12/28/12 Forecast Update

Type of precipitation (green = rain, red = freezing rain, purple = sleet, blue = snow)

UPDATE*** Please, please be careful driving tonight and tomorrow. This snow coming in will be partially melted when it hits the ground, forming a higher density (8:1 ratio instead of 10:1) slushy accumulation. Coupled with sleet, freezing rain, and eventually all snow, untreated surfaces will be very slick.
The amount of dry air in the mid levels this afternoon and evening may be the difference maker tonight in terms of what type of precipitation we see. My thoughts are this: the High Country will see a mix of rain/fat melted snowflakes to start this event (possibly starting as just rain for the foothills north of HWY 18). The degree of evaporative cooling and the precipitation rate will play a large role in determining what kind of precipitation falls. Watch the radar tonight: when a "darker" signature (heavier precipitation) shows up over you, watch the precipitation change to wet snowflakes. Evaporative cooling will cool down the surface enough to keep this as a mostly frozen mix through early Saturday morning however the precipitation rate is not high enough to keep it at all snow. Prevailing warm air aloft will create a mixed precipitation even through the night. Saturday morning does transition to all snow showers. Snow accumulations through Saturday are at just 1-3" max (dusting to 1" for foothills), with a 0.15" ice accretion total.  I am concerned about the higher ice accretions for the interior valley regions and for the eastern slopes.

   

Percent of frozen precipitation

Saturday evening a cold front pushes through with a short but concentrated burst of northwest flow snow showers. Another 1-2" max (up to 3" for high peaks along the NC/TN line) is expected. We are expecting a 2-4" grand total of snowfall.