... A weak surface front will linger over the Southeast States from the Gulf coast to the Carolina coast through the middle of the week as it slowly dissipates. Moisture will gradually increase from Thursday onward, keeping daily shower and thunderstorm chances in the forecast. Another weak front will approach from the north during the weekend.
NEAR TERM /THROUGH WEDNESDAY/... As of 140 PM: Sfc high pressure is centered just north of the Ohio River. At its southern edge, a virtually stationary front remains across GA/SC, immediately south of our CWA. The front is mainly evident from dewpoint gradient. RAP and NAM prog soundings depict a weak subsidence inversion over the CWA thru this afternoon, thanks to the sfc high. CAMs remain mostly dry other than a few blips of simulated reflectivity near the front across the southern Upstate. Small PoPs are retained in the lower Piedmont but the remainder of the area should stay dry the rest of the day.
Aloft, a broad trough extends across the northeast quadrant of the CONUS. A shortwave rounding the base of the trough will reach the southern Appalachians this evening. Coupled with eastward migration of the high, the trough should bring low-level flow around to a southerly direction, reactivating the front and pulling it north across the area. Lapse rates aloft may improve slightly, but more importantly the southerly flow makes for a small amount of viable MUCAPE. Accordingly, shower and/or weak thunderstorm chances will expand northward across more of the CWA overnight.
More typical summertime weather is on tap for Wednesday. Subsidence inversion will have been weakened, but the return of mid-upper 60s dewpoints will also make for more favorable convective profiles. Aided by weak southwesterly to westerly flow, CAMs depict convection firing over the mountain ridges, with only slow/limited propagation into the foothills/Piedmont. Presumably that is a result of weak shear and a lack of dry air aloft and thus reduced cold pool formation. The shortwave axis will be to our east and may have a negative effect in our eastern zones, as well. Will feature likely PoP for a portion of the Blue Ridge Escarpment, and mainly scattered coverage at peak elsewhere. Max temps still look to be impacted by cloud cover, but should be a tad warmer than today.
SHORT TERM /WEDNESDAY NIGHT THROUGH FRIDAY/... As of 145 pm EDT Tuesday: A stout southern CONUS ridge will return Wednesday night through Thursday, with the ridge axis migrating over the southeast coastline through Friday. Low-level southeasterly flow under the ridge will provide for higher than typical summertime cloudiness as moist layers advect back into the region. The upslope component will also produce triggering of scattered thunderstorms during peak heating, with a mainly mountain and southwest CWA focus to the activity. Better moisture pooling near the base of the southern Appalachians and the upper Savannah River area will support likely PoPs with the diurnal peak convection by Friday afternoon. The warm late week profiles suggest limited severe weather potential, but increasing precipitable water in profiles and weak steering flow will make for slow-moving convection and locally heavy rainfall at impact locations. The diurnal range of temperatures through the period should be slightly smaller than climo given the clouds.
LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/... As of 150 pm EDT Tuesday: A weakness in the eastern ridge will develop Friday night through Saturday as a deep cyclone moves east from Ontario to Quebec and heights fall from the Great Lakes to the southern Appalachians. This will permit a weak cold front to settle southward through the Ohio Valley on Saturday. Confidence is then low on the frontal progress through Sunday, but the boundary may stall in or just north of the area by Sunday afternoon. Precipitable water values should peak on Sunday when moisture convergence ahead of the front interacts with increasing Gulf and Atlantic moisture around the Bermuda high. Well-above-climatology PoPs are indicated for Sunday, and a heavy rainfall threat with training or slowly moving cells is possible. This is too uncertain to mention in the HWO at present.
Monday and Tuesday then become even more uncertain depending on the southward frontal progress, but summertime conditions will continue with a return to more prominent ridging aloft. Will feature warmer maximum temperatures and continued above climo PoPs for the early week period.