textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

The Aviation Discussion has been updated for the 12Z TAFs issuance.

KEY MESSAGES

1. Very active convective weather is expected to continue through early next week. Isolated severe storms and localized flash flooding will be possible each day through Monday. Otherwise, a cooling trend is expected, with well-below normal temperatures forecast by Monday. 2. A return to seasonably hot conditions and typical coverage of diurnal showers and storms is expected during the latter half of next week.

DISCUSSION

Key message 1: Very active convective weather is expected to continue through early next week. Isolated severe storms and localized flash flooding will be possible each day through Monday. Otherwise, a cooling trend is expected, with well-below normal temperatures forecast by Monday.

How convective trends will evolve throughout the day is about as clear as mud at this juncture, as in addition to ongoing activity across the NC mountains, there are multiple areas of loosely organized showers and storms currently stretching from the Cumberland Plateau in TN/KY western Ohio Valley into the mid-Miss Valley. These areas of convection will approach the southern Appalachians and vicinity throughout the day...as an area of height falls dives from the Corn Belt early this morning...to the TN Valley tonight. The extent of any severe storm threat today will depend largely upon how this upstream activity evolves through the morning...and the degree (if any) to which it is able to inhibit destabilization later today. Based upon the latest mesoanalysis, convection is going to continue struggling to make significant inroads into the CWA this morning, as muCAPE falls off dramatically (< 500 J/kg) east of the Blue Ridge. We therefore suspect the Piedmont and foothills should moderately destabilize this afternoon, with MCV's and/or outflows/differential heating boundaries left behind by upstream convection providing the lift to interact with this instability and produce good coverage of showers and storms across the area this afternoon into the evening...with multiple rounds of convection possible as activity farther upstream approaches the area.

While deep layer shear is forecast to remain weak, low level shear of 15-20 kts will continue to support some degree of cold pool-clustering that will pose some threat of strong-to-damaging downbursts. The threat for isolated flash flooding will also persist through the daylight hours, especially across the far western mountains...the area which stands the best chance of seeing multiple rounds of heavy rain showers and storms. Otherwise, one more day of above normal temperatures is expected today before cooler conditions evolve early next week. A few spots across the southern tier of forecast zones and the Charlotte metro area could briefly see Heat Index in the 101-103 range this afternoon, but nothing worthy of any heat headlines.

Convection-allowing and short term model guidance agree that a relative lull in the convective activity will occur tonight, before another round of fairly widespread convection develops Sunday...as an upper low begins to close off over the TN Valley. Increased cloud cover and slightly lower heights are forecast to yield one of the few near-normal temperature days we've seen over the past couple of weeks. This will result in slightly lower instability. In terms of the severe weather threat, the lower buoyancy may be somewhat offset by a brief increase in deep layer shear Sunday afternoon...so another marginal risk is appropriate.

By late Sunday, a slow-moving back door cold front is expected to begin moving into the CWA from the NE...although uncertainty about how far SW it will push remains high...owing to potential development of weak low pressure across the region. In association with this boundary, ESE upslope flow is forecast to develop across at least the northern part of the area. All this as the flow aloft weakens within a col region on the east side of the upper low. These factors spell potential for slow-moving and/or training cells...within an environment characterized by PWATs of around 2 inches and warm cloud depths well over 10 kft. The flash flood threat therefore may see a bit of an uptick in the late Sunday through Monday time frame...although instability will increasingly become a limiting factor by Monday, as extensive cloud cover and NE flow holds high temps well below climo.

Key message 2: A return to seasonably hot conditions and typical coverage of diurnal showers and storms is expected during the latter half of next week.

Weakly cyclonic upper flow early in the week is forecast to give way to rising heights and the return of ridging aloft by the middle of next week, as an anticyclone centered over the northern Great Plains/ upper Miss Valley gets squeezed from the south and broadens. Prior to this pattern change, convective chances remain somewhat elevated Tuesday, but by Wed...a more typical regime of widely scattered mountain convection...with isolated coverage elsewhere is expected...with that trend generally forecast to continue through the end of the week. Temperatures are forecast to remain several degrees below climo Tuesday before warming to near-normal Wed...the n ending the week a couple of degrees above climo.

AVIATION /12Z SATURDAY THROUGH WEDNESDAY/

At KCLT and elsewhere: Areas of weakening showers and isolated thunderstorms continue to move into western North Carolina from east TN this morning. Most of these have been dissipating shortly after moving away from the TN border, but a few of these may drift toward KAVL early in the forecast period. Chances for showers and possibly TS will increase from the west from late morning on, and a tempo for TSRA is included at KAVL beginning at 15Z. The convective potential will increase at the other TAF sites during the afternoon, and Prob30s for TSRA begin as early as 17Z at KGSP/KGMU, and generally after 18Z at the other sites. Prob30s are likely to be eventually converted to tempos at the other sites, but this will need to wait until confidence in timing improves.

Winds will remain out of the WSW...increasing to around 10 kts at most sites by early afternoon. This will create a crosswind concern at KCLT. Directions are expected to primarily favor S of due W, but periods of 270 and even 280 can be expected. For the overnight period, some convection-allowing models bring nocturnal showers and storms to the area...including to KCLT. This is far from certain at this point, and we will continue to highlight this afternoon/evening with the greatest potential. Conditions will be more favorable for fog and/or low stratus development late tonight/early Sunday, but confidence is still generally low...restrictions will be limited to MVFR at the typical locations (KAVL/KHKY) for the time being...with the potential for low stratus hinted at the other sites with FEW/SCT IFR layers.

Outlook: Greater than normal coverage of SHRA/TSRA will continue across our area Sunday into Monday as multiple rounds of storms are expected across the region. Drier conditions should develop Tuesday into Wednesday. Low stratus and/or fog will be possible each morning in the mtn valleys and in locations that see appreciable rainfall the previous afternoon/evening.

GSP WATCHES/WARNINGS/ADVISORIES

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