Area Forecast Discussion
Issued by NWS Greer, SC
Issued by NWS Greer, SC
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139 FXUS62 KGSP 220140 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 940 PM EDT Sat Sep 21 2024 .SYNOPSIS... High pressure will linger across the region through the weekend keeping above normal temperatures around. A more moist airmass arrives early this week, lingering through at least mid-week. && .NEAR TERM /THROUGH SUNDAY/... As of 935 PM EDT: Main change was the removal of thunder chances through the evening hours. Otherwise, the forecast remains on track. Seeing dry conditions return this evening, but a few isolated showers may redevelop along the shortwave in the NC Piedmont through the next several hours. Thus, have maintained the isolated convection mention across portions of the NC Piedmont this evening into the overnight hours. Should be another round of mountain valley fog with patchy fog elsewhere. Lows will be 5 to 10 degrees above normal. An upper ridge remains over the area Sunday with increasing thickness values. Warming mid levels should limit instability and put a lid on convection. Have a dry forecast for now. Highs will be around 10 degrees above normal. && .SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY/... As of 140 PM EDT Saturday: No major changes to the short term forecast this afternoon. Ensembles are coming into somewhat better agreement that on Monday, the z500 ridge axis over the Carolinas will be nudged east of the forecast area, permitting a steady increase in upper-level moisture streaming into the area from the west. On Tuesday, it`ll become further suppressed, as a lobe of shortwave energy translates up the Ohio Valley and over the ridge. This`ll also have the effect of driving a weak cold front across Tennessee and northern Alabama. The bulk of guidance depicts this feature stalling west of the Appalachians by Tuesday evening, which will keep the Carolinas locked into a warmer, moisture air mass. So, expect lots of cloud cover through the period. High temperatures will be in the mid to upper 80s in most locations both afternoons, and on Monday, a few spots across the lower Savannah River Valley may even hit 90. This would be a little on the high side of model consensus, but given the setup, it`s not unreasonable. Models also depict scattered shower coverage both afternoons, and a plume of afternoon instability to the tune of 1000 J/kg sbCAPE...enough to support some embedded thunder, at the very least. Forecast profiles aren`t especially impressive in terms of shear, lapse rates, or overall updraft strength on Monday...so severe risk doesn`t look especially notable. By Tuesday, there`s considerably more disagreement among long-range ensembles in terms of features like dry air aloft, the location of the frontal circulation, and mid-level lapse rates, so for now suffice to say there`s a potential for some thunder, with details on the nature of that thunder hopefully becoming clearer with future model cycles. && .LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/... As of 155 PM EDT Saturday: By Wednesday, even the new 12z suite of operational guidance begins to diverge on important synoptic forecast details. Like yesterday, ensemble guidance basically falls into two camps: first, those depicting a weaker trough passage that leaves a stronger ridge in place over the Atlantic coast, and second, those depicting a stronger trough that nearly or completely scours out the remnant upper ridge on Wednesday and early Thursday. The first solution, with a stronger ridge, remains the dominant one, and would drive a cold front through the region by Thursday, ushering in drier weather on the whole as we move into the end of the medium range. The second solution, which also develops an upper low over the Ozarks and central Great Plains, would result in no frontal passage an instead a strong mid-level moisture return over the Deep South, resulting in rainier conditions late next week. Of note is the development of a tropical system over the Gulf in most guidance towards the end of the medium range. Should the first solution above play out, remnants of this system could impact the Carolinas toward the end of D7 and beyond; the second solution`s deep upper low would potentially steer any remnant low away from the Carolinas or absorb it entirely. Confidence in the overall track and timing of this system, as well as any potential impacts, remains very low at this point. && .AVIATION /02Z SUNDAY THROUGH THURSDAY/... At KCLT and elsewhere: VFR (outside of mountain valley fog/low stratus daybreak Sunday) through the 00Z TAF forecast period. Mostly clear skies are in place with a FEW to SCT VFR cirrus streaming over the terminals as of 0130Z. Cirrus will continue to stream over the terminals through the overnight hours. KAVL and KHKY may see LIFR to IFR fog and/or low stratus develop around daybreak so have TEMPOs at both sites to account for this potential. Fog and low stratus should mix out by 13Z-14Z. Winds east of the mountains will generally be S/SW this evening into tonight before gradually turning N/NE on Sunday. Winds at KAVL will start out WSW before toggling WNW overnight into Sunday. Another cu field may develop Sunday afternoon/early evening. Outlook: A weak cold front will slowly approach from the west early next week before tracking over the area on Thursday. This front will keep diurnal shower and thunderstorm chances around through at least the middle of next week. Mountain valley fog/low stratus may develop each morning around daybreak. && .GSP WATCHES/WARNINGS/ADVISORIES... GA...None. NC...None. SC...None. && $$ SYNOPSIS...MPR NEAR TERM...AR/RWH SHORT TERM...MPR LONG TERM...MPR AVIATION...AR