Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 241712 AFDRAH AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 1210 PM EST WED DEC 24 2014 .SYNOPSIS...A WARM FRONT WILL LIFT NORTHWARD THROUGH THE CAROLINAS TODAY AS A DEEPENING LOW PRESSURE SYSTEM TRACKS INTO THE TENNESSEE VALLEY. A COLD FRONT WILL SURGE EAST ACROSS THE REGION TONIGHT AS THE SURFACE LOW PROGRESSES NORTHWARD THROUGH THE OHIO VALLEY INTO THE GREAT LAKES. && .NEAR TERM /TODAY AND TONIGHT/... AS OF 1050 AM WEDNESDAY... LARGE WARM CONVEYOR BELT PRECIP SHIELD STRETCHES FROM THE MID- ATLANTIC COAST TO THE GULF OF MEXICO...WITH AN EMBEDDED QLCS FROM SOUTH CAROLINA THROUGH THE FLORIDA PANHANDLE. MEANWHILE...THE WEDGE AND COASTAL FRONTS CONTINUE THEIR RETREAT THROUGH THE COASTAL PLAIN AND INTO THE PIEDMONT THIS MORNING...LEADING TO A STRONG BAROCLINIC ZONE JUST EAST OF THE US-1 CORRIDOR. TEMPS ARE STILL IN THE MID 40S NORTHWEST TO NOW UPPER 60S SOUTHEAST. THE BULK OF THE PRECIP WILL CONTINUE TO SHIFT EAST THROUGH EARLY AFTERNOON IN CONCERT WITH THE LOW-LEVEL JET....WITH SOME ENHANCEMENT ALONG THE AFOREMENTIONED FRONTAL ZONE. A LULL IN PRECIP IS ANTICIPATED THIS AFTERNOON ON THE HEELS OF THE LOW-LEVEL JET...WITH FORCING SOMEWHAT LIMITED GIVEN THAT THE POSITIVELY TILTED UPPER TROUGH OVER THE MISSISSIPPI RIVER VALLEY DOESNT MAKE ITS STRONG EASTWARD PUSH UNTIL TONIGHT. MOST OF THE CAM`S SUPPORT THIS IDEA...WITH JUST ISOLATED TO WIDELY SCATTERED SHOWERS AFTER 18Z. THE THREAT OF AN SEVERE WEATHER SHOULD REMAIN CONFINED TO THE COASTAL PLAIN...WHERE MARGINAL CAPE AND STRONG SHEAR PHASE THIS AFTERNOON. MODIFIED RAP SOUNDINGS SUGGEST 200-300 J/KG MLCAPE EAST OF I-95 IN THE TRUER WARM SECTOR... WITH 50-60KT BULK SHEAR. HOWEVER...LOW-LEVEL HODOGRAPHS WILL BECOME MORE LINEAR WITH TIME AS THE SURFACE WINDS CONTINUE TO VEER TO MORE SOUTHWESTERLY....SO UNLESS THERE IS LOCAL BACKING IN RESPONSE TO SOME PERTURBATION WITHIN THE FLOW...IT APPEARS THE CONDITIONAL TORNADO THREAT WILL BE LIMITED TO THE SOUTHERN COASTAL PLAIN THROUGH EARLY AFTERNOON. CHANGES TO THE FORECAST ARE MOSTLY COSMETIC BASED ON TRENDS ON RADAR AND OBS....WITH THE MAIN ADJUSTMENT TO THE NEAR TERM TEMPS AS WARMING FROM THE TRIANGLE WESTWARD HAS BEEN A LITTLE SLOWER THAN FORECAST. LATEST HRRR...WHICH HAS DONE A PRETTY GOOD JOB SO FAR...SUGGESTS THE WEDGE MAY NOT BREAK IN THE FAR NORTHWEST PIEDMONT UNTIL THE COLD FRONT ERODES THE AIRMASS LATER TONIGHT. HIGHS ULTIMATELY END UP RANGING FROM 54-71 NORTHWEST TO SOUTHEAST. -BLS PRECIP TONIGHT: PRECIPITATION MAY RE-DEVELOP IN THE FORM OF A NARROW CONVECTIVE LINE LATER THIS EVENING (ROUGHLY 00-06Z) AS THE SFC COLD FRONT SURGES EASTWARD THROUGH CENTRAL NC. CONFIDENCE IS LOWER THAN NORMAL WITH REGARD TO THE EVOLUTION OF PRECIPITATION ALONG THE COLD FRONT THIS EVENING...PARTICULARLY WITH REGARD TO PRE-FRONTAL THERMAL PROFILES /INSTABILITY/ DUE TO THE WIDE VARIETY OF MECHANISMS AND POSSIBLE INTERACTIONS DRIVING THE EVOLUTION THEREOF. BASED ON PREVIOUS EXPERIENCE WITH SIMILAR SETUPS AND THE LATEST MODEL GUIDANCE...EXPECT THE BEST POTENTIAL FOR ADDITIONAL PRECIPITATION ALONG AND EAST