February 16th-17th 2013 Winter Event
Well, as I have been saying all week, we couldn’t fall asleep on our storm potential this weekend. These setups where waves form along arctic fronts are always tricky and the models often times struggle with them. Thus we have seen the models mostly showing any storm development far offshore, but an occasional run would throw us a bone.
However, beginning overnight the trend has definitely been towards a closer-in development and as a result we are seeing greater potential for at least some wintry weather across areas of the Southeast tomorrow and tomorrow night.
The synoptic setup is this. A strong upper level trough is carving out a mean trough across the eastern US as we speak. This trough is currently positively tilted and is producing a line of some precipitation across the Deep South. However, this trough is forecast to become neutrally tilted tonight and even start to take on a bit of a negative tilt by tomorrow morning/afternoon. This will lead to the development of a low pressure off the SE coast.
As upper level energy feeds into this system, we will likely see an area of precipitation break out across the Deep South tonight/tomorrow morning and coalesce into a more concentrated area across Georgia/Carolinas during the day tomorrow. The major question will be how strong the low pressure will get and how close to the coast it will develop. This will ultimately determine the extent of the snowfall.
Another issue will be boundary layer warmth. Although the atmosphere will cool off quickly tomorrow with 850mb temperatures and 1000-850mb thickness values supporting an all snow event, the boundary layer remains stubbornly warm through tomorrow afternoon/evening. Now often times in these types of setups, the boundary layer will cool off quickly beneath areas of strong vertical motion and we will see temps quickly drop into the 32-34 range supporting accumulating snow. So the major question is going to be where does the strongest lift/precipitation rates set up? Beneath this, boundary layer warmth will likely not be an issue, and we will see the lift/precip rates overcome the warmth. The best chance for this will likely be over NE NC/SE Va, where the best dynamics will be and will occur Saturday night after the sun goes down.
Unfortunately the 2 major NWS models the NAM and GFS did not converge during today’s 12z cycle. The 12z GFS, Canadian, and GFS Ensemble showed a moderate to strong hit across the Carolinas and SE Virginia, while the 12z NAM showed much less precip. The 00z ECMWF is not as wet as the GFS and company, but did show a nice upward bump from recent runs. On another note, some of the high-resolution modeling used at my company are wetter than the NAM and 4km NAM.
Right now I feel compelled to side with the GFS and company although I am certainly wary until the NAM/ECMWF beef up the QPF a little and I do take them into consideration. Taken all these factors into consideration as well as my experience forecasting these types of events in the past, I have come up with the following snowfall forecast map which you can see. It is possible this event could feature 2 precip maximums one over SE Va/NE NC which is represented in my forecast by the 3-6 inch forecast. A 2nd area could be present over upstate SC and the southern piedmont of NC. The first more associated with the coastal low and the 2nd more with the upper level vort max. However, at this time I do not feel confident enough to show this 2nd maximum in great detail. I do feel like initial boundary layer warmth will hold back accumulating snow through Saturday afternoon but as the boundary layer cools, snow falling in the 32-34 temperature range will accumulate, though mostly in grassy areas. Again, this event may depend on where the heaviest lift sets up which will most likely lie NE of the Triangle near Elizabeth City and the Tide Water.
I will update again as needed.