334 FXUS62 KGSP 260317 AFDGSP AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC 1017 PM EST MON FEB 25 2013 .SYNOPSIS... NORTH OF A DEVELOPING WARM FRONT...MOISTURE WILL SPREAD ATOP OUR REGION FROM THE GULF OF MEXICO. STRONG LOW PRESSURE WILL DRIVE A COLD FRONT ACROSS OUR AREA ON TUESDAY. && .NEAR TERM /THROUGH TUESDAY/... AS OF 10 PM...LATEST RADAR IMAGE INDICATED AN AREA OF LIGHT RETURNS EXPANDING ACROSS THE UPPER SAVANNAH RIVER VALLEY. THE PRECIP APPEARS TO BE THE RESULT OF INCREASING 290K ISENTROPIC LIFT...SUPPORTED BY H9-H85 SE WINDS AROUND 40 KTS. LIGHT RAINFALL SHOULD CONTINUE TO EXPAND ACROSS THE UPPER SAVANNAH RIVER VALLEY AND ALONG THE EAST FACING SLOPES OF CENTRAL NC MTNS. TEMPERATURES AND WET BULBS UNDER THE PRECIP SHOULD REMAIN WARM ENOUGH TO SUPPORT RA. THIS AREA OF EVENING PRECIP WILL STRUGGLE TO REACH MEASURABLE AMOUNTS. REGIONAL RADAR INDICATED THAT THE SIGNIFICANT RAINFALL WAS LOCATED ACROSS SOUTHERN AND WESTERN GA. LATEST MODEL RUNS HAVE TRENDED SLOWER WITH THE ARRIVAL OF THE RA ACROSS THE WESTERN FA AROUND MIDNIGHT. WIDESPREAD PRECIPITATION IS EXPECTED TO EXPAND ACROSS THE ENTIRE CWA BY 9Z. FORECAST SOUNDINGS INDICATE THAT A DEEP WARM NOSE WILL EXIST ACROSS THE REGION TONIGHT...WARMING ABOVE 4C AT 3 KFT BY LATE TONIGHT. PRECIP THAT FALLS WILL BE EITHER RA OR FZRA...POSSIBLY SOME IP AT TIMES. OBSERVATIONS AT 10 PM INDICATE THAT NRN MTNS RANGE FROM THE MID 30S TO U20S. HOWEVER...TEMPERATURES MAY BECOME STEADY OR GRADUALLY RISE WITH THE ARRIVAL OF WAA AND LOWER CLOUD COVER. IN ADDITION...LLVL FLOW...PRECIP...DEEPENING MOISTURE SHOULD RESULT IN A GRADUAL WARMING OF DEWPOINTS OVERNIGHT. BASED ON THE MARGINAL TEMPERATURES...FREEZING RAIN SHOULD REMAIN LIMITED TO THE EAST FACING SLOPES OF THE MTNS FROM HENDERSON TO AVERY...ALONG WITH THE ADJACENT FOOTHILLS. IN ADDITION...UPSLOPE FLOW MAY AID IN COOLING TEMPERATURE ALONG THE ESCARPMENT. THESE FACTORS YIELD NEAR WARNING CRITERIA FREEZING RAIN ALONG THE EAST FACING SLOPES...BUT LIGHT AMOUNTS ELSEWHERE. I WILL KEEP THE WINTER WEATHER ADVISORY IN PLACE. WINDS TONIGHT ACROSS THE MTN WILL BECOME QUITE STRONG...PRESSURE GRADIENT ACROSS THE MTNS TIGHTENS TO AROUND 8 MBS BY 12Z. RIDGETOP GUSTS SHOULD EXCEED 50 KTS...WITH WINDS BETWEEN 30 TO 40 KTS. THE COMBINATION OF THE WINDS...WET GROUND...AND ICE ACCUMULATIONS MAY YIELD TREE AND POWERLINE DAMAGE BY SUNRISE TUESDAY. I WILL UPDATE THE NPW ALONG WITH THE HWO. AS OF 630 PM...THE FORECAST APPEARS ON TRACK. I WILL UPDATE THE T/TD/WINDS TO LATEST OBSERVATIONS. AS OF 545 PM...I WILL UPDATE THE FORECAST THROUGH TUESDAY. LATEST REGIONAL RADAR COMPOSITE INDICATES LITTLE TO NO RETURNS ACROSS THE FORECAST AREA AND ACROSS THE UPSTREAM REGION. THEREFORE...I HAVE SLOWED THE ARRIVAL OF CATE POPS UNTIL AFTER MIDNIGHT. THEN...ALL SOLUTIONS INDICATE THAT WIDESPREAD PRECIP WILL OCCUR DURING THE PRE DAWN HOURS AND EARLY DAYTIME HOURS ON TUESDAY...BEFORE A DRY SLOT ARRIVES IN THE AFTERNOON. THE SLOWING OF THE ARRIVAL OF