Scientific Forecaster Discussion

Area forecast discussion National Weather Service Greenville-Spartanburg SC 430 PM EDT Friday Aug 22 2014 Synopsis... a broad upper level ridge of high pressure will continue to bring very warm temperatures to much of the region into Saturday. A back door cold front will then push into the area from northeast ushering in cooler temperatures starting Sunday. Temperatures will gradually return to normal through next week. && Near term /through Saturday/... as of 425 PM...convection developing much slower than forecast across the area. Likely due to lack of trigger and very high level of free convection values. Have slowed the onset of scattered coverage a couple of hours but left isolated in place given the isolated coverage now developing. With very high dcape values in place over the area...any storms that form will have the potential for strong to even severe downburst winds. That said...with relatively low shear values...any severe storms should remain isolated with little organization. Still expect better coverage later when mesoscale convective system activity approaches...so no changes to later pop. Otherwise...updates for current conditions. As of 225 PM EDT...water vapor imagery shows convection developing all around the ring of fire that stretches from Kansas to Illinois to West Virginia around the strong central Continental U.S. Ridge. Little to no upper level triggering has been present in our forecast area...and it may take a run from an upstream mesoscale convective system segment to produce much coverage. The amplifying upper ridge is cooperating in this regard by turning deep layer steering flow more northwesterly to nearly with time. In addition... decent heating along and south of the Interstate 40 corridor...along with dewpoints persisting in the upper 60s in most locations...has generated surface based cape values of nearly 3000 j/kg to help prime the area. The best consensus of solutions is an mesoscale convective system running southeast from Kentucky to reach our forecast area close to 00z and transiting a portion of the region north to S before dissipating by around 06z. Will thus feature high chance to near likely northern tier probability of precipitation for the evening...and diminish southward...with just lingering western NC probability of precipitation for any additional weak overnight activity in the northwest steering flow. Expect sultry mins in the upper 60s in the mountain valleys to lower 70s east. A backdoor frontal boundary will make steady progress in our direction from the NE on Sat...but with any real thickness falls delayed until after the near term period. Nearly flow and more clouds will allow maximum temperatures to fall a couple of degrees or more despite the continued warm low level thicknesses. Another run of organized convection will be possible Sat afternoon as activity moves southeast along the backdoor front. In addition...precipitable water will pool above 2 inches across the region by Sat afternoon. However...there should be enough nearly steering flow to keep the heavier rain elements moving. Any training of thunderstorms would be the primary Hydro concern Sat afternoon. Lapse rates will actually be steeper in SW sections...but the coverage might be better in the northern tier closer to the forcing and weak mesoscale convective system tracks. Shotgun PM chances probability of precipitation seems reasonable throughout. && Short term /Saturday night through Monday/... as of 1245 PM Friday...the inherited magnitude of the expected cooldown for Sunday remains on track with the expectation of surface ridge building southwestward in the County warning forecast area. Accompanying low level easterly flow continues to Promise unsettled weather...especially across the mountains/fthls where the instability will become increasingly confined to. An influx of drier air...from NE to SW across the region...is prognosticated for Sunday night and precipitation chances will wane accordingly. Deeper layered ridging and dry air atop the County warning forecast area should provide quiet weather conditions with maximum temperatures several categories below climatology on Monday. Closer to the low level ridge periphery...slight chances for diurnal deep convection will be confined to the SW NC/north Georgia mountains && Long term /Monday night through Friday/... as of 230 PM EDT Friday...latest global models continue to indicate that a surface wedge of high underneath a persisting upper ridge of high across the southeast states will keep conditions relatively dry and cool Monday night into Wednesday. By Wednesday night...the upper pattern will flatten out as an upper trough approaches from the central Continental U.S.. the GFS suggests that the trough and an associated cold front will push through the region Thursday afternoon into Friday while the old European model (ecmwf) stalls out the front just to our north. At any rate...increasing moist return flow and marginal buoyancy aloft are supportive of low end chance for convection during this period. Temperatures will stay 3-5 degrees below normal on Tuesday and around normal Wednesday through Friday. && Aviation /21z Friday through Wednesday/... at kclt...cell going up near the Airport had some in-cloud lightning...so did a quick amend for thunderstorms and rain and an aww for cloud-to-ground strikes. Otherwise...still expect only isolated coverage through the afternoon. Any precipitation mention going forward will be associated with decaying mesoscale convective system activity arriving from the northwest this evening. Since thunder is questionable depending on how quickly the system dissipates...will advertise only vcsh at this point. Otherwise...expect northwest winds this afternoon to toggle west-southwest with weak Lee troughing through the evening hours...returning to northwest or nearly overnight into Sat. Mainly scattered cumulus are expected early...but with moisture increasing in nearly flow to permit VFR ceilings to develop tonight and persist Sat with an approaching backdoor cold front. Elsewhere...similar to kclt...will have to watch the isolated thunderstorms and rain this afternoon for any amend. Then...all eyes will be on convective activity developing from Tennessee to eastern Kentucky to WV to see what can run into or near the terminal forecast area this evening. It still appears the best chance of thunderstorms in the vicinity will range from as early as 22z in the far northern tier to perhaps as late as 06z in far southern sections. Expect mainly northwest flow in western NC...turning more nearly with time. Winds may briefly toggle west-southwest or SW in the upstate with weak Lee troughing...but with a return toward nearly overnight into Sat. Moisture will increase in the nearly flow through the morning on Sat...with VFR ceilings likely as a backdoor cold front approaches. Any fog at kavl at daybreak should be MVFR at worst. Outlook...a backdoor cold front will sag into the region through the weekend...with chances of restrictions increasing at khky and kavl through Sunday. A more diurnal pattern of early morning low clouds/fog and isolated afternoon thunderstorms may return early next week...with steady drying. Confidence table... 20-02z 02-08z 08-14z 14-20z 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 100% khky high 100% high 100% high 95% 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...joh near term...hg/rwh short term...csh long term...joh aviation...hg/rwh

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