Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 171853 AFDRAH AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 250 PM EDT FRI APR 17 2015 .SYNOPSIS...LOW PRESSURE WILL MOVE SLOWLY AWAY FROM THE SOUTHEAST US COAST THROUGH TONIGHT. HIGH PRESSURE WILL OTHERWISE EXTEND ACROSS THE MIDDLE ATLANTIC STATES THROUGH SATURDAY. A LOW PRESSURE SYSTEM WILL BEGIN TO AFFECT OUR AREA SUNDAY...BRINGING A DECENT CHANCE FOR SHOWERS LATE SUNDAY INTO MONDAY && .NEAR TERM /THROUGH TONIGHT/... AS OF 945 AM FRIDAY... UPDATE - RESIDUAL WEDGE LINGERING OVER THE PIEDMONT AND ASSOCIATED HEAVY CLOUDINESS AND LIGHT PRECIP WILL HINDER MAX TEMPS...WILL BE LOWERING MAXES ACROSS THE AREA...BUT ESPECIALLY THE NORTHWEST WHERE 70 MIGHT BE A STRETCH. WILL ALSO BE INCREASING POPS INTO VERY EARLY AFTERNOON ALONG AND NORTH OF THE PERIPHERY OF THE BOUNDARY AS WELL WITH WEAK ISENTROPIC LIFT WHICH WILL BE WEAKENING AFTER 15Z WITH 300K WINDS VEERING TO WESTERLY. PREVIOUS DISCUSSION A NEARLY SATURATED AIR MASS IN THE LOWEST 10K FT OF THE ATMOSPHERE WILL MAINTAIN EXTENSIVE CLOUDINESS ACROSS CENTRAL NC THIS MORNING. MAY STILL SEE AN ISOLATED SHOWER OR SPOTS OF DRIZZLE BUT A LIFTING MECHANISM FOR PRECIP GENERATION LARGELY ABSENT. MAIN FORECAST CHALLENGE CENTERS ON WHEN/IF WE DISSIPATE ENOUGH OF THE CLOUDS BY THIS AFTERNOON TO WARM TEMPS BACK TO NEAR NORMAL LEVELS. AS IS OFTEN THE CASE...RESIDUAL CAD AIR MASS WILL LIKELY BE SLOW TO ERODE AS NO DISCERNIBLE SCOURING MECHANISM (SUCH AS A COLD FRONT PASSAGE) NOTED. APPEARS WE WILL HAVE TO RELAY ON MIXING OF DRIER AIR FROM ALOFT TO ERODE THE CLOUDS...AND PER MODEL SOUNDINGS THIS MAY TAKE SOME TIME. CLOUD DECK APPEARS TO BE THINNER OVER THE FAR EAST- NE SO THIS REGION MAY SEE PARTIAL SUN INITIALLY. SIMILARLY...WEAK DOWNSLOPE COMPONENT TO THE LOW LEVEL FLOW MAY AID TO ERODE THE CLOUD COVER OVER OUR WESTERN PERIPHERY BY THIS AFTERNOON. THIS SUGGEST THAT AREAS CLOSEST TO THE HIGHWAY 1 CORRIDOR MAY BE THE LAST TO SEE PARTIAL SUN SOMETIME THIS AFTERNOON (PROBABLY LATE AFTERNOON). THUS...MAY SEE A FAIRLY UNIFORM MAX TEMPS FIELD WITH HIGH TEMPS IN THE MID 70S. TEMPS HIGHLY DEPENDENT ON SOME EROSION OF CLOUD DECK. IF SKIES REMAIN OVERCAST...MAX TEMPS WILL BE CLOSER TO 70-LOWER 70S. WHILE BULK OF REGION WILL BE A DRY...A WEAK S/W IN THE NORTHERN STREAM WILL PASS NORTH OF OUR REGION LATE THIS AFTERNOON. THIS FEATURE MAY DRAG A WEAK SFC TROUGH INTO OUR REGION. WEAK CONVERGENCE ALONG THIS FEATURE COUPLED WITH AFTERNOON HEATING MAY TRIGGER AN ISOLATED SHOWER OR STORM...MAINLY ACROSS THE NORTHERN COUNTIES AND CLOSER TO THE VA BORDER. TONIGHT...WHAT PARTIAL CLEARING THAT OCCURS LATER TODAY/THIS EVENING WILL DETERMINE THE EXTENT OF FOG DEVELOPMENT LATE TONIGHT-EARLY SATURDAY MORNING. CURRENTLY EXPECT ENOUGH PARTIAL CLEARING ALONG WITH A SATURATED TOP SOIL TO CAUSE FOG TO DEVELOP MOST LOCATIONS AFTER MIDNIGHT. NOT EXPECTING WIDESPREAD DENSE FOG AT THIS TIME BUT COULD SEE POCKETS OF FOG REDUCE THE VISIBILITY TO NEAR ZERO. IF CLOUD COVER