Area Forecast Discussion
Issued by NWS Greer, SC

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000 FXUS62 KGSP 191806 AFDGSP AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC 106 PM EST FRI DEC 19 2014 .SYNOPSIS...
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A PLUME OF MOISTURE WILL ROTATE ACROSS THE REGION TONIGHT INTO SATURDAY WITH DRY HIGH PRESSURE RETURNING ON SUNDAY. AN AREA OF LOW PRESSURE WILL BRING RAIN CHANCES BACK BY MONDAY.
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&& .NEAR TERM /THROUGH TONIGHT/...
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AS OF 100 PM...FOR THE 1730 UTC UPDATE...I UPDATED THE NEAR TERM GRIDS THRU SATURDAY MORNING FOR THE 18Z TAFS. SOME CHANGES TO THE DEPICTION OF WINTRY MIX OF PRECIP LATE TONIGHT IN THE NC MTNS. MODELS ARE CONTINUING TO TREND VERY LIGHT AND SPOTTY ON QPF WITH THIS SYSTEM. SO NO BIG CHANGES WITH THIS UPDATE. I WILL FINALIZE TEMPS/POPS/WX WITH THE NEXT UPDATE (2030 UTC). AS OF 930 AM...FOR THE 1430 UTC UPDATE...FCST LOOKS PRETTY MUCH ON TRACK. I TWEAKED SKY COVER THRU NOON TO REFLECT MORE SUNSHINE PER SAT TRENDS. OTHERWISE...EXPECT AN INCREASE IN HIGH CLOUDS THIS AFTN WITH TEMPS NEAR NORMAL IN THE MTNS AND A CATEGORY OR TWO ABOVE NORMAL ACRS THE PIEDMONT...THANKS TO A DOWNSLOPE WNWLY FLOW. PREVIOUS DISCUSSION... A SURFACE RIDGE NOW EXTENDING FROM THE GREAT LAKES TO THE MID MS RIVER VALLEY WILL MOVES SLOWLY EAST AND OVER THE APPALACHIANS BY TONIGHT. WITH LIMITED CLOUD COVER TODAY IN THE WAKE OF THE DEPARTING MID LEVEL ENERGY...AND SLIGHTLY ABOVE NORMAL HEIGHTS ALOFT...MAXIMUM TEMPERATURES SHOULD RUN NEAR NORMAL...DESPITE WEAK COLD ADVECTION IN NE SURFACE WINDS. THE MODELS SHOW ONLY WEAK ISENTROPIC UPGLIDE SETTING UP OVER THE SURFACE RIDGE LATE TONIGHT...SUPPORTIVE OF ONLY LIGHT PRECIPITATION DESPITE SUBSTANTIAL CLOUD COVER. MINIMUM TEMPERATURES WILL RUN SLIGHTLY ABOVE NORMAL UNDER THE CLOUDS...BUT STILL NEAR OR BELOW FREEZING IN MUCH OF THE MOUNTAINS. MODEL SOUNDINGS...WHILE COLD ENOUGH FOR ICE NUCLEATION AT THE UPPER END OF THE MOIST LAYER...HAVE EITHER A WARM NOSE ALOFT OR WARM SURFACE TEMPERATURES BY LATE TONIGHT...DEPENDING ON THE MODEL AND THE LOCATION. EXAMINING BOTH A TOP DOWN APPROACH AND THE BOURGOIUN METHOD...AND BLENDING THE NAM AND GFS...WOULD YIELD A LIGHT FREEZING RAIN EVENT IN MUCH OF THE NC MOUNTAINS...BEFORE A TRANSITION TO SLEET AND SNOW...AND THEN SNOW. ICE ACCUMULATIONS WOULD BE QUITE LIGHT...BUT WOULD STILL WARRANT A WINTER WEATHER ADVISORY SHOULD CONFIDENCE BE GREAT ENOUGH. WITH MODEL SOUNDINGS SO MARGINAL FOR ICE VERSUS SNOW...CONFIDENCE IS TOO LIMITED FOR A SECOND PERIOD ADVISORY AT THIS TIME. A MENTION OF A WINTRY MIX WILL BE MADE IN THE HAZARDOUS WEATHER OUTLOOK HOWEVER.
