textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Increased concern for severe weather and localized heavy rainfall threats Wednesday night.

Low confidence in rain potential this weekend into early next week.

KEY MESSAGES

1. Dry conditions expected again today. A Fire Danger Statement is now in effect for this afternoon for part of the North Carolina Piedmont. 2. A cold front arrives Wednesday and produces scattered showers and thunderstorms from Wednesday afternoon through Thursday night. Isolated large hail and damaging wind are possible. Localized, brief heavy rainfall is also possible, but flash flood risk appears low, and extremely isolated. 3. Rain chances continue for portions of the weekend into Monday then drying for Tuesday.

DISCUSSION

Key message 1: Dry conditions expected again today. A Fire Danger Statement is now in effect for this afternoon for part of the North Carolina Piedmont.

High pressure over the western Atlantic continues to drive mostly clear and dry conditions across the western Carolinas today. A zone of upper confluence and associated focused forcing was visible on water vapor imagery well to our northwest, over the Upper Ohio Valley and aligned with an apparent surface front extending from southern Illinois into western Ohio.

Forecast profiles depict a deep dry layer beginning at around 800mb, with only a weak capping inversion in the subsidence layer. Expect dewpoints to mix out significantly again today, resulting in widespread RHs bottoming out in the 25-30% range during peak heating this afternoon. Also expect WAA winds to be as strong or stronger than yesterday, as the influence of the approaching frontal system results in a 3-5mb pressure gradient across the forecast area. Low-end fire weather concerns are therefore in order - and NCFS has indicated the need for a Fire Danger Statement in certain parts of the NC Piedmont, where fine fuel moisture may dip below 8% again, and the meteorological conditions so described will enhance the risk of fires catching and spreading. So, a Fire Danger Statement is in effect through 8 PM this evening for a chunk of the Piedmont this afternoon.

The front will slow on approach tonight, but low-level moisture will nonetheless increase, rapidly eroding the dry slot and bringing an end to fire wx issues for the next few days. Hi-res guidance continues to even depict a subtle shortwave lifting out of the Lower Mississippi Valley this evening into tonight, resulting in some isolated showers over the Smokies and Balsams this evening.

Key message 2: A cold front arrives Wednesday and produces scattered showers and thunderstorms from Wednesday afternoon through Thursday night. Isolated large hail and damaging wind are possible. Localized, brief heavy rainfall is also possible, but flash flood risk appears low, and extremely isolated.

A cold front looks to approach the area Wednesday, but the forecast remains somewhat murky with respect to how convection will evolve on Wednesday. As a 500mb speed max amplifies over Tennessee early Wednesday morning, well-organized convection will eject off the surface front and barrel east toward the Carolinas. The 12z CAMs (including most REFS members) depict the arrival of a decaying MCS around daybreak. Shear during this time will be limited (as the MCS will likely have outrun the speed max) and any lingering instability should be elevated, and feature anemic lapse rates even then. Consequently, think the severe risk with whatever deteriorating convection makes it here early Wednesday will be low.

Model consensus, however, is for redevelopment of convection later Wednesday. 300-600 J/kg sbCAPE is supported - especially across the western half of the CWA - for the afternoon and early evening hours, where the 12z HREF depicts a 40-70% chance of sbCAPE > 500 J/kg...a notable increase since last night's 00z cycle. Shear should steadily increase as the upper speed max migrates eastward on Wednesday afternoon, eventually pushing into the NC mountains and northeast Georgia. However, synoptic forcing will be in a bit of a lull during peak heating, unfortunately making initiation more difficult to pinpoint during the period when the convective parameter space looks best. Several of the 12z CAMs support upstream initiation over northern Alabama and Georgia within an axis of low-level frontogenesis, and then translate this feature into the Upstate Wednesday evening, and through the first half of Wednesday night before the advancing front drives convection to our south and east. Though surface-based instability would become increasingly paltry going later into Wednesday night, the increased synoptic forcing and elongated hodographs are enough to make the 12z HREF's signal for severe potential believable.

Should severe storms develop Wednesday evening/night, there's support for mainly a hail/wind threat, and especially over northeast Georgia and the western Upstate. In these zones, the latest HREF and REFS guidance depict updraft helicity bullseyes arriving after 21z, supportive of the rotating updrafts needed to support large hail. Meanwhile, a wind threat would normally be a slam dunk with organized convection, but in this case may be somewhat muted as increasingly, deep layer shear looks to have a large along-boundary component. Still, would expect to see at least a few surging segments along the line. Tornado risk still looks like it should be kept in check by a lack of good low-level SRH and high LCLs.

Consequently, the bigger issue could turn out to be heavy rain; with some hi-res guidance depicting two or even three rounds of ample rainfall for the Smokies, Balsams, and northeast Georgia. 12z HREF PMMs generally depict up to 1-1.5" of rain in 6 hours, and 2-2.25" in the 12 hours leading up to daybreak Thursday. Whether this is enough to cause more than isolated, low-end hydro issues is debatable: almost the entire forecast area remains in D3 drought, with rainfall deficits of 6 inches or more...so think that antecedent conditions alone should keep the flood threat in check, though nonzero. Nonetheless, given the orientation of the boundary and the simulated reflectivity forecast from the 12z CAMs, it would appear there's some potential for training storms...and therefore will at least need to monitor hydro closely.

Key message 3: Rain chances continue for portions of the weekend into Monday then drying for Tuesday.

The cold front from Thursday stalls to our south on Friday, then a series of Gulf Coast low pressure systems move east along the front through Sunday before a scouring cold front moves through from the northwest on Monday. Confidence on the evolution of these systems is low as guidance differs on which wave will be stronger with more in the way of precip for our area. In fact, one model has little with the weekend waves saving it for the front on Monday. The model blend has the better chances Sunday night into Monday, but of course, this could and will likely change. Temps drop below normal for Friday, rise to near normal Saturday, above normal Sunday and Monday, then back to near normal Tuesday.

AVIATION /18Z TUESDAY THROUGH SUNDAY/

At KCLT and elsewhere: VFR on tap for Wednesday. Gusty SW winds will continue through this afternoon and evening, with frequent gusts of 20-25kts expected at all terminals, and occasional higher gusts possible. Cirrus will steadily increase through the evening, and a FEW/SCT cu field already visible on satellite will expand for another few hours before dissipating after sunset. Some of the statistical guidance depicts MVFR cloud bases developing tonight at KAVL and other mountain/foothill sites, but confidence remains limited so just hinted at this in the KHKY and KAVL TAFs. VCSH was added on Wednesday morning for the predicted arrival of a decaying MCS around and after 12z. Scattered TSRA looks likely again Wednesday afternoon into Wednesday night, but for the 18z TAFs, this only affects KCLT, where a PROB30 was added.

Outlook: Restrictions are likely with a cold front crossing the area from late Wednesday through early Thursday. TSRA may develop with this front. Confidence is low past this point.

GSP WATCHES/WARNINGS/ADVISORIES

GA...None. NC...INCREASED FIRE DANGER until 8 PM EDT this evening for NCZ056- 057-068>072-082-504. SC...None.