Area Forecast Discussion
Issued by NWS Greer, SC

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745 FXUS62 KGSP 281807 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 107 PM EST Sat Nov 28 2020 .SYNOPSIS...
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Dry high pressure will briefly return through tonight. A strong low pressure system approaching from the southwest will support a threat for heavy rainfall late Sunday and Sunday night. Northwest flow snow is expected to develop across the mountains in the storm`s wake from late Monday through Tuesday. Another strong low may cross the region at the end of the week.
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&& .NEAR TERM /THROUGH SUNDAY/...
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As of 1245 PM: A dry sfc high centered near the Missouri Bootheel has brought deep dry air across the upper TN Valley and southern Appalachians. Skies are virtually clear across the CWA as a result. The sfc high will migrate toward the East Coast through Sunday along with the associated upper ridge. A compact upper low crossing the ArkLaTex region will induce Gulf coast cyclogenesis in the morning; cirrus will increase ahead of this system beginning tonight. Deep subsidence will continue over our area, and while the progression of the pattern will allow southerly flow to redevelop in the low levels, precipitation chances remain near zero until late Sunday morning. Deepening moisture and upglide along with increasing upper divergence ahead of the approaching system will result in increasing PoPs from SW to NE thereafter. Model QPF has backed off a bit, and it looks like a more rapid increase in moisture won`t occur until late afternoon or evening; for these reasons the trend has been slowed. Following the airmass change that has already occurred, and increased cloud cover, max temps Sunday should be a few degrees below today`s readings.
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&& .SHORT TERM /SUNDAY NIGHT THROUGH MONDAY NIGHT/...
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As of 200 AM Saturday: Low pressure will carry the momentum into the Southeast with deep layer moisture and perhaps modest instability, especially in the southern fringe of the cwa. Model guidance has been spitting out 100-300 J/kg of SBCAPE and 0-6 km effective bulk shear of 40-50kts. The better environment for any strong to severe thunderstorm development is south and east of the I-85 corridor. Confidence is on the upward trend on the the overall severe threat staying confined to that area, while confidence remains steady with a low-end hydro threat over the southern Blue Ridge Escarpment due to upslope enhancement. As the sfc low and attendant frontal boundary pushes upstream the backdoor longwave trough that eventually absorbs the cut off low, cold NW flow will push into the area and the transition to winter weather is likely over parts of the mountain community. Heaviest amounts will be at the highest peaks and TN-NC border along the spine of the Blue Ridge where the transition is likely to occur much sooner than other areas in the higher elevations. A sharp drop in thicknesses around 18Z Monday will drop snow levels overall, but the most favorable locations will be elevations at or above 3500 ft. With a lingering moisture fetch over the OH Valley and stretching into the southern Appalachians on the backside of the low. Expect the snow to stick around through the end of the forecast period for the higher peaks in NC. Last hazard that could present weather troubles will be the tightening of the pressure gradients over the mountains once the NW component fully becomes established. CAA will enter the environment which will only increase the overall momentum at the sfc. Gusty winds and possible advisories could be considered in this scenario to go along with the expected snowfall in the mountains of NC. Near-normal temperatures are expected through Monday, but a huge dip is expected Monday night with lows in the 20s for the mountains and low 30s every where else.
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&& .LONG TERM /TUESDAY THROUGH SATURDAY/... As of 300 AM Saturday: The evolved upper low sets up shop over the eastern CONUS with the center of it just north of the area. Very cold air will be in store with thicknesses <540 dm and deep radiational cooling, expect the coldest max and min temps of the season so far with highs not getting out of the 40s anywhere and lows in the 10s across the mountains and low to mid 20s elsewhere. During this part of the medium range, temperatures will be 10+ below climo for this time of the year. Any lingering NW snow should taper off by Tuesday afternoon and dry air will filter into the cwa. Temperatures begin to rebound by the middle part of next week as upper ridging pushes higher heights into the region and the upper low ejects to the north and east. Broad sfc high controls the overall weather pattern through Thursday before more changes take place. Model guidance really diverge past Thursday due to the location and track of the next cut off low that generates over the southwestern CONUS just like the previous. Both the GFS and ECMWF show a second cut off low diving south over the High Plains while the other develops over the Southwest. The ECMWF actually phases the two together over the Southern Plains and forms a very potent upper low that moves across the southeastern CONUS by the end of the period. However, the GFS keeps the initial cut off low over the Southwest while the second cut low dives in from the High Plains and slides into the Southeast near the same time frame as the ECMWF. Not as potent, but keeps the area unsettled to say the least and kept 20 PoPs in the forecast for now due to the lack of confidence this far out. Looks as if the unsettled and amplified pattern will continue as we run through the first week of December. Outside of Tuesday, Tuesday night, and Wednesday night expect temperatures to make a return to near-normal or slightly below-normal values on the back half of next week. && .AVIATION /18Z SATURDAY THROUGH THURSDAY/...
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At KCLT and elsewhere: High pressure will shift into the area from the west, resulting in mainly NE winds thru tonight, and continued dry conditions. Easterly flow above the boundary layer and below subsidence inversion likely will produce some low-VFR cloud cover by early morning. Low stratus and fog are expected to develop as well, but remain south of the SC TAF sites. Abundant cirrus will spill back into the area overnight; low-VFR clouds should fill in as well as moist layer deepens. Winds will flirt with ESE in some areas, including KCLT, as the sfc high progresses east. Precip chances return during the afternoon, and are thus only mentioned at KCLT near the end of the period. Outlook: A round of precipitation and restrictions are expected Sunday night and part of Monday as strong low pressure crosses the region. Behind this system, wintry precipitation will be possible in the mountains later Monday into Tuesday with VFR returning elsewhere, lasting thru midweek. Confidence Table... 18-24Z 00-06Z 06-12Z 12-18Z 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 93% 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
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&& .GSP WATCHES/WARNINGS/ADVISORIES... GA...None. NC...None. SC...None. && $$ SYNOPSIS...DEO NEAR TERM...Wimberley SHORT TERM...CAC LONG TERM...CAC AVIATION...Wimberley

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