Area Forecast Discussion
Issued by NWS Greer, SC

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000 FXUS62 KGSP 292000 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 300 PM EST Sun Nov 29 2020 .SYNOPSIS... A strong low pressure system approaching from the southwest will support widespread showers and isolated thunderstorms from this evening through Monday morning. In the storm`s wake, Snow is expected to develop in the mountains near the North Carolina and Tennessee border from late Monday through Tuesday. Another strong low may cross the region at the end of the week. && .NEAR TERM /THROUGH MONDAY/... As of 1250 PM: Still expecting mainly just increasing cirrus over the area until this aftn. A relatively dry sfc airmass remains over our CWA, associated with high pressure off the mid-Atlantic coast. Deepening moisture and upglide atop this airmass, along with increasing upper divergence, will develop ahead of a shortwave and sfc low pressure now spinning up near New Orleans. Cloud bases are already lowering over the southern CWA, and we are beginning to see some light showers develop over the northern SC Midlands...actually a bit faster than any guidance suggested. This warranted a minor adjustment to the aftn PoPs. Sfc low will strengthen as it moves NE toward the southern Appalachians this evening, with the associated shortwave being absorbed into a deeper trough digging into the mid-Mississippi valley. Consensus suggests the low will skirt along the NC/TN border. As precip falls into the sfc airmass mainly ahead of and along it, weak CAD is expected to develop. This wedge will remain entrenched across the upper Piedmont and Foothills until the cold front scours it out later in the morning. S to SE 925-850mb winds will be favorable for upslope enhancement, and event totals above 2 inches appear likely along the southern Blue Ridge Escarpment and other south-facing ridges. Isolated flooding could occur in those upslope areas, but overall QPF is not expected to be much of a concern. LLJ crossing the area ahead of the cold front will produce very strong low-level shear. Minuscule sfc-based instability, even south of the wedge boundary, remains the limiting factor. Modified SHERB (MOSH) values from both NAM and GFS remain low enough ahead of the cold front that any storms that fire along it most likely will not produce damaging winds or tornadoes, although any cells developing via SBCAPE will need to be monitored closely. We do retain slight-chance thunder south of the wedge boundary and in some spots where there is MUCAPE atop the wedge. Given WAA, the LLJ is not expected to produce advisory-level gusts except perhaps at very high elevations. Some degree of dry-slotting is expected in the very late overnight or around daybreak; precip should come to an end for the Piedmont from SW to NE during the morning. Often precip ends earlier than synoptic models depict in these kinds of events, but the lack of strongly organized convection along the front makes me buy their consensus. The low clouds may take some time to erode, nonetheless. Winds will become gusty following the fropa but high elevations still don`t look to mix high enough to get advisory-level gusts. Max temps will end up a tad above normal in the Piedmont. For the mountains, the morning passage of the cold front will initiate falling temps, which along with moisture wrapping around the system on NW winds means precip will transition to snow. Profiles also will become increasingly favorable for snow crystal growth, so initially low SLRs will become pretty good by late afternoon. Snow accums will begin in the morning at higher elevations, falling to lower elevations with time. This event will continue into the short term, and significant accums are likely along the TN border. We are issuing a Winter Storm Watch with this package; an elevation split between Warning and Advisory may be needed tonight or on tomorrow`s day shift. && .SHORT TERM /MONDAY NIGHT THROUGH WEDNESDAY NIGHT/... As of 220 PM Sunday: Let Winter begin! We should be in the throes of a NW flow snow event at the transition to the short range at 00Z Tuesday. Precip-type is not in question, with deep cold air in place wrapping around the slowly departing upper low that moves up the OH Valley Monday night. Moisture should be relatively deep on Monday evening...with high RH well up into the dendritic growth zone. Backward trajectories from the NAM Nest suggest at least a few hour period with a Great Lakes moisture connection. The 850mb flow is more WNW, but that is typical. What we lack is strong upper support, but we will make that up with the potential for higher amounts because of the moisture supply in the dendritic growth zone. The upshot is that we have good potential for significant high elevation snow near the TN border, and with that in mind, precip probs have been raised in the likely/cat range on the TN border, and QPF/Snow has also been raised according to the latest model guidance. NOTE: We often like to err on the side of caution with