Area Forecast Discussion
Issued by NWS Greer, SC

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973 FXUS62 KGSP 211048 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 648 AM EDT Sat Apr 21 2018 .SYNOPSIS... High pressure over the eastern United States will keep conditions dry across the southeast for much of the weekend. However, moisture will return late Sunday through Monday as strong low pressure approaches from the lower Mississippi River Valley. Rainfall, possibly heavy at times, will likely develop by Monday and linger into Tuesday. Unsettled weather could continue through late week. && .NEAR TERM /THROUGH TONIGHT/... As of 645 AM Saturday: Main update for 12Z TAF issuance along with minor adjustments made to temperatures to coincide with latest trends, as the rest of the near term forecast remains on track. A Frost Advisory remains in effect through 9 AM for portions of the central and southern mountains and Rabun County in northeast GA. Areas across the Upstate and NW Piedmont could see areas of patchy frost this morning as well, but anticipated coverage is not widespread enough to warrant a Frost Advisory attm. It`s a quiet, second to last Saturday of April morning across the FA with nearly clear skies and light winds. Morning lows are below normal, with obs depicting temperatures ranging between the 30s to lower 40s attm. Today will be the only overall rain-free day the area will see for the next seven days. As dry sfc high pressure continues to influence the weather pattern, areas across the Carolinas and northeast GA can expect plenty of sunshine with afternoon high temperatures slightly below normal, in the upper 60s/lower 70s, cooler across the mountains. As sfc high pressure shifts east of the area, do anticipate the gradual infiltration of thin cirrus across the FA from west to east, ahead of the next approaching system. With the continuation of increasing clouds tonight, am expecting overnight low temperatures to be warmer than this morning, but still slightly below normal, with mid to upper 40s across the Upstate and NW Piedmont, into the lower 40s across the mountains. && .SHORT TERM /SUNDAY THROUGH TUESDAY NIGHT/... As of 330 AM Saturday: The early part of next week is looking quite wet across the southeast. The numerical models now feature good agreement through the short-term period, with a deep closed low pressure system moving east across the lower/middle MS River Valley Sunday through Sunday night. Meanwhile, the strengthening downstream ridge will likely develop an Omega or Rex-type blocking pattern which may contribute to very slow evolution eastward Monday into Tuesday. Ahead of the system, the leading edge of the better moisture and thicker clouds will start to push in on Sunday afternoon but any precipitation in the most southeasterly upslope- preferred areas will likely hold off until later in the day. Increasing clouds will keep Sunday afternoon temperatures at least a category or two cooler than Saturday. Strengthening southeasterly winds at lower levels will then begin to produce mountain upslope precipitation through Sunday night as surface low pressure starts to approach from the MS River Valley. Meanwhile, a strong Atlantic moisture fetch will develop Sunday night through Monday and persist into Monday night. The deepest layer Q-vector convergence will likely cross our forecast area slowly, mainly Monday through Monday night. The expectation is that surface high pressure persisting to the northeast will keep some measure of hybrid cold air damming going through the short term period, and this will keep the diurnal temperature range quite small and cause most of the forecast area to remain too stable for deep convection. Still, the robust southeasterly jet, precipitable water values running 1 to 2 standard deviations above climo, and slow movement of the system appear destined to produce at least some hydrology concerns throughout the early part of the week. The main uncertainty for our flooding concerns will be whether or not developing upstream convection along the GA/SC coasts will rob some of the moisture flux into our region. Surface low pressure will likely transition through the region to the coast on Tuesday. The heavy rainfall threat could linger as late at Tuesday night, but with greater uncertainty. An HWO mention of heavy rain and possible flooding will be needed mainly for Monday with this forecast package. && .LONG TERM /WEDNESDAY THROUGH FRIDAY/... As of 340 AM Saturday: The passing Appalachian low pressure system will finally phase and lift northeast by midweek, but with another low pressure system/trough digging through the midwest Wednesday night into Thursday. Steeper lapse rates will be possible ahead of this system, but will keep any precipitation mention as showers instead of thunderstorms for now. The GFS is at least 10 dm lower in the 500 mb height field as this trough reaches the Appalachians on Thursday, so confidence on PoPs is fairly low at this point. However, there should be some brief resurgence in lift and moisture in the consensus solution. Another deep system will sharpen up to the west on Friday. Instability could be a bit better ahead of the associated cold front. All told, unsettled weather should continue Wednesday through Friday with minimum temperatures slightly above climo and maximum temperatures slightly below climo. && .AVIATION /12Z SATURDAY THROUGH WEDNESDAY/... At KCLT and elsewhere: Expect VFR to prevail through the valid TAF period, with increasing clouds expected overnight into Sunday, but any cigs remaining VFR. Dry sfc high pressure continues to influence the weather pattern for today as light and mainly NE (though at times calm) winds will prevail through the morning hours, gradually veering to ESE/SE and increasing to 5 to 8 kts late morning/early afternoon at all TAF sites as the sfc high shifts east of the area. Do anticipate winds to become light and variable around ~00Z-02Z, continuing through the period. Skies will remain nearly clear through this afternoon, with the gradually increase of thin cirrus from the west ahead of the next approaching system this evening, and increasing mid to high-level clouds beginning overnight. Expect any precip to hold off until just past the valid TAF period (not reaching KCLT until after 06Z per latest trends). Outlook: Approaching low pressure system will bring increasing clouds and precip late Sunday through at least Tuesday, with flight restrictions anticipated. Otherwise, expect VFR. Confidence Table... 10-16Z 16-22Z 22-04Z 04-06Z KCLT High 100% High 100% High 100% High 100% KGSP High 100% High 100% High 100% High 100% KAVL High 100% High 100% High 100% High 100% KHKY High 100% High 100% High 100% High 100% KGMU High 100% High 100% 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 and ensemble forecasts are available at the following link: www.weather.gov/gsp/aviation && .GSP WATCHES/WARNINGS/ADVISORIES... GA...Frost Advisory until 9 AM EDT this morning for GAZ010. NC...Frost Advisory until 9 AM EDT this morning for NCZ048-051>053- 058-059-062>065-507-509. SC...None. && $$ SYNOPSIS...HG NEAR TERM...SGL SHORT TERM...HG LONG TERM...HG AVIATION...SGL

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