FLOOD WATCH THURSDAY (additional rain/snow may cause some high streams to overflow)
UPDATE 12:15 AM, 1/17: Forecast guidance this evening seems to be a little colder with potential earlier start time for snowfall, perhaps by midafternoon. With that, and continued guidance on amounts, I’m raising the top end of my Roanoke Valley area snow total to 3-7 inches (also east to Bedford and north to Lexington), and opening up the top of the New River Valley snow total (including Floyd County) to 5-10 inches. Staying at 6-10 but expecting some locally 12+ amounts in I-77 corridor and near West Virginia border. This is a very fluid situation that will depend largely on just when snow begins and how quickly the lower atmosphere cools. I’ll plan a morning update — first on this blog entry, and then a brand new one — to take us into a day of “nowcasting” to see what really happens with this interesting setup, not what a computer projects. END UPDATE
ORIGINAL 7 P.M. 1/16 ENTRY:
For a summary … go to the last paragraph.
Thursday may be a miserable day for hard-core snow lovers.
Before you freak out (or celebrate, if you’re inclined otherwise), I’m NOT saying it won’t snow. In fact, I think it will snow several inches (read 3+ inches) in most of our region. But, for snow lovers, it’s going to be a long day of agonizing waiting, anticipating, watching as a cold rain pelts down in puddles created by the 2-5 inches of rain we just got. The radar’s not going to look impressive at times. You’re going to think it’s too warm to snow. And for a while, you’ll be right. It’s just a matter of when and how it all comes together .. when an upper-level low closes off, goes “negative tilt,” triggers a surface low and begins the processes that will pull the freezing level lower that can turn whatever rain is falling into snow. Once it gets going, it may not take long to get flakes flying across our region, with a relatively short but heavy thump of wet snow likely late Thursday afternoon to mid Thursday evening. (Think 2-10 p.m. as the likely “prime time” for this snow event.)
The early snow lover angst deals with the inevitable back and forth of model runs. So far, the 18Z NAM’s slip south of the storm so that snow cuts off at I-64 and is somewhat reduced in much of our region is an outlier. The 18Z GFS’ massive dump of heavy wet snow on two-thirds of Virginia may be an outlier the other direction. But all of the models for several runs now have been showing a period of heavy wet snow for 3-6 hours, sometimes even longer, over Southwest Virginia. The timing of the start and the exact amounts are still a bit in flux, as they should be, considering that a variance of 25 miles in the storm’s track or a few tenths of an inch of moisture or a few tenths of a degree over a few hundred feet of the atmosphere can have substantial impact on the fine details. We’ve been through the model ping-pong and speculation game many times before with winter storms on this blog. (Pictured at left, Hydrometeorological Prediction Center map of risk for 4-plus inches, the red signifying a high risk.)
The trend I am most going to watch is not the moisture – it’s going to be there. Barring an extreme shift north or south, it’s not even the storm’s track – there is enough north- south margin that most of Southwest Virginia is going to be in some pretty heavy duty precipitation even with slight movements north or south. Some readers closer to Lexington and Buena Vista may be affected more if the 18Z NAM southerly track and northern cut-off becomes the expected course of events. The trend I am watching is the timing and depth of cold air. Both of the recent runs of the GFS and NAM bring cold air southward just a bit earlier than before. Starting the snow at 2 p.m. instead of 5 p.m. could make a huge difference in snow totals for any given location. On the flip side, the later it goes before the cold air advection and dynamic cooling take over with the developing storm system, the lesser the amounts will be.
Right now, here’s where I’m at with the snow: 3-5 inches Roanoke Valley east to Bedford and north to Lexington; 5-8 inches New River Valley and southern Blue Ridge (Floyd area, higher elevations western Franklin and Patrick counties); 7-10 inches along the I-77 corridor and West Virginia border (Grayson County, Galax, Wytheville, up to Giles, Craig, Alleghany counties); 2-4 inches from Smith Mountain Lake’s southern shore through Southside. But I’m close to being persuaded upward in some areas if colder trends continue on the 0Z forecast models later this evening. And then … mercifully … we’ll be largely past the model-watching phase and into “nowcasting.” The blessings of a storm system that we haven’t been watching run-by-run for an entire week.