textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

No changes to the Flash Flood Watch, High Wind Warning, or Wind Advisory with this update.

Updated aviation discussion for the 00z TAFs.

KEY MESSAGES

1. As Isaias tracks northward from the Gulf Coast and weakens, a flash flood threat will develop in the western Carolinas and northeast Georgia on Saturday, and continue through Sunday morning or early afternoon. Strong winds will develop in high elevations tonight, and in lower elevations Saturday morning, continuing at times through Sunday night. An isolated instance of damaging wind could occur Saturday night and Sunday mainly south of I-85 and/or east of I-77. 2. Gusty northwest winds may develop Sunday night and Monday, particularly in high elevations, as dry air builds back into the region. Dry and warm weather will last most of the work week.

DISCUSSION

Key message 1: As Isaias tracks northward from the Gulf Coast and weakens, a flash flood threat will develop in the western Carolinas and northeast Georgia on Saturday, and continue through Sunday morning or early afternoon. Strong winds will develop in high elevations tonight, and in lower elevations Saturday morning, continuing at times through Sunday night. An isolated instance of damaging wind could occur Saturday night and Sunday mainly south of I-85 and/or east of I-77.

As Isaias moves inland and weakens to a tropical storm late tonight and Saturday morning, 850mb winds will increase over the damming high and rainfall will become increasingly frequent and widespread as moisture fetch improves mainly from the Atlantic, which should reinforce/amplify CAD. Profiles are expected to quickly become moist-adiabatic as rich subtropical moisture flows in aloft, and thus lapse rates and in turn convection are expected to remain somewhat in check. Of course there remains the upslope lift near the Blue Ridge as an additional significant source of forcing. However, in addition to those two sources, diffluent flow remains fixed over the CWA, downstream of Isaias's upper low component, as the weakening system shifts northward toward TN before finally being picked up by the westerlies during the day Sunday. This results in an axis of upper divergence remaining over GA and the Carolinas Saturday and Saturday night. 00z Friday to 12z Friday runs of the global models seemed to key on that, coming into agreement on a focused axis of greater QPF from eastern GA northward to the NW NC mountains which ticked upward compared to earlier runs. And as CAMs began to incorporate that period, they largely depicted a similar axis with an embedded narrower band of substantially higher QPF than we had previously been messaging. The northward motion of Isaias should carry the best isentropic lift northward over the wedge late Saturday, though at least shallow upglide should keep the wedge in place for a time. The upper divergence however may make up for that loss. weakening the isentropic component over our area late Saturday.

In light of the above, today's QPF guidance has pretty much trended higher across the board, such that the threat of flash flooding appears higher than on earlier cycles, at least where the terrain enhancement is in play. A Flood Watch for Flash Flooding ("Flash Flood Watch") has been issued for the areas along and below the Escarpment, valid from 8 AM Saturday to 2 PM Sunday. The longest duration of upslope/upglide is expected over the NC foothills counties from Polk to Caldwell, where the uptick in QPF on this forecast cycle was the greatest. We also included the traditional NC mountain zones except Graham, Swain, and Madison, which are likely to be in the rain shadow and rainfall is much less likely to cause flooding. Haywood can't be split the way the foothills zones or Jackson can, and there is likely to be a sharp gradient in flood potential across the county, decreasing from south to north. with most of the Balsams likely to get 6" or more storm total, we felt it was best to include Haywood.

By late Saturday evening it appears Isaias's remnant will be over TN, leading 925mb winds to veer. There had appeared some potential for the wedge to begin to erode along our southeastern border at that time as a result, but with there now being hints that widespread rainfall may still be ongoing via the dynamic lift, that doesn't seem as likely, as that precip could lead to the wedge remaining locked in a bit longer. Most models do however show some SBCAPE developing in a small warm sector, in triple-point fashion, later Saturday night or Sunday morning, with sufficient helicity/shear to suggest at least an isolated threat of damaging winds or a brief, short-tracked tornado or two. Furthermore CAMs largely develop a band of convection in that same timeframe as the low's movement becomes more easterly, though that activity probably would be elevated over the mountains. Any convection in that time could provide a final, brief shot of very high rainfall rates early Sunday that could exacerbate any existing flood issues. Surface based convection still appears possible over our lower, more eastern Piedmont zones early in the day Sunday. The flash flood threat should be over by 2 PM Sunday, and if the final round of convective precip comes through earlier than expected and/or as small creeks come down, we may be able to let it go early.

