Area Forecast Discussion
Issued by NWS Greer, SC

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841 FXUS62 KGSP 200803 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 303 AM EST Mon Nov 20 2017 .SYNOPSIS... Cool high pressure will move in before a weak cold front crosses the area Tuesday night. Low pressure will develop along the Gulf Coast Thursday and up the Atlantic seaboard through Friday. Another strong cold front will arrive next weekend. && .NEAR TERM /THROUGH TONIGHT/... As of 205 AM EST: Clouds will linger along the TN border in the moist NW flow. Moisture and winds quickly diminish after daybreak as the pressure gradient relaxes with the center of high pressure moving into the area. The center of the high moves east of the area for the afternoon with light SW wind developing in response. Cirrus will begin to infiltrate the area from the west during the afternoon as well. Highs will be up to 5 degrees below normal. High pressure remains ridged into the area from the east tonight as an upper trough approaches from the west. Cirrus increases in coverage and thickens through the night, but the bigger story is the increase in low level moisture from the SW after midnight. Some of the guidance is producing precip before daybreak. There could be some mechanical upslope lift in the southwesterly flow. However, the moisture is very shallow and isentropic lift remains south of the area until after daybreak. Therefore, continued the dry forecast but did increase clouds from the SW after midnight. The guidance has come in slightly warmer for lows, and this seems reasonable given the increasing cloud cover. Lows will be up to 5 degrees below normal, but many locations will be only a couple of degrees colder. && .SHORT TERM /TUESDAY THROUGH THURSDAY/... As of 230 AM Mon: High pressure moves further off the Atlantic coast on Tuesday as an upper trough dips into the Ohio Valley. In response, moist southerly flow sets up over the Carolinas and Georgia. Similar to Saturday`s isentropic event, the flow is a bit veered relative to the pressure gradient. Clouds should easily be generated but unsurprisingly QPF response is fairly small overall. Hence only low PoPs are being included in the fcst. Mechanical lift along the south-facing Escarpment is likely to be a factor, and temperatures there are expected to be right around freezing at daybreak. However, the better moisture does not advect in until later in the morning, and therefore freezing rain is not anticipated. Max temps will remain a couple degrees below normal. The aforementioned trough swings east Tuesday night and will drive a weak cold front into the NC mtns early Wed morning. Moisture is too shallow to expect any upslope-driven precip at that time. The front weakly pushes across the remainder of the area Wednesday; downsloping and more sunshine will help temps rebound to about normal. A cold, dry sfc high expands out of the Mississippi Valley and onto the Eastern Seaboard in the wake of this front. Accordingly, with northeasterly flow, temps tumble for Thursday. Mins return to near or below freezing over the NC Mtns and NW Piedmont, and top out only 5-7 degrees below normal area-wide. There continues to be some discrepancy between global models on the evolution and timing of a shortwave trough diving into the Deep South near the end of the short-range period. The GFS cuts off the wave but continues its trend of suggesting the resulting lift and moisture advection will remain south of the area. The EC is slower and drier in its depiction. Hence the forecast remains virtually dry thru Thursday. && .LONG TERM /THURSDAY NIGHT THROUGH SUNDAY/... As of 100 AM EST Monday: The notably quiet medium range begins Thanksgiving night as an upper trough axis crosses the forecast area. At the surface, high pressure will slide in across the Carolinas as a series of coastal lows drifts across Florida and hugs the Carolina coast. Guidance has been inconsistent with the handling of the track of these low pressure systems, with previous runs depicting scenarios in which some Gulf moisture is shunted into our area. With the upper trough and cold air solidly in place over the area on Thursday night and Friday morning, any moisture able to make it this far north may present some wintry p-type issues. Confidence is increasing that the moisture will remain to the south and east of our area at the moment, though, so have kept pops unmentionable for the time being. The overall pattern will remain progressive through the medium range, with another upper trough arriving over the eastern CONUS Saturday and Sunday. The next low pressure system will pass well to our north on Sunday, with just a slight chance of wringing out enough moisture in the higher elevations for some isolated NW flow snow showers. Colder air will then advance behind what will be a dry front for the majority of our area next Monday. Temperatures will hover near or just below average for the majority of the medium range. && .AVIATION /08Z MONDAY THROUGH FRIDAY/... At KCLT and Elsewhere: Gusty northwest winds continue to diminish early this morning. Mainly light NW to N winds by day break continue through the late morning. Winds turn S to SW for the afternoon. Light S or light and variable wind expected during the evening. Cirrus will increase and thicken through the period. Outlook: Increasing low level moisture could result in cig restrictions and perhaps -RA on Tue/Tue night. Otherwise, dry/cool conditions should persist into late week. Confidence Table... 17-23Z 23-05Z 05-11Z 11-12Z KCLT High 87% High 100% High 100% High 100% KGSP High 100% High 100% High 100% High 100% KAVL High 94% High 100% High 100% High 100% KHKY High 100% High 100% High 100% Low 58% KGMU High 100% High 100% High 100% High 100% KAND High 94% High 100% High 100% High 83% 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...None. NC...None. SC...None. && $$ SYNOPSIS...Carroll NEAR TERM...RWH SHORT TERM...Wimberley LONG TERM...Carroll AVIATION...RWH

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