textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Expanded the Flash Flood Watch eastward across the Upstate. No changes to the High Wind Warning/Wind Advisory. The severe thunderstorm risk has increased slightly with this update.

Updated aviation discussion for the 06z TAFs.

KEY MESSAGES

1. Isaias is expected to track northward today, increasing the threats for flooding, landslides over eastern parts of the mountains, strong wind gusts, and isolated severe storms along and south/east of I-85 through tonight. The risks should peak between late afternoon and the early morning hours on Sunday. 2. Gusty northwest winds may develop across the higher elevations Sunday night through Monday after the remnant circulation of Isaias passes to the north. After that, dry weather with a warming trend into the middle of next week.

DISCUSSION

Key message 1: Isaias is expected to track northward today, increasing the threats for flooding, landslides over eastern parts of the mountains, strong wind gusts, and isolated severe storms along and south/east of I-85 through tonight. The risks should peak between late afternoon and the early morning hours on Sunday.

Post-Tropical Cyclone Isaias should track northward and weaken today across mainly AL, and then across middle TN tonight, raising several weather risks across the forecast area. The storm appears to be moving inland a little faster than previously thought, so the timing of precip arrival was moved up yet again. Could this be an indicator for threats increasing earlier? Any recent track changes have minimal impact on the forecast. We're still expecting precip probs to ramp up through daybreak, followed by increasing rainfall rates as we get through the middle part of the day, with the highest risk peaking between late afternoon and the early morning hours on Sunday. Some of the details remain elusive.

The QPF continues to undergo adjustments from one run to the next, but in general continues to favor very high amounts in the southeast upslope flow locations near the Blue Ridge Escarpment. Highest values have come down by about an inch, but that isn't enough to substantially change the fcst over the mtns. What is more interesting is the suggestion of a band of northward-training showers and storms will translate slowly east across the fcst area late this afternoon and this evening as Isaias makes the gradual transition to extratropical. Taking this into account with higher precip amounts farther east across Upstate SC, the Flood Watch will be expanded to include the I-26 corridor. This lines up best with the updated Watch from the CAE office. The QPF doesn't really support expansion any farther east than Spartanburg and Laurens counties at this time. This trend also suggests maybe a greater risk of landslides near the escarpment over NC and the Black Mtns, and maybe a decreasing trend over the Balsams and the escarpment near the SC/NC border. Still a large area of potential 5-8 inch storm total rainfall in those locations. The rain should lift out to the east during the overnight/early morning hours Sunday. I would like to say that confidence is better, but there are concerning differences between the global models and the CAMs, so failure modes abound, particularly where the cold air damming wedge is concerned.

Little change was made to the wind hazards, though incorporating some of the higher-end of the NBM suggests most places east of the mtns have similar chances of seeing periods of stronger wind gusts on the order of 30-40 mph through this evening. The main concern is saturated ground as rain accumulates, making it easier for the gusty winds to knock down and uproot some trees. For now, that would seem to preclude the eastern Upstate and I-77 corridor from consideration, but we will monitor for expansion if the higher precip amounts shift eastward. Elevations above 3500 feet will have the best shot at seeing wind gusts that actually meet criteria.

As for the severe storm risk, that is more conditional on the scouring of the cold air damming wedge east of the mtns. The wedge appears likely to hold through the better part of the day, meaning our risk would be very low until after sunset. After 00Z, the center of Isaias remnants would be moving past to our west and the best southerly flow at low levels would attempt to scour the wedge and move the boundary northward. The 03Z RAP does that, uncovering the northeast GA and Upstate SC zones through the pre-dawn hours Sunday with sfc-based CAPE reaching 500 J/kg and effective shear reaching 40 kt. In other words, enough buoyancy and shear to put us in the rotating storm business. The 00Z HREF does just that, with joint probs reaching 40-50% overnight along and south of I-85 and west of I-77. That raises our concerns for isolated brief tornadoes in any low-topped storms with supercell characteristics. This threat would shift eastward across metro Charlotte Sunday morning after sunrise.

The threats should come to an end by the middle part of the day on Sunday.

Key message 2: Gusty northwest winds may develop across the higher elevations Sunday night through Monday after the remnant circulation of Isaias passes to the north. After that, dry weather with a warming trend into the middle of next week.

Once the remnants of Isaias have moved past to the north Sunday evening, the wind should veer around to the NW and become gusty across the higher terrain owing to a lingering strong pressure gradient on the back side of the system. Moisture on the back side will get forced up as well, so a small precip chance will be kept through Monday morning. After that, cue the warmup as an upper ridge builds in from the west. Temps are already expected to be above normal on Monday, but make it to ten degrees above normal by the middle of next week. Confidence goes down after that. The NBM starts to drop temps back down toward normal toward the end of the week, but several of the model solutions still have us under the ridge associated with an upper anticyclone traversing the Southeast. Big changes could be in store for next weekend if the cool GFS/Canadian work out.

AVIATION /06Z SATURDAY THROUGH WEDNESDAY/

At KCLT and elsewhere: SHRA are spreading north across the area but cigs remain VFR. Expect vsby and cigs to lower to MVFR toward daybreak and IFR cigs near or shortly after daybreak. Can't rule out some heavier showers or TS during the morning, but chances are better during the afternoon and evening. IFR vsby and cigs expected during the afternoon as well. Vsby goes back to MVFR for the evening with IFR to LIFR cigs, along with lower conditions in any heavier showers or TS. NE winds become gusty through the morning with stronger winds and gusts during the day, generally around 30 kts but could be higher. KAVL may see winds flip/flop up then down valley with an easterly low level jet near the ridge tops. More sustained SE wind with low end gusts expected for the afternoon into the evening. Expect LLWS to develop at KAVL given the afore mentioned low level jet. KHKY will see LLWS in the afternoon. Chances are lower elsewhere, but may still develop.

Outlook: Convection and restrictions move east of the area by Sunday afternoon. Winds turn SW and gusty for all but KAVL where NW gusty winds develop. 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 and Wednesday, with light winds.

GSP WATCHES/WARNINGS/ADVISORIES

GA...Wind Advisory until 8 AM EDT Sunday for GAZ010-017-018-026-028- 029. Flood Watch from 8 AM EDT this morning through Sunday afternoon for GAZ010-017-018-026-028-029. NC...Wind Advisory until 8 AM EDT Sunday for NCZ033-048>053-058-059- 062>065-501-503-505-507-509. Flood Watch from 8 AM EDT this morning through Sunday afternoon for NCZ033-049-050-052-053-059-062>065-501>510. High Wind Warning until 8 AM EDT Sunday for NCZ048>053-058-059- 062>064. SC...Wind Advisory until 8 AM EDT Sunday for SCZ010-011-019-101>107. Flood Watch from 8 AM EDT this morning through Sunday afternoon for SCZ010>012-019-101>109.