textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Severe thunderstorm chances look a little better for this afternoon and again Friday afternoon as the Day 1 and Day 2 Convective Outlooks show an increase to Marginal Risk.

The aviation discussion was updated for the 18Z TAFs.

KEY MESSAGES

1. Continued hot and humid east of the mountains into the weekend, but not hot enough to reach heat advisory criteria. If you have outdoor activity planned in the afternoons through Saturday, make sure to stay hydrated and take frequent breaks to cool off and avoid heat-related stress. 2. Mainly diurnal showers and thunderstorms will be mostly confined to western North Carolina again today with an isolated severe storm possible. Coverage improves for Friday and the weekend, with better chances for loosely organized severe weather. The risk for flash flooding over the mountains will increase into the early part of next week.

DISCUSSION

Key message 1: Continued hot and humid east of the mountains into the weekend, but not hot enough to reach heat advisory criteria. If you have outdoor activity planned in the afternoons through Saturday, make sure to stay hydrated and take frequent breaks to cool off and avoid heat-related stress.

Although the Bermuda High continues to weaken and be shunted off to the south for the next few days, we retain the typical summertime heat and humidity even in the face of increasing storm chances. Beginning tonight, the western Carolinas are more under the influence of a broad and baggy mid/upper trof that will have several weak short waves rippling through it. The expectation is that storms will be scattered enough through the early part of the afternoon Friday and Saturday to allow significant warmup with temps climbing about five degrees above normal. This could easily not work out depending on upstream convection and remnant cloudiness...more on that below. The combo of forecast high temps and dewpoints is not expected to raise the heat index above advisory criteria, but it will still feel hot, especially Friday.

As a northern stream upper trof deepens over the Northeast and a low cuts off across the Midwest, a sfc front is expected to drift south into the region on Sunday. The latest guidance seems to have more of a southward push with the front, being south of the fcst area by Monday morning and then staying south as weak high pressure moves across the nrn Mid-Atlantic Monday then offshore Tuesday. The signal for cold air damming Sunday into Monday looks weak on the new model runs, so confidence is relatively low with the high temp forecast. Whether or not a true CAD unfolds might end up being semantics, as there should be enough cloud cover and a cooler N/NE low level flow from the weak high to the north to keep temps on the cool side of normal. Right now, our forecast highs from Sunday to Tuesday drop down into a range 5-10 degrees below normal. Without strong CAD support, the temps might not get that cool, though. Either way, guidance suggests this will be short-lived and the temps rebound back to typical summer heat by the end of the week.

Key message 2: Mainly diurnal showers and thunderstorms will be mostly confined to western North Carolina again today with an isolated severe storm possible. Coverage improves for Friday and the weekend, with better chances for loosely organized severe weather. The risk for flash flooding over the mountains will increase into the early part of next week.

Convective clouds are deepening at this hour across the forecast area, with showers and storms developing over the mtns. The environment isn't quite as favorable for pulse-severe storms this afternoon as the sfc-based CAPE is only modest...on the order of 1500-2500 J/kg...but the dCAPE should still be above 800 J/kg east of the mtns. Thus, an isolated severe wind gust producing storm won't be ruled out. In fact, the new Day 1 Outlook has an upgrade to a Marginal Risk east of the mtns. That being said, the CAMs have much better-looking storms well to our north. The storms should die off with the loss of heating this evening.

The situation might get more interesting through the weekend as the guidance suggests something of an MCS track from the Mid-MS valley to the Mid-Atlantic starting on Friday. A few members of the 12Z HREF show an MCS coming across the Mid-MS valley later this evening and then toward us overnight, then it would remain to be seen how long/far east it could survive. If the system forms, it could at a minimum leave behind an MCV for outflow that would be a trigger for more storms to develop over our region on Friday. Another MCS could do the same on Friday night and have a better chance for survival all the way east into the mountains. There is much uncertainty and we look to upcoming model runs for clarity. Precip probs have been modified only slightly and will start to relax the diurnal tendency. The main concern with storms will be damaging wind gusts Friday and Saturday, and the Day 2 Convective Outlook already features an expansion of the Marginal Risk across the fcst area. The greater risk will gradually shift to heavy rain and flash flooding into early next week. Sunday should have the best rain chances as the front sags southward, and moisture should be plentiful enough to support an increased risk of flash flooding, but again details remain sketchy. Precip chances should drop once the front moves farther south on Tuesday.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

At KCLT and elsewhere: Prevailing VFR through most of the period outside of scattered TSRA this afternoon/early evening. Storms have started to develop to the west and will be monitored. For the time being, a PROB30 has been added for the Upstate terminals based on that upstream convection. Meanwhile, the NC terminals stay the same. The CAMs support an increase to a TEMPO for thunderstorms late this afternoon for KCLT, but we kept the PROB30 to stay in agreement with the public forecast. An amendment is anticipated that will include the upgrade depending on the evolution of the convection to the west at issuance time. Meanwhile, scattered low clouds with a base 050 or higher, and occasional low-end wind gusts from the SW to WSW through peak heating. The storms should die off with the loss of heating, leaving behind convective cloud debris and a light SW wind. No indication of restrictions in the valley around KAVL, so that was left out. Friday convective coverage looks higher, with an earlier start possibly before 18Z. For now, confidence is too low to include, except at KCLT for late afternoon stuff. Wind may be occasionally gusty from the SW once again.

Outlook: Better coverage of SHRA/TSRA returns for all terminals Saturday into Monday. Drier conditions should develop Tuesday into Wednesday. Low stratus and/or fog will be possible each morning in the mountain valleys and in locations that see appreciable rainfall the previous afternoon/evening.

GSP WATCHES/WARNINGS/ADVISORIES

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