... A strong cold front will cross the area from the northwest today, bringing the potential for strong storms. The front will usher in cool and dry high pressure, which will build into the area through the end of the week. Dry weather and seasonable temperatures will continue into early next week.
NEAR TERM /THROUGH TONIGHT/... As of 653 AM EDT Wednesday: Showers continue to stream northward east of I-77 at this hour...on the western edge of a weak southerly conveyor belt-like flow with slightly better isentropic upglide. More shower activity...and a few thunderstorms...was noted to our west coming off the eastern edge of the Cumberland Plateau. In-between the two...only a few showers, mainly in the upslope areas of Upstate SC, but no doubt there are many patches of drizzle. The lull will end during the middle part of the morning as the pre-frontal showers and storms reach the wrn tip of NC.
Today looks active. The main player will be a mid/upper low that closes off over srn IL and then moves up the OH Valley during the day, then northward toward the Great Lakes tonight. The movement of the upper low will push a strong cold front toward the region this morning and then across the fcst area through the day, finally moving east of the fcst area in the middle part of the evening. Strong/severe thunderstorms are the main concern ahead/along the cold front, but as it looks at this time, we appear to be a few ingredients shy of having a good chance of severe. First problem is that we will begin the day with a remnant pool of relatively stable air across the fcst area. Model guidance suggests this will be moved out from the south during the afternoon ahead of the front, but perhaps only east of the Blue Ridge. Sfc-based CAPE will remain fairly modest...perhaps upwards of 1500-2000 J/kg over the eastern zones late in the day. Confidence in this happening is only average. We lack mid/upper forcing in spite of the strong front, because the best forcing lifts northeastward on the W side of the mtns and the main mid/upper trof axis stays well to our west thru the day. We also lack a low level jet, which keeps the deep layer shear relatively weak, maybe 20 kt at best. Low level convergence along the boundary looks excellent, however, and for that reason a categorical precip prob will be kept. The Marginal Risk on the Day 1 Outlook from SPC is accepted on general principles and will not be surprised to see a few severe storms, mainly along/east of I-77, but the number of severe storms will probably be limited by the modest shear. The QPF does not look great enough to suggest a flood threat and storms should be moving right along. As for timing of the storms, the model blend looks excellent and was used for the fcst. High temps will depend on precip timing. The precip cuts off quickly from west to east behind the front this evening, except in the W/NW flow upslope areas along the TN border, where some light precip could linger into early Thursday. The cooler and drier air will steadily filter into the region behind the front overnight, such that low temps Thursday morning should be about 15 degrees cooler than the last few nights.
SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/... As of 230 AM Wednesday: The short term forecast picks up at 12z Thursday with a closed low drifting slowly northeast across the southern Great Lakes. A NWLY flow on the south side of this low will be over the Southern Appalachians, resulting in lingering clouds and perhaps a few sprinkles near the TN border and breezy conditions. CAA will keep temps about 7-15 deg below normal in the mountains, and 5 deg below normal east. It will definitely feel like fall has arrived just a day after the equinox, as continental high pressure builds in and settles over the region. Clear skies and weakening wind should allow temps to fall into the 30s on the ridgetops and 40s to lower 50s in the lower elevations. There may be some frost on the ridgetops, but enough wind and low dewpts should limit the potential. There will probably be decent fog in the mountain valleys and around lakes as the cool air moves over the warmer water. Similar conditions can be expected on Friday, with lighter winds.
LONG TERM /SATURDAY THROUGH TUESDAY/... As of 230 AM Wednesday: Dry high pressure will persist over the Southeast through the weekend and into early next week. Another closed upper low looks to rotate toward the area late Saturday, bringing a reinforcing cold front across the Appalachians early Sunday. The fropa still looks dry on the 00z suite of deterministic guidance, but they now show some precip potential just to our north late Saturday. So the forecast is still dry thru the entire medium range, but cannot rule out a brief period of isolated to scattered showers with the fropa, if there ends up being more moisture than currently expected. The air mass will gradually modify thru the period, so expect a slow warming trend, with near normal temps Saturday, followed by slightly above normal readings early next week. Fortunately, humidity will remain comfortable.