POPS AND SLIGHT QPF FORECAST WILL HAVE AN IMPACT ON THE ICE FORECAST LATE TONIGHT AND TUESDAY MORNING. THE LATEST FORECAST WILL FEATURE VALUES APPROACHING 0.25 ALONG THE EAST FACING SLOPES OF THE NC NRN MTNS...ELSEWHERE ACROSS THE ADJACENT FOOTHILLS AND VALLEY...VALUES WILL REMAIN LESS THAN A TENTH OF AN INCH. THE CURRENT WSW APPEARS WELL PLACED AND TIMED FOR ADVISORY FREEZING RAIN. ONE NOTE...THE SREF GUIDANCE APPEARS VERY HIGH ON FZRA AMOUNTS. THEREFORE...THE SREF PLUMES AND ANY GUIDANCE BASED ON THE SREF WILL NOT BE CONSIDERED. && .SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY/... AS OF 230 PM EST MONDAY...THE SHORT TERM PERIOD BEGINS WITH AN OCCLUDED FRONT MOVING EASTWARD AWAY FROM THE AREA ALLOWING A DRY SLOT TO CONTINUE TO SURGE ACROSS THE REGION. HENCE...IT APPEARS THAT THE PRECIP THREAT WILL HAVE ENDED TUE EVENING. TIME SECTIONS INDICATE THAT LOW LEVELS REMAIN RATHER MOIST...SO MOSTLY CLOUDY CONDITIONS WILL ONLY GRADUALLY GIVE WAY TO PARTLY CLOUDY CONDITIONS TUE NIGHT. THERE IS SOME MODEL QPF RESPONSE IN THE SW MOUNTAINS LATE TUE NIGHT DUE TO UPSLOPE...SO WILL HAVE SOME CHC TYPE POPS THERE. THERMAL PROFILES INDICATE THAT ANY PRECIP WOULD BE LIGHT SNOW SHOWERS. AS AN INTENSE UPPER LOW TRACKS INTO THE EASTERN GREAT LAKES ON WED...A SHORT WAVE RIDGE IS FORECAST TO BUILD ACROSS THE AREA WHICH SHOULD PRODUCE MOSTLY CLEAR CONDITIONS AND NEAR NORMAL TEMPS EAST OF THE MOUNTAINS ...ALSO DUE IN PART TO DOWNSLOPING WINDS. MOISTURE/CLOUDS WILL INCREASE OVER THE MOUNTAINS BY LATE IN THE DAY AS AN UPPER TROUGH APPROACHES. SOME SCT RAIN/SNOW SHOWERS MAY DEVELOP BY LATE WED ALONG THE TENNESSEE BORDER COUNTIES. PROFILES CONTINUE TO MOISTEN AND COOL OVER THE MOUNTAINS WED NIGHT WITH THE WINDS TURING MORE NW. THIS COMBINATION ALONG WITH SOME UPPER AIR SUPPORT IS EXPECTED TO PRODUCE NUMEROUS SNOW SHOWERS ACROSS THE MOUNTAINS...ESPECIALLY IN THE FAVORED UPSLOPE AREAS. AT LEAST SCT NW FLOW SNOW SHOWERS WILL CONTINUE THROUGH THU AS WINDS AND MOISTURE REMAIN FAVORABLE...THOUGH COVERAGE IS EXPECTED TO DECREASE THROUGH THE DAY THU. SNOW TOTALS OVER THE MOUNTAINS FOR WED NIGHT AND THU SHOULD BE IN THE 1 TO 3 INCH RANGE. STAYED CLOSE TO THE METMOS FOR TEMPS THROUGH WED NIGHT. FOR THU...USED OUR LOW LEVEL THICKNESS SCHEME WHICH YIELDED TEMPS BELOW GMOS. && .LONG TERM /THURSDAY NIGHT THROUGH MONDAY/... AS OF 230 PM EST MONDAY...THE DEEP LOW PRESSURE SYSTEM OVER THE NE U.S. ON FRI WILL MOVE OFFSHORE...BUT WITH ADDITIONAL PIECES OF SHORTWAVE ENERGY MOVING SWD THROUGH THE CENTRAL AND ERN CONUS TO MAINTAIN THE UPPER TROUGH OVER THE AREA. AT LOW LEVELS...ENOUGH UPSTREAM MOISTURE WILL PERSIST IN THE COLD NW FLOW TO KEEP SCATTERED UPSLOPE SNOW SHOWERS GOING ACROSS THE WRN NC MTNS THROUGH FRIDAY. MEANWHILE...THE STRONGEST UPSTREAM SHORTWAVE WILL DIG OVER THE LOWER MS RIVER VALLEY...WITH THIS FEATURE LIKELY CROSSING THE EXTREME SOUTHERN APPALACHIANS AND ADJACENT FOOTHILLS AND PIEDMONT SOMETIME ON SAT. THE ECMWF REMAINS THE MOST