Scientific Forecaster Discussion

Area forecast discussion National Weather Service Greenville-Spartanburg SC 646 am EST sun Dec 11 2016 Synopsis... cold high pressure moves off the mid Atlantic coast Sunday as another cold front crosses the plains states. This front will cross the region Monday, with precip chances lasting into mid-week. Another blast of cold air will settle across our area during the latter half of the week. && Near term /through tonight/... as of 640 am, low clouds associated with moisture surge had already pushed into the lower SC Piedmont. This is a couple of hours faster than previously anticipated, and the sky cover forecast has been increased across much of the upstate and northeast Georgia through the morning. This also necessitated some downward adjustments to Max temps to the lower 40s across these areas. (And even this may prove to be too warm). Otherwise, surface high pressure will continue to migrate off the mid-Atlantic coast through the period, while cool/dry air circulating around the high will maintain an inverted ridge east of the Appalachians through the period. Meanwhile, falling upper heights responding to a short wave trough digging into the northern Great Plains will result in increasing S/SW flow above this cool/Dry Ridge later today, with a wall of low level moisture expected to advect across the remainder of the forecast area through the afternoon. Isentropic lift/warm advection will be quite strong btw ~925 mb and ~800 mb throughout the period. In fact, forecast soundings depict 800 mb temps warming from near freezing this morning, to +10 c by 12z Monday. While low level lift will be substantial, resulting in a nearly saturated layer up to roughly 800 mb, Thermo profiles dry out completely above about 0 c, which would argue against generation of large hydrometeors. In other words, profiles argue more for a drizzle situation than a -ra scenario, despite the fact that most deterministic models do squeeze out some light quantitative precipitation forecast from this afternoon through the end of the period. That being the case, opted to advertise small chances for measurable precip, while adding "areas of drizzle" to the forecast suite from the Blue Ridge east. The result of the thick low level cloud cover and drizzle/patchy -ra should be establishment of in situ cold air damming this afternoon through tonight, as the surface layer will remain very cool and dry. Forecast highs will be reached in most areas around noon, with temps steady or falling slightly during the afternoon. There is a concern regarding the potential for freezing drizzle or very light rain along the eastern Escarpment of the Blue Ridge, mainly across the northern mtns and foothills from late afternoon into the evening. Surface wet bulbs will likely remain below freezing in those areas through the day, so it's not out of the question that some locations could wet bulb down to sub-freezing if enough light precip develops. However, it's highly questionable as to whether precip will be heavy and/or persistent enough for that to happen. In fact, it's not entirely a given that drizzle/-ra will develop in those areas at all. Although patchy -fzdz was added to the forecast in those areas, we certainly do not have confidence to do anything with this but to add a mention to the severe weather potential statement. Chances for light precip are expected to gradually diminish in most areas this evening, as the strongest isentropic lift shifts northeast of the area. However, -dz will remain possible from the Blue Ridge east within residual wedge air mass. After midnight, shower chances will increase across the western zones, as a weakening cold front approaches from the Tennessee Valley. Temps are expected to warm slowly through the night as The Wedge gradually weakens. 800 mb winds increase to 50-60 kts across the southern Appalachians beginning this evening. However, forecast soundings depict stable air in the valleys, so it's highly unlikely that any of this higher momentum will mix down. However, the exposed peaks and ridge tops above 4kft or so will certainly be very windy, likely gusting in excess of 50 mph at times. This also warrants an severe weather potential statement mention. && Short term /Monday through Wednesday night/... as of 240 am sunday: Monday, the cold front will slowly slide across Kentucky and TN, with a large band of moisture spreading across the southern appalachian region. 850 mb winds ahead of the front are forecast to peak around 50 kts during the early sunlight hours on Monday. Forecast soundings indicate that the pre frontal environment will feature a very strong low level inversion, with temperature differences from the sfc to 2kft ranging close to 10c. The strong inversion should isolate the strong llvl winds from reaching elevations below 2500 ft. I will indicate gusts around 50 kts across the mtn. Ridges early Monday morning. Short range guidance indicates that temps will begin to warm across the high terrain before sunrise Monday. Using a blend of temp guidance, highs on Monday are forecast to range from the low to mid 50s across the mtns to the around 60 across the lakelands. I will time a band of moderate to light rain showers to sweep across the forecast area primarily Monday morning. Coverage will remain the greatest over the Tennessee border counties, then should gradually erode as the band moves into the Lee of the mtns. Monday night, the GFS continues to indicate that the cold front will generally push south of the forecast area by 12z tues. However, the European model (ecmwf) and NAM indicate that the front will stall along the southern tier, then may begin to lift north as a warm front as a frontal wave develops over MS. I will continue to use a blend of the short range solutions, favoring the wetter solutions, holding on to clouds and chance to schc pops. Lows in the upper 30s to mid 40s should support only a chilly light rain. A few pockets of wet snow may mix with the rain over the ridges of the northern NC mtns during the pre dawn hours, no accums. Tuesday, conditions across the County Warning Area will remain