... A couple of weak cold fronts will help drive scattered showers and storms each afternoon and evening through Wednesday. Some of these storms could pose a risk of severe weather this evening and Tuesday. The week will start out seasonably warm, but mild temperatures return Thursday and Friday behind the front.
NEAR TERM /THROUGH TUESDAY/... As of 947 PM EDT Monday: Did someone say busted forecast? With the last of any notable convection fizzling out over the NC-TN border, I'm prepared to call it a night. The 500mb speed max that was progged to provide a roborative slug of deep shear for what should have been ongoing discrete convection is now overhead...with nary a rumble of thunder to be heard. It seems that the afternoon's persistent cloud cover - which, by the way, *still* has not fully dissipated here at the GSP airport - sapped the environment of its ability to initiate convection. The recent 00z HRRR has come in with virtually no redevelopment later tonight, so no further activity is expected tonight apart from a sprinkle here and there across the Upstate, which at this hour will likely go as unnoticed as I wish my unfortunate forecast would.
Despite the flop in convection, guidance continues to support a round of fog across the NC mountains and perhaps the foothills/Blue Ridge Escarpment zones as well. This seems fair: with dewpoints running, if anything, higher than forecast, and lows still expected to fall up to 3 degrees below crossovers across the mountains, patchy fog is a no-brainer. Lows, by the way, will be around a category below normal.
Drier air will work in after daybreak, but mainly across NC...with the new GFS depicting the theta-e boundary stalling somewhere in the NC Piedmont north of the state line. In light of this, there will be enough lingering moisture for weak instability to develop across NE GA and the Upstate. Still, without a triggering mechanism, and sans any discernible synoptic forcing, CI will be a challenge; indeed, the latest round of CAM guidance still has slim pickings for tomorrow, even over the southern zones. Still have isolated diurnal convection for these areas, but confidence isn't great. Even with the isolated coverage and weak instability, a few severe storms will be possible as at least "better-than-negligible" shear and high DCAPE values will remain in place. Highs will be a few degrees above normal.
SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY/... As of 215 PM Monday: We remain under the cyclonic flow around the northeastern upper low component of the persistent omega blocking pattern through the middle of the week. Today's model guidance shows better agreement and timing with regard to vorticity rounding the western side of the mid-level circulation that drives yet another sfc boundary down from the north. So, after a relatively quiet Tuesday night, the boundary should approach thru Wednesday and drop into the fcst area late in the day. This boundary has a lot going for it forcing-wise, but not much thermodynamically, if the model guidance is correct. The model response is impressive enough to suggest ramping up to a likely to good chance of precip from north to south in the afternoon, so a relatively big upward adjustment was made based on the model agreement. The boundary is slow to move thru in some of the guidance and the agreement on the back end is still lacking, so we retain a chance of precip thru Wednesday night and into Thursday. The lack of instability should preclude much of a thunderstorm threat, much less anything strong to severe. Thursday precip chances are more uncertain and could easily be overdone if the boundary gets driven farther south and faster as in some of the guidance. Temps will be a category above normal for Wednesday ahead of the front, but the frontal passage cools temps off by ten degrees or so, and back below normal, for Thursday.
LONG TERM /THURSDAY NIGHT THROUGH MONDAY/... As of 121 PM Monday: The model guidance is in fairly good agreement that the frontal zone will finally be far enough south of the forecast area such that precip will end quickly around sunset on Thursday. That should give us a quiet start to the weekend as high pressure moves in Friday and then overhead through Saturday as the old eastern upper low gets kicked out of the Northeast. Temps warm back to normal from Friday into Saturday. The break in the action will probably be brief, however, as the atmosphere wants to fall right back into the omega blocking pattern starting Sunday with another vigorous short wave digging down across the Great Lakes. This wave should cut off another upper low from Sunday into Monday, in locations varying from the Midwest to the Ohio Valley and TN valley regions. So...after a weak flat upper ridge takes the sfc high overhead early in the weekend, we are right back into the cyclonic flow aloft Sunday night into Monday. Moisture and forcing both increase substantially as this happens, which threaten to spoil our Sunday. Our precip chances will increase accordingly, but for now only into the chance range for Sunday, and then above climo for Monday. For this package, that comes out to be a 60/50 mtns/Piedmont split, but one can easily imagine widespread shower activity with the pattern shown by the model consensus. The increased cloudiness and precip coverage should bring temps back down below normal for Monday.