Area Forecast Discussion
Issued by NWS Greer, SC
Issued by NWS Greer, SC
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839 FXUS62 KGSP 131804 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 204 PM EDT Mon May 13 2024 .SYNOPSIS... Low pressure brings rain and some thunder to our region on Tuesday and Wednesday. Brief high pressure brings one dry day on Thursday. Rain and some thunder returns on Friday and remains through the weekend. && .NEAR TERM /THROUGH TONIGHT/... As of 945 AM: Change is on the way as an upper ridge axis moves east of the area today. Short waves rotate around an upper low, that drops into the mid MS valley, and across the area late today and tonight. This brings an increase in deep moisture later today into tonight as deep southerly flow develops ahead of a slowly advancing warm front and low pressure system. High-based light precip has occurred over much of N GA so far this morning, but the dry sfc airmass in place over our area has so far led this precip to weaken or dissipate completely as it has advected toward our CWA. Nonetheless, seeing how trace precip has given way to measurable amounts at a few sites, saw fit to increase PoPs slightly through the remainder of the day, particularly in our southwest zones over the next 3-4 hrs. Also touched up temps a bit, with the quicker onset of precip and already abundant cloud cover, with highs being lowered a couple of degrees in many spots. Temps still will feature an unusual gradient with the precip development: almost normal over the I-40 and I-77 corridors where precip arrives later, but below normal along and west of the I-26 corridor where precip arrives earlier. Low level moisture and insentropic lift, along with increasing synoptic forcing from the short waves and upper divergence from a coupled jet streak, move in during the afternoon. Clouds will thicken and lower more quickly by that time as showers pick up and saturate the low levels. The best coverage will be west of the I-26 corridor by the end of the day with light QPF. If any CAD develops, it will be weak and late in the day despite the clouds given the slow advancement of the showers. Winds will begin adjusting to the SE where precip develops, but will be southerly through the bulk of the day. Precip coverage increases during the evening into the overnight as moisture and forcing remain over the area. Isolated TSRA will be possible overnight as weak MUCAPE develops with only a weak CAD at best and better lapse rates. Moderate QPF is expected generally from along the I-85 corridor to the Blue Ridge. Isolated heavy rainfall will be possible as well. Lows will be around 5 degrees above normal. && .SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/... As of 243 AM EDT Monday: Deep trough will close off into an upper low over the Ozarks on Tuesday, and ripples of DPVA will round the base of this low into the Carolinas throughout the day. A preexisting weak in situ wedge will be in place east of the mountains early Tuesday, but as low-level WAA intensifies it will erode, and the wedge front will gradually retreat northward. This will likely expose at least the southern third of the forecast area to some surface-based instability, to the tune of ~500 J/kg. Meanwhile, a 500mb speed max will lift out of the lower Mississippi Valley into GA and SC, allowing deep layer shear to surge to 45kts or more. Thus, confidence on some severe risk on Tuesday is increasing. In theory, lapse rates appear too meager for a widespread large hail risk; rather, wind looks to be the primary threat with any severe storms that develop. SPC`s Marginal Risk for Severe Weather on Tuesday looks reasonable. Highs will climb into the low to mid 70s...limited by widespread clouds and rain. Tuesday night will feature a lull in rainfall as diurnal instability wanes and the most potent lobe of DPVA translates east of the forecast area. Lows will fall into the low 60s. On Wednesday, instability will be more widespread and enhanced, with most operational models now depicting a plume of >1200 J/kg over the eastern zones and the LREF ensemble mean landing right around 1000 J/kg for areas east of I-26. So, coverage of convection should be widespread ahead of an advancing surface cold front, which will pass through during the afternoon hours. Sans stronger winds aloft, deep layer shear is only forecast to reach 25-30kts by the bulk of guidance sources, so severe risk should be more muted. Highs will climb into the upper 70s on Wednesday. && .LONG TERM /THURSDAY THROUGH SUNDAY/... As of 253 AM EDT Monday: Rainfall chances will drop off sharply by Thursday morning as the axis of an expansive upper ridge translsates across the Eastern Seaboard. Ensemble soundings depict the development of a weak subsidence inversion at around 700mb, which should be enough to inhibit any convective response on Thursday. Afternoon highs will surge into the 80s. The next disturbance will arrive in the form of a southern stream shortwave trough, which will carry deep moisture and ripples of DPVA forcing out of the Gulf Coast area and into the Carolinas. Lapse rates look slow to steepen on Friday, but enough instability will develop by afternoon that most guidance is indicative of scattered diurnally-driven convection. By Saturday, the operational models depict a robust frontal zone entering the Ohio Valley and spurring a round of well-forced afternoon convection across the Carolinas. Around 2/3 of the broader ensemble envelope feature a complete frontal passage followed by the arrival of continental high pressure for Sunday afternoon and beyond. The remaining ensemble members feature a slower frontal passage and lingering rainfall chances on Sunday. In general, the pattern looks dynamic enough going into next week that confidence in any particular solution is pretty low. && .AVIATION /18Z MONDAY THROUGH SATURDAY/... At KCLT and elsewhere: A wave of sprinkles will pass over the terminals between 18-21z which should promote formation of in-situ cold-air damming and create lower VFR cloud decks. Recent convection allowing models are developing shallow convection in some parts of the area after 20z, notably near KCLT, but this looks too shallow to warrant inclusion of TS. The chance of SHRA otherwise should slowly increase from W to E this evening, with continued moisture flux and increasingly frequent precip to lower cigs to MVFR. Once precip becomes steady IFR is expected to develop. KAND/KGSP/KGMU expected to see wedge-like ESE to NE winds given earlier development of cold pool, but other sites should stay SE. Heaviest precip expected in the early morning and a TS is not entirely out of the question then. Precip coverage expected to diminish with loss of nocturnal LLJ and vsby may fall to lower MVFR or IFR after daybreak. Although confidence is low as to how it unfolds, some degree of diurnal instability will develop Tue and suggests PROB30 for TSRA near the end of the period at all sites except KAVL. Slight improvement in cigs/vsby possible Tue aftn but widespread improvement appears unlikely Tue night. Outlook: Restrictions should ease Wed morning but scattered TSRA could redevelop in the aftn and cause brief return to IFR. Some nocturnal restrictions likely again Wed night anyway. Brief drying expected Thursday. Convection and associated restrictions return ahead of a cold front on Friday. && .GSP WATCHES/WARNINGS/ADVISORIES... GA...None. NC...None. SC...None. && $$ SYNOPSIS...MPR NEAR TERM...RWH/Wimberley SHORT TERM...MPR LONG TERM...MPR AVIATION...Wimberley