Scientific Forecaster Discussion

Area forecast discussion National Weather Service Greenville-Spartanburg SC 1102 am EDT Thursday Sep 3 2015 Synopsis... a weak pressure pattern will linger atop the region for the rest of the work week. Late Summer warmth and daily thunderstorm chances will be the main features of interest. A back door cold front is expected to push across the region on Friday followed by a cool and moist area of high pressure this weekend. && Near term /through tonight/... as of 1030 am...not much happening across the forecast area. Widespread middle/high cloudiness was temporarily keeping deep convection initiation in check. Will make some tweaks to the precipitation chances accordingly. The main item of interest is the area of instability that has appeared in the Savannah River valley. The Storm Prediction Center mesoanalysis shows a small area of 3000+ surface based cape...due mainly to a small zone where surface dewpoints are in the upper 70s according to observation. It could be that the observations are in error...but this seems unlikely as three of them are near each other (keba/kiiy/khqu). That raises the question about whether the precipitation chance needs to be raised across the lakelands. On the one hand...water vapor imagery shows a short wave that will pass just S...which could provide some forcing. On the other hand...dewpoints are expected to mix out this afternoon...perhaps more so in that area...which would cut back on the instability later in the day. Will keep a close eye for any sort of clearing that would permit convective initiation. Temperatures were bumped up a bit over the northwest Piedmont to agree with neighbors. Previous discussion... As of 650 am EDT...water vapor imagery shows an upper level shortwave poised to move east across the base of the southern Appalachians this morning. High level cloudiness ahead of the wave continues to keep most morning fog and low cloud problems at Bay over the area. The upper feature may bring slightly enhanced upward vertical velocities through the southern tier of the forecast area today...with the best deep layer q vector convergence likely crossing the region during the morning hours. This timing will not favor deep convective development...but 850 to 500 mb lapse rates will steepen through the day...and southern tier instability may linger through the morning hours where higher surface dewpoints have pooled. In addition to this southern tier forcing...another round of isolated to scattered ridge top shower/thunderstorms and rain development is expected with peak heating. Any activity that develops in the marginally unstable profiles may drift off the Blue Ridge into the adjacent foothills in northwest flow aloft later this afternoon or early evening...but dewpoint mixing should limit rain showers/thunderstorms and rain potential farther out into the Piedmont. Despite middle and high clouds inhibiting heating somewhat today...plenty of lower 90s temperatures are still expected east of the mountains this afternoon. Any lingering shower activity should generally wane tonight with the loss of instability and the persistent north to NE flow from surface high pressure to the north. Additional 500 mb shortwave energy will drop southeast toward the southern Appalachians overnight...but with limited moisture. Will confine any overnight slight chances of precipitation to the northern mountains. && Short term /Friday through Saturday/... as of 245 am Thursday...the 500 mb ridge will build across the Great Lakes region with a broad trough across the southeast Continental U.S.. the center of a 1025+mb high is forecast to develop across the New England states...ridging southwest across the middle and southern Atlantic states. A back door cold front is expected to reach the Piedmont and foothills during the daylight hours. The combination of widespread weak instability and a advancing surface boundary should trigger scattered rain showers and thunderstorms and rain...especially near the east facing slopes. I will forecast probability of precipitation to range from near 40 to 50 percent near the east facing slopes to around 30 percent east. Using a blend of preferred guidance...I will indicate highs ranging from the middle 80s within the mountain valleys to low 90s east. The GFS shows a elongated 500 mb 500 mb vorticity sliding over the region Friday night. The combination of lingering instability and the passage of the middle level disturbance should yield scattered rain showers and thunderstorms and rain overnight. On Saturday...GFS indicates that Piedmont winds will strengthen to marginally gusty NE winds after sunrise Saturday. The NE winds...broken sky cover...scattered to num shra/tsra...should provide cooler temperatures on Saturday. The 2m temperatures from the European model (ecmwf) and GFS have high temperatures in the low 80s on Sat. I will maintain cool high for Sat...generally 1 to 2 degrees warmer than the raw model values. && Long term /Saturday night through Wednesday/... as of 3 am Thursday...on Sunday...NE winds appear as strong as Sat...but cloud cover appears thinner. High temperatures will remain below normal...but likely 1 to 2 degrees warmer than Sat. Labor Day...the surface high center shifts over the western Atlantic...resulting in weaker winds across the region. I will limit chance probability of precipitation to the mountains...foothills....and upper Savannah River valley. Highs temperatures may range within a couple of degrees of normal on Monday. On Tuesday...middle level ridge will build across the deep south over the southeast Continental U.S....with a broad surface high across the southern appalachian region. Forecast soundings indicate weak lapse rates within the middle levels...yielding weak values of instability. I will indicate diurnal schc to chance probability of precipitation...with coverage favoring the mountains highs should range very close to normal. On Wednesday...the 500 mb ridge is expected to strengthen across the southeast...resulting in mild middle level temperatures across the County Warning Area. High temperatures will likely range within a degree or two of normal. However...cape values may remain less than 1000 j/kg due to the warm temperatures aloft. I will forecast diurnal schc to chance probability of precipitation...coverage favored over the mountains && Aviation /15z Thursday through Monday/... at kclt and elsewhere...middle and high level moisture just ahead of an approaching shortwave will provide occasional altocumulus and cirrus ceilings through the day...but with lower level convective cumulus slow to build given the anticipated dewpoint mixing. The best precipitation chances during the taf period will be associated with the upper level shortwave crossing the southern tier of the terminal forecast area...mainly south of kand...and with scattered ridge top development over the higher terrain. Confidence is too low to feature any rain showers/thunderstorms and rain at the terminals yet. Expect mainly light northwest to NE winds throughout during the period...possibly toggling west near kand this afternoon and evening. Higher level clouds could get reinforced overnight as another shortwaves drops south toward the southern Appalachians. Outlook...a pattern of diurnal isolated to scattered showers and thunderstorms is expected to continue through Friday. By the weekend...a moist easterly flow should develop...bringing better chances for showers and restrictions. Low clouds and fog will be possible each night in the mountain valleys. Confidence table... 15-21z 21-03z 03-09z 09-12z kclt high 100% high 100% high 100% high 100% kgsp high 100% high 100% high 100% high 100% kavl high 100% high 100% high 100% high 94% khky high 100% high 100% high 100% high 100% kgmu high 100% high 100% high 100% high 100% kand high 100% 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 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...Ned near term...hg/PM short term...Ned long term...Ned aviation...hg

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