Scientific Forecaster Discussion

Area forecast discussion National Weather Service Greenville-Spartanburg SC 1215 am EST Sat Feb 28 2015 Synopsis... cold Canadian high pressure will remain across the area through the weekend. The atmosphere will moisten considerably ahead of an approaching cold front that is expected to cross the area on Monday. Another high will slide over the region in the fronts wake. The high will be short lived...however...as a more substantial cold front crosses the region by late Wednesday or early Thursday. && Near term /through today/... 0515 UTC update...winds were updated with a blend of the latest NAM and adjmav. Sky cover was updated from satllite imagery. 930 PM update...clouds have yet to form over the County warning forecast area or even immediately upstream within the building wedge. Newest guidance /00z NAM/ still shows development of a low cloud layer after midnight...so though skies have been kept clearer this evening...an increasing trend later still is appropriate. Drier air is filtering into the Piedmont on The Wedge flow as dewpoints are generally trending downward. Depressions are still pretty large and profiles not conducive to widespread fog development...but if anywhere the southern end of the County warning forecast area could see some light fog develop in a few spots. This area is still seeing the lighter winds and higher dewpoints...that is...The Wedge has not yet moved in there. Revised temperatures through morning based on latest observation and short-term consensus product...but values are not much different from the previous expectations. At 200 PM...high pressure will move from the Midwest into the Middle- Atlantic States overnight. By Saturday sunrise...the high will have wedged SW along the east slopes of the mountains. This will produce increasing NE flow and cold advection. Also...model time heights show plenty of moisture below h850...so expect clouds to develop overnight as boundary later temperatures cool. On Sat...The Wedge looks like it will remain in place. Hence...it looks like a chilly day east of the mountains along with considerable cloudiness. Maximum temperatures will range from the middle 30s over portions of the northern NC Piedmont and foothills to the middle 40s over parts of NE Georgia and the western upstate. The mountains will also remain chilly with the warmest temperatures occurring in the SW mountain valleys outside The Wedge where highs will reach into the 40s. && Short term /tonight through Monday/... as of 235 PM Friday...guidance in good agreement that a hybrid cold air damming event will be ongoing at the beginning of the period. The guidance also agrees that low level isentropic lift will develop Sat night and continue into Sunday as southerly flow develops above the cold dome. However...there still remains a split in the timing of precipitation development as this lift gets going. The European model (ecmwf) and GFS are slower...with the European model (ecmwf) the slowest...while the NAM and sref are faster...with the NAM the fastest. Again...have used a model blend for timing as even the slower guidance shows a significant increase in moisture and isentropic lift Sat night. Therefore...end up with chance pop by daybreak Sunday across NE Georgia...much of the upstate and the southern NC mountains and foothills...with slight chance elsewhere. Now for temperatures...the NAM has warmed...but still has below freezing wet bulb temperatures for much of the area at onset. The GFS is colder even with the slower onset...showing freezing rain/drizzle developing before quickly warming after onset. Once again used a blend to develop the temperature forecast which indicates freezing precipitation at onset then quickly warming with all liquid by noon Sunday. Have also included drizzle or freezing drizzle all areas as that could be the predominant precipitation character early on. The good news is that quantitative precipitation forecast will be very light leading to only a very light accretion...mainly across the mountains and foothills at onset. That said...cold temperatures could become locked in across portions of the area and higher accretions could develop. Lows Sat night will be 5 to 10 degrees below normal while highs Sunday will be 10 degrees below normal across the mountains and up to 20 degrees below normal elsewhere in the cad region. Precipitation tapers off outside of the mountains Sun night as the best isentropic lift moves northeast of the area. Still expect rain and drizzle to continue through the night however. The cad erodes Monday as a cold front moves in from the west. Precipitation chance tapers off even further to slight chance outside of the mountains that said...precipitation chance remains high across the mountains Sun night and early Monday with the front. Temperatures remain warm enough all areas for precipitation to remain liquid through the period. Lows Sun night will range from near normal to as much as 10 degrees above...while highs Monday warm to near normal levels. && Long term /Monday night through Friday/... as of 230 PM Friday...an active pattern expected in the medium range. Upper ridge will build over the eastern states Tue-Wed...as a trough deepens over the west. The trough will begin to consolidate and progress out into the plains on Wednesday. At the surface...a 1032 mb high will quickly translate east from the Great Lakes to off the East Coast by Tuesday evening. The guidance seems to be in good agreement on increasing isent lift throughout the llvls...with moisture spreading in from the SW atop a developing wedge. The middle-upper levels look fairly neutral for supporting uvm. However...given the strong 850 mb warm air advection...and plenty of moisture...I think high-end chance to likely probability of precipitation looks good by daybreak Tuesday...continuing through the day. Any amount of precipitation/drizzle should help lock in an in situ cold air damming wedge...keeping temperatures about 10 degree below normal. Tuesday night through Thursday...the 12z runs of the GFS and European model (ecmwf) still differ on the timing of a slow-moving anafront from the northwest. The GFS continues to be on the fast side of guidance...while the European model (ecmwf) is catching up but still slower. Going with the wpc preference (slower than gfs)...Wednesday looks warm (up to 15-20 degree jump from Tuesday/S temps) under decent southwesterly flow. Precipitation will move in from northwest to southeast during the day (or evening if European model (ecmwf) is right)...with potential for a soaking rain. Cannot rule out some thunder along the front across the Piedmont...as both the GFS and European model (ecmwf) show instability in the warm sector. But severe threat should be minimal. The bigger concern may be heavy rain. Although...the guidance has backed off the quantitative precipitation forecast somewhat from previous runs. Still plan to leave any mention out of the severe weather potential statement. Temperatures will be above normal overnight then only rise a few degree on Thursday...as clouds/precipitation give way to northwesterly cold air advection flow. The 850 mb temperatures will fall such that the highest elevations may see some snow on back edge of the the precipitation shield. Some minor accums may be possible. Thursday night through Friday night...both the GFS and European model (ecmwf) show dry polar high pressure build in from the northwest...returning our area to below normal temperatures and dry conditions. Highs in the 40s to lower 50s Friday with lows back below freezing for most areas. && Aviation /05z Saturday through Wednesday/... at kclt...high clouds will spread in from the west overnight... but before daybreak a model blend would bring an MVFR ceiling to the field. The ceiling would rise to low VFR by midday...remaining there through this evening. Guidance does not support a visibility restriction. Winds will favor the NE through the forecast as cool high pressure remains along the eastern Seaboard. Elsewhere...increasing high cloud cover from west will occur early in the forecast. Guidance bring MVFR ceilings to kavl and the SC sites before dawn...with low VFR at khky. By midday the SC sites and kavl rise to low VFR...while khky falls to MVFR. The khky ceiling rises to low VFR this evening...while kavl and kand fall to MVFR. Moist flow from the south over cool high pressure will spread precipitation over the Savannah River valley late this evening...but the chance of rain at kand is too low to mention. Guidance does not favor visibility restrictions. Winds will favor the NE at foothills sites as high pressure remains along the East Coast. Kavl winds will favor the southeast. Outlook...cold air damming strengthens into Sunday. Flight restrictions will be possible during this timeframe...particularly late Sat night and sun due to increasing moisture and lift over The Wedge. Rain chances linger into Wednesday as several fronts cross the area...but confidence is limited on restrictions. Confidence table... 13-19z 19-01z 01-07z 07-12z kclt high 95% high 100% high 100% high 100% kgsp high 100% high 100% high 100% high 100% kavl medium 74% high 100% high 100% high 100% khky medium 77% 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...jat/lg/Wimberley short term...rwh long term...Arkansas aviation...jat

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