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&& .SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/... AS OF 320 AM EST FRIDAY...THE LATEST 00Z SHORT RANGE MODELS HAVE TRENDED DRIER AND A BIT COLDER WITH THE PASSAGE OF UPPER SHORTWAVE ON SAT. THEREFORE...HAVE MADE REASONABLE CHANGES TO THE POPS/P-TYPE ACCORDINGLY. A WEAK SOUTHERN STREAM UPPER SHORTWAVE IS FORECAST TO PASS OVERHEAD ON SAT...SHEARING OUT AS IT PUSHES OFF THE CAROLINAS COAST BY SAT NIGHT. H5 HEIGHTS WILL RISE OVER THE SE BY SUN AS WEAK UPPER RIDGE BUILDS IN FROM THE WEST. SURFACE PATTERN AND SENSIBLE WEATHER...A WEAK CAD WILL SET UP OVER THE CWA ON SAT AS HIGH PRESSURE CENTERED OVER THE OHIO VALLEY SHIFTS EAST. MEANWHILE...A CYCLOGENESIS WILL OCCUR OFF THE SC COAST AND THIS WILL AID IN MOISTURE ADVECTION TO OUR SE ZONES OF THE CWA. THE LATEST NAM AND GFS HAVE BACKED OFF ON THE QPF RESPONSE...ESPECIALLY OVER THE SC PIEDMONT AS DEEPER MOISTURE WILL REMAIN SOUTH WHILE BETTER UPPER FORCING PASSES NORTH OF THE AREA. THIS UPPER FORCING WILL PROVIDE BETTER LIFT OVER THE NORTHERN HALF OF THE CWA...WITH MODELS SHOWING LIGHT QPF RESPONSE LESS THAN A TENTH OF AN INCH. THERMAL PROFILE IS SUPPORTIVE OF A BRIEF PERIOD OF SNOW OVER THE NC MTNS/VALLEYS AND PORTIONS OF THE NORTHERN NC PIEDMONT ON SAT MORNING...TRANSITIONING TO RAIN BY LATE SAT MORNING. ARAS OVER THE NOTHERN NC MTNS WILL SEE A SLIGHT CHANCE OF FREEZING RAIN ON SAT NIGHT AS SURFACE TEMPS DIPS INTO THE FREEZING LEVEL WHILE MID-LAYER WARM NOSE ALOFT DEVELOPS. AFTER COORDINATING WITH NEIGHBORING OFFICES...HAVE MENTIONED LOW END LIKELY POPS OVER THE NC MTNS AND PORTIONS OF THE NC PIEDMONT WITH 40/50S ELSEWHERE ON SAT MORNING...RAMPING DOWN INTO THE CHANCE RANGE BY SAT 18Z. CONDITIONS SHOULD DRY OUT BY SAT EVENING AS THE SHORTWAVE PUSHES OFFSHORE AND MOISTURE ASSOCIATED WITH THE LOW OFF THE CAROLINAS COAST PULLS FARTHER NE. SUN...NE FLOW WILL CONTINUE IN WEAK CAD REGIME OVER THE REGION AS 1028 MB HIGH BECOMES CENTERED OVER THE NEW ENGLAND. HAVE GONE WITH MOSTLY DRY FORECAST (SLIGHT CHANCE POPS AT BEST) UNDER THE WEAK RIDGING ALOFT DURING THE DAY ON SUN...WITH POPS RAMPING UP FROM WEST TO EAST INTO THE SOLID CHANCE/LIKLEY RANGE BY LATE SUN NIGHT AS MOIST ISENTROPIC LIFT INCREASES AND BETTER UPPER DIFFLUENT SWLY FLOW SETS UP. NO FROZEN PRECIP IS EXPECTED ON SUN NIGHT GIVEN WARMER THERMALPROFILE. TEMPS ON SAT WILL RUN 5-7 DEGREES BELOW NORMAL...WITH A FEW DEGREES BELOW NORMAL ON SUN. && .LONG TERM /MONDAY THROUGH FRIDAY/... AS OF 230 AM