textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

There is now an isolated tornado threat late this afternoon into this evening across northeast Georgia and the western South Carolina Upstate.

KEY MESSAGES

1. A cold front will bring the potential for strong to severe storms late this afternoon into this evening with the main hazard being damaging wind gusts. However, there is now a low tornado threat across northeast Georgia and the western South Carolina Upstate through this evening. The severe threat will gradually wane tonight into daybreak Thursday but the locally heavy rainfall threat will gradually ramp up through Thursday morning as convection lingers over the area. 2. Drier conditions Friday and Saturday before rain chances increase Sunday and into Monday. Dry Tuesday and Wednesday.

DISCUSSION

Key message 1: A cold front will bring the potential for strong to severe storms late this afternoon into this evening with the main hazard being damaging wind gusts. However, there is now a low tornado threat across northeast Georgia and the western South Carolina Upstate through this evening. The severe threat will gradually wane tonight into daybreak Thursday but the locally heavy rainfall threat will gradually ramp up through Thursday morning as convection lingers over the area.

The first round of convection is currently pushing east of the GSP CWA with a brief lull in activity expected through late this afternoon or early this evening. Seeing cloud cover scatter out across portions of the forecast area but especially across northeast Georgia and the South Carolina Upstate per the latest visible satellite imagery. Any thinning of cloud cover will allow us to destabilize this afternoon/early evening thanks to better insolation. If we manage to get enough daytime heating before the next round of convection, then the strong to severe storm potential will increase late this afternoon into this evening. For now it appears that northeast Georgia and the western South Carolina Upstate will have the best chance to destabilize per the latest satellite and high-res model trends. The SPC mesoanalysis page shows 500-1,000 J/kg of surface instability developing during peak heating and the 16Z HRRR shows updraft helicity swaths mainly across northeast Georgia and the western South Carolina Upstate late this afternoon into this evening. There will also be plenty of shear to work with, with 50-60+ kts of deep layer shear expected ahead of the cold front. Although high-res guidance has been trending more towards a low-end tornado threat across northeast Georgia and the western South Carolina Upstate, the limiting factor appears to be 0- 1 km bulk shear <40 kts during peak heating per model soundings. So, all this to say, the severe threat looks highly conditional and confidence remains low.

There is arguably a slightly higher concern for locally heavy rainfall through Thursday morning. Although this will be a beneficial rain for most locations with rainfall totals ranging from mostly 1-2", locally higher amounts are possible from training of convection which could lead to a localized flash flood threat. This potential looks highest across the southern North Carolina mountains, extreme northeast Georgia, and the northwestern South Carolina Upstate as the 12Z REFS the 95th percentile shows the potential for 3-4"+ across these zones. However, the REFS only has a roughly 15% to 30% chance of rainfall >3" across these zones. So confidence on the heavy rainfall threat is also low.

Dry conditions will return for most locations by late Thursday morning or early Thursday afternoon but another round of showers and thunderstorms should develop right along the cold front Thursday afternoon/early evening, mainly along/near I-77. There does not appear to be a severe or heavy rainfall threat with this activity as the atmosphere will be mostly worked over by this time and as convection will not last long. Highs this afternoon will end up near normal to just below normal across the mountains and near normal to just above normal east of the mountains. Lows tonight will end ~10 degrees above normal thanks to rain and cloud cover. Highs on Thursday will end up a few degrees below normal area-wide behind the front. Lows Thursday night will be much colder, dropping to around 5 degrees below normal. There is the potential for some patchy frost to develop Thursday night in the North Carolina mountains, especially where winds decouple.

Key message 2: Drier conditions Friday and Saturday before rain chances increase Sunday and into Monday. Dry Tuesday and Wednesday.

Guidance continues to agree on the overall pattern but differs in the details with run to run and model to model inconsistencies. In general, dry high pressure moves in Friday as the Thursday front stalls near the Gulf Coast. Waves of low pressure form along the front and move east Saturday and Sunday. A cold front crosses the area from the northwest on Monday, then dry high pressure returns for Tuesday and Wednesday. Most of the guidance, but not all, agree that Saturday will be dry with better precip chances Sunday or Monday. That said, the latest ECMWF is coming in with only some spotty light rainfall each of those days. Obviously, despite what the model blend has for the forecast, confidence remains low for this period. Below normal temps Friday, rise to around normal for the weekend, back to the low side of normal Monday, and below normal for Tuesday.

AVIATION /18Z WEDNESDAY THROUGH MONDAY/

At KCLT and elsewhere: Messy 18Z TAF period ahead thanks to a cold front tracking across the western Carolinas. The first round of SHRA/TSRA has pushed east and we should see a brief lull before another round of SHRA/TSRA returns late this afternoon into this evening. Cannot entirely rule out a few strong to severe storms, with damaging winds being the main hazard. However, KAND will also have an isolated tornado threat through this evening. SHRA and isolated TSRA will linger across the terminals through mid to late morning Thursday before drier conditions return. However, did add a PROB30 for TSRA at KCLT for Thursday afternoon as the TAF goes out 30 hours and scattered SHRA/TSRA should develop directly along the front. Should see mostly MVFR to IFR restrictions develop with SHRA/TSRA through the period, but any terminals that see heavier precipitation develop could see restrictions as low as LIFR. MVFR restrictions should linger through the 18Z TAF period across most of the terminals. Low-end wind gusts will stick around through early this evening before gradually diminishing. Wind direction will be mainly SW ahead of the front through tonight east of the mountains but will likely go VRB overnight into Thursday morning. Winds at KAVL will be mainly S/SSW through early this evening before going VRB. Winds will flip NW at KAVL around daybreak Thursday.

Outlook: Scattered SHRA/TSRA will develop along a cold front Thursday afternoon/early evening, mainly impacting KGSP, KGMU and KCLT. Dry and VFR conditions return Thursday evening into at least Friday night. Dry conditions may linger through Saturday night but confidence is low. Another cold front impacts the region late this weekend into early next week bringing another round of rain and restrictions.

GSP WATCHES/WARNINGS/ADVISORIES

GA...None. NC...None. SC...None.