textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

The aviation forecast was updated for the 00Z TAF issuance.

KEY MESSAGES

1. Scattered strong thunderstorms are possible this evening. There is some potential for a few of the storms to produce damaging wind gusts. More widespread coverage of showers and thunderstorms is expected Friday afternoon, with a threat of locally heavy rainfall. Drier weather returns Sunday though the pattern will become somewhat favorable for storms to reach the area from the northwest early next week.

DISCUSSION

Key message 1: Scattered strong thunderstorms are possible this evening. There is some potential for a few of the storms to produce damaging wind gusts. More widespread coverage of showers and thunderstorms is expected Friday afternoon, with a threat of locally heavy rainfall. Drier weather returns Sunday though the pattern will become somewhat favorable for storms to reach the area from the northwest early next week.

A stationary front is present across the lower Piedmont from Virginia to eastern Georgia, seemingly just east of our CWA as of early afternoon. A weak lee trough is providing some degree of low level convergence over the CWA. An elevated mixed layer overhead seems to have capped off convective development so far. Diurnal mixing is expected to help erode the associated inversion, and a weak vort axis looks to move through the longer trough centered to our north later this afternoon. Hence, some development of showers/storms still is expected. The 12z CAMs came into consensus that these elements would come together near peak heating, supporting the idea that storms would fire earlier than previously thought (when the main driver for convection was an MCV or cold pool arriving from upstream). Recent HRRR runs have however trended back toward later onset, so confidence on timing remains relatively low. With the pattern being ill-defined overall PoPs have been kept largely 20-50% this evening. There appears adequate DCAPE to support marginally severe winds from any cells that do develop, and with 0-6km shear on the order of 20 kt along with the potentially linear forcing of the convergence axis, can't rule out that storms will organize loosely into clusters posing a somewhat broader-scale wind threat than we would see with pulse convection. SPC updated the Day 1 outlook to include a Slight Risk for a portion of our Piedmont zones, possibly a nod to the aforementioned model consensus. The threat does not look particularly higher than it had this morning when we were in the Marginal Risk, but one could argue greater coverage of storms translates to greater likelihood of severe weather. The setup for organized/severe weather does still look better to our northeast and southwest. With potential cold pools or outflow boundaries entering our CWA from adjacent areas, increasingly cyclonic flow aloft overnight as well as the stronger cold front approaching from the northwest, can't say the threat of overnight showers/storms is nil.

A second vort lobe over the Ozarks as of early Thu afternoon will progress through the shortwave and by Friday appears to phase with another vort lobe nearer the Great Lakes. These together lead to the trough sharpening slightly. As noted above, a better defined cold front should move thru the lower OH/TN Valleys overnight, ending up somewhere across the Carolinas by Fri afternoon. Moisture pooling invof the front will lead to high PWATs over 2 inches across much of our area; warm-cloud layers are very deep and MBE vectors are largely short due to E to SE sfc flow and westerly flow aloft. These alone suggest a threat of locally heavy rainfall. There are hints at a surface low developing along the front due to the interaction with the shortwave which could additionally focus convective development. WRF/NAM family models feature very high SBCAPE Friday afternoon and evening on the warm side of the front, near our southeastern border. HREF and REFS PMMs feature bullseyes of 3"+ QPF in 6 hours or less, particularly near and east of Charlotte. However, models vary in the pattern of convection which limits confidence on where the peak threat is, and may artificially inflate the PMMs. Right now it appears the greater concern for excessive rainfall will be just to our east. Nevertheless some consideration to a short-fused Flood Watch may be given tonight if the ingredients look to come together farther west. Aside from the heavy rain threat, deep layer shear again will be sufficient that storms may organize and pose a damaging wind threat again, though cold pool formation may be somewhat inhibited by the abundant moisture.

While the trough-y upper pattern persists over the area into Saturday, synoptic models are in agreement that the axis shifts east just enough to allow drier air to work into the western CWA Friday night; the front will slowly drift south and east. Not totally confident we will be fully stable on Saturday, with models suggesting the front may not truly clear the area and still with soupy dewpoints and SBCAPE of 1500-2000 J/kg possible in the lower Piedmont. By Sunday we look to be in a northwest flow pattern. While we will be initially stable, the airmass will modify to our west and we appear not to be out of the woods for MCS(s) to make a run at the area sometime early next week.

AVIATION /00Z FRIDAY THROUGH TUESDAY/

At KCLT and elsewhere: Isolated showers and thunderstorms are expected to develop this evening, mainly along the Blue Ridge Escarpment and back over in eastern Tennessee. Guidance is all over the place as far as overall coverage, but has consistently shown activity moving into western North Carolina. Kept a PROB30 for TSRA and associated restrictions at KCLT/KAVL/KHKY, while taking the previous PROB30 out at the Upstate TAF sites. Lingering cloud debris is expected through a good portion of the overnight period. Light and variable winds overnight, picking up out of the SE late Friday morning. There is some speculation of some morning stratus near KCLT/KAVL/KHKY around daybreak Friday, but confidence wasn't high enough for a prevailing BKN/OVC IFR/MVFR cig mention at this time. A cold front will move into the area later on Friday, with winds turning the dial behind the front with a northerly component. More scattered showers and thunderstorms are likely Friday afternoon, so placed a PROB30 for TSRA and associated restrictions at all TAF sites.

Outlook: Afternoon SHRA/TSRA may return Saturday. By Sunday, a drier pattern should arrive and last through the first half of next week, possibly keeping SHRA/TSRA to a minimum those days. Mountain valley fog and low stratus will be possible each morning.

GSP WATCHES/WARNINGS/ADVISORIES

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