textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Adjustments were made to precip chances early in the week and temperatures during the second half of the upcoming week.

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 /18Z SATURDAY THROUGH THURSDAY/

At KCLT and elsewhere: Satellite imagery shows a well-developed stratocu field across the region with cloud bases hovering between 025 and 035, which means that a temporary MVFR ceiling restriction will be possible at most terminals for the next few hours. By 20-22Z the boundary layer deepens and raises the ceilings to VFR if they exist. Meanwhile, showers and thunderstorms are developing over the southern mountains of NC as shown by the HRRR, which puts KAND at the greatest chance for a storm through early evening. They get a TEMPO as does KAVL for later development over the mtns, but the other sites warrant only a PROB30 until a clearer indication of storm development is seen. Wind should stay mainly NE. Think the recent ESE flow at KCLT is a temporary fluctuation and the direction will come back around to N of E. Tonight, the northeasterly flow at low levels is expected to help a low cloud ceiling develop during the early morning hours, falling down through MVFR to IFR, especially close to the Escarpment. Mountain/valley fog and stratus should result in LIFR or VLIFR. All of this should mix out by 13Z or so. We should then do it all again in the afternoon under a similar setup.

Outlook: Shower and thunderstorm chances continue each afternoon through Tuesday. Mountain valley fog and low stratus possible each morning.

GSP WATCHES/WARNINGS/ADVISORIES

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