the first event of the season, thus the Winter Storm Watch that extends to midday Tuesday along the TN border. The deeper moisture starts to pull out around daybreak Tuesday, but the low level moisture departs more slowly, such that there is a high chance that snow shower production will continue past sunrise and into the middle part of the day along the TN border. The snow should taper off completely by the end of the daytime period. East of the mtns, it should remain dry for the most part. There is some indication that a small area of low level convergence downwind of the Balsams could result in some snow flurries along the NC/SC border, mainly over the nrn Upstate, on Monday evening. Both operational models have some small response east of the GSP metro area, but for now, it bears watching but not including in the official forecast. As for temps, needless to say, they will be the coldest of the season and very much like winter across the entire region. As for the rest of the period into the middle part of the week, after the upper low moves away Tuesday night, we should be in for a lull with cool sfc high pressure over most of the Southeast. A slowly dampening upper ridge in-between the old upper low and another upper low moving out over the central/srn Plains will move overhead Wednesday. The ridge axis should move past Wednesday night and we begin to come under the influence of the cyclonic flow/warm advection ahead of the next upper system. However, the clouds and precipitation associated with the next one are not expected to arrive from the west before daybreak Thursday. && .LONG TERM /THURSDAY THROUGH SUNDAY/... As of 240 PM Sunday: Forecast confidence deteriorates rapidly as the end of the week approaches. At issue is the organization and track of the next upper low, and what, if any, cyclogenesis takes place to the lee of the Appalachians on Friday or Saturday. In spite of timing differences, there is some agreement that the upper low will move slowly east and begin to shear out over the Midwest on Thursday and Thursday night, or perhaps Friday if the ECMWF timing is correct. Precip chances will ramp back up during this period but remain in the chance range, while overnight temps remain borderline for rain/snow. The main difference comes along Saturday with the new ECMWF being slower and deeper with one upper low moving along the VA/NC border inducing lee cyclogenesis over NC. If that scenario plays out, we could have another rain/snow event over the mtns at least on Saturday. For now, the fcst will remain a blend between the two solutions which comes up decidedly vague with an unmentionable precip chance. At least we are reasonably confident that temps will remain below normal. && .AVIATION /20Z SUNDAY THROUGH FRIDAY/... At KCLT and elsewhere: Cirrus will be seen thru this afternoon, with low VFR then MVFR cigs pushing in generally S to N during this time as low pressure approaches from the SW. Deeper moisture will follow, and IFR to LIFR cigs will set in soon after steady precip arrives, as CAD forms. Mainly LIFR is expected overnight within this pattern; chances for SHRA peak overnight as low tracks across the southern Appalachians. Can`t rule out isolated TSRA forming either above the wedge or along its southeast periphery, but likely too isolated to mention in TAFs. Likewise, LLWS could occur atop the wedge but too hard to pin down to warrant mention. There remains some question as to how fast cold front will scour out low clouds and precip in the early part of the day. Even if precip ends as early as suggested by convection-allowing models, it may be hard to erode the restrictive cigs. Have included PROB30 at all sites except KAND to reflect the possibly lingering restrictions at that time. Scattering and VFR should return by midday. KAVL will begin to see clouds associated with NW flow over the mountains by midday; periodic restrictions from these may develop later in the day. Brisk wind gusts will develop in the late overnight around the region, and these will continue/strengthen as winds flip to W or NW behind the front. Outlook: Conditions improve Monday as the cold front crosses the area, but windy conditions develop. Behind this system, wintry precipitation will be possible in the mountains later Monday into Tuesday, with VFR elsewhere. VFR all locations by midweek. Confidence Table... 20-02Z 02-08Z 08-14Z 14-18Z KCLT High 95% Med 63% Med 79% High 81% KGSP High 92% High 87% Med 75% High 84% KAVL High 85% High 85% High 92% High 80% KHKY High 100% High 82% High 95% Med 74% KGMU High 82% High 89% Med 68% Med 79% KAND High 90% Med 72% Med 71% High 82% 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: && .GSP WATCHES/WARNINGS/ADVISORIES... GA...None. NC...Winter Storm Watch from Monday morning through Tuesday morning for NCZ033-048>052-058. SC...None. && $$ SYNOPSIS...DEO NEAR TERM...Wimberley SHORT TERM...PM LONG TERM...PM AVIATION...Wimberley is the U.S. government's official web portal to all federal, state and local government web resources and services.