OF HWY 1 IN THE 01-07Z TIME FRAME. FOR EXAMPLE...THE HIGH-RES WRF-ARW SUGGESTS DRY CONDITIONS IN THE WESTERN PIEDMONT WITH A NARROW LINE OF CONVECTION DEVELOPING IN VICINITY OF THE TRIANGLE AROUND MIDNIGHT...EXITING THE COASTAL PLAIN BY 2-3 AM THU MORNING. WHETHER OR NOT CONVECTION WILL ACCOMPANY THE COLD FRONTAL PASSAGE LATE THIS EVENING INTO TONIGHT ALSO REMAINS UNCLEAR. THE 00Z WRF-ARW SUGGESTS THAT CONVECTION WILL DEVELOP ALONG THE FRONT AS IT PROGRESSES FROM THE TRIANGLE INTO THE COASTAL PLAIN BETWEEN 01-07Z. AN ISOLD SEVERE STORM CAPABLE OF PRODUCING DAMAGING WINDS CANNOT BE RULED OUT IN THAT TIME FRAME...SHOULD CONVECTION DEVELOP. -VINCENT && .SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/... AS OF 255 AM WEDNESDAY... THERE MAY BE A FEW SHOWERS IN THE PROCESS OF EXITING THE FAR EASTERN ZONES CHRISTMAS MORNING... OTHERWISE EXPECT A WEST TO EAST CLEARING TRENDING IN THE WAKE OF THE EXITING COLD FRONT. ADDITIONALLY...DEEPENING SURFACE LOW LIFTING NEWD INTO SE CANADA WILL RESULT IN BREEZY CONDITIONS ON CHRISTMAS DAY...WITH WEST- NORTHWESTERLY WINDS GUSTING INTO THE UPPER TEENS TO LOWER 20S...STRONGEST DURING THE FIRST HALF OF THE DAY. WEAK CAA BEHIND THE FRONT WILL STILL ALLOW FOR SEASONABLY MILD TEMPERATURES. HIGHS IN THE MID 50S NW TO LOWER 60S SE. LOWS IN THE LOWER TO MID 30S. && .LONG TERM /SATURDAY THROUGH TUESDAY/... AS OF 255 AM WEDNESDAY... SATURDAY NIGHT INTO MONDAY...A PATTERN CHANGE WILL BEGIN TO OCCUR AS THE UPPER RIDGE OVER THE SE STATES BREAKS DOWN AND A LARGE MID/UPPER TROUGH DEVELOPS INTO THE WEST-CENTRAL THEN CENTRAL STATES. THE LEAD COLD FRONT IS EXPECTED TO PUSH INTO NC FROM THE NORTH LATE SUNDAY OR SUNDAY NIGHT. ALL BETS ARE OFF ON HOW FAR SOUTH THIS INITIAL FRONT WILL BE ABLE TO SAG INTO NC OR SC. ABOUT HALF THE GFS ENSEMBLE MEMBERS SUPPORT A STRONG ENOUGH SURFACE HIGH OVER A FAVORED GREAT LAKES LOCATION TO PUSH THE FRONT INTO SC. THIS IS NOT SUPPORTED BY THE LATEST OPERATIONAL EC AND HALF THE RELATED ENSEMBLES. THE STRENGTH OF THE SURFACE WAVE SLATED TO DEVELOP OVER THE LOWER MISSISSIPPI VALLEY AND THE SUBSEQUENT TRACK (EITHER JUST TO OUR WEST AND NORTH... OR VERY NEAR OR OVER OUR REGION) IS ALSO CURRENTLY DEBATABLE. WPC HAS SPLIT THE MIDDLE TAKING THE AVERAGE APPROACH WHICH WOULD KEEP MOST OF OUR REGION IN THE WARM SECTOR INITIALLY (UNTIL THE INITIAL WAVE OF LOW PRESSURE PASSES). THIS WOULD ENSURE ALL RAIN AND MILDER TEMPERATURES THAN IF CAD WERE TO DEVELOP PER SOME OF THE GFS MEMBERS. FOR NOW... WE WILL TREND A BIT COOLER AND WETTER... AND TOWARD THE COOLER SOLUTIONS AND GIVE IT A FEW MORE RUNS BEFORE COMMITTING EITHER WAY. THIS WOULD LEAD TO A CHANCE OF RAIN LATE SUNDAY INTO MONDAY WITH LOWS IN THE MID 30S TO MID 40S AND HIGHS IN THE MID 40S TO LOWER 60S N TO SE. A TURN TO SHARPLY COLDER WEATHER IS EXPECTED BY MID NEXT WEEK WITH LOWS BACK INTO THE 20S AND HIGHS IN THE 40S. && .AVIATION /18Z WEDNESDAY THROUGH SUNDAY/... AS OF 1210 PM WEDNESDAY... PRIMARY AVIATION CONCERNS THIS PERIOD: IFR/LIFR CONDITIONS THROUGH THIS EVENING... AND LOW LEVEL WIND SHEAR POTENTIAL THROUGH THIS AFTERNOON. PERIODS OF