EST FRIDAY...THE LATEST 00Z GLOBAL MODELS ARE IN GOOD AGREEMENT WITH REGARD TO THE OVERALL SYNOPTIC PATTERN AND THERMAL THROUGH THE MEDIUM RANGE PERIOD. SOUTHWESTERLY UPPER DIFFLUENT FLOW WILL SET UP BY MON AS A LOW AMPLITUDE UPPER RIDGE AXIS PUSHES OFF THE EAST COAST WHILE AN UPPER TROUGH DIGS INTO THE UPPER MID-WEST. A LOW IN THIS UPPER TROUGH WILL CLOSE OFF AS IT DIGS INTO THE LOWER MISSISSIPPI VALLEY THROUGH TUE NIGHT...BECOMING NEGATIVELY TILED AS IT SWINGS ACROSS THE SOUTHEAST BY WED. NWLY FLOW ALOFT WILL SET UP BY THU AS THE UPPER TROUGH PUSHES OFF THE ATLANTIC COAST. AT THE SURFACE...1030MB HIGH CENTERED OVER THE NE WILL BE RESPONSIBLE FOR CAD OVER THE CWA ON MONDAY...WITH A CYCLOGENESIS OCCURRING OFF THE CAROLINAS COAST AT THE SAME TIME. GIVEN THE COMBINATION OF STRONG MOIST ISENTROPIC LIFT...UPPER DIFFLUENT FLOW AND GOOD MOISTURE ADVECTION IN THE WAKE OF THE CYCLOGENESIS...PRECIP WILL BE ONGOING ACROSS THE CWA ON MONDAY. DESPITE THE CAD WITH DECENT WARM NOSE ALOFT...SURFACE TEMPS WILL RUN ABOVE FREEZING LVL...SUGGESTING ALL LIQUID PRECIP FOR MOST LOCATIONS ATT. WITH DECENT QPF RESPONSE (0.25-0.75")PER 00Z GFS AND ECMWF...HAVE RAISED POPS INTO THE LOW LIKELY RANGE DURING THE DAY ON MONDAY...RAMPING DOWN INTO THE LOW END CHANCE RANGE THRU MON NIGHT AS THE LOW OFF THE NC COAST PULLS FARTHER NE AWAY AND THE CAD WEAKENS. TUE...MODELS AGREE THAT IN SITU CAD AND MOIST ISENTROPIC LIFT REGIME WILL SUPPORT LOW END CHANCE FOR PRECIP THROUGH THE AFTERNOON BEFORE POPS RAMP UP FROM WEST TO EAST INTO THE LIKELY RANGE AS A SURFACE LOW AND ASSOCIATED COLD FRONT APPROACH FROM THE WEST. THE LOW IS EXPECTED TO LIFT NE ACROSS THE TN VALLEY AND INTO KY TUE NIGHT INTO WED MORNING...PUSHING A STRONG COLD FRONT THROUGH THE FORECAST AREA. ALTHOUGH THERE WILL BE STRONG LLVL WIND SHEAR/UPPER FORCING...SEVERE WEATHER CHANCES LOOK MINIMAL GIVEN ZERO SBCAPE. THE MAIN ISSUE WILL BE STRONG CAA FLOW BEHIND THE FRONT...SUPPORTING RAIN SHOWERS TRANSITIONING TO SNOW BY TUE NIGHT OVER THE NC MTNS. WITH WRAP AROUND MOISTURE AND PREVAILING CAA NW FLOW...SNOW WILL BE ONGOING OVER THE TN BORDER COUNTIES THROUGH WED NIGHT. SNOW ACCUMS WILL BE LIMITED TO THE HIGHER PEAKS OF THE MTNS. ELSEWHERE...CONDITIONS SHOULD DRY OUT BY WED AFTERNOON AS THE FRONT AND ASSOCIATED MOISTURE MOVE OFF TO THE EAST AND DRY HIGH PRESSURE BUILDS IN FROM THE WEST. TEMPS WILL BE 5-7 DEGREES BELOW NORMAL ON MON...NEAR NORMAL TUE...AND 2-5 DEGREES BELOW NORMAL WED/THU. && .AVIATION /18Z FRIDAY THROUGH WEDNESDAY/...
