Scientific Forecaster Discussion

Area forecast discussion National Weather Service Greenville-Spartanburg SC 112 am EST Monday Dec 22 2014 Synopsis... a moist air mass will remain across the region before a cold front crosses east on Wednesday. Dry Canadian high pressure will build in Thursday and persist into the weekend. && Near term /through tonight/... as of 1245 am EST...a line of enhanced cold cloud tops on infrared imagery stretching from the MS Gulf Coast through the southern Appalachians to the Delaware-Maryland-Virginia nicely depicts a 140 knots upper jetlet overhead. Weak upper divergence southeast of this feature is slightly enhancing moist upglide over the surface cold air damming layer in place...and light rain and drizzle will result over much of the area this morning until the upper jet axis slides southeast and weakens. Any lingering spotty freezing drizzle along the northern Blue Ridge should be very isolated and brief. At the surface...1028 mb high pressure over eastern New York will continue to ridge southward through the western Carolinas to keep cold air damming in place through the day despite the gradually weakening isentropic lift. This should hold temperatures temperatures in the upper 30s to middle 40s throughout the region today...except for some lower 50s in the SW mountains outside the cad regime. Meanwhile...a deep trough stretching from the northern plains to East Texas will see heights closing off over the central Continental U.S. Through tonight. The main downstream result of this will be resurgent upglide tonight along with slightly deeper moisture. Will allow probability of precipitation to rebound to high chance to likely for mainly light rain...and temperatures will generally remain steady through the night. && Short term /Tuesday through Wednesday/... as of 230 PM EST Sunday...Monday night through Tuesday night...latest 12z models continue to indicate that surface cold air damming wedge will continue across the area as 1030mb high will remain anchored over the NE. Meanwhile...strong south-southwesterly flow aloft develops across the southeast by Tuesday as a deep upper trough moves east across the central Continental U.S. While an upper ridge amplifies over the western Atlantic. This will lead to increasing 290k isentropic lift/Atlantic moisture advection over our County Warning Area through Tuesday. Hence...have carried chance probability of precipitation for light rain with drizzle Monday night into Tuesday morning...ramping up into the categorical probability of precipitation with steady rain towards Tuesday night. With ongoing precipitation within the cad...afternoon temperatures on Tuesday will run 7-10 degrees below normal over the Piedmont with 3-5 degrees below over the western NC mountains Wednesday...the upper trough over the Mississippi Valley will advance eastward...becoming positively tiled as it pushes through the eastern Continental U.S.. this will lift a surface low into the Ohio Valley by 18z...pushing its associated cold front through the forecast area between 18z Wednesday and 00z Thursday. Although there will be strong low level wind shear along and ahead of the front...the best upper forcing will not reach the area until 00z Thursday and areas SBCAPE regime will be shunted to the south. Therefore...high shear Low Cape induced severe event appears unimpressive at this time. Have inherited a chance of afternoon thunder over the western NC Piedmont...portions of the upstate SC...NE Georgia and extreme western NC mountain zones to account for the presence of weak elevated buoyancy. As for quantitative precipitation forecast...models still depict widespread convective activity along the Gulf Coast/southeast coast...which could block deep moisture transport into our area. With the uncertainty...have gone with the model consensus quantitative precipitation forecast...yielding 1-2 inches across the region with up to 3" over the Blue Ridge Escarpment. Categorical probability of precipitation were highlighted through 18z Wednesday... quickly ramping down to the chance range from west to east through 00z Thursday as the front pushes to the east. With increasing warm air advection flow...afternoon temperatures will warm into the u50s/60s east of the mountains and 50s over the mountains && Long term /Wednesday night through Sunday/... as of 200 PM Sunday...the late week will see the sharp upper trough push off the East Coast...leaving quasi-zonal flow over the eastern third of the Continental U.S.. dry high pressure will cross the southeast before heights fall over the Mississippi Valley as next low rolls off The Rockies and across the Great Lakes. Differences between the GFS and ec are minor in terms of sensible weather Christmas day and Friday...both indicating northwest flow precipitation tapering off Christmas morning followed by dry weather most of Friday. In cold advective pattern higher elevations will likely see breezy/gusty conditions on Christmas. The cold front associated with the Great Lakes low will arrive by Friday night albeit without much oomph...the GFS pretty much washing the front out over the Carolinas. The ec however diverges at this point...stalling the front along the coast and eventually developing a coastal low near the Outer Banks Sunday in response to a southern stream shortwave trailing the front. Both the 21/00z and 21/12z runs feature this evolution. So while the GFS would suggest next weekend will be quiet and dry...ec indicates some ptype issues may exist Sunday with wintry partial thicknesses and thermal profiles. Temperatures will be at or a little above climatology from Thu-Sat...and below normal Sunday if the clouds/precipitation materialize. && Aviation /06z Monday through Friday/... at kclt...radar shows light returns moving northward over the airfield this morning...and either very light rain or drizzle should continue in the upglide over the surface cad layer through much of the day. Given lowering IFR ceilings over the midlands of SC...will advertise tempo IFR through daybreak and then fairly solid IFR through most of the day as drizzle and light rain gradually wane from the west. Expect a steady NE flow at less than 10 knots through the period. Drizzle or light rain will return with resurgent upglide by late evening. Elsewhere...upglide moisture over the surface cad layer will yield light rain or drizzle throughout the region through most of the morning hours...before tapering off from the west this afternoon. More solid IFR ceilings will likely develop from S to north...except for rather quick IFR development at kavl in southeast flow early this morning. Some brief recovery to lower end MVFR is possible this afternoon as the drizzle wanes. Expect continued NE flow at less than 10 knots through the period at the foothill sites...except southeast winds at kavl. Drizzle and light rain will return this evening along with IFR ceilings. Outlook...moisture will deepen late tonight through Tuesday night ahead of the next approaching cold front. Low restrictions and widespread rain showers are expected. A cold front will cross the region from the west on Wednesday. Dry high pressure will return through late week. Confidence table... 06-12z 12-18z 18-24z 00-06z kclt high 91% medium 76% high 80% medium 78% kgsp high 97% high 83% high 100% high 83% kavl high 88% medium 79% high 100% high 88% khky high 95% high 89% high 92% high 87% kgmu high 97% medium 78% high 100% high 86% kand high 95% high 80% high 95% high 83% 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: (must be lower case) Www.Weather.Gov/gsp/aviation_tables && Gsp watches/warnings/advisories... Georgia...none. NC...none. SC...none. && $$ Synopsis...sbk near term...hg short term...joh long term...Wimberley aviation...hg

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