textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Forecast was updated to reflect the recent 06Z TAF issuance.

High temperatures were lowered slightly for Tuesday based on current conditions and recent model trends.

KEY MESSAGES

1. Mostly quiet conditions continue through Tuesday morning, although several flood products remain in place for the next few hours due to previous rainfall. Widespread showers and storms are expected to redevelop on Tuesday afternoon, presenting both a low-end severe threat and a low-end hydro threat. 2. Daily chances for showers and thunderstorms continue into the weekend. Drier conditions may briefly return on Friday, but there is not a signal for more robust drying until at least Sunday.

DISCUSSION

Key message 1: Mostly quiet conditions continue through Tuesday morning, although several flood products remain in place for the next few hours due to previous rainfall. Widespread showers and storms are expected to redevelop on Tuesday afternoon, presenting both a low-end severe threat and a low-end hydro threat.

The forecast area is now mostly clear of showers, as the upper vort lobe and associated stronger forcing have shifted into the NC Piedmont. With what few showers remain, rates have dropped significantly, and aren't expected to pose much of an issue the rest of the night. That said, we still have several flood advisories and a lone flash flood warning in effect across the NC Blue Ridge Escarpment, where streamflows remain elevated, and some low-lying areas are inundated with previous rainfall. Will continue to monitor these products through the course of tonight, but based on current trends, the worst has passed and things should improve from here on out. One wrinkle is the trend in many of the 00Z CAMs, as well as recent RAP cycles, toward a brief rebound in upglide across the Smokies and Balsams after 5AM, which, if it came to pass, would manifest as a few light showers there. QPF response appears to be on the order of a few hundredths of an inch, and guidance doesn't suggest even these light showers will impact areas that were hit most heavily by this evening's rainfall, so such a scenario shouldn't cause significanty impacts. Like the last few nights, the potential for patchy dense fog also exists through daybreak, but there's not a strong signal for more widespread issues that would prompt any dense fog products to be issued; rather, dense fog is expected to be more isolated in nature and confined to those areas which received the most rainfall over the last 12 hours.

We can expect a brief period of quiet conditions early Tuesday. Whatever weak, shallow in situ wedge is in place by dawn will gradually mix out through late morning. The lull won't last long, however. Hi-res guidance rapidly destabilizes most of the Deep South, depicting widespread convective initiation there as early as 11AM...and all that convection will gradually arrive in the Upstate through early afternoon, with most of the 00Z HREF members featuring widespread showers and embedded thunder across the region throughout the afternoon and early evening. It's increasingly looking like the severe threat will be nonzero; the HRRR as well as most of the CAMs feature a plume of some 1500-2000 J/kg sbCAPE across the area in an environment of 20-25kts of deep layer shear. It's by no means a slam dunk, but a couple of strong to severe storms are well within reason. About the only thing they have going against them is the lack of any strong forcing mechanism and thus fairly weak mid-level lapse rates that will result in updrafts which are taller, but weaker.

Meanwhile, the environment continues to feature 1.75-2.0" PWs and very warm, very moist profiles. Any deeper showers and storms that develop could produce locally heavy rainfall, and given antecedent conditions, some locations may not take long to realize an isolated nuisance flood risk, perhaps even a low-end flash flood. Indeed, while the 00Z HREF depicts generally less-impressive QPF response across the area than it did for Monday, it continues to key on a corridor across the western two-thirds of the Upstate receiving another localized 1.5+ inches through the afternoon.

Key message 2: Daily chances for showers and thunderstorms continue into the weekend. Drier conditions may briefly return on Friday, but there is not a signal for more robust drying until at least Sunday.

By Wednesday, long-range guidance begins to depict a subtle pattern shift, with slight height rises early in the day and a more WSW direction to low-level flow. The result is more of a Deep South moisture fetch from that point forward, with trajectories crossing the lower Tennessee Valley rather than advecting moisture to us directly from the Gulf. With no more shortwaves featured in synoptic guidance, low-level upglide tends to falter in most of the ensemble output...and so the CAMs that go out that far (as well as the operational guidance) feature considerably lower coverage of afternoon showers on Wednesday afternoon...with anything more than isolated activity confined to the Appalachians. That said, there does appear to be potential for another low-end severe risk on Wednesday afternoon: height falls over the Ohio Valley and Mid-Atlantic will result in an area of confluent flow aloft that'll actually strengthen the 500mb wind, so whatever convection does develop will do so in an environment of moderately-better deep layer shear than we've had the last several few days (and Tuesday).

The remainder of the week remains somewhat of a question mark. LREF members are tending strongly toward a scenario where a weak backdoor cold front arrives sometime Thursday...likely too late in the day to prevent a band of frontally-focused showers and storms from crossing the area that afternoon...and briefly ushering in some drier air. However, that front is variously depicted stalling in or just south of the Upstate, then acting as a focus for renewed isentropic ascent, possibly even another hybrid CAD event, as the parent high moves off the New England coast and another upper trough reactivates the boundary. That means - you guessed it - more rain for Friday night and Saturday.

There remains very high uncertainty for the rest of the weekend and early next week. There are really two camps among long-range ensembles: one in which we get a strong cold backdoor cold front Sunday night, ushering in a dry cP air mass that persists through the end of D7, and another in which we get a much weaker backdoor cold front and only modest, short-lived drying at best. Unfortunately, the stronger/drier scenario is currently the less-favored among ensembles...but at this time range and with how unsettled the atmosphere has been, there's really no confidence to be had.

AVIATION /06Z TUESDAY THROUGH SATURDAY/

At KCLT and elsewhere: Convection has mostly exited the area tonight, with some lingering showers north of I-20 on the extreme southern fringe of the terminal forecast area...but otherwise just some patchy drizzle. Statistical and hi-res guidance continues to feature IFR to LIFR ceilings developing againt tonight - and indeed they've begun to in a few places, but much less widespread and with much less consistency than on previous nights. Nonetheless, given the amount of rain that much of the area has received, and the redevelopment of a very weak, very shallow in situ wedge, most of the area is at least under the gun for intermittent lowered restrictions through daybreak. An isolated shower or two could also crop up anywhere with little warning.

Conditions should improve to VFR in most locations by late Tuesday morning. Afternoon convection will again develop across the entire area, with SHRA and embedded TSRA possible at all six TAF sites. Confidence is limited on Tuesday night's forecast, as restrictions will be heavily dependent on the evolution of afternoon convection.

Outlook: The pattern remains unsettled through the workweek with at least scattered diurnal convection expected each day (possibly persisting into the overnight hours) and lowered visibility and ceilings forecast each night.

GSP WATCHES/WARNINGS/ADVISORIES

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