textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

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

KEY MESSAGES

1. Fair weather will persist today with near normal temperatures. 2. Widespread rain is expected to move into the region on Saturday and continue through early Sunday. Excessive rainfall will be possible Saturday afternoon through Sunday morning, along with a small risk for severe thunderstorms. 3. Gusty winds expected across the mountains of North Carolina Saturday through early Sunday morning, especially above 3500 ft.

DISCUSSION

Key message 1: Fair weather will persist today with near normal temperatures.

One last quiet day is expected before the situation deteriorates tonight. A cold front analyzed moving south across the fcst area will continue to slip southward this morning, and in its wake we can expect a weak and dry cold air damming wedge to develop thanks to sfc high pressure moving past across the central Appalachians and Mid-Atlantic region. As a result, temps drop back to around normal as the wedge develops and continues to be overspread by high cloudiness exhausted from Isaias moving northward across the Gulf.

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

Hurricane Isaias is forecast to move 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. Forecast confidence in rain continues to increase as a tropical remnant moving northward west of the mountains is highly favorable for widespread rainfall across our region, though confidence in amounts remains elusive.

Excessive rain and localized flash flooding are still the main concerns. If we follow the trend shown clearly by the CAMs, the tropical moisture will move in earlier than shown previously, such that the leading edge of the rain will be working its way into the dry air across our region as early as sunset today. It might take a few hours for the light rain to reach the ground, but it is looking more like rain will be falling by daybreak Saturday across the western two-thirds of the forecast area, and then everywhere by the early afternoon. Once the rain starts to fall, it should lock in the cold air damming wedge early in the day, keeping the region roughly ten degrees below normal and resulting in a low diurnal range in temperature. Getting back to the excessive rain potential...the situation obviously has a lot going for it, with deep tropical moisture moving up and over the wedge and then encountering the mtns, along with excellent forcing as what's left of Isaias moves northward to the west of the mtns Saturday night. In fact, the trend in the model QPF is upward on this cycle, but the question remains if the amounts will be enough to raise more of a flooding threat. Fortunately, we enter this event on a dry spell. The potential for heavy rain ramps up Saturday afternoon as a low level jet with 40-60kt of upslope flow at 850 mb arrives. The 00Z run of the RRFS suggests that rain will begin to pile up in the headwaters of the French Broad and Savannah River basins by late afternoon, with the best 6 hour probability matched mean values Saturday evening and early Sunday morning over the southeast facing slopes of the Balsam and Black Mountains, and near the Escarpment. The fcst QPF reflects more of a focus toward the Blue Ridge Escarpment and southeast upslope areas, while raising amounts slightly in those locations, with maxima in the 4-6 inch range over a 60 hour period. Confidence is 50/50 as to whether or not this will be enough to trigger flooding to warrant issuing a Watch this early, so we are leaning away from it and prefer to wait one more forecast cycle. The new guidance moves the system through quicker, so the steadier precip is already moving out to the north by late morning Sunday. Precip chances are pared back to the TN border by Sunday evening as the flow turns out of the west and northwest.

The secondary concern will be the severe weather potential, mainly on Sunday. Once the main rain shield moves out, a strengthening SW flow at low levels will attempt to scour out the remnant wedge, which is most likely to happen up into parts of the Charlotte metro region. What is interesting is the LREF joint probs of CAPE above 500 J/kg and shear above 30, which shows some likelihood reaching Union County NC in the afternoon as the dry slot reaches our region with the remnant low making its closest pass to our NW. Unfortunately, the CAMs don't extend that far in time, so we shall have to wait until we see the next two model cycles to get a better handle on this potential.

Once the system lifts out Monday, the rest of the forecast looks quiet as an upper anticyclone moves over the Southeast.

Key message 3: Gusty winds expected across the mountains of North Carolina 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 increase across the region. The aforementioned low level jet could reach upwards of 40-60 kt, mainly late Saturday and Saturday night, but this will be moving over top of the wedge east of the mtns. Those locations will have a cool northeasterly breeze. However, the high elevations will stick upward enough to be impacted. The forecast as it stands will show wind gust potential not quite high enough to warrant an advisory, but it would be too early to issue a Wind Advisory at this time anyway. We will continue to monitor further developments.

AVIATION /12Z FRIDAY THROUGH TUESDAY/

At KCLT and elsewhere: VFR conditions expected into the evening with deteriorating conditions overnight. OVC cirrus continues through the morning with increasing mid clouds during the afternoon. VFR stratocu spreads in through the evening with cigs falling to the low VFR to MVFR range near daybreak. MVFR vsby in scattered showers also possible near daybreak. Wind picks up speed through the morning with low end gusts possible. KAVL the exception with light SE wind developing. Increasing NE wind with gusts developing overnight. KAVL sees S to SE wind.

Outlook: Widespread rain and associated restrictions develop Saturday and begin improving by late Sunday. Gusty NE wind, SE at KAVL, expected through Sat night with lighter, but still gusty SW wind on Sunday. VFR with gusty NW wind on Monday. VFR outside of mountain valley fog Tuesday with light winds.

GSP WATCHES/WARNINGS/ADVISORIES

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