textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Storm total rainfall forecast amounts have trended higher along the upslope areas of the Blue Ridge Escarpment, but down across most of the Piedmont.

Wind gusts have trended higher Saturday through Saturday night in the mountains above 3500 ft.

The Aviation Discussion has been updated for the 18Z TAF issuance.

KEY MESSAGES

1. Quiet weather with a slight cool-down back to normal temperatures expected 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. 3. Gusty winds expected across the North Carolina Mountains Saturday through early Sunday morning, especially above 3500 ft.

DISCUSSION

Key message 1: Quiet weather with a slight cool-down back to normal temperatures expected for Friday.

One last quiet day expected on Friday before effects from Hurricane Isaias enter from the south. A weak cold front will cross the forecast area tonight 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 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.

Hurricane Isaias is forecast to drift north and make landfall along the central Gulf Coast Friday night, then quickly transition to an extratropical low as it drifts inland toward western TN on Saturday. Refer to the latest guidance from the National Hurricane Center for more details. The leading edge of deep moisture and precip chances ramp up steadily from the south in the pre-dawn hours Saturday and reach the categorical range everywhere by early Saturday evening.

Heavy rain and localized flash flooding are still the main concerns with tropical moisture spreading over a cold air damming wedge that sets up before rain begins. Periods of light to moderate rain are expected to occur thru the day Saturday, with conditions likely too stable for thunder. The period of greatest concern for excessive rainfall looks to be Saturday night thru Sunday morning, as persistent easterly to southeasterly flow will allow for rainfall totals to add up in the upslope areas near the Blue Ridge Escarpment. Outside of that, overall the guidance is still hinting that a lot of deep convection will be ongoing across the Carolina Coast and nearby waters, which may interrupt moisture transport further inland and to the Southern Appalachians. The NBM continues to show broader rainfall totals across the Piedmont, and lower values (1-2 inches lower) across the upslope areas compared to the official WPC QPF. Given the flow staying more easterly longer and the concern of a rain shadow downstream of the coastal convection, the WPC amounts look more reasonable.

The remnants of Isaias will turn toward the east and track over the Ohio Valley Sunday. Low-level flow will turn out of the south, then southwest and bring a dry slot into the forecast area Sunday morning. This will work to erode the wedge and uncover sfc-based instability with some overlap of 0-3km helicity. The NAM and RRFS show sig tor parameter value approach 1.0 within this area along and south of the wedge front, and the 12z RRFS even has a few rotating updrafts, especially Sunday aftn across the NC Piedmont. All this to say that there looks to be a non-zero tornado threat from early Sunday morning thru Sunday aftn, roughly associated with a broken line of showers or low-topped thunderstorms tracking from SW to NE. With the dry slot punching in, the excessive rain/flood threat should wind down, but some main-stem rivers may continue rising thru the day.

As Isaias remnants cross the central Appalachians and into the Mid-Atlantic, flow will turn out of the NW and some upslope showers may linger along the TN border into Sunday night. But otherwise, dry weather should return to the area by Monday morning and continue thru late in the week. Temps will be cool on Saturday due to the CAD, then rebound to near normal Sunday, before warming to above normal Monday. Above normal temps will likely continue thru the week, as a stout upper ridge builds over the Southeast.

Key message 3: Gusty winds expected across the North Carolina Mountains Saturday through early Sunday morning, especially above 3500 ft.

As the circulation associated with the remnants of Isaias interact with a strong sfc high over the northern Mid-Atlantic, easterly flow will ramp across the Carolinas. The latest models are in decent agreement on the 850 mb flow increasing into the 50-70 kt range starting early Saturday morning, peaking Saturday evening, as flow veers to southeasterly. While an inversion associated with cold-air damming will limit these winds from reaching the sfc, the higher terrain could see frequent gusts of 35-55 mph, with 60 mph or higher possible above 4500-5000 ft elevations. These winds may combine with saturated soils to knock down some trees and cause isolated power outages, mainly at the highest elevations.

AVIATION /19Z THURSDAY THROUGH TUESDAY/

At KCLT and elsewhere: VFR conditions expected thru the period. A gradual increase in high clouds will thicken and lower tonight thru Friday, as Isaias drifts north across the Gulf. These clouds along with warmer overnight temps should limit mountain valley fog and lake fog across the area compared to last couple of nights. Will keep a 6sm BR at KAVL during the usual hours around sunrise with the 18z TAF. Winds will be generally LGT/VRB thru this evening, then should start to increase slightly and be more consistently out of the NE (except at KAVL).

Outlook: VFR conditions prevail through Friday. Widespread rain and associated restrictions develop Saturday and begin improving by late Sunday.

GSP WATCHES/WARNINGS/ADVISORIES

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