PERSISTS WELL INTO THE AFTERNOON-EVENING HOURS...THEN FOG PRODUCTION MAY BE LIMITED. MIN TEMPS MILD IN THE UPPER 50S-LOWER 60S. && .SHORT TERM /SATURDAY AND SATURDAY NIGHT/... AS OF 300 PM FRIDAY... SATURDAY...MIXING OF DRIER AIR ALOFT SHOULD AID TO QUICKLY ERODE ANY FOG/LOW CLOUDS BY MID MORNING SATURDAY. WITH LOW LEVEL THICKNESSES SEVERAL METERS ABOVE NORMAL...SHOULD SEE TEMPS CLIMB ABOVE 80 DEGREES MOST LOCATIONS WITH SOME SPOTS IN THE SOUTH APPROACHING 85 DEGREES. AFTERNOON INSTABILITY MAY SUPPORT AN ISOLATED SHOWER OR STORM IN THE FAR SE ASSOCIATED WITH AN INLAND MOVING SEABREEZE. A WEAK SFC COLD FRONT MAY SLIDE INTO OUR FAR NORTHERN-NE COUNTIES LATE IN THE DAY SATURDAY OR EARLY EVENING. FRONTAL PASSAGE SHOULD BE UNEVENTFUL THOUGH COULD SEE SFC DEWPOINTS DIP INTO THE UPPER 40S OVER THE FAR NORTH-NE SATURDAY EVENING. OTHERWISE...MAY SEE CLOUDS INCREASE/THICKEN AFTER MIDNIGHT OVER THE FAR SOUTH AND WEST AHEAD OF THE NEXT APPROACHING LOW PRESSURE SYSTEM. THIS WILL LEAD TO MILD OVERNIGHT TEMPS IN THE 55-60 DEGREE RANGE. && .LONG TERM /SUNDAY THROUGH FRIDAY/... AS OF 245 PM FRIDAY... SUN-MON NIGHT: A VERY WET AND POTENTIALLY STORMY PERIOD. THE NORTHERN STREAM SHORTWAVE TROUGH OVER SW CANADA WILL HAVE PICKED UP THE OPENING LOW THAT IS NOW OVER THE FOUR CORNERS REGION... WITH THE FORMER FEATURE AMPLIFYING OVER THE UPPER MIDWEST WHILE THE LATTER FEATURE IS TOSSED NORTHEASTWARD TOWARD LOWER MI. A BATCH OF MID LEVEL VORTICITY AND SUBTLE TROUGH NOW OFF THE COAST OF NW MEXICO WILL TRACK QUICKLY EAST THEN NE WITHIN THE STRONG SUBTROPICAL FLOW... AND IT IS THE MERGING OF THIS WAVE WITH THE OLD SOUTHWEST LOW INTO A DISTINCT TROUGH AXIS THAT WILL HELP TRIGGER WIDESPREAD RAIN OVER NC SUN INTO EARLY MON. THE MODELS HAVE TRENDED STRONGER WITH THESE TWO MERGED FEATURES AND THEIR RESULTANT TROUGH... AND THUS ARE STRONGER WITH AN ASSOCIATED SURFACE LOW NEAR THE MO BOOTHEEL SUN MORNING... EXPECT TO MOVE QUICKLY NORTHWARD THROUGH WRN MI THROUGH SUN NIGHT. MEANWHILE... THE BACKDOOR FRONT THAT DROPS SOUTHWARD TOWARD THE NC/VA BORDER -- PROPELLED BY AN INTENSE MID LEVEL SHORTWAVE TROUGH EXITING NEW ENGLAND AND COLD DENSE SURFACE HIGH PRESSURE CENTERED OVER SRN QUEBEC PUSHING SOUTHWARD -- NOW APPEARS LIKELY TO MAKE AT LEAST SOME PROGRESS DOWN THROUGH THE NC PIEDMONT. AND IT LOOKS LIKE THIS FRONT WILL MERGE WITH THE WEAK FRONTAL ZONE TO OUR SOUTH AS THE LATTER MOVES NORTHWARD... YIELDING A STRONGER SURFACE FRONTAL ZONE THROUGH NC BY LATE SUN. CONFIDENCE IS HIGH THAT WE`LL SEE A STRONG SURGE OF LOW LEVEL GULF- AND ATLANTIC-SOURCE MOISTURE INTO NC... STARTING FIRST IN SW NC AND SPREADING ENE SUN/SUN NIGHT IN CONJUNCTION WITH THE PRIMARY LIFT MECHANISMS FORCING ASCENT... INCLUDING MID LEVEL DPVA... UPPER DIVERGENCE... AND LOWER LEVEL (850-925 MB) MASS CONVERGENCE. MOISTURE SHOULD BE DEEP AND PLENTIFUL WITH PW WELL OVER 200% OF NORMAL AND A DEEP LCL-TO-0C LAYER NEAR 3.5 KM SUPPORTIVE OF WARM RAIN PROCESSES... AND A PERIOD OF HEAVY RAIN APPEARS LIKELY. OVERALL