VIGOROUS WITH THIS WAVE...BRIEFLY CLOSING IT OFF ON SAT...BUT THE GFS/CMC ARE ALSO EXHIBITING BETTER TROUGHINESS AND MOISTURE WITH THE WAVE. EVEN THE LESS AMPLIFED MODELS APPEAR TO HAVE A FAIRLY POTENT VORT LOBE CROSSING THE REGION...AND 850 TO 500 MB LAPSE RATES UP TO 6.5 DEG C/KM MAY SUPPORT ISOLD TO SCT SNOW SHOWERS EAST OF THE MTNS SAT. POPS HAVE BEEN ADJUSTED UP SLIGHTLY FROM THE PREVIOUS FORECAST. A RETURN TO NW FLOW UPSLOPE MOISTURE WILL YIELD ADDITIONAL LIGHT MTN SNOW SHOWER ACCUMS INTO SUNDAY AS THE UPPER TROUGH AXIS BECOMES REINFORCED BY ADDITIONAL SHORTWAVE ENERGY. BETTER DRYING SHOULD THEN SET IN THROUGH MON AS HIGH PRESSURE BUILDS IN FROM THE WEST. WILL CONTINUE TO FEATURE MINS 5 TO 10 DEGREES BELOW NORMAL...WITH MAXES 10 TO 15 DEGREES BELOW NORMAL...THROUGH THE PERIOD. && .AVIATION /03Z TUESDAY THROUGH SATURDAY/... AT KCLT...WINDS WILL REMAIN BETWEEN 070 TO 060 DEGREES AND 8-14 KTS OVERNIGHT AND THROUGH THE EARLY AFTERNOON. WINDS SHOULD BACK TO AROUND 300 DEGREES AND DECREASE TO LESS THAN 5 KTS WITH THE PASSAGE OF A COLD FRONT BETWEEN 18Z TO 21Z. THE FORECAST CHALLENGE WILL BE THE TIMING OF MVFR CLOUDS...MADE MORE DIFFICULT BY EXPANSIVE CIRRUS CLOUDS...EXPECTED TO ARRIVE AROUND 6Z. CLOUD BASED SHOULD LOWER TO IFR BY 12Z AND WILL LIKELY REMAIN IFR UNTIL DRIER AIR ARRIVE IN THE FINAL HOURS OF THIS TAF PACKAGE. IN ADDITION...VIS MAY FALL TO IFR BETWEEN 18Z TO 21Z WITH THE GREATEST RAINFALL RATES AND CHC FOR TSRA. ELSEWHERE...MVFR CLOUDS HAVE STARTED TO SPREAD ACROSS THE UPSTATE OF SC AND NC MTNS. ALL WESTERN CAROLINA TAF SITES SHOULD REACH MVFR THIS EVENING...KHKY MAY NOT SEE MVFR UNTIL LATE TONIGHT. ALL SITES WILL SEE CLOUD BASES LOWER TO IFR AROUND SUNRISE WITH THE ARRIVAL OF DEEP MOISTURE AND LLVL LIFT. IFR CEILINGS AND POSSIBLY VIS WILL CONTINUE THROUGH MID DAY. MODELS INDICATE A DRY SLOT WILL PUSH ACROSS THE UPPER SAVANNAH RIVER VALLEY ANS WESTERN NC MTNS AROUND MID DAY...REACHING THE PIEDMONT BY LATE AFTERNOON. KHKY MAY SEE MVFR RESTRICTIONS DEVELOP AROUND 22Z AND CONTINUE THROUGH THE REMAINDER OF THE TAF PERIOD. OUTLOOK...CLEARING SKIES ARE IN STORE TUESDAY NIGHT WITH VFR EXPECTED THROUGH THURSDAY EAST OF THE MOUNTAINS. SOME POTENTIAL FOR MVFR ACROSS THE NC MTNS WITH SNOW SHOWERS MID TO LATE WEEK. CONFIDENCE TABLE... 03-09Z 09-15Z 15-21Z 21-03Z KCLT HIGH 89% HIGH 90% HIGH 87% MED 72% KGSP HIGH 91% HIGH 81% HIGH 93% MED 70% KAVL HIGH 91% HIGH 97% HIGH 83% HIGH 91% KHKY HIGH 96% HIGH 100% HIGH 83% MED 78% KGMU HIGH 88% HIGH 81% HIGH 88% MED 66% KAND HIGH 96% MED 76% HIGH 91% HIGH 90% 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... GA...NONE. NC...WINTER WEATHER ADVISORY UNTIL 9 AM EST TUESDAY FOR NCZ033- 048>050-052-053-064-065-501-503-505-507-509. HIGH WIND WARNING UNTIL 6 PM EST TUESDAY FOR NCZ033-048>052- 058-059. WIND ADVISORY UNTIL 6 PM EST TUESDAY FOR NCZ033-048>052-058- 059. SC...NONE. && $$ SYNOPSIS...CSH NEAR TERM...NED/WIMBERLEY SHORT TERM...LG LONG TERM...HG AVIATION...NED