textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Increasing chances for severe storms over northern portions of the area this evening.

Updated the aviation discussion for the 18Z TAFs.

KEY MESSAGES

1. Severe thunderstorms are possible across northern portions of the area this evening. Additional rounds of storms may push into the area overnight through Wednesday, but confidence is low at this time. 2. Heat builds by late week into the weekend with continued daily chances for mainly afternoon/evening thunderstorms.

DISCUSSION

Key message 1: Severe thunderstorms are possible across northern portions of the area this evening. Additional rounds of storms may push into the area overnight through Wednesday, but confidence is low at this time.

Broad upper ridging extends from the Gulf States through the Southern Plains and into the Four Corners region. Deep-layer northwest flow remains entrenched on the northern flank of the ridge from the Northern Plains across the Midwest and through the Ohio Valley and into the Southern Appalachians. This synoptic pattern is synonymous with late summer "ridge rider" patterns where complexes of storms develop across the Midwest and drop into the Appalachians. This exact scenario is already ongoing with two notable MCSs with the lead complex of storms dropping southeast across portions of eastern Kentucky into southern West Virginia and western Virginia. Farther upstream, an intense convective line is dropping southeast across Indiana into western Ohio. A fairly quiet afternoon is expected outside of a the potential for a few isolate to widely scattered pulse type storms. Attention quickly shifts to the lead complex of storms as it pushes towards the western Carolinas this afternoon. Storms will likely track along a notable instability gradient and will make a run towards at least the northern portion of the forecast area along and north of I-40. Moderate instability and plenty of DCAPE will support the potential for damaging winds with any linear segments that survive into the area. Southern extent of any severe threat remains uncertain, but cannot be ruled out as far south as the I-85 corridor or even the entire area should a more robust line of storms be realized. The expanded slight risk down to the NC/SC state line and marginal for the remainder of the area is well placed for now.

Forecast confidence wanes by the overnight into early Wednesday morning as the second complex of storms drops out of the Midwest and towards the area. Guidance is split as to how far south any storms survive during the overnight instability minimum and where exactly they track. The lead storms this evening will likely lay out a cold pool with the instability gradient shifting farther west in response. This could keep additional rounds of storms just to our west, but that's far from certain with some members of guidance bringing more convection into the area during the early morning hours. Any severe threat should be isolated with any overnight storms, but an isolated flash flooding threat cannot be ruled out should storms train down the remnant outflow boundary from earlier activity. Of course the potential for any storms Wednesday afternoon will be highly dependent on how things evolve overnight.

Key message 2: Heat builds by late week into the weekend with continued daily chances for mainly afternoon/evening thunderstorms.

By Thursday into Friday, a prominent ridge is forecast to build across the Deep South with the western Carolinas and northeast Georgia situated on the northeast flank of the ridge and on the southern periphery of the deeper northwest flow regime. The ridge further strengthens through the weekend as it expands into the Southern Plains. The result will be the potential for a hotter stretch of weather with afternoon highs creeping into the low to upper 90s across much of the area. A humid airmass will also remain in place and will support increasing heat indices with low 100s to around 105 possible. It's not out of the question that a day or two of heat advisories may be needed, but confidence is low to moderate at this time owing to uncertainty with how much dewpoints mix out and any degree of convection and debris clouds. Given the persistent northwest flow, a continued threat for complexes of storms to drop into the area will persist, especially Thursday and Friday, along with the potential for more typical diurnally driven afternoon/evening storms. Given the hot/humid airmass, any storms that do develop could become severe with a threat for damaging winds.

AVIATION /18Z TUESDAY THROUGH SUNDAY/

At KCLT and elsewhere: Forecast confidence through this afternoon and evening is lower than usual as multiple complexes of thunderstorms are evolving to our northwest and may make a run at the area. Should these storms make it, typical convective restrictions can be expected, generally between ~21-02z. Regardless at least a couple isolated storms cannot be ruled out ahead of any line of storms approaching from the northwest. The overnight should be fairly quiet and mountain valley fog cannot be discounted, but confidence is low owing to what could be copious amounts of lingering cloud cover from decaying storms. Winds will generally be light and out of the northwest.

Outlook: A few showers or thunderstorms will be possible each afternoon with flight restrictions possible for low stratus and fog each morning in the mountain valleys and locations that received rain the day before.

GSP WATCHES/WARNINGS/ADVISORIES

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