Scientific Forecaster Discussion

Area forecast discussion National Weather Service Greenville-Spartanburg SC 921 am EDT Wednesday Apr 1 2015 Synopsis... surface ridging will prevail on Wednesday ahead of increasing precipitation chances on Thursday as moist southerly flow returns. Another cold front will push through northeast Georgia and the western Carolinas on Friday evening ahead of drier high pressure for the weekend. Moisture will return for the start of the next work weak as southerly flow once again commences over the region therefore increasing precipitation chances. && Near term /through tonight/... as of 915 am...made some adjustments to temperature trends. Otherwise...the current forecast looks on target. As of 630 am...after a near miss from the northern extent of the decaying precipitation associated with the convection moving across Georgia...we should remain dry this morning. Still a very small chance of a shower developing over the lakelands/Savannah River basin with the front drifting S across that region...but it will not be included in the forecast. Temperatures continue to run warmer than forecast...giving US a bit of a milder start to the day. Over the next 24 hours...a progressive upper pattern will see the upper trough axis move eastward out over the western Atlantic...and an upper ridge building eastward over the Appalachians/western Carolinas. Weak high pressure will build in from the middle-Atlantic region behind the front this morning. High temperatures should be on the order of five degrees cooler than yesterday with the subtle air mass change. The upper ridge axis will move east across the region late tonight and that should push the surface high offshore...setting the stage for moisture return after midnight. A dampening short wave riding up the ridge will approach from the west after 06z Thursday and will bring some Gulf moisture with it...while the flow around the departed surface high will allow some Atlantic moisture to return as well. Prefer the slightly less enthusiastic precipitation development of the GFS near the Blue Ridge west of I-26 around daybreak Thursday...as it seems the model guidance develops the upslope precipitation a bit too fast. Will cut down the precipitation chances a bit. Min temperatures should be at or above normal. && Short term /Thursday through Friday night/... as of 300 am EDT Wednesday...the short term forecast period initializes Thursday morning amidst short wave ridging aloft over the southeast Atlantic coast...while region of height falls approaches from the west. At the surface...high pressure will be in control of the pattern across northeast Georgia and the western Carolinas...however quickly retreating out to sea as a warm front approaches from the southwest driven by potent southern stream upper wave and associated surface cyclone over the Central Plains. A northern stream upper wave will be sliding through the northern Great Lakes with a surface trough/front stretched to the south along the Mississippi River valley. Models continue to agree on improving warm air advection regime with dewpoints surging across the southern Apps through the day on Thursday. Therefore the forecast features increasing probability of precipitation through morning across northeast Georgia...southwest NC...and the western upstate as upslope/upglide forced precipitation prevails early on ahead of maximum heating/instability. Probability of precipitation will then spread out further east as model soundings indicate modest/improving lapse rates aloft as the upper ridge shifts east...with steepening llv lapse rates with heating and moisture influx. Soundings also indicate some directional sheer in the profile Thursday afternoon/evening across these zones with approx 30km bulk shear present...however speed shear looks to be lacking as no primary low level jet/mlj is present. All said...would not be surprised at all to see a few deep/organized thunderstorms with hail/damaging winds being the primary threats as fz levels remain below 10kft. Beyond that...southerly upslope flow will veer southwesterly somewhat and weaken into the overnight hours. This combined with heating loss warrants lowered probability of precipitation due to lack of any forcing in the return. On Friday southerly flow will continue...however according to model guidance...llv dry air from remnant 850 mb ridge over the Gulf will be entrained in the mean flow thus advecting a region of lower dewpoints northward. This drier air at the surface on Friday would be a substantial limiting factor for any convection ahead of the frontal passage Friday night. Thus...probability of precipitation actually lower to slight chance levels across the NC/SC Piedmont as soundings look rather unimpressive. Models continued to favor at least high end chance probability of precipitation over the high terrain thus that was accepted in the forecast. Meanwhile...a line of convection just ahead of the front will likely be marching east through the Tennessee Valley...eventually arriving into the western NC and northeast Georgia mountains by the 00z timeframe. Expecting this line to move through the region overnight...therefore likely probability of precipitation are feature over most of the forecast area with thunder mentioned. Due to the above mentioned stable airmass over the southern Apps...would not be surprised to see any strong/severe convection along this line to weaken as it enters the forecast area. By periods end Saturday morning...the primary upper trough axis will swing through thereby motivating the front along or just east of the Interstate 77 corridor. && Long term /Saturday through Tuesday/... as of 345 am EDT Wednesday...the medium range forecast period initializes Saturday morning amidst a departing cold front and intruding high pressure beneath quasi zonal flow aloft. Mentionable probability of precipitation will be in the forecast initially...however will lower quickly to non mentionable levels by middle afternoon. Skies will clear out as the high builds in thus allowing for abundant sunshine amongst falling thicknesses. Temperatures on Saturday will be a few degrees under climatology...with slightly cooler temperatures prevailing on Sunday where frost conditions are possible over the mountains during the early morning hours. The surface ridge will migrate east into Monday allowing for the return of southerly warm air advection beneath continued zonal flow aloft. Modest baroclinic zone across the arklatex...east into the midsouth will build ahead of another potent cold front. Increasing moisture over the southern Apps Monday into Tuesday will yield reintroduction of chance probability of precipitation by Monday evening as upglide maxima region advects overhead along the western periphery of the departing surface ridge. Probability of precipitation will remain at low end chance levels on Tuesday as improving warm air advection regime yields dewpoints in the upper 50s to lower 60s thus providing ample moisture for convection. Temperatures on Monday will return to normal levels...with warmer temperatures forecast on Tuesday. && Aviation /14z Wednesday through Sunday/... at kclt...VFR through the period. Some thicker cirrus/convective debris cloudiness will move overhead through middle morning. North wind early this morning courtesy of high pressure over the middle Atlantic region will come around more to the NE through midday...then eventually shift to southeast late in the afternoon/early evening as the high center moves offshore. Went with a wind shift at 22z...when some scattered low clouds were introduced. Improving moist upslope will bring more low clouds into the region...but at this time it appears any restrictions will be closer to the Blue Ridge. Elsewhere...a weak front will slip S across upstate SC this morning...which should bring the wind direction around to north by middle morning...then NE during the afternoon. Conditions should remain VFR through the evening hours. Similar to kclt...the wind should come around to southeast during the early evening hours. Moisture return from the Gulf/Atlantic will help the development of low clouds near the Blue Ridge by the early morning hours on Thursday...perhaps near kand by late in the taf period. Think any light precipitation will not restrict visibility and will be confined to the Blue Ridge through 12z Thursday. Outlook...an approaching warm front and resultant moisture return will bring better chances for precipitation/restrictions across the area beginning on Thursday morning. Expect low clouds to develop with a southeast upslope flow Thursday night and Friday morning. A cold front will affect the area Friday into early Saturday...bringing another round of rain/possible thunderstorms and restrictions. VFR will return for the rest of the weekend. Confidence table... 13-19z 19-01z 01-07z 07-12z 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 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 89% 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...cdg near term...lg/PM short term...cdg long term...cdg aviation...PM

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