textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Timing of rain onset for tomorrow's showers and storms was refined, and the temperature forecast was updated based on the latest round of hi-res guidance's handling of a backdoor cold front and cold-air damming development.

Rainfall totals were updated for this weekend's system.

The aviation forecast was updated to reflect the recent 18Z TAF issuance.

KEY MESSAGES

1. Quiescent weather continues today and tonight, with little more than a token shower or two west of I-26 expected this evening. 2. Widespread rainfall is expected on Saturday and Sunday, potentially causing localized flooding in a few spots. Confidence remains limited in details. 3. Dry and cool conditions persist most of next week. Temperatures will steadily warm back up through Friday.

DISCUSSION

Key message 1: Quiescent weather continues today and tonight, with little more than a token shower or two west of I-26 expected this evening.

Broad high pressure off the Atlantic coast continues to drive light S/SW flow over most of the forecast area today, while aloft, a low- amplitude shortwave trough remains trapped over the Ozarks. The bulk of better moisture transport appears to be confined to the TX/LA Gulf coast, north into the Ozark Plateau and western Tennessee Valley...but some steady increase in moisture over our area continues nonetheless. Make no mistake...dewpoints are currently in the mid- to upper-60s across most of the area, and with a shrinking low-level dry slot, many locations along the I-85 corridor and south may see dewpoints hit 70 degrees this afternoon.

Hi-res guidance continues to key on the potential for some isolated showers, perhaps even a rumble of thunder, across the upper Savannah River Valley and into the Smokies/Balsams. These zones may wind up realizing several hundred joules of surface-based instability during peak heating this afternoon...a far cry from the environment needed for any strong updrafts, but enough to support some weak convection should ridgetop triggering get a cell or two going.

Key message 2: Widespread rainfall is expected on Saturday and Sunday, potentially causing localized flooding in a few spots. Confidence remains limited in details.

Increasingly active conditions are forecast to develop over the weekend as a dynamic upper system arrives. Deterministic guidance depicts the remnants of a shortwave trough, initially impressive over the Great Plains and Midwest, becoming smeared across the Ohio Valley and steering a surface cold front to our western doorstep by early Saturday. A backdoor cold front will begin to settle down the Virginia Piedmont by mid- to early-afternoon, but for our part, we seem likely to remain within a warm sector-like environment for most of Saturday. Most of the CAMs feature initiation of showers by noon or so on Saturday, within a plume of 500-900 J/kg sbCAPE, based on the spread of the latest-available REFS ensemble. Though a stray strong to severe storm can't be entirely ruled out...the thinking remains that absent much good deep-layer shear, more than a few generic thunderstorms will struggle to develop.

Saturday evening and into the overnight, the environment will change dramatically as the backdoor cold front settles south across the area. Within just a few hours, forecast soundings depict the onset of a cold-air damming wedge concurrent with strong surface CAA behind the front. As a consequence, the potential for surface-based convection will diminish, although some weakly-enhanced convective rain rates may persist, by virtue of elevated instability above the wedge layer. Rain, though, seems increasingly likely to continue through the night and into at least the first half of Sunday, as a second and more well-defined shortwave crosses the Ohio Valley and incites stronger moist upglide over the wedge. This should have the added effect of further reduce Sunday's temperatures...keeping them two to three categories below normal for most of the area.

The jury is still out, then, on exactly how any heavy rainfall threat will pan out. The case seems very limited initially. Through Saturday night, the most impressive signal for heavy rainfall comes in the 2" or greater PWs developing mainly south of I-85...which, to wit, is above the 95th percentile of historical data for this time of year; paired with even weak updrafts, this could result in impressive rain rates. However, upper flow should be sufficient to keep showers and storms moving, so limited residence time should keep widespread hydro issues at bay. The notable exception is locations which receive heavier showers early in the day in the warm- sector portion of the event, and then get a second round of rain as the backdoor front arrives in the evening. Recent runs of the HREF and REFS all feature an axis of higher QPF consistent with this thinking, but vary wildly in terms of where that axis sets up.

The forecast becomes murkier after 12Z Sunday. Few of the CAMs extend out that far, but the deterministic guidance supports improving synoptic forcing as the second shortwave crosses the area through Sunday night. In response to the shortwave, the surface front will finally push east across the area; long-range ensembles remain split on how quickly this will take place, though the 12Z suite of guidance seems to have trended toward a slower evolution, and a later end to rainfall. More generally, think the threat of heavy rain on Sunday will hinge on how much rain certain areas receive on Saturday/Saturday night and whether/how long any convective rates persist into Sunday. Continued rainfall on Sunday may result in a more of a cumulative, slow-burn flood threat for some locations. Confidence remains limited, but should begin to improve over the next 12 to 18 hours as models hopefully develop a consensus on the first half of the event, and as more of the hi-res guidance begins to capture the latter half of the event.

Key message 3: Dry and cool conditions persist most of next week. Temperatures will steadily warm back up through Friday.

Lingering cold air damming should begin to drain on Monday as flow aloft slackens. Dry and cool cP air will settle into the Carolinas behind a cold front, and that air mass will largely sit and modify through most of the week. It should be another dry and unperturbed week...an afternoon breeze on Tuesday, followed by cool temperatures each day, steadily warming back to around normal by Thursday or Friday as the air mass modifies. The latest forecast has trended a tad warmer for Tuesday night and Wednesday morning...but could still see isolated spots across the high terrain of the northern Blue Ridge approach freezing before dawn.

AVIATION /18Z FRIDAY THROUGH WEDNESDAY/

At KCLT and elsewhere: VFR on tap for the bulk of the 18Z TAF period, with mainly thin cirrus and a mid-level cu field developing this afternoon and evening across the terminal forecast area. SW winds expected to persist this afternoon and evening, with most sites going light and variable overnight tonight. Thickening cirrus tonight as better moisture moves in, and guidance increasingly keying on the development of a FEW/SCT mid-level cloud deck above 8 kft, after midnight...mostly altocu based on forecast soundings. Low confidence on the fog/stratus forecast at KAVL on Saturday morning; no restrictions developed this morning, and although moisture may be marginally better Saturday morning...cloud cover will also be considerably thicker. The statistical guidance continues to feature IFR fog and low stratus at all the mountain terminals...but this forecaster is skeptical, and only included MVFR restrictions at KAVL in a TEMPO.

Otherwise...isolated showers and perhaps a rumble of thunder could develop as early as 15-16Z Saturday. Winds should remain out of the SW initially (except in the immediate vicinity of any stronger shower or storm, where they could be gusty and erratic), but quickly toggle to the NE as a backdoor cold front moves from the I-40 corridor south and west across the area Saturday evening. Concurrently, coverage of SHRA and embedded TS will increase, and restrictions will develop as a cold-air damming sets up for the latter portions of the TAF period and beyond. This only really falls within the 18Z TAF at CLT...but will become an issue for future iterations of the other TAFs.

Outlook: Widespread SHRA and associated flight restrictions are expected across the area on Saturday night and the first part of Sunday. Improvement to VFR is expected by Monday, and will persist well into next week.

GSP WATCHES/WARNINGS/ADVISORIES

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