... A line of showers with embedded thunderstorms will cross the area this morning, posing a risk of isolated strong to severe wind gusts especially southwest of Interstate 26. Very windy conditions will persist the remainder of the day following the passage of a cold front. Chances for showers and thunderstorms will occur periodically Sunday through Tuesday, with temperatures remaining mild through that time.
NEAR TERM /UNTIL 6 PM THIS EVENING/... As of 249 AM EDT Saturday: A potent negatively tilted trough will lift through the Ohio Valley today and into New England by tonight. At the surface, a deepening low will lift into the Great Lakes region with an associated frontal boundary pushing into the Southern Appalachians. The first focus of the forecast period will be ongoing upstream convection in the form of a QLCS that extends from just west of Knoxville to northern Alabama as of this writing. This line of storms will continue to surge east into the Carolinas and northeast Georgia through the early morning hours. Latest observations depict a reservoir of upper 50s to near 60 dewpoints across north Georgia into the Carolinas which is supporting several hundred J/kg of MLCAPE in a highly sheared environment. A 50-60kt low-level jet will translate across the area and help enhance hodograph curvature within the lowest 1km. A marginal threat for severe storms will continue across the western two thirds of the area as the line crosses the CWA. the main threat will be for damaging winds with a secondary threat for line embedded tornadoes. 0-3km shear vectors are quite robust and have supported numerous upstream mesovortices within northeast surging line segments. With time, the line will weaken with eastward extent and will likely become broken as well. What's left of the line should push out of the area by the mid morning hours.
Conditions will quickly clear through late morning into the afternoon as the frontal boundary ushers in much drier air. This will not be a noticeable cold front as a belt of deep southwesterly flow aloft remains draped from the Southern Plains into the Mid- Atlantic. The front will become oriented parallel to the upper flow which will allow it to stall in the vicinity of the South Carolina Midlands as upper support wanes. In fact, this flow regime will keep warm 850mb temperatures in place with afternoon high temperatures surging back into the upper 70s to mid 80s within a very deeply mixed airmass. Deep mixing up to ~700mb will also support the transport of gusty winds to the surface, thus the ongoing wind advisory remains well placed. A fire danger statement will also remain in place along the I-40 corridor where relative humidity values will fall to near critical values.
The 00z suite of CAM guidance depicts at least a low-end potential for convection to redevelop within the vicinity of the stalled boundary late this afternoon and into the overnight hours. Confidence in this scenario is low owing to widely varying model solutions and nebulous forcing as the main wave will have lifted well away from the area. However, parameter space on the warm side of the boundary will be quite favorable for severe convection IF a rouge storm(s) is able to initiate. Will carry low PoPs across the lower Piedmont, but will not advertise a severe threat for now owing to low confidence.
SHORT TERM /6 PM THIS EVENING THROUGH MONDAY/... As of 1230 AM Sat: Weak high pressure will be present over the Mid-Atlantic states Sunday, beneath quasi-zonal upper flow on the northern periphery of a broad subtropical high. Temperatures will fall back perhaps a category from Saturday's readings, and dewpoints probably will dip several degrees lower within the high. Winds will remain seasonably breezy.
By Sunday morning the cold front will have stalled along or south of the I-20 corridor; convection looks to be ongoing near the front in MS/AL. A jet streak will amplify over the Mid-Atlantic coast, providing some upper divergence over our CWA within its right-rear quadrant. This will help to reactivate the front, resulting in PoPs spreading back into the CWA from the south during Sunday. Synoptic models generally depict vortices emanating from the MS/AL convection and advecting toward the Carolinas during the day. The NAM seems to respond to those vortices more than the other models, leading it to bring PoPs back several hours sooner. Of note, the NAM also depicts a swath of SBCAPE on the immediate warm side of the boundary, which in its depiction includes some of our southernmost zones. This would coincide with large deep shear parameters afforded by the tight gradient aloft. SPC Day 2 retains that part of our CWA in Marginal Risk, which appears contingent on a NAM-like solution. Elevated storms above the warm front certainly are a possibility so we will carry a broad area of mentionable thunder chances.
Model consensus continues to suggest the best PoPs will be Sunday night or Monday morning, as a weak low ejects from the Western trough, crosses the Midwest, and backs the flow over the front around that time. Lapse rates improve by the same token, and a few elevated t-storms will remain possible during that time. The jet streak and Midwest low will both track eastward during the day Monday; the upper divergence will exit our area accordingly, and the frontal boundary, which will effectively be stalled, will then sag southward again. Model solutions begin to diverge beyond that. Trailing the low, a cold front will push southeast across the Ohio Valley but models/ensemble members are not yet in good agreement as to whether it reaches our area at all. It does look like PoPs should taper off for a time. Cloud cover lingering thru much of the day will have some impact on temps even if we do not feel any impact of CAA.
LONG TERM /MONDAY NIGHT THROUGH FRIDAY/... As of 115 AM Sat: The GFS and NAM actually come back into agreement Monday night, both depicting dry high pressure settling south thru the CWA, and prolonging a lull in precip chances into Tuesday morning. That high would reinforce a stalled boundary to our south, although there will be some vestige of the earlier boundary in that area regardless. The next feature of interest will be another shortwave moving thru the Midwest Tuesday. This wave is better defined than the previous couple, but models vary in whether it reactivates the front, and/or how quickly the associated precip returns to the CWA. The ECMWF, GDPS, and a number of GEPS members bring precip back to the area Monday night and Tuesday by that mechanism. The models reconverge Tuesday night in depicting the shortwave axis passing the Appalachians along with a stronger cold front, This may result in NW flow PoPs as well as strong wind gusts in higher elevations, which could warrant at least a Wind Advisory. The incoming airmass will bring temps back to near seasonal normals and PoPs to near zero for Wed, and likely Thu as well. The sfc high is still depicted as being transitory, migrating to the East Coast by Thursday night. By that time a very deep Western trough will have evolved, supporting Plains cyclogenesis Thursday and Friday. Moisture will return ahead of the resultant cold front, which could reach the area as soon as Friday night.