Winds will increase tonight, with gusts of 46 mph or higher becoming possible in the high elevations southwest of the French Broad Valley as soon as midnight tonight; gusts increase, expand further northeast, and to lower elevations later Saturday morning. Gusts of 55-60 mph become possible during the day on the higher ridgetops above 3500 ft. Because of that, we issued a High Wind Warning above 3500 ft beginning at midnight tonight, with Wind Advisory below 3500 ft and in the mountain zones along the eastern Escarpment. Gust potential will taper off early Sunday in the higher elevations as the LLJ shifts off to the northeast so the High Wind Warning and Wind Advisory currently are set to expire at 8 AM. Near the periphery of the wedge in the western Upstate and NE GA, there is some potential for gusts of 40-46 mph mainly during the day Saturday. For simplicity of messaging, and for the possibility of gusts with the retreating wedge boundary early Sunday morning, those areas were included in the Wind Advisory using the same timing.

Key message 2: Gusty northwest winds may develop Sunday night and Monday, particularly in high elevations, as dry air builds back into the region. Dry and warm weather will last most of the work week.

Once the remnant low from Isaias moves by to our north, winds will turn west and then northwest, eroding the wedge. Skies trend clearer east of the mountain spine, and temperatures warm on downslope flow. Light rainfall may continue into Sunday night or early Monday along the TN border and wind gusts of 25-35 mph still are expected across the mountains before weakening late Monday. Otherwise the ridge will maintain dry weather for most of the work week, with light winds and temps trending well above normal Tue-Wed. A dry cold front should bring temperatures closer to, but still above, normal Thursday.

AVIATION /00Z SATURDAY THROUGH WEDNESDAY/

At KCLT and elsewhere: VFR will continue through most of tonight, though under thickening and lowering bases. Guidance has trended slightly faster in lowering cigs and onset of -RA overnight. Low MVFR cigs develop from south to north beginning by daybreak and lower quickly to IFR during the day Saturday under cold air damming. Periods of +RA will become possible Sat afternoon and evening. Isolated thunderstorms will be possible Saturday evening, so PROB30 groups have been added. NE winds will increase into the 12-15 kt range with gusts 25-30 kt expected late tonight thru Saturday morning. Gusts at KCLT but may exceed 30 kt at times late Sat morning to afternoon. Winds will be tricky at KAVL, expected to be ENE to ESE and gusty. Strong LLWS is expected over KAVL starting around sunrise Sat morning and continuing into Sat evening with easterly winds around 50 kt possible within 2000 ft AGL.

Outlook: Widespread rain and associated restrictions continue into Sunday morning. Locally stronger or damaging winds associated with +SHRA or TSRA cannot be ruled out Saturday night. Gusty NE winds--SE at KAVL--are 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...Wind Advisory from midnight tonight to 8 AM EDT Sunday for GAZ010-017-018-026-028-029. Flood Watch from 8 AM EDT Saturday through Sunday afternoon for GAZ010-017-018-026-028-029. NC...Wind Advisory from midnight tonight to 8 AM EDT Sunday for NCZ033-048>053-058-059-062>065-501-503-505-507-509. Flood Watch from 8 AM EDT Saturday through Sunday afternoon for NCZ033-049-050-052-053-059-062>065-501>510. High Wind Warning from midnight tonight to 8 AM EDT Sunday for NCZ048>053-058-059-062>064. SC...Wind Advisory from midnight tonight to 8 AM EDT Sunday for SCZ010-011-019-101>107. Flood Watch from 8 AM EDT Saturday through Sunday afternoon for SCZ101>104.