Area Forecast Discussion
Issued by NWS Greer, SC

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480 FXUS62 KGSP 191800 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 100 PM EST Sat Jan 19 2019 .SYNOPSIS... A strong low pressure will cross Tennessee today and reach New Jersey early Sunday morning. The associated strong cold front moves from west to east across our area tonight. Strong northerly wind on Sunday will make temperatures feel much colder. The wind will gradually diminish Sunday night with a clearing sky. Another low pressure system is expected to impact the region on Wednesday and Thursday. && .NEAR TERM /THROUGH SUNDAY/... As of 1250 pm: Rain shield continues to gradually overspread the area from the west and SW, and steady ramping up of pops from west to east throughout the afternoon into the evening still seems to be the right idea. Weak/in-situ cold air damming is more or less stuck in place across the area with little evidence of warm sector/southerly surface flow intrusion into the area. This should gradually change this afternoon into the evening, as southerly gradient continues to intensify across the Southeast. Latest 12Z guidance doesn`t offer much of anything new regarding threats later today. The very strong/deep meridionally-oriented forcing depicted along surging cold front and very strong (50+ kts) 0-1 km shear are certainly alarming, and would be a red flag for high shear/low CAPE severe convective potential IF there was even a modicum of instability present. However, short term guidance remains insistent in shunting ribbon of very low sbCAPE south of the forecast area. Nevertheless, there will probably be plenty of interesting convective features within the frontal band that will need close monitoring late this afternoon/evening. Also could not rule out some isolated damaging winds from showers bringing down the 60kt+ momentum from aloft via plain old convective mixing. Otherwise, mountain gradient winds will ramp up in the warm advection this afternoon, and a high elevation Wind Advisory remains in effect above 3500 feet for the southern NC mountains. Locally excessive rainfall will be possible tonight, with the biggest threat in the southern mountain upslope areas where localized two inch storm QPF totals are possible. With river forecasts a bit below flood stage, and 3-hr FFG values on the order of 2.75 to 3 inches, a Flood Watch will not be posted yet. But, one could be needed later if QPF rises any. Snow levels will quickly plunge across the mountains from the west 09Z to 12Z behind the frontal passage. There could be a brief window of mixed ptypes across the mountains, but with solid snow supported in the W to NW upslope flow areas from 12Z Sunday morning onward. The upper trough axis will cross the region from the west circa 18Z Sunday, and the peak cold advection winds will develop. Another mountain wind advisory could be needed on the cold advection side of things Sunday, but will handle the warm advection part first this morning. Outside of the mountains, mid-level dry air will wrap in quickly Sunday morning to rapidly end PoPs from the west. The northwest flow upslope snow period will be intense and cold. The short-lived nature of the event Sunday will be a limiting factor on snow accumulations, but there is also strong evidence of a Great Lakes moisture connection to parcels arriving in the NC mountains. Advisory-level snow accumulations will be a possibility across the TN border counties and the higher ridges by late Sunday. A Winter Weather Advisory could be needed for the NC mountains at some point, particularly with the flash freeze potential with temperatures falling quickly into the 20s across the mountains through the day. && .SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY/... As of 300 AM Sat: Very cold high pressure will continue to filter into the area Sunday night, with the pressure gradient still sharp across the mountains at the start of the period. Temperatures in the upslope layer will have already cooled enough that despite the moisture having become rather shallow, any moisture that remains will be at favorable temperatures for snow crystal production. So some very light additional snow accumulation is possible, though with such small QPF output even the most favored areas only look to get a couple tenths after 00z. Flurries however may be seen in the mtns thru early Monday morning. The gradient will weaken gradually overnight, and while winds will diminish to unremarkable speeds east of the mtns by the wee hrs Monday morning, brisk northwesterly winds/gusts will continue across the higher mountain elevations into the day. Looks like this will be the coldest morning of the season so far in the mountains and NW NC Piedmont, and likely at least