Scientific Forecaster Discussion

Area forecast discussion National Weather Service Greenville-Spartanburg SC 231 PM EDT Monday Sep 22 2014 Synopsis... cool and dry high pressure will remain over the region through middle week. Moisture and rain chances increase as we go into the weekend. && Near term /through Tuesday/... as of 230 PM EDT Monday...this afternoon and evening...areas of showers and thunderstorms were developing along and in the vicinity of the surface boundary to our south this afternoon. A deep moisture axis just to our south will continue to bring considerable low-middle level cloudiness to the southern half of the County Warning Area through this evening. Conditions should remain mostly dry for most locations as a Canadian high pressure continues to build in from the N/NE. The exception will be over the extreme southern zones of the forecast area where slight to low end chances for showers were warranted at least through late this afternoon. Tonight and tomorrow...latest near term models continue to indicate that a potent upper shortwave energy will round the base of the eastern Continental U.S. Trough...closing off as an upper low as it moves across the region tonight. This upper feature is expected to slowly pull NE away from the region tomorrow. Despite this upper feature...conditions should remain dry as a cool Canadian high pressure builds in from the NE and the best upper forcing remains south and east of the County Warning Area. With strong cold air advection in north/NE flow (h85 temperatures of 10c-12c will push south into the upstate sc) temperatures...overnight lows will plunge to the upper 30s/low 40s over the higher terrain and 50s over the Piedmont. Afternoon highs for tomorrow will remain 5-10 degrees below normal given persistent cool NE winds. && Short term /Tuesday night through Thursday/... a closed upper low over the southeast U.S. Will gradually fill through the short term leaving a weak upper trough over the area by Thursday. At the surface...high pressure intially over New England will be displaced initially to the east by a weak low moving northward from off the Carolina coast. By Thursday...the high is forecast to re-develop to the west over the eastern Great Lakes and ridge southward again along the east slopes of the Appalachians. The net result initially will be an increase in deep layer moisture across the the western Carolinas and NE Georgia on Wednesday into Wednesday night thanks to some isentropic upglide as the coastal low develops to the east. Hence...cloud cover will increase during Wednesday. As we get into Wednesday night...the models show some light quantitative precipitation forecast response mainly east of I-26. Hence..will bump probability of precipitation upward into the chance category anticipating some patchy light showers. The showers should tend to wane on Thursday as the coastal low moves NE. However...model time-heights indicate that cloud cover will be slow to dissipate. Temperatures will remain below climatology through the period. && Long term /Thursday night through Monday/... as of 2 PM Monday...the medium range forecast picks up at 00z on Friday with broad and steep upper ridging persisting across most of the Continental U.S.. the ridge is expected to begin breaking down over the weekend as some sort of complex...split upper flow pattern develops over the central Continental U.S.. because the polar jet will likely remain well to the north...the upper flow will remain weak over much of the central and eastern Continental U.S.. this makes it difficult to predict exactly how the upper pattern will evolve as the ridge breaks down on days 6 and 7. The older 00z run of the European model (ecmwf) closes off an 500 mb low over Arkansas by early Monday and moves that low to our doorstep by early Tuesday. The newer 12z GFS just maintains some degree of upper swv energy over the same region and does not form a closed low. At the surface...high pressure will be centered to our north with cool northeasterly low level flow over the forecast area. Over the next few days the latest guidance keeps the high nearly stationary...except for the European model (ecmwf)...which moves it slightly westward. The European model (ecmwf) still remains more bullish with respect to deeper layer moisture persisting over the County warning forecast area from the south and east on Friday and Sat while the latest 12z GFS...and CMC...continue to keep the higher relative humidity values just to our south through at least Sat. By sun...the models try to spin up some sort of Gulf of Mexico low which spreads deeper moisture over most of the County warning forecast area by late sun. Monday appears to have the best chance for any widespread precipitation with deeper moisture over most of the region and some degree of upper level support just to our west. As for the sensible forecast...I kept probability of precipitation at just a slight chance or below on Friday and Sat with values increasing to solid chance over the most of the County warning forecast area by late sun and higher end solid chance by Monday afternoon. Temperatures were largely unchanged from the previous forecast with the coolest values expected at the beginning of the period on Friday with some minor warming going into the weekend. && Aviation /19z Monday through Saturday/... at kclt...VFR conditions will persist through the taf period as a Canadian high pressure continues to build in from the N/NE. NE winds will continue tonight and into tomorrow with some gusts to around 15kt possible after sunrise on Tuesday due to tighter surface pressure gradient. A line of convection to our south will spread considerable middle/high clouds to the terminal this afternoon into tonight. Elsewhere...dry conditions will continue over much of the region through the taf period as a Canadian high pressure continues to build in from the N/NE. A line of convection to our south will bring considerable middle/high cloudiness to the region through this evening. Latest guidance suggests VFR conditions mainly east of the mountains with IFR to MVFR visby/ceiling stetting up over the mountains/valleys toward daybreak on Tuesday. Hence...have mentioned tempo for IFR/ceilings at kavl between 9z and 12z Tuesday. Otherwise...expect NE winds to continue with occasional wind gusts to around 10-15kt possible across the region after sunrise Tuesday as a tight surface pressure gradient sets up. Outlook...dry high pressure will build over the region through the week...with easterly flow moisture possibly returning from the coast this weekend. Confidence table... 18-24z 00-06z 06-12z 12-18z kclt high 100% high 100% high 100% high 100% kgsp high 100% high 100% high 100% high 100% kavl high 100% high 100% high 83% high 91% 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...jpt near term...joh short term...lg long term...jpt aviation...joh

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