Forecast Discussion
Summary
SYNOPSIS
... Very warm temperatures return again today. Preceding a frontal system, a line of showers with embedded thunderstorms is expected to cross the area Saturday morning. Dry and warm weather will return for Sunday. Small chances for rainfall are expected Monday and Tuesday with an additional frontal system.
NEAR TERM /THROUGH TONIGHT/... As of 238AM EDT Friday...Early morning water vapor imagery continues to depict a stout sub-tropical ridge parked over the Straits of Florida while a shortwave trough begins to eject out of the Desert Southwest and into the Southern Plains. A resulting broad belt of deep southwesterly flow is entrenched from central Texas into the Ohio Valley with a conga line of convection anchored on a similarly placed warm front. Farther east, across the Carolinas and northeast Georgia, warm moist advection will ramp up on the western flank of surface/upper ridging with return flow trajectories emanating from the Gulf of Mexico. Today will feature mostly sunny skies with only a few mid- and high-level clouds. Anomalously high heights and low- level temperatures will support well above normal high temperatures in the upper 70s to mid 80s. A few readings pushing into the upper 80s cannot be completely discounted across the lower Piedmont and the Charlotte metro. Deep mixing will also help bring down gusty winds late this morning through the afternoon.
As we head into tonight, attention quickly turns to what will likely be an upstream severe weather outbreak. A significant severe weather episode will be ongoing from the Lower Mississippi Valley into portions of the Tennessee Valley as the aforementioned shortwave trough takes on a negative tilt as it lifts into the Ohio Valley. An attendant ~990mb surface low will in turn lift through the Mississippi Valley and into northern Indiana by Saturday morning. A mixed convective mode within a pre-frontal surface trough/confluence zone will eventually grow upscale into a robust QLCS across northern Alabama and central Tennessee. This solid line of storms will race east, especially once a deep cold pool becomes well established. As such, have leaned the forecast more in line with the HRRR/HRW FV3 which bring convection into the mountains around 4am and to the I-77 corridor by daybreak Saturday morning.
The next question pertains to the severity of convection as it pushes into the area during the early morning hours. Continued warm/moist advection will bring a plume of upper 50s to near 60 dewpoints across much of the area, which should be sufficient to support a narrow swath of 100-500 J/kg of MLCAPE. Strong forcing/height falls aloft will further act to augment low values of instability. There will be no lack of vertical wind shear as a 50- 60kt low-level jet translates across the area, however, flow will be quite veered above the lowest 1km with predominately unidirectional southwest flow. This will confine the vast majority of hodograph curvature and effective SRH below 1km, which wouldn't necessarily preclude a severe weather threat as the 0-500m layer plays an important role in tornado potential. Regardless, the parameter space will be supportive of maintaining at least a marginal threat for severe weather across the western two thirds of the area with the main threat being damaging winds. A secondary threat for tornadoes cannot be completely ruled out if any bowing line segments can surge northeast in alignment with hefty 0-3km shear vectors. These segments would be prime candidates for mesovortices and line embedded tornadoes. CAM guidance does depict a noticeable weakening trend with eastward extent during the predawn hours, which lowers confidence in how far east any low-end severe threat would persist. Ultimately, the magnitude of any severe weather threat will be dictated by upstream convective evolution which won't be apparent until the evening/overnight hours. Thus, the forecast will likely change between now and then and the reader is encouraged to stay up to date with the latest forecast through the evening.
SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/... As of 145 AM Fri: The Bermuda High will maintain strength over the Southeast coast through the short term period; the parent low driving the front will skirt by to our northwest, and 500mb heights change little thru Sunday. We get a glancing blow of upper divergence on Saturday, but the synoptic scale DPVA looks to remain nearer the track of the upper low. The tight height gradient surrounding the low will support very strong winds through most of the column.
PoPs Saturday are pretty much entirely dependent on the prefrontal line of convection, which likely will be weakening/decaying around 12z. Surface-based convection would be exposed to remarkable shear/SRH, particularly further north and west in the CWA. Given the expected timing, that does not look especially likely. A narrow plume of SBCAPE is possible along the line, and current Day 2 SPC Marginal Risk certainly looks warranted, and it would not be surprising to see it extended further north into the CWA on subsequent updates. Timing varies perhaps a tad more than usual. The HRRR and HRW-FV3 lead the pack, depicting the line in the eastern half of the CWA by 12z Saturday, whereas the HRW-ARW and NAMNest are more closely aligned with the synoptic models, depicting it crossing the Smoky Mountains at that time. Given the strong linear forcing, we are leaning in favor of the faster solutions. To that effect, PoPs will peak early in the day, diminishing in value from west to east; precip is expected to have ended in most of the CWA by 18z. The actual surface front will reach the CWA a few hours following the convective line. Though winds may be gusty ahead of the front, deep mixing should develop behind it, and near-advisory gusts will result in higher elevations and likely some lower elevation areas of the Foothills and upper Piedmont. That is, winds are more likely to be impactful toward the north and west.
The front will stall Saturday night just to our south, likely extending from the central Gulf Coast to the SC Midlands, as the parent low becomes increasingly distant. A weak post-frontal high will settle into the area, making for a more seasonably breezy day. With the aid of developing upper divergence, the boundary will reactivate fairly quickly on our southern fringe, which should offer our next round of PoPs. The NAM is most aggressive, bringing it northward before the end of the day Sunday, whereas most other guidance shows that occurring Sunday night and early Monday; this seems to be the result of the NAM depicting much stronger cyclogenesis with a weak shortwave crossing the Midwest.
Temps over the weekend won't be quite as warm as Friday, but will remain on the order of 10 degrees above normal for mins and maxes. That said, records look unlikely to be jeopardized.
LONG TERM /MONDAY THROUGH THURSDAY/... As of 245 AM Fri: Upper divergence along the stalled front will diminish Monday; a weak shortwave also will eject from the western CONUS trough around that time, and induce slight drying aloft. As a result, PoPs taper off during the day as the boundary again shifts south, leaving us with at least a short period of dry weather. A better defined shortwave will pass to our north circa Tuesday. Some guidance depicts that wave as activating the stationary front yet again, possibly with a sfc wave developing along it near the NC/SC coast. However, the better agreed upon solution simply brings a weak cold front across the Appalachians Tuesday or Tuesday night. Either way, we advertise only slight-chance to chance range PoPs to reflect these scenarios. Behind the cold front, continental high pressure will bring more seasonable air back to the region; it is not out of the question precip will end as light NW Flow snowfall along the TN border Tue night. Near-normal temps are expected Tue and Wed, moderating above normal by Thu. The high remains transitory, quickly migrating to the East Coast, and by the end of next week it could be in position to initiate return flow and small PoPs.