Scientific Forecaster Discussion

Area forecast discussion National Weather Service Greenville-Spartanburg SC 926 am EST Sat Feb 13 2016 Synopsis... the leading edge of an Arctic airmass will push into the region today and settle solidly over the area on Sunday. A low pressure system will approach from the west on Monday...cross the region Monday night...and depart to the east on Tuesday. A wintry mix of precipitation is likely with this system. Another fast moving system will cross the southern Appalachians middle week...with drying and warming expected late in the week. && Near term /through tonight/... 1430 UTC update...only minor adjustments were made to winds and temperatures. Sky cover was reduced per satellite imagery. Areas of snow were trimmed back near the NC borer per radar and satellite trends. As of 630 am...snow shower activity is on a downward trends across eastern Kentucky/Tennessee and southern Appalachians early this morning. While minor additional accums will be possible through late morning...for the most part things are winding down. Roads are likely snow covered and slick in the usual northwest flow areas this morning. Plus high elevations are seeing wind chills dipping below -5 degrees. So we have allowed the Winter Weather Advisory to expire...but keeping the wind chill advisories going. Overall...temperatures are still running warmer than forecast...and looking current maximum temperatures...I opted to bump them up a degree or two east of the mountains where downslope should negate the cold air advection. As of 200 am...a deep trough continues to dominate the eastern Continental U.S....with the axis now shifting just east of the County warning forecast area today. Confluent northwesterly flow on the back side of the trough will usher in a large Arctic high pressure system into the region today through tonight. Strong low level winds and cold air advection is resulting in Wind Advisory criteria winds across the northern NC mountains also...regional radar mosaic still showing healthy snow showers across eastern KY/TN. So expect continued likely to categorical coverage of snow along the Tennessee border for the next few hours. Then snow should taper off fairly quickly starting right before daybreak through the rest of the morning. Temperatures are not falling as quickly as expected. Probably due to turbulent mixing and clouds/snow adding a little heat. Still...the combination of wind and falling temperatures should result in sub-zero wind chills for much of the elevations above 3500 feet this morning. By midday...snow showers should be winding down to just a few flurries and lingering stratus along the Tennessee border...while winds slowly weaken...but remain breezy. Temperatures will only hold steady or rise slightly in the northern mountains...so wind chills will hover in the 0 to -5 range through the day. Elsewhere...it will be cold with temperatures ranging from 15 to 30 degrees below normal...despite full sun outside the northwest flow stratus. Tonight...the center of Arctic high pressure will settle over the central Appalachians...while a weak Alberta clipper dives into the plains states. The upper flow will flatten out somewhat as the clipper system approaches...allowing some middle and high clouds to start streaming atop the area late. Still...it looks like a decent radiational cooling night...with some of the coldest temperatures of the winter expected by daybreak Sunday. Lows will be generally 20-25 degrees below normal. Even though winds will be substantially weaker in the northern mountains...the colder temperatures will result in dangerous wind chills continuing through the night. && Short term /Sunday through Tuesday/... as of 245 am EST Saturday...considerable uncertainty persists with the wintry weather forecast for Sunday night through Tuesday. One question centers around how quickly the cold and dry Arctic airmass present at onset Sunday night will retreat/modify through the period. Uncertainty also arises over the surface low track forecasts...with the GFS/European model (ecmwf) Camp starting off the surface low west of the mountains and tracking it through the central/southern Appalachians...while the NAM/Gem Camp depict more of a redeveloping Piedmont low to the southeast. These differences will need to be resolved before enough confidence exists to Post a Winter Storm Watch. For the specifics...a northern stream system moving across the upper Midwest on Sunday will extend a warm frontal feature across the southern Appalachians through the day...with associated upglide moisture arriving at middle levels. Lower levels...however...remain mostly dry through late day. Meanwhile...1034 mb surface high pressure will move east through the middle Atlantic region Sunday and offshore Sunday night. The low level thickness trough remaining across the western Carolinas will be cold and dry and favorable for hybrid cold air damming as the precipitation starts ahead of an approaching southern stream wave. The models have trended less amplified and more progressive with this southern wave which sharpens up over the MS River Valley on Monday. A consensus of the varying model profiles promotes a slow warmup with gradual cad erosion late Monday into Monday night. The faster solutions now bring a middle dry slot in quickly behind the wave late Monday night through Tuesday morning...with heaviest precipitation shutting off more quickly. Upglide moisture should get started in earnest across the forecast area before daybreak Monday. Still anticipate nearly all snow at onset with the 850 mb zero degree isotherm along the southern periphery of the forecast area. However...the warm nose aloft should steadily expand and move northward through the day to permit a transition to sleet and then freezing rain throughout most areas by Monday evening. The middle level drying and associated removal of ice nuclei may keep any overnight precipitation rain or freezing rain/freezing drizzle in any lingering colder mountains pockets. Then...northwest upslope moisture on Tuesday morning will diminish through Tuesday afternoon...with a brief lull in precipitation likely. All told...at least advisory snow or ice accums look quite possible for a significant chunk of the forecast area...with mainly the northern mountains at risk of winter storm snow/ice at this point. Winter storm watches could be needed for Monday as early as this afternoon. && Long term /Tuesday night through Friday/... as of 245 am EST Saturday...the next upstream shortwave rounding the broad eastern trough will cross the southern Appalachians Tuesday night. A resurgence of upslope moisture will accompany the wave as colder temperatures briefly return...and some scattered snow shower activity could break containment east of the mountains in the early morning hours Wednesday. Behind the departing the wave...heights will build in from the west later Wednesday through Thursday. Timing differences exist on the next upstream wave late week in the progressive flow Patten...and no precipitation will be advertised through Friday. The main feature of the forecast Wednesday through Friday will be drying and warming conditions with maximum temperatures finally climbing above climatology by Friday. && Aviation /15z Saturday through Wednesday/... at kclt and elsewhere east of the mountains...VFR...with generally sky clear or few250 the day...then increasing high clouds that will thicken and lower to a middle cloud deck from west to east overnight. Winds will remain out of the northwest...with some low-end gusts expected as mixing deepens middle-late morning through early afternoon...especially across the NC sites. Winds veer to NE tonight...generally 4-6 kts. At kavl...gusty north-northwest winds and MVFR stratus will continue due to moist northwest flow channeled up the French Broad valley this morning. Should see gradually clearing late morning with VFR expected rest of the 12z taf period...but winds will be slow to subside through the day...finally losing gusts late evening. Outlook...a large area of Arctic high pressure will settle over the region on Sunday. Then a strong low pressure system will develop and cross the area late Sunday night through Tuesday...bringing widespread clouds/precip. This system may produce significant winter impacts...especially in the NC mountains Confidence table... 14-20z 20-02z 02-08z 08-12z kclt high 100% high 100% high 100% high 100% kgsp high 100% high 100% high 100% high 100% kavl medium 77% high 100% high 100% high 100% 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 100% The percentage reflects the number of guidance members agreeing with the schedule taf issuance flight rule category. Complet hourly experimental aviation forecast consistency tables and ensemble forecasts are available at the following link: Www.Weather.Gov/gsp/aviation && Gsp watches/warnings/advisories... Georgia...none. NC...Wind Chill Advisory until 10 am EST Sunday for ncz033-049-050. Wind Chill Advisory until noon EST today for ncz048-051-052- 058-059. SC...none. && $$ Synopsis...hg near term...Ark/jat short term...hg long term...hg aviation...Arkansas

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