Area Forecast Discussion
Issued by NWS Greer, SC

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997 FXUS62 KGSP 180006 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 706 PM EST Sun Feb 17 2019 .SYNOPSIS... A moist warm front will continue to move north from Georgia tonight as low pressure crosses Tennessee. Expect Canadian high pressure ridge down into our area from North Dakota by late Monday giving us a short dry period. Rain returns on Tuesday as a series of low pressure systems will track from southwest to northeast across the region, resulting in a persistent moist pattern through most of the week. && .NEAR TERM /THROUGH MONDAY/... As of 650 PM EST Sunday: Precipitation coverage has ramped up very quickly this evening, and deeper forcing and moisture will move in from the west through the late evening hours. Have thus hastened the west to east increase in PoP trends this evening, maximized the coverage early overnight, and boosted QPF values a bit in the southwest NC mountains for this update. Also lowered minimum temperatures a touch, but these are of no real consequence as mins are occurring right now and temperatures should rise slightly overnight throughout. Otherwise, the cold air damming wedge will remain locked in across the region into the overnight hours now that a chunk of the parent high has broken off over the Mid-Atlantic region. Low-level isentropic lift will improve through late evening in the 295-300K layer, which should drive the development of a large area of moderate to occasionally heavy rain that is expected to move across the fcst area through at least 06Z. The improving SW flow should focus the heavier precip over the SW-facing mtns, meaning the biggest threat for excessive rainfall will be in the Little TN River valley. There is some concern about precip amounts and how that might relate to flood potential. As it stands right now, our QPF is only on the order of half the 1hr FFG, so think we should be able to tolerate what falls. That being said, it will only serve to further increase our potential for flooding later this week. The precip should end steadily from the west with the passage of a sfc wave and the ending of the isentropic lift. Will hold onto some higher chances on the TN border a bit past daybreak, but that should also end quickly. Downslope quickly ensues by midday Monday, so we should clear out and warm up nicely for Monday afternoon. Might be the only decent weather we get all week. && .SHORT TERM /MONDAY NIGHT THROUGH WEDNESDAY/... As of 255 PM EST Sunday: The fcst will start off dry across the area as a broad Canadian high builds south across the Glakes region and begins to ridge down the east coast. Temps will drop to arnd freezing across the NC mtns while a developing warm front begins to approach the area arnd daybreak. There is some uncertainty with the level of moisture with this front as the NAM continues to have very little qpf response by 12z Tue. Likely a null chance of any wintry precip Tue morning as any light precip shud not arrive until temps warm abv freezing. Temps wont rise much during the day...probably remaining 40s all locales as sfc-based CAA continues and cloud cover increases. The fcst becomes quite tricky Tue night as a parent 1040 mb high sets up across the ern Glakes. The high config looks to be classical in nature and bad for wintry precip across the colder NC mtn areas as a stg llvl sw/ly jet brings in a sigfnt warm nose aloft. Model temp guidance has trended down overall so mixed in lower values with the previous fcst numbers. This gave more areas for accum fzra over the NC mtns and even a -ra/fzra mix across the nrn fthills and nrn NC piedmont. Temps look to hover just north of freezing generally outside the nrn mtns...but a degree or two difference in sfc Tw/s could create wintry/icy precip. The amt of precip is not totally certain as there could be robbing convec south and the overall depth of saturation will likely be limited to arnd h7. Therefore...ice nuclei may not be activated esp as the morning progresses...per the latest GFS/NAM soundings. Thus...will anticipate an onset of freezing precip arnd 02z with likely a switch to -fzdz...before a changeover to all rain occurs arnd 14z. Expect a light glaze of ice at most across the non/mtns...and the best ice accum potential of a couple tenths over the nrn NC mtns and high elev nrn fthills...and perhaps even further south across the Black Mtns. Right now...this event is certainty adv level and as the fcst changes warning level conds could develop. A light cold rain will continue thru period with temps only reaching the L40s north and L50s over the far sw/rn zones as the wedge is slow to lift out of the area. Another issue Tue night into Wed will be the potential for stream flooding across the sw/rn NC mtns. WPC has the area in a Day 3 slight risk of flooding the favored upslope sw-facing regions will likely recieve arnd 2 inches to near 3 inches by then in continued high saturated soils. && .LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/... As of 200 PM Sun: An active pattern is still depicted across the CWFA throughout the medium range, still with a strong ridge centered over the eastern Caribbean and deep troughing over the southern Rockies and vicinity. As low pressure traverses the Great Lakes region early Thursday, a cold front will move thru the lower Mississippi Valley, then pivot across the Appalachians as the sfc low moves thru New England. That will promote erosion of the midweek CAD affecting our area, though with the boundary lingering, unsettled weather will persist. Furthermore, global models depict yet another seasonably strong sfc high moving eastward north of the boundary, suggesting CAD will return by Friday. Temperatures within the weekend CAD event currently do not look cold enough for wintry precip, so the main impact initially may just be to prolong the already cloudy/damp conditions. Model consensus shows pattern change finally occurring next weekend as a shortwave ejects from the Rockies, cyclogenesis occurs somewhere to our northwest, and a cold front swings thru the Southeast. This front is most likely to reach our area on Sunday. The European and Canadian solutions depict a stronger low and better defined cold front than does the GFS, though all of them show at least modest instability in the warm sector preceding the front, while CAD persists for some of our area. These trends are worth watching as they imply some severe weather threat may result for those parts of our area along/south of the wedge boundary. All in all it is difficult to identify any meaningful break in precip chances between Wednesday evening and Saturday morning, so we will advertise at least likely PoPs for nearly the whole area during that timeframe. A limited diurnal range is expected due to cloud cover and/or wedging, but overall temps will remain near climo. Chances were allowed to drop a bit by Saturday afternoon as warm front shifts northward ahead of the developing low. && .AVIATION /00Z MONDAY THROUGH FRIDAY/... At KCLT and elsewhere: With the steadier rainfall now spilling east across the terminal forecast area, cigs are already locked into IFR and falling into or near the LIFR range at issuance time. Expect mainly steady LIFR cigs and IFR vsby as the heavier rain passes through 06Z or 08Z east. Conditions will begin to improve from the west before daybreak as flow turns westerly and a light downslope flow develops east of the mountains. It may take some time, perhaps through mid morning, to scatter the shallow wedge and lingering boundary layer moisture, but once it starts happening it should scatter quickly through noon. Anticipate some low end gusts developing in the westerly flow with mixing near the end of the period, with perhaps more robust gusts well into the 20s at KAVL in the French Broad Valley flow. Outlook: After brief improvement through Monday evening, a series of low pressure systems will move through or near the region through much of the rest of the week. This will produce precipitation and associated restrictions for long periods of time during each wave of precipitation. Confidence Table... 00-06Z 06-12Z 12-18Z 18-00Z KCLT High 83% Med 72% Med 73% High 100% KGSP High 84% Med 62% High 88% High 100% KAVL Med 66% Med 70% High 96% High 100% KHKY Med 69% Med 70% High 91% High 100% KGMU Med 74% Med 62% High 87% High 100% KAND Med 75% Med 69% High 99% 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...None. NC...None. SC...None. && $$ SYNOPSIS...DEO NEAR TERM...HG/PM SHORT TERM...SBK LONG TERM...Wimberley AVIATION...HG/PM

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