Scientific Forecaster Discussion

000 fxus62 kgsp 191944 afdgsp Area forecast discussion National Weather Service Greenville-Spartanburg SC 244 PM EST Tue Feb 19 2019 Synopsis... Canadian high pressure will ridge down over the forecast area into Wednesday as a series of low pressure systems moves up from the Gulf. The combination of cold air and abundant moisture will result in an area of wintry precipitation for the mountains and foothills through early Wednesday. The moist pattern will continue well into the weekend until a cold front finally moves through the region and dries things out. && Near term /through Wednesday/... as of 205 pm: frontogenetically-forced precipitation continues to impact much of upstate SC, northeast GA, and southern NC this afternoon. Precip type has been rain for the most part, but sleet continues to mix in across the Piedmont and foothills, with some locations across the mtns (esp along the southern Blue Ridge Escarpment from Cashiers/Lake tox east) having reported accumulating sleet. Farther north, where wet thermodynamic profiles are more supportive of frozen precipitation, there hasn't been a great deal of penetration of the precip band up to this point, but light radar returns have developed over the northern mtns, and more significant precip currently over southeast Tennessee is poised to overspread much of western NC over the next couple of hours. A Winter Storm Warning remains in effect for the northern mtns through much of tomorrow, where up to an inch of sleet and snow is possible into early evening, especially along the Escarpment. Meanwhile, with short term model guidance in good agreement that a prominent warm nose will develop from the southwest this evening, the character of the event will transition to more of a rain/ freezing rain rather quickly this evening. Precip across much of the current advisory area (except in aforementioned southern Blue Ridge area) will therefore hinge on cold air advection developing in association with development of classical cold air damming tonight, as 1035-ish mb Arctic high pressure settles into southern New England. For the official forecast, we've leaned heavily on the cold NAM, which has done rather well with diabatically enhanced cad events throughout the winter. Thus anticipate freezing/sub-freezing wet bulb temps to pull as far south as the I-40 corridor in the Piedmont and foothills after midnight, although confidence in this is not especially high at this time. Confidence is high that cold air will be able to lock in near the Blue Ridge, especially in the barrier jet location along and near the eastern Blue Ridge Escarpment. In addition to the cooling from the maintenance of the cad, very strong southeast upslope flow at the top of the developing cold dome will both enhance the cooling while providing additional forcing to augment precip rates. In fact, with the frontogenetical forcing shifting north of the forecast area this evening, upslope flow will become the primary source of lift tonight into Wednesday. All that being the case, our confidence in significant ice accretion remains highest in areas along the eastern Blue Ridge Escarpment north of I-40, where the warning is currently in place. Other areas of concern will be farther southeast, especially in the upslope areas of Polk/Rutherford mtns, as well as Henderson and Transylvania counties. I suspect that some locations in these zones will see warning criteria ice, esp along and South/West of The Divide, but confidence is low in heavy ice accretion being widespread enough to warrant an upgrade. Meanwhile, the strength of the southeast upslope flow and developing cad creates a concern that areas along the Blue Ridge in northwest SC and extreme northeast Georgia will see light icing develop toward daybreak Wed, and Pickens/Oconee mtns in SC, as well as Rabun County in Georgia will be added to an advisory. Otherwise, Wed will see gradually diminishing precip chances, with most areas likely warming enough to see just rain/drizzle during the afternoon. Temps will be almost 20 degrees below climo under the influence of mature cold air damming. && Short term /Wednesday night through Friday/... as of 215 PM EST tuesday: upper jetlet divergence will increase over the mountains Wednesday night, with 500 mb channeled vorticity crossing the region as well, enhancing upward vertical motion over the forecast area. Meanwhile, a surface cold front west of the Appalachians Wednesday night will push east into the mountain chain before laying over into the region through the day on Thursday. The uptick in forcing and deeper moisture along and ahead of the frontal zone will raise the qpf, and the associated Hydro threat, starting Wednesday night. It's quite possible that a Flood Watch will be needed soon, especially for the southwest NC mountains and the northeast Georgia mountains. The narrow zone of deeper moisture around the frontal boundary may well stay parked over the