Area forecast discussion National Weather Service Greenville-Spartanburg SC 350 am EDT Tuesday may 21 2013 Synopsis... an approaching upper level trough will continue to influence the region for most of the week. Southerly flow will keep the area in a moist and generally unsettled airmass with storm chances each afternoon. Rain chances diminish after a cold front crosses through by the end of the week. && Near term /through tonight/... at 300 am EST Tuesday...a weak but persistent upper low will drift northeast and away from our area today and tonight...while an upper ridge axis crosses the southern Appalachians...and an upper trough moves from the plains to the MS River Valley. Model time heights show lingering moisture in the wake of the departing trough...but with diminished upper forcing...precipitation will be driven by diurnal heating. Instability in excess of 1000 j/kg will support thunderstorm formation this afternoon if a weak low level cap can be overcome. Dry air aloft and beneath cloud bases will support evaporative cooling and downdraft production. Shear will decrease through the day...limiting storm organization. A weakening steering flow will mean slow moving thunderstorms may produce locally heavy rainfall. Convection will linger into the evening before the low levels stabilize. Temperatures will run several categories above normal under the upper ridge. && Short term /Wednesday through Thursday night/... as of 245 am Tuesday...short waves rotating around an upper trough move into the area Wednesday and Wednesday night. The upper trough then sharpens and moves into the area Thursday night. At the surface...a weak ridge extending from the Atlantic is over the area Wednesday morning. The ridge weakens as a cold front approaches from the west. This will allow southerly flow to develop bringing moisture back into the area. Instability develops...but remains weak. However...forcing will increase as the short waves move in. Best precipitation chance remains over the mountains and diminishes south of I-85. Given the forcing and weak but increasing shear...isolated severe storms cannot be ruled out. As the short waves move east and atmos stabilizes overnight...precipitation chance diminishes with chance pop mountains and slight chance elsewhere. Cold front jumps into the developing Lee trough on Thursday. This brings best forcing to the northern and eastern County warning forecast area...where highest pop will be forecast. Precipitation chance retreats to the mountains overnight with lingering moisture and northwesterly upslope flow. Highs will be a little above normal each day. Lows around 5 degrees above normal Wednesday night drop to near normal Thursday night in the cooler air mass moving in behind the front. && Long term /Friday through Monday/... as of 345 am Tuesday...the County warning forecast area remains in northwesterly flow aloft as a trough slowly moves out over the Atlantic and an upper ridge moves east across the central Continental U.S.. short wave energy then moves across the area by early next week as the flow becomes more zonal. At the surface...dry high pressure settles in from the northwest for Friday and Sat. A weak warm front develops to our west on sun and moves toward the area through Monday. The GFS and European model (ecmwf) disagree on the amount of moisture and lift that develop as the front moves east. However...they do agree that precipitation may develop for Sun night and Monday. Have slight chance pop for now. However...the bigger story is that the air mass moving in with the high is quite cool. In fact...highs will be around 5 degrees below normal Friday and Sat...rising to a couple of degrees below normal sun and Monday. Lows will be around 5 degrees below normal Friday and Sat nites...rising to near normal Sun night. && Aviation /08z Tuesday through Saturday/... at kclt...no precipitation will be carried overnight as heating has abated and the atmosphere has stabilized. A blend of model guidance supports only low VFR visibility in fog toward dawn...despite moist low levels. Confidence on ceilings is quite low due to model variance. Clouds heights will generally be based on current observations... with a daybreak dip to MVFR...which is supported by the middle of the Road met guidance. Instability will support robust convection on Tuesday...mainly in the afternoon...but coverage will be rather limited as upper level support drifts to the east. Light south winds will persist. Elsewhere...radar trends support leaving precipitation out of the tafs overnight in a stabilizing atmosphere. A guidance blend supports a daybreak IFR ceiling at kavl...low VFR at kgsp...and MVFR at all other sites. Confidence is low on ceilings...especially at daybreak...as model guidance varies considerably...and there is still some doubt cloud coverage will increase enough by dawn in the wake of a departing upper system. MVFR ceilings will be carried toward daybreak at kgmu...kgsp and khky...which is supported by less extreme met guidance. Although convective coverage will be limited Tuesday as upper forcing departs to the east...instability will support vigorous updrafts. Light south winds are expected. Outlook... ceiling and visibility restrictions in scattered showers and thunderstorms again Wednesday and Thursday aftn/eve. Generally VFR weather Friday and Saturday. Confidence table... 08-14z 14-20z 20-02z 02-08z kclt low 57% high 90% high 100% high 100% kgsp medium 64% high 100% high 100% high 100% kavl low 51% low 59% low 58% medium 62% khky medium 74% high 100% high 100% high 100% kgmu medium 77% high 97% high 100% high 100% kand medium 74% high 97% 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...jat short term...rwh long term...rwh aviation...jat