Scientific Forecaster Discussion

Area forecast discussion National Weather Service Greenville-Spartanburg SC 1009 am EST sun Nov 29 2015 Synopsis... high pressure will weaken and move east before a frontal boundary sinks southward into the area late today. A cool and wet air mass will settle in behind this front for Monday and Tuesday. A second cold front is likely to bring additional rain Wednesday...before dry high pressure arrives Wednesday night. && Near term /through tonight/... as of 1000 am EST...the main changes for the morning update were to sharpen up the west to east pop gradient the rest of the day...with light rain falling from higher cloud bases reaching the ground in many mountain locations...but with mostly virga still east of the high terrain. Temperatures appear on track with a warm start but with increasingly cloudy skies limiting insolation. Otherwies...the longwave trough over the western Continental U.S. Will move eastward and nudge a frontal zone with precipitation ahead of it into the gsp County Warning Area through tonight. Higher rain chances will spread eastward Sunday night. Some boundary layer upslope into the mountains will place the heaviest precipitation into the western North Carolina mountains tonight. Midlevel ridging over the area ahead of the long wave trough will produce extensive cloudiness but keep temperatures above normal and with a modest diurnal range. As system develops...surface high pressure strengthens over the northeast and begins to create a wedge of high pressure up against the mountains by Monday morning which will maintain and high relative humidity conditions. && Short term /Monday through Wednesday/... as of 300 am Sunday...cad wedge will strengthen Monday as continued precipitation allows diabatic cooling. Quantitative precipitation forecast rates appear likely to peak during the day as the layer of moist isentropic lift reaches maximum depth. Also a distinct pocket of enhanced upper forcing does move over the area around this the diffluent flow downstream of a High Plains closed upper low. That feature wobbles its way into the upper Mississippi Valley Monday afternoon and night...and surface low wraps up ahead of it. For our County warning forecast area the effect will be to induce a lull in precipitation rates Tuesday morning as the deep baroclinic zone essentially shifts northward. However some upglide will continue particularly in the very low The Wedge looks to remain entrenched through Tuesday. Erosion will eventually occur Wednesday with the cold frontal passage. Forcing is pretty robust Monday. Precipitable waters are prognosticated to peak around 1.25 inches in our area...these being over 2 Standard dev above climatology. Deep saturation however appears to preclude convection...and wind profiles are such that precipitation elements will likely remain moving along. However quantitative precipitation forecast rates do not currently appear likely to cause flash flooding. The second prong of the event will be the frontal passage Wednesday. Appreciable q-vector convergence is noted over our County warning forecast area due to upper divergence associated with SW-NE oriented jet streak crossing the upper south. Once again the wind orientation largely perpendicular to the boundary /and lack of a good low level jet ahead of it/ are limiting factors for heavy quantitative precipitation forecast. For now we will advertise generally 2 to 3 inch storm totals over the mountains...and 1 to 2 inches Piedmont. Any Hydro issues are more likely to result from the duration of rainfall. Our office and our northern/western neighbors have collaborated with wpc regarding our inclusion in the slight risk area on the day 2 and 3 excessive rainfall outlook products. We came to agreement that a few rivers or poorly drained basins do have a chance of experiencing the risk area was expanded east of the mountains both days. We will also add heavy rain wording in the severe weather potential statement this morning. The Wedge will hold maximum temperatures in the 50s Monday and Tuesday over most of the area /with maximum Monday likely being in the morning/. The mountains will begin to see the cooling effect of the front Wednesday...but the Piedmont will warm into the lower 60s once again. && Long term /Wednesday night through Saturday/... as of early Sunday morning...cold front will clear our area by the end of the day Wednesday allowing dry high pressure to build back in for the latter part of the week. Upper pattern remains active across the Continental U.S....with a series of shortwaves traversing the northern tier as well as one crossing the Southern Plains Thursday. Some previous model runs have shown this wave interacting with the front in vicinity of the Florida Panhandle...developing a surface wave and spreading precipitation northward into the Carolina Piedmont Friday. However...while the latest GFS and ec continue to suggest some interaction they have trended weaker with their depicted surface features and keep precipitation off to our south and east. Current runs lend confidence to a forecast for continued dry weather through Saturday. Temperatures should be seasonably cool and near normal. && Aviation /15z Sunday through Thursday/... at kclt and elsewhere...with the frontal system approaching from the west...rain and reduced ceilings and visibility will reach all taf sites by tonight. Rain will begin over the mountains of western North Carolina today and spread southeastward across the area the rest of the period. Cloud cover currently around fl100 to 120 will continue to deepen with ceilings lowering to around fl070 this afternoon and down to MVFR or lower this evening. As rainfall increases tonight... visibility will also decline to MVFR or lower. The front and surface pressure troughs are poorly defined east of the mountains...and surface winds will be light veering from southwesterly through this afternoon to northerly tonight. Outlook...moisture will increase across the area tonight into Monday as a backdoor cold front settles southward into the region. Unsettled conditions will continue Tuesday before a drying cold front crosses the area on Wednesday. Confidence table... 15-21z 21-03z 03-09z 09-12z kclt high 100% high 100% medium 78% high 95% kgsp high 100% high 100% low 48% high 84% kavl high 100% high 90% low 50% medium 77% khky high 100% high 100% medium 75% high 100% kgmu high 100% high 100% low 59% high 84% kand high 100% high 100% low 51% 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 and ensemble forecasts are available at the following link: (must be lower case) Www.Weather.Gov/gsp/aviation && Gsp watches/warnings/advisories... Georgia...none. NC...none. SC...none. && $$ Synopsis...Wimberley near term...hg/wjm short term...Wimberley long term...Wimberley aviation...hg/wjm

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