forecast area Thursday. Anticipate some 500+ j/kg SBCAPE values over the lower piemdont southeast of the front, with temperatures throughout being quite difficult to pin down given uncertainty over the frontal position. Anticipate some very modest height rises over the southeast Thursday night through Friday. The strongest upper jet energy continues to wrap around to the north and west of the region for most of late Thursday through Friday. However, low level southwest flow refocuses once again over the mountains through Friday while upglide fans out to the east along a baroclinic zone. This will cause the Hydro threat to either persist or return through the period. Although 1032 mb high pressure will set up to the north, low level temperatures do not look quite cold enough to support mixed ptypes at present, so will keep the robust pops all associated with liquid rain for now. && Long term /Friday night through Tuesday/... as of 230 PM EST tuesday: a southern stream closed low will move from The Four Corners region of the Desert Southwest to the Southern Plains Friday night through Saturday, before lifting over the Great Lakes on Sunday. This will batter down heights across the southeast over the weekend. Low-level southwesterly flow will strengthen Saturday night ahead of an approaching cold front, and it's good bet that Hydro problems will persist and develop through a good chunk of the weekend. There is good consensus on a cold fropa from the west on Sunday morning. This timing will be unfavorable for much instability along the passing front, keeping any thunder in check. Upper flow then becomes more zonal late Sunday through Monday. Any shot of shallow northwest flow moisture into the NC mountains Sunday night behind the departing cold front will be very short-lived. Mid to upper flow backs Monday night through Tuesday, with weak shortwaves moving from the western Gulf Coast to the southern Appalachians. The GFS holds off on upglide moisture returning across the southeast until Tuesday afternoon, while the European model (ecmwf) brings moisture back a bit faster late Monday night. Will feature a consensus of increasing pops for rain on Tuesday. Max temps some 10 degrees above climo on Sunday will fall back below climo by Tuesday. && Aviation /20z Tuesday through Sunday/... at kclt and elsewhere: precip band near the NC/SC border, and impacting all terminals except kavl/khky, continues to lift slowly north this afternoon. While the primary precip type has been -ra, ice pellets has mixed in at times, and in some cases has completely changed to ice pellets in association with heavier precip rates. Indications are that precip rates will diminish a bit during late afternoon/early evening, which should result in less of a chance for ip, and tempos are therefore ended around 21z at most terminals. Heavier precip rates may well return this evening, but warming aloft should preclude any mixed precip concerns at that time. Meanwhile, -ra mixed at times with sleet should develop shortly at kavl, and by late afternoon or so at khky. Colder air will begin filtering into the area tonight in response to development of strong cold air damming. -Ra is therefore expected to transition to -fzra at kavl/khky and persist into Wed morning. Can't rule out -fzra developing as far south as kclt, but probability appears low at this time. Otherwise, cigs will gradually lower through the afternoon and into the evening, likely reaching MVFR at most sites late this afternoon and IFR by mid/late evening. Visby will be obstructed, at least occasionally during periods of heavier precip. Low cigs/ restrictions, along with NE winds at 10-15 kts should persist through the end of the period as cold air damming locks in. Outlook: freezing rain in the mountains should gradually change to all rain during Wednesday afternoon. A series of low pressure systems will continue to move through the region through the end of the week. This will produce precipitation and associated restrictions for long periods of time with each system. Confidence table... 19-01z 01-07z 07-13z 13-18z kclt high 87% low 51% Med 67% Med 69% kgsp Med 74% low 55% low 59% Med 60% kavl high 85% Med 66% Med 64% high 90% khky high 88% Med 64% Med 69% high 90% kgmu Med 78% low 53% Med 60% high 80% kand Med 72% low 48% Med 64% Med 60% 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... Georgia...Winter Weather Advisory from 6 PM this evening to 4 PM EST Wednesday for gaz010. NC...Winter Weather Advisory from 6 PM this evening to 4 PM EST Wednesday for ncz035>037-048-052-056-057-059-063-502-504- 506-508-510. Winter Weather Advisory until 4 PM EST Wednesday for ncz053- 064-065-507-509. Winter Storm Warning until 4 PM EST Wednesday for ncz033-049- 050-501-503-505. SC...Winter Weather Advisory from 6 PM this evening to 4 PM EST Wednesday for scz001-002. Winter Weather Advisory until 4 PM EST Wednesday for scz003. && $$

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