textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Humidity trending lower each afternoon on Thursday and Friday.

Updated details on what is looking increasingly likely to be an all- rain precipitation event Saturday night through Sunday night.

KEY MESSAGES

1. A cold front brings morning rain and gusty winds before dry conditions return this afternoon into Friday. Well above normal temperatures stick around today before cooler temperatures and lighter winds return behind the front Thursday into Friday. 2. Widespread rain still looks likely over the weekend, with precipitation most likely Saturday night and Sunday. This generally should be beneficial rain for most of the area. Localized heavier rainfall is possible, which could lead to nuisance flooding of poorly drained areas, and/or isolated bankfull conditions on area streams.

DISCUSSION

Key message 1: A cold front brings morning rain and gusty winds before dry conditions return this afternoon into Friday. Well above normal temperatures stick around today before cooler temperatures and lighter winds return behind the front Thursday into Friday.

Latest radar mosaic shows light rain tracking east across eastern TN ahead of an approaching cold front this morning, with some light rain already pushing into the southern North Carolina mountains. Rain chances will continue to increase from west to east through daybreak as the front tracks across the forecast area, with rain expected to linger through the morning hours. However, areas along/north of I-40 in the North Carolina foothills and Piedmont could miss out on the rain this morning as the NBM and CAMs have been consistent showing very little rain, if any, developing. Thus, PoPs will remain lowest across these locations (less than 25%). QPF will be light, with rainfall accumulations of 0.25" to 0.50" expected along the NC/TN border as well as elevations above 4,000 ft. Locally higher amounts from 0.75" to 1.0" are possible along the southern NC/TN border. Locations elsewhere will be lucky to pick up 0.05" to 0.10" of rainfall (if any). Lows this morning will end up ~20-25 degrees above normal thanks to cloud cover and rain limiting radiational cooling.

Dry conditions return this afternoon as surface high pressure builds in from the northwest behind the departing front. Highs will be slightly cooler this afternoon but will still end up ~5-10 degrees above normal across the North Carolina mountains and ~10-14 degrees above normal elsewhere. Much drier air will filter in behind the front this afternoon, allowing minimum RH to fall near or below 25% for most locations east of the mountains. However, morning rain should will limit fire weather concerns.

Gusty winds stick around today in association with the cold front. Winds start out W/SW ahead of the front, turning NW behind the front. Gusts east of the mountains gradually diminish this evening, but will linger across the mountains through tonight before gradually diminishing Thursday. Wind gusts are expected to remain well below advisory criteria, ranging from 20 to 30 mph across the lower elevations and 30 to 40 mph across the higher elevations.

The cooling trend will linger through Friday as dry high pressure remains over the region. Highs on both Thursday and Friday afternoon will end up just a few degrees above normal. Much cooler lows are then expected each night through Friday night, ending up only a few degrees above normal.

Although lighter winds and cooler temperatures return Thursday into Friday, minimum RH will drop near or below 25% from the Blue Ridge Escarpment and east Thursday afternoon and near or below 30% across the lower elevations Friday afternoon. The NBM has trended slightly lower with RHs both afternoon compared to the previous forecast and if this trend continues low-end fire weather concerns may return.

Key message 2: Widespread rain still looks likely over the weekend, with precipitation most likely Saturday night and Sunday. This generally should be beneficial rain for most of the area. Localized heavier rainfall is possible, which could lead to nuisance flooding of poorly drained areas, and/or isolated bankfull conditions on area streams.

Deep Pacific shortwave still set to take an eastward track across the southern Rockies by Saturday morning, and induce cyclogenesis in the southern Plains into the lower MS Valley. ECMWF, GFS and their ensemble counterparts have come into closer agreement on the sfc low moving across the Deep South, whereas earlier runs of those models had hinted at a more northerly track across the Ozarks and KY/TN. The operational Canadian remains a bit farther north than the GFS/EC, though it too has trended south. Precip is expected to develop ahead of the low along the warm front, with a slight chance PoP warranted in our CWA SW of I-26 Saturday afternoon, with chances increasing and expanding west to east through daybreak Sunday. Peak coverage is still expected Sunday, tapering off again from west to east Sunday night and Monday morning.

