textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Updated Aviation Discussion.

KEY MESSAGES

1. The potential for isolated heavy rainfall and flash flooding returns tonight into Saturday, as a front moves back into the area. One last round of convection is anticipated Monday before the front finally clears the region. Temperatures trend below normal for the weekend. 2. Dry and cooler than normal weather is expected Tuesday through at least Thursday.

DISCUSSION

Key message 1: The potential for isolated heavy rainfall and flash flooding returns tonight into Saturday, as a front moves back into the area. One last round of convection is anticipated Monday before the front finally clears the region. Temperatures trend below normal for the weekend.

A brief period of NE low level flow will continue to advect slightly drier and cooler air into the forecast area through today, and max temps are forecast to be around 5 degrees cooler than Thursday...or right around normal. Much of the forecast area is expected to be relatively stable today, and PoPs through 00Z are primarily confined to the southern and western periphery of the CWA...closer to the old frontal boundary that has been meandering across the region for days.

Otherwise, an upper low will remain centered near New England through much of the weekend, with a trough extending southward near the East Coast. An upstream, high-amplitude ridge extending from the Deep South into the Canadian Prairie is forecast to weaken over the Southeast today, as a short wave trough ejects from the Southern Plains into the TN Valley by tonight. This will allow the frontal boundary to nudge northeast back into the western part of the CWA tonight, with the boundary and increasing synoptic forcing allowing for development of at least scattered convection across roughly the western half of the area. Precipitable water values re- surging to around 1.75", combined with mean cloud-bearing winds roughly parallel to the boundary suggests the possibility of training of heavy rain-producing cells. Additionally, shear parameters should be adequate for organization of convection into small clusters, with potential for back-building clusters adding to the locally heavy rainfall potential. The threat for flash flooding still looks too localized to justify Watch consideration, especially since instability will be weak given the time of day. Nevertheless, a flash flood event or two is very much in the realm of possibility tonight/early Saturday.

Convection should increase in coverage (or redevelop) across at least the SW half of the CWA as the atmosphere destabilizes Saturday afternoon. Depending upon how much instability is realized (which will depend largely on the coverage of convection and/or extent of convective debris cloudiness Sat morning), shear parameters are expected to be adequate for a low-end severe storm threat, along with the continued potential for isolated flash flooding Sat afternoon/evening.

A surface high building behind a short wave digging through the mean trough across the Northeast and Mid-Atlantic late Saturday will strengthen low level NE flow across our area Saturday night/early Sunday, pushing the frontal zone back southwest of the CWA in backdoor fashion. Resultant advection of cooler and drier air into the CWA (Sunday max temps 10 or more degrees below climo) is expected to result in another pause in convective activity. A more substantial short wave trough/upper low digging into the Northeast Sunday night/early Monday will sweep a cold front through the region Monday, bringing one final round of (mostly) scattered convection before cooler/drier/stable air with more staying power arrives for the remainder of next week.

Key message 2: Dry and cooler than normal weather is expected Tuesday through at least Thursday.

An upper low is forecast to be established off the Mid-Atlantic coast by mid-week, with an anomalously low heights forecast to cover much of the Eastern Seaboard until later in the week. This is expected to result in temperatures around 5 degrees below climo from Tuesday through the end of the period. These cooler temperatures along with surface dewpoints generally in the 50s are expected to preclude the potential for any diurnal instability/convection Tuesday through Thursday.

AVIATION /12Z FRIDAY THROUGH TUESDAY/

At KCLT and elsewhere: Fog/low stratus will remain confined to the mountain valleys west of KAVL this morning, with VFR forecast through at least this evening at the TAF sites. FEW/SCT cumulus or stratocu mainly in the 030-050 range are possible at just about any time, but especially during the afternoon. Generally light NE winds this morning steadily weakening, possibly becoming light SSW by this evening.

A frontal boundary will approach from the southwest this evening, resulting in increased coverage of showers and some thunderstorms, mainly after midnight and mainly in western areas. Most TAF sites receive a Prob30 at some point late tonight/early Saturday...mostly for SHRA, although TSRA is carried at KAND. Can't rule out TS at the other sites, but chances are too low for a TAF mention. Cigs will begin to lower from the west overnight and MVFR cigs are forecast at all sites by daybreak except KCLT and KHKY (and can't rule it out there either.)

Outlook: Chances for showers, thunderstorms, and associated restrictions continue into Saturday, especially during the afternoon. area. After another brief drying period Sunday, chances for mainly diurnal convection return Monday before a longer term drying pattern becomes established beginning Tuesday,

GSP WATCHES/WARNINGS/ADVISORIES

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