Here is what can be said with some degree of confidence about Wednesday’s winter storm potential:
* It is likely that a large portion of the central/southern Appalachians and Mid-Atlantic region, probably centered on Virginia, will be affected by a high-impact winter storm on Wednesday, possibly beginning as early as late Tuesday night especially in the higher elevations of western Virginia.
* Forecast model guidance suggests that 6-inch+ snowfall amounts will likely occur over a large portion of the area described above, and 12+ amounts, either widespread or localized, are quite possible. Exactly where the bigger amounts occur is uncertain.
* It is probable that much if not all of Southwest Virginia will see accumulating snowfall on Wednesday.
* The potential exists for a MAJOR to even HISTORIC winter storm to affect our region … but there is still a possibility that the heaviest snowfall will shift north of our region, or that the system will be too fast or too mild for the heaviest possible amounts to be realized.
* Power line and traffic disruptions are very likely to occur in and/or near our region on Wednesday.
The driving force in this winter storm threat is a pocket of upper-level energy called a “vort max,” (short for “vorticity maximum” and often just called a “vort”) its position Wednesday morning projected on the 12Z GFS at left. You can just call it an upper-level disturbance if you like. As this energy dives southeastward the eastward, it will help deepen a surface low-pressure system over North Carolina (probably) and the lift created by it will cause heavy precipitation and cooling temperatures north of it, especially 100-250 miles or so north of it. The vort’s track is highly important to snow chances. Generally speaking, snow chances in the Roanoke-Blacksburg area are maximized if the vort passes over or south of Interstate 40 in North Carolina. The center of the vort is approximated by the X at left, which on this model, has it diving into South Carolina. The 18Z GFS had it farther north in North Carolina at the same time — that model still showed heavy snow in Southwest Virginia, especially along and west of Interstate 81, but a track much farther north than it depicted would cut into the totals. Tracking where this vort dives will be critical in snow forecasts for Wednesday. What is likely to transpire is that cold rain will develop Tuesday night and Wednesday, perhaps changing to snow gradually from northwest to southeast and from higher elevations to lower ones through the night and early morning. Once this disturbance passes by to our south, a rapid change to snow will occur areawide — recall how fast it changed from rain to snow on Jan. 17. As the surface low deepens rapidly to our southeast, winds will pick up, and an large area of snow will wrap around the backside of the low across western Virginia. Forecast models vary on the speed of the low, but it does appear it will track fairly slowly east, so snow may continue for several hours once it gets going. For whatever locations get in the heaviest bands, that could mean 1-2-inch-per hour snowfall rates (probably some thundersnow, too, considering the strong dynamics of the system) that could pile up fast.
Remember that, as I type this, we are still likely more than 72 hours away from this event. I don’t expect huge changes in storm evolution or large jumps in the storm track from this point onward, but nudges north or south or slight changes in speed of the system could mean several inches difference up or down in the snow potential at your location. We’ll track the trends in guidance and see where this goes. It’s probably our last potential widespread winter storm to track in the 2012-13 snow season. If you’re a winter fan, try to enjoy it, as nerve-wracking as it may get. If you’re not, there is hope of spring just around the corner.