A volatile severe weather situation is setting up for a large area from the Upper Midwest to the Mid-Atlantic and Northeast on Wednesday and Thursday. Southwest Virginia, however, is NOT in the bullseye of this setup. One unusual factor that is changing the playing field from what we would typically expect in early June is an unseasonably strong surface-low-pressure system (depicted at 992 millibars or 29.30 inches of mercury for Thursday evening on 0Z NAM at left) expected to track eastward from Nebraska to near D.C. This low is going to focus a lot of the strong storm dynamics — the best shear, moisture, lift and instability — well to our north. Surface winds backing to the east and southeast just east and north of this low’s track may spur an unusually high tornado risk up that direction — this dangerous possibility may have got lost in some of this week’s derecho hype. A trailing cold front will be cutting into hot (some 90s possible Wednesday in Roanoke and points south and east, maybe even locally mid-upper 90s) and increasingly moist air farther south — its speed and placement will determine a lot about our storm chances.
In short, here are Southwest Virginia’s three best chances of seeing storms, some of them possibly severe, through Thursday. The picture is a little blurry, but actually leans AGAINST a significant severe weather outbreak in our region.
(1) Scattered storms, Wednesday. This appears more likely to happen to our north near a stationary front. But there may be some chance of storms developing Wednesday afternoon and evening in our region as moisture increases and heating and terrain factors produce instability and lift. Downsloping northwest winds will be an inhibitor for this, at least in the Roanoke and New River valleys and close to the Blue Ridge. Sometimes a “lee trough” develops in this situation that helps fire storms some distance east of the Blue Ridge. UPDATE 8:30 AM: Radar is picking up some storms along I-77 in West Virginia, which may work into some of SW Virginia later today. END UPDATE
(2) Storms moving in from Ohio Valley late Wednesday evening, early Thursday. This is where whatever’s left of one or more MCS’s (mesoscale convective systems — storm clusters or lines) could move in from the northwest, with some locally strong wind gusts possible. The timing of this several hours past peak warming and the tendency for drying out and weakening by downslope flow over the mountains argues strongly against the “d-word” — it’s doubtful high winds could be maintained widely for the length of time necessary to reach us, unlike a certain 100+ degree afternoon/early evening in late June of last year. There is a fair amount of forecast model evidence suggesting this storm cluster will either (a) stay north of Interstate 64, maybe by a wide margin, or (b) fall apart crossing the mountains or (c) both. Sometimes, though, these storm clusters seem to get minds of their own, as they develop their own cold pools from outflow winds behind them that propels them in directions that can defy expectations. A June 29, 2012-like widespread damaging derecho event, however, is HIGHLY UNLIKELY in Southwest Virginia.
(3) New development on Thursday. At this point, this is the most unclear of the three vague possibilities, as it depends on the timing and movement of everything that happens Wednesday. A slower arrival to the cold front could lead to strong storms developing in or even west/northwest of our area and moving through on Thursday; a faster arrival favors development east of the Blue Ridge. Outflow boundaries (mini-fronts produced by winds of old storms), subsidence (sinking air behind the storm that inhibits convection) and other products of Wednesday evening’s storms will also have a big say on what happens Thursday. The strong possibility exists that storms will die or substantially weaken crossing the mountains and then re-fire well east of the Blue Ridge on Thursday, leaving Southwest Virginia totally out of a severe threat.