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AT KCLT...VFR CONDITIONS ARE EXPECTED THRU THE PERIOD. A MID LVL DISTURBANCE WILL RIPPLE ACRS THE AREA LATE TONIGHT THRU SATURDAY WITHIN ZONAL WESTERLY FLOW. THIS FEATURE WILL SPREAD PLENTY OF MID AND HIGH CLOUDS THIS AFTN THRU THE EVENING. SOME LLVL MOISTURE WILL APPROACH FROM THE SW AROUND DAYBREAK SATURDAY...LOWERING CIGS TO SOMEWHERE AROUND 3500-5000 FT. CANNOT RULE OUT A BRIEF PERIOD OF MVFR CIGS...BUT CONFIDENCE IS LOW. ALSO...THERE IS A CHC FOR SPOTTY - RA AND SPRINKLES SATURDAY MORNING. WILL ADD A PROB30 FOR -RA...BUT KEEPING IT VFR. WINDS WILL BE LIGHT THRU THE PERIOD...GENERALLY NE THRU THE AFTN...THEN E/SE OVERNIGHT...THEN BACK TO NE BY DAYBREAK SATURDAY...HOWEVER...WILL BE VARIABLE THRU THE PERIOD. ELSEWHERE...GRADUALLY THICKENING AND LOWERING CIRRUS WILL PREVAIL UNTIL THIS EVENING...WHEN LOW LEVEL MOISTURE ARRIVES IN ADVANCE OF A MID LVL DISTURBANCE FROM THE WEST. LOW VFR CIGS ARE EXPECTED LATE THIS EVENING AT KAVL AND THE SC SITES...WITH LIGHT PRECIPITATION AS WELL. A WINTRY MIX OF RA/SN/FZRA MAY FALL AT KAVL LATE TONIGHT... BEFORE A CHANGE TO LIGHT SNOW BY DAYBREAK. OTHER THAN POSSIBLE VSBY RESTRICTIONS IN PRECIP AT KAVL...THE MAIN CONCERN FOR RESTRICTIONS WILL BE MVFR CIGS CREEPING IN FROM THE SW TOWARD THE UPSTATE AROUND DAYBREAK SATURDAY. A CONSENSUS OF THE GUIDANCE DOES PREVAIL MVFR AT KAND FOR SATURDAY MORNING. FOR NOW...I KEPT ALL THE OTHER SITES VFR. THE LOW CLOUDS WILL LIKELY PERSIST THRU 18Z. OUTLOOK...LINGERING LOW LVL MOISTURE MAY KEEP SOME FOG AND LOW STRATUS AROUND SATURDAY NIGHT THRU SUNDAY. A WETTER SYSTEM WILL THEN TRACK INTO THE AREA FOR MONDAY LIKELY BRINGING WIDESPREAD CLOUDS AND PRECIP RESTRICTIONS. YET ANOTHER STORM SYSTEM IS EXPECTED TO CROSS THE AREA LATE TUESDAY OR EARLY WEDNESDAY...BRINGING ANOTHER ROUND OF PRECIP. CONFIDENCE TABLE... 18-24Z 00-06Z 06-12Z 12-18Z KCLT HIGH 100% HIGH 100% HIGH 100% HIGH 100% KGSP HIGH 100% HIGH 100% HIGH 100% HIGH 100% KAVL HIGH 100% HIGH 100% HIGH 100% HIGH 95% KHKY HIGH 100% HIGH 100% HIGH 100% HIGH 100% KGMU HIGH 100% HIGH 100% HIGH 100% HIGH 94% KAND HIGH 100% HIGH 100% HIGH 94% MED 79% 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
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&& .GSP WATCHES/WARNINGS/ADVISORIES... GA...NONE. NC...NONE. SC...NONE. && $$ SYNOPSIS...JOH NEAR TERM...ARK/JAT SHORT TERM...JOH LONG TERM...JOH AVIATION...ARK

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