Area Forecast Discussion
Issued by NWS Greer, SC

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000 FXUS62 KGSP 031103 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 703 AM EDT Mon Aug 3 2020 .SYNOPSIS... Rain chances increase across the region through today as tropical moisture from Tropical Storm Isaias interacts with a stalling front. The storm will pass near the Georgia and Carolina coasts today and then through eastern North Carolina tonight, enhancing our risk of heavy rainfall and isolated flooding. A more typical summertime pattern of afternoon and evening thunderstorms will then return for the remainder of the week. && .NEAR TERM /THROUGH TONIGHT/... As of 655 AM: A stalling cold front over East TN will serve to focus convective activity this morning, driven by diurnal destabilization and enhanced by an amplifying jet streak oriented along the Appalachians. PoPs ramp up to categorical first over the mountains near the TN border as a result. While the Piedmont is currently under a lull in forcing, the approach of Isaias is promoting moist upslope flow that should expand coverage near the Escarpment. It also will bring likely to categorical chances back for much of the area this afternoon. CAPE is expected to stay a little below typical August values. However, robust mid to upper level flow surrounding the trough will produce 0-6km shear of 30 to 40 kt over the NW half of the CWA. Therefore SPC has included a D1 Marginal Risk across that part of our area. Loosely organized multicells producing damaging wind would appear to be the most likely route that severe wx would occur. The continued weakening of the front and slightly more eastward trend in the Isaias track no longer appears as likely to support a sharp axis of heavy rainfall. There will be at least an isolated risk of flash flooding across much of the area today. The Flash Flood Watch for the northern mountains/foothills is being maintained, however, as it is not yet totally clear the convective trend is downward. The trough to our west will begin to make eastward progress tonight, creeping the front eastward, and some drying is expected both due to that development and as winds turn more downslope around Isaias as it crosses eastern NC. The circulation of the storm will be near enough to support a mixed E to NE flow late tonight in the low levels along and east of I-77. Breezy conditions likely will develop, with gusts perhaps up to 25 mph at times. Significant impacts are unlikely aside from a few trees falling in saturated soil; given flash-flood guidance being above 3 inches in 6 hours, and total QPF remaining well under 2 inches by and large in those areas, confidence is fairly low that the soils will be wet enough for much uprooting. && .SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/... As of 305 am EDT Monday: The center of circulation of Isaias should move rapidly away to the northeast during the day on Tuesday. After lingering morning showers and thunderstorms, mainly along and east of I-77, a more typical summertime pattern of afternoon and evening convection will return as the lingering, stout upper jet axis moves slowly east across the forecast area. Low-level flow will quickly become a westerly downslope with the passage of Isaias to the east, but the downsloping will likely not counteract the upper forcing, moist boundary layer, and lingering frontal zone stalled in the area. Solid scattered coverage should easily return through the afternoon hours in spite of the westerly flow. General troughiness will persist west of the Appalachians Wednesday through Wed night as additional shortwaves move east of the Mississippi River Valley, and the stalled surface boundary slowly dissipates in place. Better insolation will permit SBCAPE values to recover above 2500 J/kg during peak heating to permit scattered to numerous afternoon and evening storms to return on Wednesday. Temperatures will return to nearly normal early August values. && .LONG TERM /THURSDAY THROUGH SUNDAY/... As of 310 am EDT Monday: The upper trough axis west of the Appalachians on Thursday will cross the region through Friday, with another round of above climo PoPs for diurnal showers and thunderstorms expected both days. Temperatures will be near or just below climo, with a bit more clouds under the passing trough. A broad and flat central CONUS ridge will try to build east over the weekend, but with persistent northwesterly flow and associated shortwaves crossing the southeast. This will keep decent, diurnal, scattered or better coverage of showers and thunderstorms going through the weekend. Temperatures will return to near climo under building heights aloft. && .AVIATION /11Z MONDAY THROUGH FRIDAY/... At KCLT and Elsewhere: SHRA/TSRA are beginning to line up along the southern mountains and Blue Ridge Escarpment as moist upslope flow occurs ahead of TC Isaias. MVFR to IFR vsby is possible in the heavy showers. Activity should increase in coverage in this area thru the morning. Some mountain valley IFR fog/stratus exists aside from the precip, and KAVL might see this fill back in between downpours during the first hr or two of the period. Diurnal destabilization will eventually lead to scattered SHRA/TSRA for the Piedmont as well, mainly midday to mid-afternoon. Bases should be mostly VFR except during heavier precip. Isaias will make its nearest pass to our area tonight, with precip coverage waning following the loss of heating, with the remaining chances being near KCLT/KHKY for outer bands. It will also bring low-end gusts to KCLT in the late evening to very early morning hours. As thicker cloud cover clears out radiational stratus could form, mainly at KAVL and the upstate sites. Outlook: Daily rounds of afternoon/evening SHRA/TSRA will likely continue most of the week. Morning restrictions can`t be ruled out. Confidence Table... 11-17Z 17-23Z 23-05Z 05-06Z KCLT High 100% High 100% High 100% High 85% KGSP High 90% High 100% High 100% High 100% KAVL Med 65% Med 68% High 95% Med 68% KHKY Med 75% Low 59% Low 54% High 80% KGMU High 95% High 95% High 100% High 100% KAND High 100% High 100% High 100% High 100% The percentage reflects the number of guidance members agreeing with the scheduled TAF issuance flight rule category. Complete hourly experimental aviation forecast consistency tables are available at the following link: www.weather.gov/gsp/aviation && .GSP WATCHES/WARNINGS/ADVISORIES... GA...None. NC...Flash Flood Watch through Tuesday morning for NCZ033-049-050- 501>506. SC...None. && $$ SYNOPSIS...HG NEAR TERM...08 SHORT TERM...HG LONG TERM...HG AVIATION...08

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