Sunrise 6:14 AM   Sunset 8:23 PM Moonrise 2:57 PM   Moonset 2:39 AM

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FXUS62 KGSP 181108 AFDGSP

Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 708 AM EDT Sat May 18 2024

SYNOPSIS
A weak low pressure system will track east across North Carlina today, bringing another rounds of showers and thunderstorms to the area. The low exits off the Carolina Coast on Sunday, with weak high presure building in from the north. Somewhat drier conditions return early next week resulting in only isolated afternoon and evening showers and storms. A cold front will bring better shower and thunderstorm chances back into the forecast late in the week.

NEAR TERM /THROUGH TONIGHT/
As of 630 AM Sat: Ill-defined, somewhat unusual pattern in place over the region, with synoptic warm front edging northward into the CWA ahead of a very weak low pressure system in the Mississippi Delta region. Southwesterly low-level flow continues between the associated upper low and a flat ridge centered off the SE coast. Very light precip is occurring over the southern half of the CWA associated with the warm front.

Although there is some MUCAPE on short term analyses, prog soundings depict a deep inversion. 925-850mb flow has veered to SW via the orientation of the ridge but also following the shortwave to our north. 850-700mb lapse rates are shown to increase with the warm front after daybreak along with slight increases in sfc dewpoints; thinning higher cloud decks should allow diurnal warming and perhaps even a few breaks in the stratocu nearer the sfc. These developments suggest the inversion will mix out and/or be overcome by sfc parcels by late morning, so PoPs ramp up from the south from then into early afternoon. The mountain ridges also will destabilize via the warming and the westerly flow may kick off a few showers there as soon as late morning. Abundant cloud cover will persist and looks to keep temps a little below normal once again.

Height falls will occur later in the afternoon as shortwave arrives from the west. CAMs are in rather good agreement on more vigorous convection developing over the Escarpment around 20-21z, expanding in coverage through early evening and propagating SE. 700-500mb lapse rates will already be fairly good but should be maintained by the wave, and altogether 1000-1500 J/kg SBCAPE is likely to be in play at that time. Near-sfc winds will be light, and midlevel flow is initially modest but weakens under the shortwave, so the aftn convection could be fairly pulsey. Models vary as to how dry the midlevels remain, and that is likely key to the level of severe risk. Sfc-midlevel delta-theta-e values look lower than we usually see with severe pulse storms. SPC Marginal Risk is certainly sufficient for this low-end threat. Storms appear to be very efficient rainfall producers and given consensus in initiation time/area, PMM QPF is quite healthy with areas of 1-2" storm totals likely. Some storms could produce tree damage simply from weak organization along cold pools if they develop, and trees may topple more easily with many areas having saturated soils.

Pattern remains murky for tonight. A tilted ridge develops to our north in the wake of the shortwave, and a weak sfc high will build down the Eastern Seaboard. This looks to bring a wedgelike backdoor front into the area, likely encountering enough sfc moisture to create a low stratus deck and possibly also initiate elevated convection, with lapse rates still good above the associated inversion. PoPs thus diminish from north to south across the Piedmont overnight, although see fit to keep a slight chance all night there; chance values for the GA/SC zones and Escarpment. Lows will be several degrees cooler than this morning despite continuing clouds.

SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/
As of 230 AM EDT Saturday: The positively tilted upper trough axis will reach the Carolina Coasts by late Sunday, ushering in strong height rises atop the forecast area. The associated sfc low pres system be off the coast by 12z Sunday, with wrap-around moisture on the north side of the circulation from the Mid-Atlantic SW into the western Carolinas. Fcst soundings across the NC Piedmont and eastern Upstate show potential for a stratus deck to gradually scatter out thru the day, with a decent inversion atop the cloud layer. However over the mountains and across the Upper Savannah Valley, sbCAPE may still be realized with little to no CIN. This looks almost like wedge setup, albeit not from CAD. So have trimmed PoPs back within the wedge-like air mass, with solid chc PoPs in the mountains, but only slight chc across most of the I-77 corridor. Any tstms that do develop will have fairly weak shear and a somewhat suppressive synoptic environment, so severe threat will be low. With steering flow roughly parallel to the Blue Ridge out of the NE, training convection could lead to isolated heavy rainfall. Highs will be below normal, especially across the I-77 corridor.

Dry air continues to filter in from the N/NE Sunday night thru Monday, but there may be some redevelopment/expansion of stratocu overnight, before dissipating during the day Monday. Subsidence from an upper ridge should cap most deep convection, except for the higher terrain. There, enough CAPE and mountain-top convergence could lead to scattered showers an perhaps a few garden-variety tstms. Temps return to near normal under partly to mostly sunny skies.

LONG TERM /TUESDAY THROUGH FRIDAY/
As of 300 AM EDT Saturday: A warming trend with generally dry wx is expected thru Wednesday, as an upper ridge builds over the Southeast. Guidance continues to back off PoPs, and it looks like even the high terrain may be largely capped from deep convection. Will keep some slight chc PoPs in the NC mountains for now, both Tue and Wed aftns. With instability remaining weak and a lack of upper support, severe threat should remain low. Highs will be a category or two above normal, while lows will be 2-3 categories above normal.

A cold front will approach the forecast area from the NW Thursday, as shortwave energy rides over the eastern CONUS ridge. Thanks to the influence of the ridge, the front will struggle to reach the area, and guidance seems to be trending drier for the latter half of next week. There still should be some uptick in PoPs, as shortwave energy ejects out of the Rockies and works to break down the eastern ridge, but the National Blend of Models may be overdone, given the model trends. Temps will continue to be above normal with modest humidity.

AVIATION /11Z SATURDAY THROUGH WEDNESDAY/
At KCLT and elsewhere: Murky forcing regime results in lower confidence than usual for TAFs. Patchwork cigs at various levels at issuance time with -DZ or sprinkles across the SC sites, north of a weak warm front. Warm front will ooze northward and promote low level warming, promoting mixing and allowing destabilization. Light SW'ly winds will continue today. Cigs should lift to MVFR or low VFR. Convection could develop south of the warm front or over the mountains as soon as midday, but more likely will develop in the mid to late afternoon with peak heating as well as DPVA from shortwave to the west. Any storm will be capable of +TSRA and IFR vsby, but mention of IFR is limited to where confidence greater on TS coverage and timing. Storms that develop along cold pools will be capable of strong to damaging wind gusts as well. Convection should work W to E from the mountains to the Piedmont, likely ending by around 02z. Backdoor cold front settling into the area tonight may however spawn an isolated elevated SHRA or TS. More likely impact is IFR cigs and flip to NE winds overnight.

Outlook: SHRA/TSRA remain possible Sunday and perhaps even Sunday night until stronger high pressure settles over the region Monday. Late night/early morning restrictions in low stratus and/or fog will remain possible Sunday night; isolated mountaintop convection still appears possible Monday.

GSP WATCHES/WARNINGS/ADVISORIES
GA...None.
NC...None.
SC...None.

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