textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Lowered thunder chances this afternoon, and in the northern portion of the CWA, lowered high temperatures today.

No major changes otherwise.

KEY MESSAGES

1. Scattered showers this afternoon near a weak cold front, producing only light rain. Any thunderstorms would be highly isolated and not pose a risk of severe weather. 2. Another weak cold front may bring a few more showers and thunderstorms to the western Carolinas Wednesday. Dry and briefly cooler Thursday and Thursday night, followed by a warming trend Friday through the weekend.

DISCUSSION

Key message 1: Scattered showers this afternoon near a weak cold front, producing only light rain. Any thunderstorms would be highly isolated and not pose a risk of severe weather.

Cold front appears to have passed through most of the CWA as of 2 PM EDT, with most obs sites having experienced a shift to gusty NE winds. RAP soundings from around the CWA depict a shallow layer of instability near or just behind the front, which has led to low stratocu breaking out, but so far little in the way of new shower activity. Band of radar returns near the TN/NC border is associated with a shortwave trough axis advecting eastward into NC/VA. CAMs generally still depict a few showers forming along either the front or the trough axis through the afternoon and evening. We continue to advertise 20-30% PoPs similar to what was shown on the previous forecast cycle, but in light of models having been somewhat overdone so far today, and the shallow nature of the convective layer on prog soundings, felt it was best to cap thunder chances below slight-chance (~10%). Can't entirely rule out a stray storm getting tall enough to produce thunder, particularly near the sfc front, SE of I-85. With instability remaining so limited, not expecting any severe weather to result.

The front will continue to sag southward tonight; abundant mid to high altitude cloud cover will linger through the evening but clearing should occur from N to S late in the night. Drying will occur only slowly at the surface, and as winds turn light in the northern CWA, some fog or low stratus may develop there. Sunnier skies Tuesday should result in max temps trending a few degrees warmer compared to Monday in our northern zones, but a little cooler in the south. Drier dewpoints will mix down to the surface across the area Tuesday and some locations will see RH dip to near 30% by mid-afternoon.

Key message 2: Another weak cold front may bring a few more showers and thunderstorms to the western Carolinas Wednesday. Dry and briefly cooler Thursday and Thursday night, followed by a warming trend Friday through the weekend.

Deep shortwave will begin to cut off over the Great Lakes Wednesday; while the best DPVA looks to occur to our north, height falls occur over our area as a result, and a cold front will approach the southern Appalachians during the day. Moisture return ahead of the front appears limited, partly owing to the good mixing/drying expected Tuesday, and veered low-level winds Tue night and early Wed. Nevertheless, models generally still do depict a narrow band of above-normal PWATs advecting into the area just prior to the fropa, with similar peak values in our Piedmont, though varying in the timing of that peak. The 12km NAM and its typically similar Canadian model are more aggressive in bringing Gulf air above the boundary layer early Wed morning, and accordingly are appreciably higher with afternoon dewpoints and SBCAPE than the GFS/EC operational models. LREF probs of 500 J/kg SBCAPE have actually decreased in our CWA on the latest available cycle, but that appears to be related to a slightly earlier depiction of the fropa. Not much change in NBM PoPs versus the overnight cycle, which seems reasonable in light of model spread mainly being in timing and little change in CAPE potential. LREF still shows mean values of 50-60 kt sfc-500mb bulk shear, with even the 10th percentile being near 40 kt. If the front ends up being timed to induce convection east of the mountains near peak heating, and we manage the higher-end CAPE, we could be looking at a low-end severe threat Wednesday afternoon, mainly for damaging wind, with potential for some linear organization. The moisture/CAPE still appears the limiting factor.

Between the cutoff low over the NE CONUS, and upper ridge in the lower MS Valley, along with CAA, gusty NW winds develop in the wake of the front Wednesday night over the mountains and skies clear. Temps trend noticeably cooler for the mountains Thursday, but downsloping appears to offset that trend for the Piedmont. Dewpoints look to tank across the CWA, with continuing gusts likely. Winds decline Thu night into Fri morning as sfc high shifts over the area from the NW. The enhanced radiational cooling conditions potentially will bring morning mins around 10 degrees below normal; some portions of the mountains could dip into the upper 30s, but probably still too warm for frost. Temps rebound a bit Friday as upper low departs and heights rise under mostly sunny skies.

Another low will pass north of the Great Lakes Saturday, which could bring another weak front into the lower OH and TN valleys, reflected in the return of small PoPs to the mountains then and Sunday. However, assuming cloud cover or precip does not impact temps, temps remain on an upward trend, particularly Sunday. Max temps Sunday may hit 90 in a number of spots.

AVIATION /18Z MONDAY THROUGH SATURDAY/

At KCLT and elsewhere: A shift to N-NE winds has occurred at all TAF sites as a frontal boundary has settled southward through the area. Occasional gusts can be expected this afternoon, potentially peaking AOA 25 kt. The front has led to a stratocu deck and cigs near the MVFR-VFR threshold across KCLT and KHKY. The other sites are expected to see only FEW-SCT clouds at that level, and any cigs should lift to VFR by mid-afternoon, scattering out in the evening. Gusts could persist into evening with N to NE winds still prevailing until decoupling occurs overnight and winds go light/VRB in parts of the NC Piedmont. There is potential for IFR level clouds to develop prior to daybreak Tue at KCLT and KHKY, though with low confidence have only mentioned these as FEW-SCT for now. Winds veer to SE at KCLT and KAVL prior to 18z, with the other sites either still light thru 18z, or veering late enough not to warrant an additional line to reflect the shift this far out. Likely too dry in the boundary layer to expect diurnal cumulus to develop Tue.

Outlook: After the front, dry conditions persist through early Wednesday. A clipper-type low may cross to our north late Wednesday, but guidance suggests rainfall will be paltry with this system, and it's questionable whether any flight restrictions will develop.

GSP WATCHES/WARNINGS/ADVISORIES

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