textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Dewpoint was lowered this afternoon and Wednesday afternoon to keep Heat Index below 105 across the Piedmont owing to low confidence. No changes to severe thunderstorm risk for today and Wednesday. Updated the aviation discussion for the 06Z TAFs.

KEY MESSAGES

1. A Marginal Risk of severe thunderstorms for late afternoon and evening, with damaging wind gusts the main concern. Otherwise, mainly isolated to scattered diurnal convection will continue each day through the week, with some potential for isolated pulse severe storms that produce damaging wind gusts and localized excessive rainfall. 2. Hot and humid for at least the next seven days. Some Piedmont/Upstate locations could have a heat index between 100-105 each afternoon...especially Thursday and Friday.

DISCUSSION

Key message 1: A Marginal Risk of severe thunderstorms for late afternoon and evening, with damaging wind gusts the main concern. Otherwise, mainly isolated to scattered diurnal convection will continue each day through the week, with some potential for isolated pulse severe storms that produce damaging wind gusts and localized excessive rainfall.

A complicated and inherently uncertain situation will develop later today as some of the near term model guidance continues to show MCS action moving from the OH Valley region to the Mid-Atlantic through tonight in more than one wave. The first batch coming eastward across WV overnight should weaken across VA this morning and should not affect our region, but a second trailing batch is shown by the 00Z HRRR to develop and move effectively along the outflow boundary left behind by the first batch. If this happens, this second round would be pointed at western NC. Of course, subsequent runs of the HRRR have lost the plot, but other HREF members support the idea that loosely scattered convection will develop into the area from the NW late in the day. The 03Z RAP shows a favorable air mass to the south of whatever boundary might be out there, with a pool of sfc-based CAPE greater than 4000 J/kg across the NC foothills and NW Piedmont, which would be supportive of convection being able to cross the mountains. Not time to mash the button yet, but the potential will be there for some strong storms, so this will bear watching. Wednesday holds more potential for some organized thunderstorms as the HRRR shows additional MCSs dropping in from the NW, so the Marginal on the new Day 2 appears warranted. More of the same is possible again Thursday. Beyond Thursday, the expansion of an upper anticyclone over the lower MS/TN valley regions may push the MCS track back to the north or northeast of the region and we will return to more of climatological diurnal storm setup.

Key message 2: Hot and humid for at least the next seven days. Some Piedmont/Upstate locations could have a heat index between 100-105 each afternoon...especially Thursday and Friday.

Not much change to the previous thinking regarding the heat over the next seven days. It would appear that the dog days have set in. For today, the light WNW flow may favor some mixing out of the dewpoint this afternoon, and once again we undercut the model blend which in turn keeps the heat index forecast in check. Hot...yes...but not to advisory level...and probably not Wednesday either...even as the heat index that day makes it into the low 100s over most of the area outside of the mtns/foothills. The model guidance continues to show a good potential for convective cloud debris, MCS remnants, or perhaps even something organized on an outflow boundary from MCS action dropping in from the NW. All of this is to say that confidence in reaching heat advisory criteria is way too low to entertain any heat advisories at this time. Thursday and Friday might be different, as dewpoint creeps upward with less of a chance to mix out, but the confidence will remain low as long as the guidance shows the potential for afternoon convection. The weekend still looks hot as the upper anticyclone moves east and envelops our region for a few days.

AVIATION /06Z TUESDAY THROUGH SATURDAY/

At KCLT and elsewhere: The ever-present mtn valley low stratus and fog was developing nicely at this hour, so once again KAVL will have to contend with variable flight conditions thru daybreak. Confidence is low as to how it will settle in, so I went with the IFR stratus in a TEMPO for now. Expect several amends. It should mix out by 13Z to 14Z. Prevailing VFR at all sites after that time, with scattered cu and a light WSW wind outside of the mtn valleys. Convection this afternoon looks problematic. We will be watching the progression of organized storms to our NW and N to see if an outflow boundary moves southeast into our area to trigger afternoon/evening storms. Will include a PROB30 at all terminals but KAND based on the HRRR model output and will amend off that. More of the same tonight.

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

GSP WATCHES/WARNINGS/ADVISORIES

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