textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

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

KEY MESSAGES

1. Quiet weather with above-normal temperatures again today, then a cool-down back to normal for Friday. 2. Widespread rain is expected to move into the region on Saturday and continue through Sunday. Excessive rainfall will be possible Saturday night and Sunday, along with a small risk for severe thunderstorms mainly Sunday afternoon.

DISCUSSION

Key message 1: Quiet weather with above-normal temperatures again today, then a cool-down back to normal for Friday.

Very little change on the front end of this forecast, other than perhaps the center of the reinforcing high coming a little farther south and into the fcst area on Friday. High temps today will be seasonally warm and better than five degrees above normal, aided by some modest downslope warming on the leading edge of the mid/upper anticyclone approaching from the west. A weak cold front will cross the area Thursday night and usher in a slightly cooler air mass for Friday as the position of the reinforcing high favors the development of a dry cold air damming wedge. A stream of thickening high clouds exhausted from TS Isaias will also help to knock the temps back to normal. It is possible the high temp forecast for Friday afternoon could be on the high side if high clouds arrive thicker and sooner. The quiet weather will come to an end after midnight Friday.

Key message 2: Widespread rain is expected to move into the region on Saturday and continue through Sunday. Excessive rainfall will be possible Saturday night and Sunday, along with a small risk for severe thunderstorms mainly Sunday afternoon.

Our attention turns quickly to the southwestern to central Gulf where Hurricane Isaias is expected to strengthen to a hurricane later today and take a more northward track toward the northern Gulf Coastal region tonight and Friday. Refer to the latest guidance from the National Hurricane Center for more details. The storm isn't expected to start affecting the western Carolinas until the pre-dawn hours Saturday at the earliest, as moisture surges northward as the center comes ashore in the middle of the northern Gulf Coast. So, precip chances ramp up steadily from the south in the pre-dawn hours Saturday and reach the categorical range everywhere by late that evening.

The excessive rain potential is the main concern, what with a current forecast favorable northward track west of the mountains. That being said, there's a lot of factors that could come into play, and some fairly big differences are seen in the QPF from various sources, some still pointing to a more beneficial rain and others suggesting something more problematic. That we are going into the weekend from a dry stretch will be a limiting factor, and the dry air mass in place could delay the precip onset early on Saturday morning. Mid/upper forcing may dominate early on Saturday, but the low level forcing and isentropic upglide will take over into Saturday evening as moisture is transported over top of the wedge. Saturday night looks like it holds the best collection of ingredients for heavy rain, including a strengthening southeasterly low level jet, deep tropical moisture, and plenty of forcing. The QPF has trended slightly upward with this cycle, particularly over the western Piedmont, but the NBM still shows less than WPC. The NBM has only modest chances of getting greater than 3 inches of rain in a 24 hour period, and that is centered on the usual southeasterly flow upslope maximum in the headwaters of the French Broad and Savannah River basin. The Friday runs of the CAMs should be much more revealing, and it might take until we see that guidance before watches are contemplated.

Assuming the track forecast works out as shown in the latest Advisory, we will start to lose the best low level forcing early Sunday as the flow at 850mb veers more S to SW. A dry slot may start to work in from the west and southwest later in the day. By Sunday afternoon, the situation could get interesting as the circulation associated with Isaias allows for the wedge boundary to lift northward into the forecast area. The LREF indicates some favorable overlap of buoyancy above 500 J/kg and greater than 30 kt of shear Sunday afternoon in the area south and east of I-85. This is highly uncertain, but worth monitoring. Eventually, as we head into Monday, precip chances move off to the east as sfc/upper ridging moves in behind the system.

AVIATION /12Z THURSDAY THROUGH MONDAY/

At KCLT and elsewhere: Derived satellite imagery AND METARS reveal fog/low stratus is fairly widespread within the mountain valleys, as well as over some foothills river valleys and lakes early this morning. Although LIFR/VLIFR conditions developed at KAVL and KHKY earlier this morning, this has since retreated, although satellite indicates the fog/stratus is bank is no more than a couple of miles away from both sites. Tempos for MVFR visby and SCT003 are carried at both sites through 13Z. Meanwhile, LIFR stratus associated with fog over Lakes Keowee/Hartwell has wafted over KAND, and a LIFR tempo is also necessary there through 13Z.

Otherwise, VFR is forecast through the period, although brief restrictions cannot be ruled out this morning at KCLT/KGSP/KGMU, but this is looking increasingly unlikely. Winds will generally remain light/variable until this evening, when winds will begin gradually increasing out of the NE.

Outlook: VFR conditions prevail through Friday. Mountain valley fog and low stratus are possible each night. Widespread rain and associated restrictions develop Saturday and begin improving by late Sunday.

GSP WATCHES/WARNINGS/ADVISORIES

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