Scientific Forecaster Discussion

Area forecast discussion National Weather Service Greenville-Spartanburg SC 348 am EST Wednesday Nov 26 2014 Synopsis... low pressure will intensify along a stalled cold front off the Atlantic Seaboard and move up the coast today. An upper level system will cross western North Carolina early Thursday. Dry high pressure will return Friday and linger into the weekend. && Near term /until 6 PM this evening/... precipitation event is unfolding more or less as planned...with one caveat...there/S no cold air below about 4000 feet at this time... and it/S becoming increasingly difficult to figure out where it/S going to come from. Temperatures are in the 40s across much of the mountain valleys...and even in Avery County there is nothing but liquid falling beneath the elevation of Beech Mountain. Latest 88d indicates heavier precipitation rates setting up across the foothills at this time (which looks scarier than it actually is on radar due to bright banding)...so I suppose its not out of the question that additional cooling brought about by these heavier rates could result in a transition to snow along the Escarpment...but even this isn/T appearing too likely at this time. The latest NAM guidance continues to pound The Table re: a fairly widespread snow event this morning across the mountains...but it has thus far verified much too cold...and again...based on upstream observations...I just Don/T see where all this cold air is going to come from. It has also gotten to the point where the NAM appears to be a lone voice in The Wilderness...as the latest rap/GFS guidance are very warm this morning...and even the 00z European model (ecmwf) has continued its warming trend. Having said all that...in collaboration with mrx/rnk...we/ve decided to take a wait and see approach before making radical changes to the west-southwest suite. One action we will take is to change the advisory for Madison/Buncombe counties back to a high-elevation product. Otherwise...precipitation will begin to end rather quickly after 12z this morning...as upper level vorticity maximum comes slamming through the area...and pushes developing surface cyclone up toward the middle- Atlantic coast. Western areas will likely see some sun by afternoon... allowing temperatures to warm to the Lower/Middle 50s across the upper Savannah River valley...with middle/upper 40s expected in the mountain valleys and across the NC Piedmont. && Short term /6 PM this evening through Friday night/... as of 300 am Wednesday...just as quickly as the coastal low pulls away from the Piedmont this afternoon...the next system enters from the west in the form of a clipper shortwave rolling through the region tonight. Models continue to show a brief but potent shot of q-vector convergence and Omega with this wave along with fairly deep moisture. Profiles will be cold enough for all snow in higher elevations at the onset of the forcing and probability of precipitation. Most of the area however will still be too warm for snow and remain so through the event. Trends in the low level flow are a little complex with a very Short Ridge and weak warm air advection coming between the two troughs. There was some indication from previous model runs that this warm air advection might cause a small warm nose to develop and allow for sleet/freezing rain in some of the foothills. Latest runs indicate this warm air advection is too weak to have such an effect and we will advertise an all rain/snow event. Snow levels are expected to drop somewhat steadily overnight with wet bulb effects providing only marginal enhancement. With or just after the clipper axis passes early Thanksgiving morning...winds veer to northwest and remain brisk through the lowest few thousand feet which should provide a jump in precipitation rates. The northwest winds continue to be strong through Thursday albeit with moisture slowly becoming more shallow. Despite the strong forcing model quantitative precipitation forecast is not that strong but I did favor the higher and more terrain-oriented NAM quantitative precipitation forecast as part of a blend of raw model and wpc quantitative precipitation forecast. Total accums of 1 to 2 inches are expected in the smokies and balsams...and 2 to locally 3 inches in the mountains north of the French Broad. Half of that will probably fall in a short period when the two forcing mechanisms overlap. Given the Holiday and potential travel impacts a Winter Weather Advisory will be hoisted for our tenn border zones in addition to northern Jackson County midnight to noon Thanksgiving day. Enough warming should occur during the day that most valleys will see rain mix in or a complete transition. Light shsn could continue through Friday morning though moisture is insufficient to expect much additional accumulate. Dry high pressure will filter in Thursday night into Friday. Min temperatures Friday morning will be in the l20s mountains and u20s to around 30 in the Piedmont. Skies will be mostly clear by this time...but continued northerly winds will keep temperatures from rising out of the 40s over most of the area Friday afternoon. Slight return flow sets up Friday night which will keep temperatures from plunging as much...also introducing a little cloud cover into the area. && Long term /Saturday through Tuesday/... as of 220 am Wednesday...the medium range forecast picks up at 12z on Saturday with a relatively zonal upper pattern over most of the Continental U.S.. as we move into sun...another broad 500 mb trough digs down over the far northern states and then over the Great Lakes. At the same time...flat upper ridging will persist spread over the Gulf of mex and persist over the region through most of the period. The long range guidance actually has the ridge amplifying towards the very end of the medium range. At the surface...high pressure will be centered over the forecast area at the start of the period with warmer southerly flow increasing thicknesses and temperatures over the weekend. The next frontal system will approach the area by early Monday and lay down across our northern fringe. Strong high pressure behind the front will slide eastward and appears to set up a potentially strong cad pattern to our NE by day 7. The European model (ecmwf) and Canadian operational models remain more moist with this front and subsequent wedge pattern...yet as we have seen many times before...models tend to overestimate the extent of the deeper layer moisture in these scenarios. With that said...the only probability of precipitation I have are slight to low end solid chance from late sun Onward. The best chances are still over the higher terrain and along the NC/tenn border region. Temperatures will warm through Monday with values exceeding climatology. Values should cool again as we go into middle week and the flow becomes more northeasterly. && Aviation /09z Wednesday through Sunday/... at kclt...light to moderate rain will continue through the overnight hours into the morning...before tapering off by early afternoon. Observation sites to the south of have already developed IFR/LIFR ceilings...and kclt recently detected few007. Therefore...have included a tempo for IFR ceilings through 09z...with categorical thereafter until middle-morning or so...with VFR expected to early afternoon. Occl IFR visby will also be possible. Otherwise...N/NE winds will gradually turn toward the northwest as surface low pressure moves up the Carolina coast. Winds may even return to light SW by evening...in advance of a fast moving upper air disturbance. Elsewhere...light/mod rain will continue to overspread the terminals throughout the early morning hours...with MVFR conditions eventually developing as the low level air mas moistens. Occl IFR conditions will be possible...especially at kavl...where some snow maximum mix in with the rain toward daybreak. There may even be a brief change over to all snow before the precipitation ends at kavl during middle-late morning. No snow is expected outside the mountains...and the rain/snow mix has been removed from khky. Conditions should improve to VFR by early afternoon at the latest. Outlook...another round of scattered rain showers south and rain/snow showers north will impact the area Wednesday night into Thursday morning. A dry air mass will then return to the area Thursday night through the weekend. Confidence table... 08-14z 14-20z 20-02z 02-06z kclt medium 73% high 87% high 100% high 100% kgsp medium 75% high 93% high 95% high 100% kavl medium 71% high 89% high 100% high 100% khky high 85% high 83% high 100% high 100% kgmu medium 71% high 85% high 100% high 100% kand medium 77% high 87% 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...Winter Weather Advisory until 4 PM EST this afternoon for ncz501-503-505. Winter Weather Advisory from midnight tonight to noon EST Thursday for ncz033-048>052-058-059. Winter Storm Warning until 4 PM EST this afternoon for ncz033- 049-050. Winter Weather Advisory until noon EST today for ncz048-052- 053-059-063. SC...none. && $$ Synopsis...jdl near term...jdl short term...Wimberley long term...jpt aviation...jdl

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