unsettled as the old front remains across the central and east Carolinas and another cold front approaches from the west. The path of the frontal wave should remain generally east of the forecast area, possibly south of the I-20 corridor. I will indicate that pops will range from high chance across the lower Piedmont to schc across the I-40 corridor by the afternoon. High temperatures are forecast to range very close to the mild values expected for Monday. Tuesday night into Wednesday, deja vu as the Monday night into Tuesday features. However, the sfc front will likely not slow or stall until reaching the deep south. The path of the frontal wave should occur further south as well, yielding less quantitative precipitation forecast than the Monday night and Tuesday event. Temperatures should range 3-5 degrees cooler, especially over the mtns. Wednesday night, guidance indicates that the core of an Arctic airmass will build over the Great Plains, with sfc pressure rising across the County Warning Area through the night. Winds will remain out of the NW, advecting cold 850 mb temps across the region. Cold air advection should drop 850 mb temps from 2c across the Tennessee border of the NC northern mtns at 0z thurs to -12c by 12z thurs. Gusty northwest winds and limited moisture may support a few weak nwfs showers, but remaining schc. Low temps should easily fall into the teens across the northern NC mtns to mid 20s within the mtn valleys, low to mid 30s should be common along and east of I-85. Dry and clearing east of the mtns. && Long term /Thursday through Saturday/... as of 230 am EST sunday: the medium range picks up Thursday morning with surface high pressure building into the southeast and an upper longwave trough swinging through the eastern Continental U.S.. a few light snow showers over the NE Georgia high terrain will taper off through daybreak Thursday, with the entire area drying out briefly during the day on Thursday and into Thursday night. This is where the interesting section of the forecast commences. A cad event will begin to take shape during the day on Friday, though whether it will be a "classic" onset (ecmwf) or "hybrid/in-situ" event (gfs) remains to be seen. Guidance has been in poor agreement over the last few days on some details related to this system, and the current forecast cycle is no exception. The main discrepancy is the amount of moisture available for overrunning associated with a "miller b" style low ejecting over the Midwest, and when that moisture arrives in our area. The European model (ecmwf) remains the wetter solution with moisture arriving earlier (overnight friday), while the GFS is basically dry for our region until Saturday morning. The European model (ecmwf) solution would therefore present a longer period over which non- liquid precip may fall across the area, while the GFS version would have a much smaller window in which to produce frozen precip before the transition to all rain takes place. This forecast takes The Middle Ground between the two as far as pops/quantitative precipitation forecast through Saturday morning, so it includes a fairly short window of rain/wintry mix before daybreak Saturday before changing to all rain outside of the mountains, while the higher terrain hangs on to frozen precip for a few extra hours before a transition to all liquid occurs by noon on Saturday. Profile and surface temperatures are marginal for most of this event as well, so p-type early Saturday morning is still a mixed bag. The key message remains that this forecast is still quite low-confidence, and that much could change regarding the amount and type of precip Friday night and Saturday morning. The balance of the medium range then becomes surprisingly straightforward, as widespread showers/rain ahead of the main cold front will continue through Saturday night and into Sunday morning. Temperatures will be below average Thursday and Friday (the coolest day), and near average on Saturday as the area comes under the influence of substantial warm air advection. && Aviation /12z Sunday through Thursday/... at kclt and elsewhere: a very moist low level air mass will push into the area from the south this morning into early afternoon, and this is expected to result in initially low VFR cigs overspreading the forecast area, becoming MVFR during early/mid afternoon. Additionally, raw model output is producing precip across much of the area this afternoon into evening. However, based upon vertical profiles, it appears to be more of a drizzle scenario, and -dz will be maintained in the terminal forecasts, as cigs fall through MVFR to the 010-020 range during the evening, with most sites expected to see IFR by the end of the period. Visby may also begin to deteriorate during the evening, although this will depend largely upon the intensity of any dz/-ra. MVFR should suffice for the time being. Winds will remain light, but generally favor a light southeast direction this morning, possibly becoming light NE during the afternoon and evening. Light winds persist within cool surface wedge overnight, but accelerate aloft, and this is expected to result in low level wind shear of 35-40 kts at most terminals by the end of the period. Outlook: restrictions and light precip will remain possible late Sunday night into Monday as a weakening cold front crosses the area. Conditions could remain unsettled through much of the week as the front stalls just to the south of the area and moisture returns. Confidence table... 11-17z 17-23z 23-05z 05-06z kclt high 100% high 100% high 80% Med 66% kgsp high 100% high 100% high 95% high 100% kavl high 100% high 100% Med 75% high 100% khky high 100% high 100% high 80% Med 66% kgmu high 100% high 100% high 100% high 100% kand high 100% Med 65% 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 and ensemble forecasts are available at the following link: Www.Weather.Gov/gsp/aviation && Gsp watches/warnings/advisories... Georgia...none. NC...none. SC...none. && $$ Synopsis...level near term...jdl short term...Ned long term...level aviation...jdl

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