textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

No major changes to overall forecast thinking. Temperatures could trend cooler Saturday if decaying thunderstorms move over the area in the morning as some models depict. Confidence on forecast timing and trends Sunday-Tuesday has diminished slightly due to a new trend in how a low pressure system may evolve.

KEY MESSAGES

1. An active period of convective weather will continue into at least Monday, with showers and thunderstorms still most likely in the afternoons and evening, but also being possible at other times each day, especially Sunday night. A few strong or severe thunderstorms and isolated flash flooding are possible each day through Monday. 2. Another hot day today, before a cooling trend sets in over the weekend, with below normal temperatures forecast early next week. If you have outdoor activity planned through Saturday, make sure to stay hydrated and take frequent breaks to cool off and avoid heat-related stress.

DISCUSSION

Key message 1: An active period of convective weather will continue into at least Monday, with showers and thunderstorms still most likely in the afternoons and evening, but also being possible at other times each day, especially Sunday night. A few strong or severe thunderstorms and isolated flash flooding are possible each day through Monday.

Broadly cyclonic upper-level flow is already in place across the eastern US and will amplify over the weekend as ridge builds over the Rockies and Plains. A well defined shortwave now over the central Plains will advect southeastward tonight into Saturday and possibly even partially cut off into an closed upper low over KY/TN by Sunday. This pattern, and abundant subtropical moisture intially over our area, are expected to result in a period of active weather lasting into Monday, with potential for heavy rain and thunderstorms not necessarily on the diurnal trend typically seen this time of year.

Convective complexes which developed Thursday night in the mid-MS and lower OH valleys created weak upper disturbances or MCVs which should advect toward NC this afternoon and evening. Seasonably unstable conditions over our CWA should develop this afternoon, with slightly more low-level shear than normal due to the westerly flow aloft. These conditions combined with the MCVs could result in some loosely organized clusters of storms producing a damaging wind threat this afternoon and early evening.

Last night's MCSs also left behind a cold pool which, in the face of southwesterly flow around the Bermuda ridge, should effectively place a warm front in place over KY/TN through tonight, and could provide a focus for overnight covective development upstream. Cold pool boundaries could remain from earlier convection in our CWA as well. As a result we will maintain 60-80% PoPs near the NC/TN border tonight and at least a 15-30% PoPs east of the mountains into the very early morning hours. A number of CAM runs also develop another MCS this evening somewhere over the lower Ohio Valley and the warm front would provide a route for such an MCS to make a run at the Appalachians near daybreak Saturday. Thus such development would appear to have more implications tomorrow, with potential for a cold pool to stabilize parts of the area and/or promote initiation of a new round of diurnal storms.

The amplifying trough will be associated with an east-west oriented cold front, which should progress southward along the East Coast in backdoor fashion and potentially will induce drier and cooler weather circa Monday or Tuesday. While this still appears possible, confidence has decreased somewhat on the cooling/drying due to some guidance depicting the cutoff low developing to our west, which suggests the front will stall near or north of the CWA. A period of easterly low-level flow still is expected to develop Sunday near the front, supplying Atlantic moisture, while the cloud layer flow is likely to remain westerly. 60-90% PoPs are forecast over most of the area then into Monday. Slow-moving or training convection remains possible along the front and thus this still looks like a period of greater concern for flash flood potential. The upper low could however turn the mean flow more southerly, which might result in the threat being more localized. PoPs do not start to trend downward until Monday night, doing so from north to south. The potential for a lingering upper low west of the area hinders confidence Tuesday and beyond, but otherwise heights should rise over the area as the upper ridge expands eastward, and we would expect a return to more typical mountain-focused convection each day.

Key message 2: Another hot day today, before a cooling trend sets in over the weekend, with below normal temperatures forecast early next week. If you have outdoor activity planned through Saturday, make sure to stay hydrated and take frequent breaks to cool off and avoid heat-related stress.

Temperatures are running above normal this afternoon with dewpoints in the upper 60s to near 70 for the most part, resulting in heat index reaching the low 100s in many spots. An isolated occurrence of heat index over 105 is possible.

Outflows or cold pools associated with convection could offer some relief for the heat this afternoon. There remains potential for convective activity in the TN Valley late tonight or Saturday morning, which could bring additional cold pools and/or a high altitude cloud shield over the area. Based on partial thickness values, heat potential is similar Saturday compared to today, though enough models depict the cooling effects of convection impacting temperatures Saturday that NBM values trend a little cooler and back to about normal in most locations. Heat index accordingly will be more comfortably below advisory criteria. A moist/cloudy pattern is even more likely Sunday, but there is also the potential for some cooler air to arrive with the backdoor cold front, so temps trend slightly lower that day also. Temperatures are expected to top out below normal Monday and Tuesday, with Monday still looking like the coolest day with highs near 10 degrees below normal.

As noted, there is some uncertainty into the overall trend due to the potential for a cutoff low to impede the progress of the upper ridge from the central into eastern CONUS toward the middle of next week, but the consensus remains that heights will trend upward and the cooler post-frontal airmass will quickly modify. As such temps trend back to normal Wed and even slightly above normal Thu.

AVIATION /18Z FRIDAY THROUGH WEDNESDAY/

At KCLT and elsewhere: TEMPOs very early in the period at all sites (except PROB30 at KAVL) with loosely organized clusters of SHRA/TSRA scattered around the region. Lower MVFR vsby is mentioned but there is potential for brief IFR. Some of the existing small clusters could grow upscale into a larger line which would be associated with more widespread or longer duration aviation impacts, so monitor for AMDs. A shortwave trough remains over KY/TN and will drift eastward into and across the terminal area later this afternoon/evening, so there is potential for a second round of convection, except perhaps where the first round consumes the available energy for storms. This warranted a PROB30 this evening at all sites except KAVL. SHRA appear more likely in this second round, except at KAND which is nearest the track of a developing line of TSRA moving into GA as of 1830z. Gusts of 25+ kt possible with any thunderstorms this afternoon/evening. Aside from TS impacts, VFR is expected through evening. Cannot rule out restrictive cigs/vsby forming tonight where heavy rain falls in the late aftn or evening. Not yet confident enough to make any mention of convection on Saturday, except KAVL, which is most likely to see precip due to decaying convection moving out of TN in the morning, prompting PROB30.

Outlook: Greater than normal coverage of SHRA/TSRA continues for all terminals Saturday into Monday as multiple rounds of storms move towards and across the area. 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.