textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

PoPs were updated this evening based on latest radar and high- res model trends.

The aviation discussion was updated to reflect the recent 00Z TAF issuance.

KEY MESSAGES

1. Thunderstorms are possible each afternoon and evening through Monday, with a few of them severe today and Monday. Damaging wind gusts and excessive rain are the main threats. 2. A pattern change during the middle part of the upcoming week is expected to bring in cooler temps right on schedule for the start of autumn.

DISCUSSION

Key message 1: Thunderstorms are possible each afternoon and evening through Monday, with a few of them severe today and Monday. Damaging wind gusts and excessive rain are the main threats.

Our weather remains a bit unsettled over the next few days as we are under what amounts to an atmospheric col region in-between the old upper anticyclone over the southern Plains, the Atlantic Subtropical Ridge, a weak mid/upper low over the FL peninsula and eastern Gulf, and the northern stream. Within this regime, a old quasi-stationary boundary across the middle of the forecast area today should wash out over the next 24 hours. That will leave us with ample deep moisture across the region that will fuel increasing mainly diurnal shower and thunderstorm chances that expand from the mountains to the whole forecast area through Tuesday. We will lack mid/upper forcing the next two days but will make up for that with a light easterly flow and thermodynamics.

The main concern for the next few days will be a few severe thunderstorms with damaging wind gusts and excessive rain, mainly in the late afternoon and evening. Note that SPC has included mostly the mountainous parts of the forecast area in a Marginal Risk today and Monday. This seems reasonable with the expectation of 2000-3000 J/kg of sfc-based CAPE each afternoon and water-loaded downdrafts falling through sufficient dCAPE to allow for strong outflows. This afternoon might be the poorest of the three days as we appear to be in a dCAPE minimum for the most part, but we have the benefit of the old boundary draped across the region. Guidance suggests that buoyancy will improve a bit each day, so our chances for severe might also improve slightly. By Monday and Tuesday, the risk for excessive rain also ticks upward as warm cloud depth increases and storm motions are more sluggish. Note that WPC has the region in the Marginal Risk for Excessive Rainfall for Monday into Tuesday.

As for temperatures, they will remain above normal through Monday, but then drop down close to normal for Tuesday assuming increased cloud cover and rain coverage keeps them in check.

Key message 2: A pattern change during the middle part of the upcoming week is expected to bring in cooler temps right on schedule for the start of autumn.

As we get into the first part of the upcoming week, the northern stream becomes more blocky over the northern Plains/Great Lakes/Midwest, with a mid/upper low consolidating over the Midwest Tuesday into Wednesday. This upper low combined with cyclogenesis off the Mid-Atlantic coast would bring a back door cold front down Tuesday and establish a cold air damming wedge by Wednesday that would persist into Thursday at least. The cool northeasterly flow and light precip potential will help to drop temps down to about ten degrees below normal for the end of the week. Confidence in the details is not great, but it looks like a noticeable change of seasons at any rate. The upper low may ultimately swing through to our north Thursday or Friday, after which we come more under the influence of the drier ridge to our west, but temps would stay below normal through the end of the period.

AVIATION /00Z SUNDAY THROUGH THURSDAY/

At KCLT and elsewhere: Convection is expected to linger across foothills and parts of the Piedmont thru late evening. Most of this activity is expected to stay between KGSP/KGMU and KCLT, with an area roughly from KHKY south thru KEHO to KGRD. Will keep a TEMPO for TS at KHKY, but just VCSH for the Upstate sites. Showers or TS outside these areas is expected to be very isolated. Once convection wanes overnight, an MVFR to possibly IFR stratus deck is expected to form across most of the area. Also, mountain valley fog looks likely at KAVL, which could go VLIFR. Mountain valley fog should improve after 13z, but the Piedmont stratus may take a while to lift to VFR. Latest guidance suggests MVFR cigs holding on until 18-19z across the Upstate, while improving to VFR at KHKY and KCLT by around 15z. Convection is expected to develop again across the mountains by early aftn, and generally drift E/SE into the Piedmont. There is uncertainty on how far off the escarpment showers and storms survive, so only PROB30 will carried, and not at KCLT with the 00z TAFs. Some guidance is starting to trend toward higher chances at KCLT, so TS mention may be added with a later TAF.

Outlook: Shower and thunderstorm chances continue each afternoon through Tuesday. Mountain valley fog and low stratus possible each morning. A wedge may develop and keep lower cigs around starting Tuesday, continuing thru Thursday.

GSP WATCHES/WARNINGS/ADVISORIES

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