the more sheltered spots across the Charlotte metro area and the GA/SC zones. Exceptionally cold 850mb temps bottom out below -15C over the northern mtns. These are expected to result in sfc temps in the single digits there, which combined with the continuing wind will produce wind chills below -5F across much of the zones. Current thinking is that this hazard is likely to be wrapped into an eventual Winter Weather Advisory along with the precipitation-related threats. A few power outages from brisk wind gusts are possible early Monday to boot. Downslope flow and sunshine occurring Monday will only be able to bring temps back to within 10-12 degrees of normal over most of the area, though winds will abate with the center of the high drifting nearer the area. By Tuesday it will have centered over the Mid-Atlantic coast, and return flow will get underway in the midlevels; the last hour of the NAM suggests it may be hanging on to CAA a bit longer, lending some uncertainty to max temps Tue. Temps still are expected to moderate gently, a few degrees warmer than Monday. Clouds will increase late in the day, but no PoP is expected prior to 00z Wed. && .LONG TERM /TUESDAY NIGHT THROUGH FRIDAY/... As of 130 AM Saturday: Starting at 00Z Wednesday with cold high pressure centered near Cape Hatteras and the next cold front crossing the Mississippi Valley. Southerly low level upslope flow will initiate showers over the higher terrain from near Clayton GA to near Table Rock and Caesars Head SC. This low level flow will gradually turn more SW on Wednesday and decrease the upslope effect. Areas of low pressure and good Gulf moisture helps to enhance rainfall rates from late Wednesday into early Thursday. The current EC has the main low tracking slower than the GFS and nearly up the mountain chain. One to two inches of rain for the event may be conservative. Still very little if any instability with the EC having a 100 or less CAPE early Thursday PM. The GFS has all precip ending at 18Z Thursday while the EC delays it til Thursday evening. The EC maintains a brief NW Flow event into Thursday night but ends that by Friday morning. A clipper type system rapidly approaches from the NW Friday night into Saturday. Timing differs in the models as the GFS brings this quick shot of snow across our forecast area early Saturday while the EC has an evolving larger system Saturday night. The EC has a deep cold vortex passing just north of our area late in the weekend. We have a while week to sort this out. Temperatures are forecast to be several degrees above normal into Thursday. Max Temps fall below normal Friday and Mins around normal late in the week. && .AVIATION /18Z SATURDAY THROUGH THURSDAY/... At KCLT and elsewhere: LIFR cigs along with IFR/LIFR (and in some spots VLIFR) visby have settled over the forecast area this afternoon in response to weak cold air damming. The boundary between cold air damming conditions and warm/humid conditions will attempt to lift north in response to increasing southerly flow in advance of strong storm system/associated cold front. This would be accompanied by improving flt conditions as well as gusty southerly winds, but timing this is going to be very tricky. As such, LIFR conditions are forecast to persist until at least 00/01Z. While some improvement to MVFR is forecast at that time, widespread moderate rain, with periods of light and heavy rain will become widespread across the terminal forecast area this afternoon into the evening, and is not expected to end at most terminals until late evening or the early part of the overnight. Isolated TS are possible this evening, but confidence is too low to include a forecast mention attm. Additionally, low level wind shear is expected at most terminals, as a very strong jet core just above the surface translates across the area. Strong cold front blasting through the area will begin clearing skies while turning winds to the NW and seeing them become quite gusty between 09 and 12Z, with gusts in excess of 30 kts appearing likely at KAVL, and more like 15-25 kts at the other terminals. Outlook: Windy conditions and cold temperatures will continue into Monday. The next storm system approaches by mid-week. Confidence Table... 18-24Z 00-06Z 06-12Z 12-18Z KCLT Med 75% Med 75% Med 64% High 88% KGSP Med 61% Med 67% Med 76% High 100% KAVL Med 78% High 84% Med 73% High 94% KHKY Med 68% Med 65% Med 60% High 100% KGMU Med 66% Med 69% High 80% High 100% KAND Med 76% High 81% High 80% High 99% 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...Wind Advisory until 10 PM EST this evening for NCZ051-052-058- 059-062>064. SC...None. && $$ SYNOPSIS...DEO NEAR TERM...HG/JDL SHORT TERM...Wimberley LONG TERM...DEO AVIATION...JDL

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