Area Forecast Discussion
Issued by NWS Greer, SC

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000 FXUS62 KGSP 050253 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 953 PM EST Fri Dec 4 2020 .SYNOPSIS... Low pressure will rapidly cross our area this evening, bringing rain to most areas and light snow to the mountains. Another fast-moving system will bring light precipitation to the region Monday. Seasonably cool and dry weather is otherwise expected for the remainder of next week. && .NEAR TERM /THROUGH SATURDAY/... 930 PM Update...The last of the heavier rain showers with embedded tstms are exiting the FA currently. Not anticipating new convec development while the system continues to push NE and a dry slot is moving in from the SW. A few showers are developing across the srn NC foothills underneath the cold h5 trof, but development shud remain low-end with perhaps a period of isol graupel. The flow across the NC mtns is moistening up and expect -snsh to begin across the highest peaks and lower thru the evening. Still expect sub-adv level accums, yet some localized areas across the Smokies and TN spine could receive arnd 3 inches before the snow tapers off Sat morning. As of 237 PM Friday...Scattered showers will move across the Upstate and wrn Piedmont/foothills of NC for the next few hours, out ahead of a developing band of precip moving in over the wrn mountains. We expect that as the forcing increases rapidly late this afternoon, the band of precip will blossom further as it spreads east over the fcst area. The forcing will come courtesy of a compact and dynamic upper low moving steadily from the mid-MS valley this afternoon to middle TN/KY early this evening. A strengthening sub-tropical jet streak across the southeast will bring a left exit region overhead this evening as the upper low approaches and acquires something of a negative tilt. Strong mid-level dpva will arrive concurrent with the upper divergence to result in strong synoptic-scale lift. A low level jet out ahead of the system will strengthen dramatically as it translates northeastward this evening as well. All this forcing will contribute to a rapid deepening of a surface low to the lee of the Appalachians tonight. There are a number of concerns to address. The system should move through rapidly, which will cut down on precip amounts. At the present time, it does not appear that we will be able to realize even the most meager of sfc-based CAPE, which in this situation, is a very good thing given the amount of shear that will develop over the wrn Piedmont this evening. Will not rule out some elevated storms, but without any of them rooted to the boundary layer, our chances for severe weather are very limited. Model guidance keeps a shallow stable boundary layer across metro Charlotte, that will effectively provide some insulation from the winds which are expected to stream overhead at around 3k-5k feet. We will not rule out some heavier showers bringing some gusty winds to the sfc briefly. This will have to be watched carefully for the development of a gravity wave east of the mtns, which sometimes occurs in the wake of a strongly-forced and deepening sfc low. Winds will likely become strong at the highest elevations this evening as the low passes, but not to the extent that an advisory is warranted at this point. Colder air moving in behind the low will change precip over to snow at the high elevations along the TN border this evening, but the moisture pulls out rapidly on Saturday, which should keep snowfall below advisory criteria. We should clear out rapidly later tonight east of the mtns. Saturday looks like a sunny and breezy winter day as high pressure builds in from the west. Temps should be right around normal. && .SHORT TERM /SATURDAY NIGHT THROUGH MONDAY NIGHT/... As of 230 PM Friday: Overall, there have not been significant changes made to the previous forecast in the current package with respect to the southern stream system that impacts our area late Sunday night through early Tuesday morning. Disagreement amongst the model guidance has not changed appreciably from the overnight runs with notable differences in timing and forcing still reducing overall confidence in the sensible weather impacts. Therefore, the forecast still advertises a period of mountain snow beginning overnight Sunday as the attendant surface low treks across the Deep South within the southern stream jet. This light snowfall will transition to a modest northwest flow event Monday afternoon as a potent 540mb closed low drops into the central Appalachians. East of the mountains, without appreciable CAA ahead of the southern stream system, and wet bulb temperatures above