... A stationary front and low pressure brings much-needed rain to the forecast area through Tuesday, after which it lifts north as a warm front on Wednesday. A cold front is expected to cross the region from the west by Thursday night, bringing an end to the rain on Friday. Dry high pressure prevails for the weekend.
NEAR TERM /THROUGH TONIGHT/... As of 654 AM EDT...KGSP radar shows patches of light rain moving slowly out of the western Piedmont of NC associated with the weak sfc low analyzed over the NW Piedmont. As long as the sfc low remains, more shower activity is possible, but precip chances will be relatively low across most of the region, and below 15% across most of northeast GA/Upstate SC where light downslope flow had taken over.
Another complicated forecast for today as the weak sfc low should lift out to the east with a weak short wave. As this happens, and a short wave ridge moves up from the southwest, the circulation around the weak low should combine with the sfc ridge trailing down to the lee of the Appalachians to finally establish a north/northeast flow that will bring the onset of late-season cold air damming. The timing of this development will have a great bearing upon the high temps for today. The NW Piedmont is expected to stay cloudy and cool and on the order of ten degrees below normal because the developing wedge gets there early in the day. Meanwhile, the Upstate and northeast GA will have some opportunity to warm up this morning as the lingering downslope flow affords some sunshine. The lower Piedmont could still easily make the low/mid 80s. In both places, there is a high potential for a temperature forecast bust. The push of the developing wedge boundary will be a focus/trigger for some shower activity this afternoon/evening, with that focus eventually shifting to the easterly upslope areas along the Blue Ridge Escarpment after dark. Guidance affords some modest sfc-based CAPE ahead of the boundary so thunderstorms could happen, but not nearly to the extent and severity as yesterday. Low temps tonight should remain close to normal, with light precip/drizzle continuing mainly over the mtns.
SHORT TERM /WEDNESDAY THROUGH FRIDAY NIGHT/... As of 300 AM EDT Tuesday: Wednesday will feature the deepening of an already-potent Z500 trough across the south-central Great Plains, fueling cyclogenesis in the interior of the country. Meanwhile, a broad surface high will linger over extreme New England into Quebec, allowing in situ cold-air damming to continue through much of Wednesday. 300K charts depict weakening isentropic lift over this wedge, which should allow for only very limited PoPs through Wednesday. South of the wedge boundary, it's possible that a few showers may develop, but relatively low CAPE and anemic lapse rates should limit severe weather chances even there.
As the influence of the deepening Plains cyclone begins to increase Wednesday night and the receding Quebec high becomes increasingly distant, CAD will become increasingly shallow and the atmosphere will begin to look more and more like a warm sector regime, with the remnant wedge boundary lifting north as a warm front. Thus by Thursday, the environment may be ripe for convective development ahead of the attendant cold front, which looks to slide across the Ohio Valley Wednesday night into Thursday morning. PoPs tick up after daybreak Thursday, with an associated convective risk in the afternoon. sbCAPE values climb to only 600-900 J/kg Thursday afternoon - a notable downward trend in model output - yet as the upper pattern tightens up with the approaching uppper low, deep layer shear still looks to surge to 30+ kts. This warrants a severe risk, as well as an increased excessive rainfall risk now that models are trending toward a slower frontal passage and more prolonged precipitation lasting into Thursday night.
Temperatures on Wednesday will be a category or two below normal as a result of the ongoing CAD event and widespread cloud cover limiting insolation. Temperatures will creep closer to normal by Thursday as the CAD erodes, yet still should remain a degree or two below climo.
LONG TERM /SATURDAY THROUGH MONDAY/... As of 215 AM EDT Tuesday: By daybreak Friday, a Z500 cutoff low will be situated somewhere over the western Ohio Valley; what sensible weather unfolds for the weekend will be ultimately dependent on how far north this feature and its surface reflection set up. To wit, the GFS solution continues to keep this feature farther south and less progressive, resulting in a slow, overnight cold frontal passage which will still be underway by the first part of Friday and which will be followed by a decent slug of additional moisture through the day as the parent low occludes. GEFS members are beginning to converge on this solution after days of waffling about. Yet, the ECMWF and CMC solutions remain more progressive, bringing the front entirely south and east of the forecast area by early Friday afternoon and keeping any wraparound moisture less robust and confined to the NC mountains. So, safe to say there's too much uncertainty now to make much more than conjecture on how Friday will unfold.
It does, however, look like the weekend will be dry, with any residual moisture from the departing low becoming increasingly shallow by Saturday. Profiles should be fairly stable by this point as deep-layer subsidence sets up across the Carolinas, so clearing skies and warming temperatures are expected as thicknesses tick up across the area. The operational GFS becomes an outlier at this point in keeping moisture socked into the Carolinas well into Saturday evening, as most of its ensembles do not depict this, nor do any of the other global guidance sources. Temperatures will begin to tick back up in the wake of this activity, starting off around a category below normal on Friday afternoon and rising each day before reaching at least a category or two above normal by Monday.
Monday looks to carry a modest convective risk - some models suggest that as high pressure slides off the Eastern Seaboard and Atlantic/Gulf moisture begins to once again impinge on the forecast area, the influx of low-level moisture will generate modest sbCAPE values on the order of 500-1000 J/kg in the afternoon. Much too early to begin harping on the details, but this will need to be monitored in the coming days.