The GFS and EC also show dry and relatively weak sfc high pressure across the Eastern Seaboard Saturday morning. That high is depicted as shifting off the East Coast in advance of the approaching low, but both models develop a slight diabatic cooling response as precip develops Sat night and Sunday, which makes sense as the pattern favors in-situ cold-air damming. Those models and ensembles still suggest temperatures will remain warm enough for the precip to be only rain in our CWA. The Canadian does remain consistent with its earlier runs in showing a reinforcing, stronger high developing over the St Lawrence Valley Saturday night, suggesting more of a hybrid CAD event. This meant it was more effective at keeping cold air in place and thus had continued to suggest wintry precip impacts (mainly due to FZRA) in our area even after the other models trended warmer. That said, even the Canadian has now trended warmer so it would now also suggest all rain. The Canadian GEPS ensemble does still have the highest probability of 0.01" FZRA along the northern Blue Ridge Escarpment, nearly 50%, substantially higher than the GEFS or EC ENS probs. It's not entirely out of the question at least brief freezing rain could occur there, though the consensus pattern and overall guidance trend continue to point to an almost if not entirely rain event. Forecast temps Sat night and Sunday reflect a blend of NBM and raw model guidance which reduces the diurnal range compared to the straight NBM, and removes cold temp-bias artifacts in high elevations that were producing spurious occurrences of snow and freezing rain.

Potential exists for at least minor hydrologic impacts Sunday into early Monday, one mitigating factor being that antecedent conditions are dry. The 12z global ensembles each featured similar PWAT anomalies peaking near +2 SD just ahead of the low, and 90th percentile total QPF in the 2-3" range. There do remain subtle differences in the low track/timing as well as in each ensemble's handling of CAD, which resulted in each ensemble's mean QPF being focused in a slightly different location. Perhaps no surprise, the Canadian has the strongest signal for terrain-enhanced QPF due to its members leaning toward stronger CAD, and more northerly low track resulting in a stronger LLJ oriented more favorably for upslope enhancement into the Blue Ridge. The other ensembles certainly don't discount that possibility, but the signal is strongest among the GEPS members. Also seeing slightly more favorable profiles for MUCAPE than was seen 24 hours ago, though lapse rates generally look very weak owing to deep moisture. As previous shift pointed out, the potential for a narrow band of frontogenetically enhanced QPF along the wedge boundary is more of an indicator of localized heavy rain (and flood) threat than is the small amount of MUCAPE potentially in play. MMEFS and HEFS output for our river forecast points still suggest a less than 10% chance of seeing bankfull conditions develop in the SW NC valleys or the Savannah and Broad river basins, but the chance would be higher on small tributaries. Can't rule out localized development of nuisance flooding (e.g. road closures) in poorly drained areas below the Escarpment, or in the more floodprone urban areas.

AVIATION /06Z WEDNESDAY THROUGH SUNDAY/

At KCLT and elsewhere: VFR and dry through most of the 06Z TAF period outside of -RA developing ahead of a cold front later this morning. -RA may allow MVFR restrictions to develop across the terminals but confidence is low as guidance has been slowly backing off on MVFR cigs/vsbys. Only went with a PROB30 at KHKY for -RA and associated restrictions as confidence is low on whether this terminal will actually see precipitation and restrictions develop. Confidence is higher elsewhere so went with TEMPOS to account for the onset of -RA followed by prevailing -RA once the TEMPO ends. -RA should linger through the late morning hours before dry conditions return by early this afternoon. Winds at KAVL will turn NW behind the front around daybreak. Winds east of the mountains will remain SW through daybreak before gradually turning NW late this morning into early this afternoon behind the cold front. Intermittent gusty winds will linger through early this evening for most locations east of the mountains before tapering off late this evening. Wind gusts at KAVL will linger through the end of the TAF period. Wind speeds will generally range from 7-14 kts with gusts ranging from 18-24 kts. BKN to OVC cloud cover should stick around through this afternoon before gradually diminishing from west to east this evening.

Outlook: Dry and VFR through Friday night before precipitation chances and associated restrictions return this weekend ahead of a low pressure system.

GSP WATCHES/WARNINGS/ADVISORIES

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