Wednesday, January 16, 2008

Weather update: From marginal, to minor, to moderate, to major?

It seems very clear that this storm is going to be stronger than many expected. I will be shocked if the NWS does not upgrade us to a Winter Storm Warning.

The cold trend in the models from overnight and early today is now being verified (if not STILL underdone) in current conditions. Snow is breaking out over central Alabama and Georgia.

Winter weather advisories have been extended far to our south - as far south east as Orangeburg, S.C. and as far southwest as Augusta, Ga.

Also, after trending yesterday to a more icy event, there is now a move to a sleetier and especially snowier event.

Here's how the event should unfold, based on the observations of a number of mets who post on message boards and model trends -- we will experience a considerable amount of what is called "virga" late this afternoon and tonight. Virga is precipitation that can often be picked up on radar, but which dries before it reaches the ground.

This begins the process of cooling the lower levels of the atmosphere (see earlier post on evaporational cooling and wet bulb temps).

By around midnight, give or take 2 hours, precipitation will start reaching the ground. It's possible this will be as rain for just a few minutes before snow kicks in.

Right now, it looks like snow will fall for at least 4 hours and as long as 8 or possibly even 10 hours. After that time, whatever it is, we will see a transition to sleet, then perhaps freezing rain. It's possible the CAD will be so strong that we stay below freezing for the entire event.

Here is the final percentage breakdown:
No precip: 0 percent
All rain: less than 1 percent
Marginal event (less than 1 inch of snow; very light icing): 10 percent
Minor event (1-2 inches of snow; .1 inches of icing): 25 percent
Moderate event (2-4 inches of snow; .1-.25 inches of icing) 45 percent
Major event (4 or more inches of snow; .25 or more of icing) 20 percent

Look for more updates throughout the day and night and morning!

No comments: