m. Tuesday through midnight for about 1"-3" of snow and sleet mixed accumulation. A Winter Storm Watch remains in effect for the Northern Neck counties highlighted in blue, where 3"-5" of snowfall accumulation is possible by pre-dawn Wednesday. Here is the latest snowfall forecast map:

A mix of snow and sleet should move into Virginia's southernmost counties tonight after dark, and gradually spread northward through Tuesday morning into the rest of central Virginia. Southeast Virginia will likely see more of a mix of sleet and freezing rain during the day and afternoon Tuesday before colder air rushes in Tuesday night and brings a brief period of light snow. Up to a tenth of inch of ice accumulation is possible in southeast Virginia where freezing rain occurs. However, with sleet mixing in with the snow in much of the rest of the Advised area, slick and hazardous road conditions are also likely Tuesday.

Winter Weather Update: Monday 6:40 AM

A Winter Weather Advisory will be in effect for much of Central Virginia from 4 a.m. Tuesday through Midnight as a snow and sleet mix of precipitation moves in from the south late Monday night. You can see the counties included in purple here:

Those counties shaded in blue in the Northern Neck are under a Winter Storm Watch, where higher snowfall accumulations are possible than for the rest of us. Here's the latest forecast snow map from late Monday through pre-dawn Wednesday:

A winter storm continues to track through the Southeast U.S. early this Monday morning, with cloud-cover ahead of the storm already overspreading Virginia. We will remain cloudy the rest of today and cold with highs about ten degrees below normal in the mid 30's. A wintry mix of sleet and snow will move into southern Virginia this evening and then overspread the entire area from south to north during Tuesday morning. Periods of sleet and snow will persist in Richmond through Tuesday evening (1 to 3" accumulation). There will be more mixing in south central and southeastern Virginia on Tuesday, resulting in lower overall winter precipitation accumulations in that part of the state. Rain, freezing rain and sleet will be possible in these areas (up to a tenth of an inch of ice accumulation). Precipitation form should be mostly snow for areas to the west and north, where accumulations look to be the highest (2 to 3"). The Northern Neck could receive higher totals closer to 5" of snow before sunrise Wednesday as a result of the low pressure system offshore beginning to intensify as it transforms into a nor'easter headed for New England Wednesday.