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FXUS62 KGSP 032145 AFDGSP

Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 545 PM EDT Fri May 3 2024

SYNOPSIS
Dry and warm high pressure moves off the Carolina coast as a front approaches from the west then stalls over our region for the weekend. This front will result in periodic thunderstorms and showers starting today and lingering through the weekend. A general summertime pattern sets up early next week, which will likely support scattered thunderstorm development each afternoon.

NEAR TERM /THROUGH SATURDAY/
As of 520 PM EDT: Major change at this update is to reflect precip coverage in the next few hours, generally supporting lower PoP for most of the area. Convection has been on a downward trend in the last hour or so, along the persistent axis near I-77, seemingly being outflow dominant without enough instability or forcing to foster new development. Water vapor imagery and RAP analysis depict weak vort max over the CWA, which maintains some deeper cu west of that activity. This activity will move into the rain-cooled region and likely won't amount to much more, but remains worthy of an isolated or widely scattered shower mention. In the more unstable area from the Savannah Valley, Smokies and westward, area of rain with embedded storms will linger, but diminish nocturnally to some degree. At this point, main impacts are expected to be locally heavy rainfall, with whatever severe threat we had having peaked. The outflow wreaked havoc on our temp and dewpoint grids thru the evening; it is always tricky to get the grids back on track when guidance reflects current conditions as poorly as it did at the top of the hour, but we have done the best we could using HRRR and short-term consensus blend to make believable trends for the evening. Confidence is too low to say exactly how, but the increased sfc moisture from the activity could enhance low ceiling and fog potential in some areas toward dawn. For now we retain patchy fog wording in most zones.

CAM guidance still shows an uptick in shower activity overnight in the continued moist and weakly forced atmos, with continuing southerly upslope flow. Coverage then continues to pick up through the day Saturday as forcing becomes better. Instability and shear will remain on the weak side, while forecast profiles show an even more saturated atmosphere. This suggests and even less chance of strong to severe storms, with a slight uptick in heavy rain potential. That said, the overall flood risk remains low given the relatively dry antecedent conditions. Highs will be near normal due to clouds and precipitation.

SHORT TERM /SATURDAY NIGHT THROUGH MONDAY/
As of 240 PM EDT Friday: The rest of the weekend looks active right into the first part of next week. A baggy upper trof overhead on Saturday night slowly lifts northeast with a short wave passage, and this bit of lingering forcing will be able to keep shower/thunderstorm activity on a slow wane into the early morning hours. Guidance suggests that it will never die off entirely, so the fcst retains some fairly high precip probs. That sets the stage for Sunday. Models show less forcing, but an uptick in the available buoyancy in the afternoon. That should make for a more thermodynamically-driven day, with better than climo coverage of showers and storms. Forecast soundings off the NAM show some pulse-severe wind damage potential with mid-level dry air and a shallow inverted-V, so it would not come as a surprise if it was a busier day than Saturday. A stronger short wave lifting northeast over the TN/OH Valley regions on Monday will continue to support diurnally-enhanced convection, so we continue with above climo precip probs. Temps look to be on the order of five degrees above normal.

LONG TERM /MONDAY NIGHT THROUGH FRIDAY/
As of 143 PM EDT Friday: The bulk of the medium range looks like a summer preview. After a baggy upper trof moves past Monday night, we should spend the remaining week underneath a low amplitude, relatively fast, WSW flow aloft. Within this flow, the model guidance shows a steady train of short waves moving through, mostly around peak heating each day, that will enhance our convective coverage each afternoon/evening. Overall, the air mass shows little change from one day to the next, as a wavy surface front will be strung out to our north in the WSW flow. Buoyancy will be modest and shear will be light, which suggests that chances for severe storms will be relatively low, but in such a pattern we eventually seem to manage to crank out a few pulse severe storms each day. Precip will be spotty and showery, which keeps the heavy rain threat at bay, but eventually there could be a few spots that start to see an increased risk of flash flooding, especially if a more coherent, stronger short wave comes along, such as what the GFS shows next Friday. Lots of uncertainty, though. Temps will remain in that 5-10 degree above normal range for most days, perhaps on the high side of that either Wednesday or Thursday, which one of those days might get a break in the wave train and thus reduced shower coverage and higher temps.

AVIATION /22Z FRIDAY THROUGH WEDNESDAY/
At KCLT and elsewhere: Convective activity diminishes somewhat this evening but returns overnight. Have PROB30s in place for that. Cig and vsby restrictions develop overnight as well, so have MVFR to MVFR before 12Z and IFR after 12Z. Light and variable wind expected overnight with S to SE wind for all but KAND, where it will be ENE, Saturday.

Outlook: Enhanced moisture will combine with a weak/stalled front through early next week, resulting in a period of active/mainly diurnal convective weather each day. There will also be the potential for restrictions associated with widespread fog and/or low clouds each morning.

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

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