textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Updated for 00z aviation discussion.

KEY MESSAGES

1. Isaias will move north into the lower Ohio Valley tonight. Periods of rainfall will continue across the western Carolinas and northeast Georgia through tonight, and will be heavy at times. Flash flooding is possible and a few rivers also are likely to reach flood stage. Landslides remain possible over eastern parts of the mountains. Strong wind gusts remain possible this evening mainly in high elevations. Isolated severe storms also may develop along and south/east of I-85 through tonight. The risks should diminish by Sunday morning. 2. Gusty northwest winds may develop across the higher elevations Sunday night through Monday as a drier airmass builds back into the region. After that, dry weather with a warming trend into the middle of next week.

DISCUSSION

Key message 1: Isaias will move north into the lower Ohio Valley tonight. Periods of rainfall will continue across the western Carolinas and northeast Georgia through tonight, and will be heavy at times. Flash flooding is possible and a few rivers also are likely to reach flood stage. Landslides remain possible over eastern parts of the mountains. Strong wind gusts remain possible this evening mainly in high elevations. Isolated severe storms also may develop along and south/east of I-85 through tonight. The risks should diminish by Sunday morning.

Post-Tropical Cyclone Isaias is now centered over Alabama as of 3 PM Sat and will track northward into the lower Ohio Valley tonight, west of the Appalachians, before beginning to turn northeast as it interacts with the westerlies. Cold-air damming remains locked in east of the Appalachians. Rainfall has remained showery over the wedge, with the dry slot around Isaias having taken a bit longer to shift over the CWA than once expected, despite the LLJ seemingly having weakened and/or shifting further north. Rainfall rates so far today have generally not been more than an inch in 3 hours, but given the duration, some areas along the southeast-facing Blue Ridge Escarpment have already seen totals around 3 inches. Creeks and streams are already responding, and as similar rates look to continue for a while longer, those responses should continue and eventually a few mainstem rivers are expected to reach flood stage, especially the French Broad and Pigeon. Can't rule out flooding on the Catawba/Broad either, as those drain the east-facing Escarpment.

There remains potential for convective bands of heavier precip to develop and pose a flash flood risk where the rain will have saturated the soil. Some clearing has occurred to our SW in Georgia along the periphery of the wedge, which has led to some bands of convection starting to take shape. These initially will be limited in their northward motion as they ride over the wedge and likely weaken with diminishing instability. However, over the course of the evening, 925mb winds over the wedge still should veer ahead of the effective cold front wrapping around Isaias on its south flank. CAMs continue to show bands of mainly elevated convection over the wedge, forming via the enhanced upglide. Such convection still appears capable of 1-2" accum in as little as an hour which could pose a risk of flash flooding where soils have become saturated, and could push accums over the threshold of 5"/24hr, which is associated with an increased risk of landslides. See our Public Information Statement issued Friday evening for preparedness information on landslides. Increased confidence in the possibility of short-term heavy rain tonight has pushed storm total QPF a bit higher in the central CWA, particularly in the east-facing Escarpment areas. Flash Flood Watch is unchanged with totals having remained similar if not come down slightly in some areas outside the watch.

The onset of the strong southerly near-sfc flow should begin to erode the wedge after sunset; sfc temps may warm slightly in our SW areas as that occurs, and as the aforementioned cold front shifts eastward through the CWA overnight. 0-1km shear may exceed 30 kt and 0-3km helicity above 200 m2/s2, so any meager sfc-based CAPE that is able to develop still could result in a risk of damaging winds and brief and short-track tornadoes. Over the night the retreat of the wedge boundary may continue up to around the I-85 corridor and so those areas are in the SPC Day 1 marginal risk area. Per HREF/REFS timing a line of convection may not clear the eastern CWA until 12-15z Sunday, which is reflected in our eastern Piedmont being included on the Day 2.

The gradient wind risk associated with Isaias generally looks to have peaked over the SW NC mountains, NE GA, and Upstate, with the LLJ now having focused farther north on the forward flank of the storm; gusts at most obs sites are now below 30 mph. There remains the possibility of better mixing in those areas as the wedge becomes more shallow later this evening and pending coordination with neighboring offices we likely will be able to let the Wind Advisory and high-elevation Warning go during the evening. Not as confident that the northern mountains and eastern Escarpment areas can be let go just yet.

Expect gradually lifting clouds from west to east from late Sunday morning onward, though with moisture wrapping around the system some cloud cover is likely to be seen most of the day. Temps should return back to near normal for afternoon maxes.

Key message 2: Gusty northwest winds may develop across the higher elevations Sunday night through Monday as a drier airmass builds back into the region. After that, dry weather with a warming trend into the middle of next week.

Light rainfall may continue into Sunday night or early Monday along the TN border and wind gusts of 25-35 mph may redevelop across the mountains, gradually tapering off Monday. Otherwise the ridge will maintain dry weather for most of the work week, with light winds and temps trending well above normal Tue-Thu. A cold front late in the week should bring temperatures back near or below normal.

AVIATION /00Z SUNDAY THROUGH THURSDAY/

At KCLT and elsewhere: Widespread RA/SHRA this evening, with generally IFR restrictions as moisture from the remnants of Isaias continues to filter in. Expect little change overnight and into Sunday morning, with cigs/vsby varying only slightly between pockets of rain. Have continued trend of PROB30 TSRA at most sites just to cover any isolated lightning. Winds remain generally out of the E to SE, but gusts have dropped off for all but KCLT where the gradient from the wedge boundary remains a little higher. Winds will continue to swing around to SW into Sunday. Restrictions will be slow to lift on Sunday, with MVFR cigs remaining into the afternoon hours, clearing from W to E.

Outlook: VFR with gusty NW wind on Monday. VFR outside of mountain valley fog Tuesday and Wednesday, with light winds.

GSP WATCHES/WARNINGS/ADVISORIES

GA...Flood Watch through Sunday afternoon for GAZ010-017-018-026- 028-029. NC...Flood Watch through Sunday afternoon for NCZ033-049-050-052- 053-059-062>065-501>510. Wind Advisory until 8 AM EDT Sunday for NCZ033-048>053-058-059- 062>065-501-503-505-507-509. High Wind Warning until 8 AM EDT Sunday for NCZ048>053-058-059- 062>064. SC...Flood Watch through Sunday afternoon for SCZ010>012-019- 101>109.