freezing, any precip that develops on the northern extent of the system early Monday morning is expected to be a chilly light rain at this time. Of course, this will continue to be watched. The ECMWF/Canadian/NAM solutions remain faster than the GFS and move southern stream precip by the area early Monday afternoon which is about 3-6 hours faster than the GFS. The forecast generally supports the consensus with regards to timing. Lacking appreciable moisture flux, all models are in agreement that QPF will be light. With the Ohio Valley closed low dropping rapidly SE into the Appalachians, northern stream phasing still looks to occur generally east of our area Monday afternoon. As northwest upslope flow forcing establishes itself over the mountains Monday afternoon, lapse rates may be steep enough, when combined with favorable vorticity advection, to support convective shower activity over the Piedmont, primarily where any convergence zones set up downstream of the mountain ridges. These zones will be highly dependent on wind direction, but the GFS and the NAM are suggesting fingers of very light precip developing from the Savannah River Valley to the I-26 corridor Monday afternoon and evening. By this time, with CAA starting to develop, NAM/GFS profiles and wet bulb temperatures suggest that a transition to snow cannot be ruled out within these showers should they develop, especially north and east of the Saluda River Valley. Overall, low-level dry air is rather unimpressive at this time to support appreciable wet bulbing, and SBCAPE values remain below 100 J/kg (a decent threshold to watch for stronger dynamic cooling), but we are still several days out and this will bear some watching. For the short-term period, we are currently expecting light snow accumulations over the mountain ridges through early Tuesday morning of up to 2-3" in isolated locations with no accumulations elsewhere. Temperatures will remain a few degrees below normal Sunday and chillier on Monday, especially where CAA will be strongest behind the southern system over the mountains, where temperatures will peak about 10 degrees below normal. East of the mountains Monday`s high temperatures are forecasted to peak in the upper 40s north and lower 50s south, but this was below model blends and we expect the forecast highs to trend cooler with successive packages. && .LONG TERM /TUESDAY THROUGH FRIDAY/... As of 230 PM Fri: With the departing upper low on Tuesday, conditions will improve quickly and set us up for a seasonably cool and dry remainder to the work week. Models are in general agreement that another potent shortwave will dive SE within the NW flow aloft and cross the region sometime Tuesday evening through Wednesday morning. However, there will be no moisture to work with and outside of the possibility of some gusty winds mixing down over the higher elevations Tuesday night, the shortwave will be a non-event. Thereafter, widespread ridging will overtake the eastern US, and allow our temperatures to moderate nicely into Thursday and Friday, with highs possibly reaching 4-6 degrees above normal, which is equivalent to the upper 50s to near 60 east of the mountains. && .AVIATION /03Z SATURDAY THROUGH WEDNESDAY/... At KCLT and elsewhere: A band of light to moderate showers crossing the fcst area is lowering CIGS and VSBY into the MVFR/IFR range while areas across the west are seeing lowering CIGS in a highly moist airmass behind a sfc low. Expect these CIGS to improve as drier air mixes in from the NW thru 04z or so. Not expecting a great thunder threat at KCLT as the main convg line moves in, but sw/ly to nw/ly winds will likely gust in the low to mid 20 kt range thru 03z. Improving flight cats across all TAF sites after midnight, with moderate nw/ly gusts likely at KAVL. Winds shud remain strong enuf to offset a morning fog threat over most terminals, yet MVFR to IFR VSBY may occur occasionally before sunrise at KAVL. Outlook: VFR for most of the weekend. Moisture will return Sunday night with a ceiling restriction likely on Monday as another low pressure system lifts out of the Gulf and up the Atlantic Coast. Confidence Table... 03-09Z 09-15Z 15-21Z 21-00Z KCLT High 80% High 100% High 100% High 100% KGSP Med 75% High 100% High 100% Med 74% KAVL High 80% Med 75% Med 65% Med 61% KHKY High 91% High 100% High 100% High 100% KGMU Med 60% High 100% High 100% High 100% KAND High 88% High 100% High 100% 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 && .GSP WATCHES/WARNINGS/ADVISORIES... GA...None. NC...None. SC...None. && $$ SYNOPSIS...JMP NEAR TERM...PM/SBK SHORT TERM...JMP LONG TERM...JMP AVIATION...SBK

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