Scientific Forecaster Discussion

Area forecast discussion National Weather Service Greenville-Spartanburg SC 1056 am EDT Friday Oct 31 2014 Synopsis... moisture will increase today out ahead of an approaching cold front. This front...and a strong middle level area of low pressure is forecast to track across the region tonight into Saturday. A strong surface low pressure system will develop offshore on Saturday...then track quickly north by Saturday night. Circulation around the coastal low will bring a cold Canadian air mass to the region through Monday. && Near term /through tonight/... as of 1030 am...water vapor imagery shows a lead shortwave diving into the trough axis along the Appalachians late this morning. The main player at 500 mb is digging into in/IL. The forecast is on track for today...except that Western Mountain probability of precipitation for light rain showers will be increased along the Tennessee border through midday with the early upper waves. Otherwise...the strongly dynamic upper low will drop down across the western Carolinas tonight to a position over the western upstate at daybreak on Saturday. This compact system should have all manner of low/mid/upper forcing that comes into play this evening and overnight. Moisture should also be fairly deep...so the pop ramps up to categorical over the mountains this evening...and into the likely range across most of the foothills and western Piedmont...and eventually categorical over the western Piedmont...where forcing is maximized on the north side of the upper low. Precipitation was limited to the chance range over the western upstate and NE Georgia as some downslope flow may inhibit precipitation development. The distribution of partial thickness accompanying the upper low will be decidedly winter-like. Strong cold advection this evening should allow precipitation to change over from rain to snow from ridgetops down to the valleys across the NC mountains by the early morning hours. East of the mountains...the latest GFS suggests a slight upward trend in temperature and dewpt by daybreak Saturday perhaps because the operational model was wetter...which was generally followed. That should bump the surface wet bulb temperatures up a bit more...meaning less of a chance of seeing any snow east of the foothills through 12z Saturday. Will give a nod to the cooler NAM BUFKIT profiles that suggest a possibility of a wet snowshower around daybreak over the NC foothills and western Piedmont...but this might not yet make the cut and appear in the forecast. Snow still looks like a good bet over the mountains mainly near the Tennessee border...with upwards of 2-4 inches possible by sunrise. Some of the snow may wrap around into the upper Savannah River basin and NE Georgia around sunrise. However...those locations have the lowest probability of any precipitation reaching the ground...at the low end of the chance range. An advisory will not be issued yet for the rest of the southern mountains of NC and NE Georgia...instead will take a look at the morning model runs for more guidance and confidence. Freeze warnings remain out for tonight with the mountain growing season ending tomorrow. && Short term /Saturday through Sunday night/... as of 320 am EDT Friday...the latest 00z short range models continue to indicate that an anomalous upper low just south of our County Warning Area 12z Sat will move off the southeast coast between 21z Sat and 00z sun with an unseasonably cold airmass filtering in behind the low in its wake. Models have trended a bit wetter...especially east of the mountains on early Sat as better deformation zone over the northern periphery of the upper low sets up over the western Carolinas Piedmont. However...latest model temperatures out side of the mountains appear a bit warmer than the previous model runs. Hence...have updated weather grids to highlight more of liquid precipitation over the Piedmont. Still with colder temperatures over the mountains...expect heavy snow accums to occur through early sun mainly along the Tennessee border. Total snowfall from this event will range from 8" over the northern NC mountains to 12" over the smokies. Elsewhere over the NC mountains will see between 1 and 2 inches of snow. Hence...will continue to highlight a Winter Storm Warning over the Tennessee border counties and a Winter Weather Advisory over Buncombe...northern half of Jackson...and Graham counties. Gusty northwest winds of 20-40 miles per hour will develop on Sat in light of tightening surface pressure gradient between a developing surface low off the southeast coast and strong high pressure to our west. As the upper low moves off to the east...probability of precipitation taper off from west to east through Sat evening. Temperatures on Sat will remain well below normal (around 15-20 below normal). Sun looks mostly dry as the Canadian high settles over the region. Northwest snow over the NC mountains will taper off by early sun as dry airmass moves in. Morning low temperatures on Sunday will be quite cold...ranging from low to middle 30s over the Piedmont to 20s in the mtns/valleys. With the presence of decent northwest cold air advection winds...wind chill reading will remain in the 10s in the mountains with 20s elsewhere. However...temperatures on Sun afternoon will be warmer than Sat...but still looking around 7-15 degrees below normal. && Long term /Monday through Thursday/... as of 250 am EDT Friday...latest global models continue to agree that an upper ridge axis is forecast to cross the forecast area on Monday and off to the East Coast by Tuesday. Southwesterly flow aloft will then set up by Tuesday night as a positively tiled upper trough approaches from the central Continental U.S.. this upper trough will cross the region on Thursday followed by northwesterly flow Thursday night. At the surface...a dry high pressure centered over the southeast states Monday will gradually shift offshore through Wednesday as a weak cold front approaches from the west. The front will cross the region around Wednesday night and off to our east by Friday morning. Models indicate that the front will dry out as it reaches the mountains hence...have carried slight to low end chance pop mainly along the Tennessee border during the frontal passage. Otherwise...conditions look dry throughout the medium range period. Temperatures will quickly warm up to near climatology by Tuesday. && Aviation /15z Friday through Tuesday/... at kclt...brief morning MVFR ceilings scattered out very quickly with heating...so VFR will remain through the day as middle and high clouds increase. By middle/late afternoon...the wind should be light and variable and may change between northwest and SW often. At that time... a low cloud ceiling should move in from the northwest ahead of the approaching low...but still VFR. The latest model guidance is in good agreement with developing light precipitation into the clt metropolitan area around sunset. A consensus of the guidance suggests a prevailing light rain...but with still a VFR category visibility...after 02z. Wind at that time should settle into a north component. No issues with precipitation type until after 06z Saturday...with snow showers likely mixing in at times early Sat morning. Elsewhere...expect VFR through the daylight hours today. Light and variable winds and increasing middle/high clouds are expected through middle morning. Low VFR and middle clouds increase through the day with low VFR ceilings expected by afternoon at kavl. Winds turn west-southwest this afternoon ahead of the approaching cold front. However...winds remain northerly at kavl. Kavl also has the best chance of seeing any rain showers by late in the afternoon...which was included as a vcsh at this time. The steadier rain should move into the mountains by sunset. Have included the rain prevailing after 02z at kavl...at which time the ceiling should drop to MVFR. Preliminary indication is that precipitation could change over to snow showers at kavl as early as 05z. A similar progression is noted at khky...but precipitation was kept liquid. Other taf sites only have a vcsh after 02z or 03z...owing to low confidence. Outlook...light precipitation will develop across the entire area late Friday night. Some restrictions are possible...especially in the mountains where even some snow showers are expected late Friday night and Saturday. VFR beyond Sunday. && Gsp watches/warnings/advisories... Georgia...freeze warning from 11 PM this evening to 10 am EDT Saturday for gaz010. NC...Winter Storm Warning from 8 PM this evening to 8 PM EDT Saturday for ncz033-048>052. Freeze warning from 11 PM this evening to 10 am EDT Saturday for ncz033-049-050-053-063>065. Winter Weather Advisory from 8 PM this evening to 8 PM EDT Saturday for ncz053-058-059. SC...none. && $$ Synopsis...joh near term...hg/PM short term...joh long term...joh aviation...hg/PM

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