Scientific Forecaster Discussion

Area forecast discussion National Weather Service Greenville-Spartanburg SC 607 am EDT Tuesday Jul 29 2014 Synopsis... Canadian high pressure over the area will dominate the surface pattern through midweek...leading to well below normal temperatures more typical of late Summer or early autumn. The surface high will weaken and moisture will return for the weekend...but temperatures will remain below normal. && Near term /through tonight/... as of 530 am EDT Tuesday...rather nice morning across the region as most locations are reporting clear skies and temperatures in the upper 50s to lower 60s. Removed what little patchy fog that was in the forecast for the mountain valleys as even the little Tennessee River valley remains fog free. Dewpoint depressions across these regions remain in the 1-3 degree mark with light/vrb winds. Latest satellite difference product indicates areas of low stratus along the Tennessee border with scattered middle level stratus across the NC/SC Piedmont regions this morning. Tweaked temperatures and dewpoints in the extreme near term to reflect latest surface observations and left the remainder of the forecast as is. Previous discussion... As of 300 am EDT Tuesday...rather pleasant near term forecast ahead for northeast Georgia...upstate SC...and western NC. Broad upper eastern Continental U.S. Trough will remain nearly stationary through the entire period leading to no change in the overall pattern. At the surface...high pressure continues to dive southward across the northern plains and into the middle Mississippi River valley. Cp airmass will continue to spread across the southern/central Apps leading to below normal temperatures and drier conditions. Dewpoints across the region have already fallen into the upper 50s for most locations with even more drying in store this afternoon. That said...forecast soundings indicate a thin layer of moisture trapped beneath low/middle level inversion which will likely saturate this afternoon amongst heating. Therefore the forecast will feature partly cloudy skies as fair weather cumulus prevails. Lastly...northerly winds will be weaker today vs yesterday as pressure gradient has relaxed...and low level winds remain weak thereby negating mixing effects this afternoon. As mentioned above...temperatures will be rather comfortable today with highs nearly a degree of magnitude under normal levels. Likewise...overnight lows will fall into Lower/Middle 60s across the low terrain while some of the higher peaks and ridgetops experience lows in the upper 40s to lower 50s. && Short term /Wednesday through Thursday night/... as of 305 am...the weather will continue to be dominated by a deep upper trough over the eastern Continental U.S.. a series of weak short waves move through the trough taking the trough axis east of the area Wednesday...with the axis rebuilding to the west on Thursday. At the surface...weak high pressure builds into the area Wednesday and remains in place Thursday. Also on Thursday...a frontal system south and east of the Gulf and Atlantic coasts moves north toward the coast. Although warm middle levels will keep atmos generally capped...there will be enough low level moisture and weak instability for isolated diurnal rain showers to develop over the southwestern NC mountains low level S to southeast flow increases Thursday into Thursday night as middle level flow becomes southwesterly and gradient increases between the surface high and approaching frontal system. This brings increasing low level moisture. Deep moisture increases as well in the developing southwesterly middle level flow. Upper divergence develops as the right entrance region of an upper jet sets up over the southeastern Continental U.S.. middle level cap remains in place...but weakens allowing weak instability to develop. All this will lead to an increased chance of scattered rain showers Thursday into Thursday night. Best chance for isolated thunderstorms and rain will be across the mountains where better instability develops. Quantitative precipitation forecast will generally be light. Highs will be around 10 degrees below normal with low thicknesses...increasing clouds...developing wedge like surface pattern and potentially light precipitation. Lows will be around 5 degrees below normal. && Long term /Friday through Monday/... as of 320 am...guidance shows generally good agreement early on during the medium range. Upper trough axis remains to our west Friday and Sat as weak short waves move across the area in the southwesterly flow. However...this changes by the end as the GFS keeps the trough axis west of the area sun...then moves it across the area Monday. The European model (ecmwf) moves it across on sun with the sub-tropical ridge building west into the southeastern Continental U.S. Monday. At the surface...high pressure remains over the area in a wedge like pattern Friday and Sat as the frontal system remains near the coast. This keeps the moist S to southeast low level flow across the area...along with the chance of precipitation. Although the guidance disagrees on the upper pattern...the surface pattern is similar. The high pressure weakens and frontal system weakens. However...the moist low level southerly flow remains in place...keeping the chance of precipitation in place as well. Highs remain around 10 degrees below normal...but show a slight warming trend through the period. Lows around 5 degrees below normal slowly warm to near normal by the end of the period. && Aviation /10z Tuesday through Saturday/... at kclt and elsewhere...VFR conditions will prevail through the taf cycle. High pressure and downsloping flow will dominate through the period leading to weather free conditions. Moisture trapped beneath low/middle level inversion will likely condense by late morning into the afternoon hours across the region leading to few/scattered low VFR fair weather cumulus. Winds will remain in the 5-8kts range and northerly through the period. Outlook...dry high pressure prevails through Wednesday with the only possible restrictions being potential early morning fog/stratus in the mountain valleys. Increasing moisture on Thursday and into Friday will yield enhanced precipitation/fog/stratus chances into the weekend. Confidence table... 10-16z 16-22z 22-04z 04-10z kclt high 100% high 100% high 100% high 100% kgsp high 100% high 100% high 100% high 100% kavl high 89% high 100% high 100% high 100% 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_tables && Gsp watches/warnings/advisories... Georgia...none. NC...none. SC...none. && $$ Synopsis...rwh near term...cdg short term...rwh long term...rwh aviation...cdg

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