POOR AVIATION CONDITIONS WILL DOMINATE THROUGH THIS EVENING. A WARM FRONT ORIENTED ROUGHLY NE TO SW THROUGH CENTRAL NC WILL PUSH SLOWLY WNW TODAY THROUGH THIS EVENING... BEFORE A COLD FRONT SWEEPS IN FROM THE WEST FROM LATE EVENING THROUGH THE OVERNIGHT HOURS. THE WARM FRONT HAS ALREADY PUSHED WEST OF FAY/RWI... WHERE SURFACE WINDS ARE MAINLY FROM THE SSW... AND THE FRONT SHOULD MOVE THROUGH RDU THIS AFTERNOON. AT THESE LOCATIONS (RDU/FAY/RWI)... VSBYS/CIGS WILL WAVER BETWEEN IFR/LIFR AND MVFR THROUGH MIDNIGHT... WITH HIGH UNCERTAINTY AS TO WHICH CATEGORY WILL DOMINATE. AT INT/GSO... THERE IS A GOOD CHANCE THE WARM FRONT WILL NOT REACH THESE SITES... SO EXPECT THESE IFR/LIFR CONDITIONS TO DOMINATE NEARLY CONTINUOUSLY THROUGH MID-LATE EVENING. STRONG WINDS FROM THE SSW AT 30-50 KTS IN THE 800-1600 FT LAYER WILL BRING A RISK OF LOW LEVEL WIND SHEAR THROUGH THIS AFTERNOON... WITH THE HIGHEST LLWS POTENTIAL AT RDU WHERE THE VECTOR WIND DIFFERENCE BETWEEN THE SURFACE AND AROUND 1000 FT AGL WILL BE GREATEST. COLD FRONT PASSAGE THIS EVENING INTO THE OVERNIGHT HOURS WILL MEAN A RAPID W-TO-E TREND OF CIGS FROM MAINLY IFR/LIFR TO VFR... MOST LIKELY AT INT/GSO BETWEEN 02Z AND 04Z AND AT RDU/FAY/RWI BETWEEN 04Z AND 07Z... WITH A SHIFT OF WINDS TO WSW THEN WNW. VFR CONDITIONS WILL THEN DOMINATE FROM LATE TONIGHT THROUGH THE END OF THE TAF VALID PERIOD... AS HIGH PRESSURE BUILDS IN FROM THE SW. LOOKING BEYOND 18Z THU... VFR CONDITIONS ARE EXPECTED TO HOLD FROM CHRISTMAS AFTERNOON INTO SUNDAY... ALTHOUGH SOME GROUND FOG IS POSSIBLE EARLY SAT MORNING AT ALL CENTRAL NC TAF SITES. A STORM SYSTEM WILL APPROACH FROM THE WEST AND SW ON SUN... BRINGING INCREASING CHANCES FOR SUB-VFR CONDITIONS AND RAIN FOR LATE SUN THROUGH MON. -GIH && .HYDROLOGIC DISCUSSION /THROUGH THURSDAY/... AS OF 210 PM TUESDAY... THERE HAVE BEEN THREE FAIRLY SIGNIFICANT RAINFALL EVENTS OVER CENTRAL NORTH CAROLINA DURING THE PAST TWO WEEKS...WITH HEAVIER RAINFALL AMOUNTS (>1 INCH TOTAL) GENERALLY FROM THE TRIANGLE AND FAYETTEVILLE AREAS EAST TO THE COAST. AS A RESULT...RELATIVE SOIL MOISTURE IS UNIFORMLY HIGH THROUGH A 200 CM DEEP COLUMN...WHICH WILL PROMOTE FASTER RUNOFF AS WELL AS A HIGHER PERCENTAGE OF RAINFALL-TO- RUNOFF FROM THE UPCOMING EVENT TONIGHT THROUGH CHRISTMAS MORNING. CURRENT QUANTITATIVE PRECIPITATION FORECASTS RANGE FROM 1.25 INCHES TO NEARLY 2 INCHES...AND HAVE BEEN TRENDING DOWNWARD WITH SUCCESSIVE MODEL RUNS. AT THIS TIME...OUR PREFERRED RAINFALL FORECAST IS FOR WIDESPREAD AMOUNTS OF LESS THAN 1.5 INCHES...WHICH WILL CAUSE MAINSTEM RIVERS TO RISE SEVERAL FEET FROM CHRISTMAS DAY INTO THE WEEKEND...BUT RIVER FORECAST POINTS WOULD CREST BELOW MINOR FLOOD STAGE. WIDESPREAD RAIN NEARER THE HIGH END OF THIS RANGE (1.75 TO 2 INCHES) WOULD LIKELY PRODUCE MINOR FLOODING ON EASTERN RIVERS... PARTICULARLY THE HAW...NEUSE...AND TAR RIVERS. && .RAH WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SYNOPSIS...VINCENT NEAR TERM...VINCENT/BLS SHORT TERM...BADGETT LONG TERM...BADGETT AVIATION...HARTFIELD HYDROLOGY...BADGETT

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