textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Slightly lowered high temperatures for Wednesday.

Raised rainfall totals by up to a quarter inch in the Piedmont and Upstate for Thursday afternoon.

Updated the aviation discussion for the 06Z TAFs.

KEY MESSAGES

1. Diurnal thunderstorms return on Wednesday, but with a lower heavy rain threat overall than the last few days. Coverage is expected to be mainly confined to North Carolina. A low-end severe weather risk is possible, with damaging wind the main hazard. 2. Diurnal rain and thunder chances continue through at least the end of the week. The pattern becomes murky, but likely still unsettled, through the beginning of next week.

DISCUSSION

Key message 1: Diurnal thunderstorms return on Wednesday, but with a lower heavy rain threat overall than the last few days. Coverage is expected to be mainly confined to North Carolina. A low-end severe weather risk is possible, with damaging wind the main hazard.

A very weak shortwave evident on water vapor imagery is finally exiting the area tonight and the bulk of showers have moved north in response. A few light popup showers continue across the Piedmont and Upstate, but rain rates thus far remain unimpressive, so concern for additional hydro issues the remainder of the night is low. A few flood advisories remain in place across the area as a result of rainfall sustained Tuesday evening, and we continue to closely monitor area river gauges for signs of worsening conditions...but the day's flood threat has largely concluded. As with the previous several nights, rainfall has produced a weak and shallow in situ wedge that'll result in some low stratus and some patchy dense fog across the area. Statistical guidance and the latest hi-res models keep this widely scattered enough that any dense fog products probably won't be necessary.

Deterministic guidance depicts an area of height falls over the upper Ohio Valley later tonight into the first part of Wednesday, and in response to that heights should actually fall subtly over the forecast area and rise slightly over the Tennessee Valley to our west. As a consequence, mid- and low-level flow will become a bit more zonal, and a weak confluence zone will develop over central NC through this afternoon. There is precious little synoptic support for convective initiation over most of the area today, though some weak ripples of DPVA may make it to the I-40 corridor on the southern periphery of the Ohio Valley system. CAMs tend to instead suggest that topography will provide the main driver for initiation over the mountains today, and that storms will then gradually migrate east through the afternoon following the mean flow. They'll encounter a decent convective environment: the 00z HREF depicts a plume of surface-based instability resulting in some 1200-2000 J/kg sbCAPE across the mountains and Piedmont, and upwards of 2000 J/kg sbCAPE across the Upstate (though sans any driver for initiation, that Upstate instability will remain largely unrealized). As storms push eastward, they'll also encounter an increasingly sheared environment featuring 25-30kts of bulk shear along I-77, beneath the confluent zone discussed above. This shear will be largely unidirectional in nature, so think we could see a few strong to severe storms this afternoon...which is well in line SPC's MRGL risk in the Day 1 Outlook.

Owing the more WSW flow in the mid- and low-levels, moisture fetch will likely be more out of the Tennessee Valley and Deep South than directly out of the Gulf as it's been in days past, and PWs should fall this afternoon accordingly...to more in the 1.5-1.8" range across most of the CWA...much closer to climo for late May. Lapse rates may also steepen somewhat in response to the Ohio Valley system, lowering the freezing level...so despite profiles remaining very moist through the depth of the troposphere, think that both lower overall coverage depicted today and the convective environment itself make for a lower heavy rain/hydro threat compared to Monday and Tuesday. Agree with WPC's MRGL ERO in the Day 1 Outlook.

Key message 2: Diurnal rain and thunder chances continue through at least the end of the week. The pattern becomes murky, but likely still unsettled, through the beginning of next week.

Area of height falls over the Ohio Valley should steadily sharpen into a deep 500mb trough that will slide across the New England and the upper Mid-Atlantic through Thursday. At the surface, ensembles are in good agreement that this will drive a weak backdoor cold front across the Carolinas by late Thursday. Models finally seem to be congregating around a solution that delays the FROPA late enough for the atmosphere to destabilize across most of the area; indeed, all of the 00z HREF members depict a broken line of convection making its way south across the area Thursday afternoon. Absent any other good forcing mechanisms, the thinking remains that this convection should be mainly focused along the advancing frontal boundary, and indeed most of the CAMs suggest coverage will be limited. The convective parameter space looks similar to Wednesday's...perhaps a little less stable and a little more sheared...so think there's a low-end severe risk, again with wind the dominant threat. But certainly not expecting anything out of the ordinary for late May.

Confidence is increasing that in the wake of the front, a dry air mass will move into the upper/central Ohio Valley, but that the front will settle only just to our south. Ensembles continue to favor reactivation of this boundary sometime Friday, meaning additional rainfall looks increasingly likely by Friday night. Confidence is lower on the nature of this rainfall; many LREF members feature more of a CAD-like scenario (and all the long-range deterministic guidance depicts easterly winds that would support synoptic cold-air damming development beginning Friday), but a sizeable chunk of ensembles instead support the boundary lifting back north as a warm front and exposing us to a more warm sector-like environment Friday afternoon and Saturday. The jury is still out on which camp will prevail, but either way at least some additional rainfall looks like a strong possibility for Friday and Saturday.

The remainder of the period remains a big question mark. There's a signal in the long-range ensembles for a stronger backdoor cold front sometime in the Sunday to Monday timeframe, but guidance doesn't quite have a handle on it yet...and confidence is limited on whether it'll bring a definite end to our wet spell or not. The larger-scale upper pattern looks to remain progressive through the end of the seven-day forecast, so even if the front gives us a "reset" we can still expect more typical springtime weather for the foreseeable future.

AVIATION /06Z WEDNESDAY THROUGH SUNDAY/

At KCLT and elsewhere: A few showers remain in place across the Piedmont and Upstate tonight, but for the most part, we've transitioned to just patchy drizzle and intermittent MVFR to IFR fog across the area. Ceilings in a few spots have already begun to crash, and statistical guidance suggests this'll continue through daybreak, with widespread IFR and perhaps some LIFR expected by morning. Can't rule out a few popup showers continuing through the night, as we've had the last several nights. Convective coverage should be much lower on Wednesday than the last two days, with SHRA and embedded TSRA expected to develop over the mountains during the early afternoon, then pivot east across the Piedmont through the early evening hours. Based on recent hi-res models, actually couldn't justify more than VCSH at the Upstate sites for Wednesday afternoon, but kept PROB30s for TSRA at the three NC TAF sites. Guidance is hinting that as a result of Wednesday's much lower coverage, Wednesday night may see considerably fewer restrictions than we've had the last week. Winds will remain mostly calm tonight (except in the vicinity of any isolated showers that develop), and will be steady out of the SW at 5-9kts this afternoon. Some low-end, intermittent gusts up to 18kts are possible at all the terminals during afternoon peak heating.

Outlook: The pattern remains unsettled through the rest of the week with at least scattered diurnal convection expected each day. Vsby and cig restrictions will remain possible each night.

GSP WATCHES/WARNINGS/ADVISORIES

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