Monday, January 10, 2011
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.