Forecast Discussion
Summary
SYNOPSIS
... A stalled front across Georgia and South Carolina will focus the development of two rounds of showers and thunderstorms, one today and another tonight or early Monday. Another round of light showers may occur Tuesday with a passing upper level disturbance. More seasonable, mild temperatures are expected Tuesday and Wednesday.
NEAR TERM /THROUGH TONIGHT/... As of 917 AM EDT Sunday: Still trying to get out from behind the 8-ball this morning with ongoing convection. Have made some upward adjustments to the precip probs to handle the showers and storms over the foothills/wrn Piedmont of NC. Outflow from this activity was briefly pushing the frontal boundary back to the south. Over the next few hours, we will have to keep an eye on rogue storms developing on the buoyancy gradient that runs along/S of I-85. Cooler air aloft was helping to foster a favorable environment for hail in these storms, altho nothing severe has been reported. The new Day 1 Outlook brings the Marginal Risk up to I-85 which is a reasonable scenario. Meanwhile, we cast a wary eye to the south where severe storms, including a few tornadic supercells, were moving along a corridor of favorable buoyancy/shear. Convective trends suggest the severe storms will continue to move to the east along the I-20 corridor, while a stratiform rain region with embedded thunderstorms expands on the north side of the deep convection training across central GA. The expansion/arrival of the stratiform region may effectively eighty-six our chances for severe storms over our northeast Georgia/Upstate SC for the better part of the daytime hours, until we can move it out to the east and recover the air mass.
Otherwise...The synoptic pattern will be characterized by a belt of strong west-southwesterly flow draped from the Southern Plains into the Mid-Atlantic between a departing shortwave trough lifting into New England and a broad trough situated over the Desert Southwest. At the surface, a largely stalled frontal boundary oriented parallel to the upper flow extends roughly along the I-85 corridor with low to mid 60s dewpoints on the warm/moist side of the boundary and upper 40s to 50s dewpoints to the north. Cool temperatures aloft overspreading the warm sector south of the boundary will promote steep lapse rates and moderate instability amidst very strong deep-layer vertical wind shear.
Morning convection and lingering showers will eventually shift east of the area with a lull in activity through much of the afternoon into the evening. Confidence in the temperature forecast remains low as any lingering clouds that are slow to clear out would result in cooler afternoon high temperatures. Have already lowered temperatures a degree or so below the national model blend, but further adjustments may be needed if cloud cover and/or showers linger longer into the afternoon.
Attention then shifts to the second round of convection expected overnight tonight. A subtle lead perturbation will eject out of the parent Desert Southwest trough this morning and lift through the Lower Mississippi Valley this afternoon and into the Appalachians tonight. This wave, while subtle, will provide enough forcing to initiate intense supercells across Mississippi and Alabama within potent parameter space further enhanced by strengthening low-level wind fields. This activity will grow upscale into supercell clusters and linear segments as it also propagates northeast along and south of the stalled boundary. Storms are currently forecasted to be knocking on the southwestern CWA border around ~Midnight to 2am Monday morning. Plentiful deep-layer vertical wind shear will be in place, but the thermodynamics are more uncertain. Airmass recovery and placement of the boundary behind this morning's storms will be important. If the airmass can recover and the boundary is far enough north a severe threat could materialize for at least the lower Piedmont with a threat for hail and damaging winds. Even with a farther south placement of the boundary, sufficient MUCAPE could still support elevated hailers and the potential for a few stronger wind gusts to penetrate a shallow stable layer. The Storm Prediction Center paints a marginal risk for severe thunderstorms generally along and south of the I-85 corridor for both this morning and then again tonight. This appears well placed for now, but further adjustments to the area and threat level may be needed once the mesoscale environment and convective evolution can be assessed.
SHORT TERM /MONDAY THROUGH WEDNESDAY/... As of 130 AM Sunday: Sfc/upper low center will pass just north of the CWA on Monday morning, with stalled boundary remaining across northern GA and SC. Low-level winds will veer on the back side of the low, effectively activating the boundary as a cold front, which will be driven southward thru the remainder of the day. Some elevated convection will remain possible during the morning and early afternoon, originating at or above the inversion incurred by the WAA. That likewise should preclude any surface based convection. CAPE profiles are narrow but overlap with 40+ kt of effective shear on some prog soundings; an elevated storm producing small hail would not be a big surprise. Confidence in the Monday forecast is limited by model spread: the NAM is more stubborn than the GFS, depicting mesolows along the boundary and taking until much later Monday to show post-frontal drying/clearing. Nonetheless, we can advertise diminishing PoP from NW to SE, starting late morning. Very minor northwest flow precip could redevelop along the TN border Monday afternoon or evening, but likely of little to no impact, especially as temps will be too warm for snow.
Some degree of airmass change will occur with the front Monday night, bringing temperatures back to near normal Tuesday. Another shortwave will cross the lower Ohio Valley Tuesday, phased with a lobe of the vortex then present over central Canada; jet will amplify downstream of these features and that alone suggests high altitude cloud cover Tuesday. Small PoPs will be advertised as a nod to those guidance members which either depict very light, high-based precip ahead of the shortwave, or that again reactivate the front to our south and pull precip northward into our area. Low-level flow will back ahead of that wave, which appears to offset drying; dewpoints still are expected to be 10-15 degrees lower in much of the area, compared to Monday. The passage of this second wave Tuesday night should bring a better chance for NW Flow precip, this time with temps cool enough that it could change to snow before ending. It would also reinforce near-normal temps, as calm sfc high migrates over the Southeast. Currently it appears temps will remain too warm for freeze concerns in the areas where the Frost-Freeze program is active, though frost can't entirely be ruled out in the more northern zones. Highs Wednesday should be similar to, if not a little cooler than Tuesday.
LONG TERM /WEDNESDAY NIGHT THROUGH SATURDAY/... As of 230 AM Sunday: 500mb heights begin to rise Wed night, with upper ridge migrating east and strengthening over the Eastern Seaboard through Thursday night. Attendant sfc high will drift toward the East Coast during this timeframe, supporting near-zero PoPs, and a warming trend lasting into the weekend.
A deep upper low will move across the western CONUS, and should lead to Plains cyclogenesis Friday. The incipient sfc low looks likely to track roughly from Kansas to Michigan between Friday morning and Saturday morning, pulling a cold front across the lower Mississippi Valley late Friday, likely producing severe weather in its warm sector. Current progs suggest that the front will cross our CWA sometime Saturday, but timing varies a bit between the global models and especially ensemble members. It is a bit too early to say with much confidence how much instability it will encounter given the timing differences. Peeking at the global models' ensembles, a few members of each depict a far stronger low than what is shown on deterministic runs, although there seems reasonably good agreement on the track. If a stronger low verifies, that should only further ensure that strong gradient winds would be present aloft and thus sufficient shear for a severe weather threat for the area; that might also improve moisture flux into the region, so for now it looks wise to stay abreast of the forecast for Saturday.