RAINFALL IS EXPECTED TO BE 1-2 INCHES... WITH THE LESSER VALUES EAST AND HIGHER TOTALS WEST. GIVEN SUCH DEEP LIFT AND ANOMALOUSLY HIGH MOISTURE CONTENT... AND IF MODELS TREND EVEN WETTER WITH HIGHER QPF OVER A LARGE AREA... WE MAY NEED TO CONSIDER A FLASH FLOOD WATCH ON TOMORROW`S SHIFTS. MODEL TIMING HAS BEEN TRENDING A BIT SLOWER WITH PRECIP ONSET BUT NOW APPEARS TO BE ZEROING IN ON A COMMON SOLUTION... AND WILL CONTINUE TO RAMP UP POPS TO LIKELY WEST/CHANCE NE SUN MORNING... GOING TO CATEGORICAL WEST/LIKELY CENTRAL/GOOD CHANCE EAST SUN AFTERNOON... AND CATEGORICAL EVERYWHERE LATE SUN AFTERNOON THROUGH SUN NIGHT... BEFORE TRENDING POPS DOWN WSW TO ENE MON MORNING. WHILE THE LARGE SCALE MODELS DON`T DEPICT MUCH INSTABILITY AT ALL SUNDAY WITH PRIMARILY MOIST ADIABATIC FORECAST PROFILES... WE DO HAVE SOME CONCERN ABOUT POCKETS OF CONVECTION AND AN ISOLATED THREAT OF A QUICK WEAK SPIN-UP ALONG AND NEAR THE AFOREMENTIONED BACKDOOR/WARM FRONT ACROSS NRN/WRN NC... ESPECIALLY LATE SUN AFTERNOON THROUGH SUN EVENING AND INTO THE EARLY OVERNIGHT HOURS... WHEN THE SURFACE FRONT SHOULD BE RETREATING BACK TO THE NNE... PROVIDING A FOCUS FOR ENHANCED LOW LEVEL VORTICITY. THIS CONCERN PEAKS SUN EVENING WITH THE COMBINATION OF 30-40 KTS OF DEEP LAYER SHEAR... AN UPPER DIVERGENCE MAXIMUM OVER THE SRN APPALACHIANS AND WRN/CENTRAL NC... INCOMING DRIER AIR IN THE MID LEVELS... AND A 40-50 KT 850 MB JETLET FROM THE SSE... AND FORECAST SOUNDINGS SHOW A STRONGLY CURVED HODOGRAPH. WILL CONTINUE TO MONITOR. WILL TAPER POPS DOWN MON MORNING WSW TO ENE AS THE MID LEVEL TROUGH AXIS AND CONVEYOR BELT OF PEAK MOISTURE PUSH TO OUR NORTHEAST... PLACING US FULLY IN THE WARM SECTOR WITH A PUNCH OF MID LEVEL DRY AIR ARRIVING FROM THE SW AND FLOW-MID LEVEL FLOW BECOMING MORE UNIDIRECTIONAL. MUCAPE IS EXPECTED TO REACH 1500-2500 J/KG MON AFTERNOON (AND COULD BE LOCALLY HIGHER IF WE GET MORE HEATING)... WITH BULK SHEAR NEAR 30-35 KTS AND SW 850 MB WINDS AROUND 30 KTS. DYNAMIC FORCING FOR ASCENT WILL HAVE DECREASED BY MON AFTERNOON... BUT WE`LL STILL BE BENEATH CYCLONIC MID LEVEL FLOW WITH WEAK DPVA... AND ANOTHER ROUND OF DECENT UPPER DIVERGENCE IN THE LEFT EXIT REGION OF THE STRAIGHT 140 KT SUBTROPICAL JET ACROSS TX AND THE GULF STATES. SO STILL EXPECT A FEW STRONG CELLS MON AFTERNOON INTO EARLY EVENING WITH STRAIGHT LINE WINDS THE PRIMARY THREAT. HIGHS SUN FROM THE MID 60S TO LOWER 70S WEST OF HIGHWAY 1... AND LOW-MID 70S FROM HIGHWAY 1 EAST. LOWS UPPER 50S NW TO MID 60S SE. STILL EXPECT HIGHS IN THE UPPER 70S TO LOWER 80S MON. LOWS MON NIGHT MID 50S NW TO LOWER 60S SE WITH COLD FRONTAL PASSAGE EXPECTED AFTER MIDNIGHT WITH DIMINISHING PRECIP CHANCES. TUE THROUGH FRI: A MAINLY QUIET PERIOD. LARGE AND DEEP MID LEVEL LOW PRESSURE CENTERED OVER AND NORTH OF THE NRN GREAT LAKES WILL COVER MUCH OF CENTRAL AND EASTERN NOAM... BRINGING SEASONABLY COOL TEMPS AS IT SITS AND SPINS THROUGH MID WEEK. HIGH PRESSURE WILL BUILD INTO OUR AREA FROM THE WEST TUE... PASS OVERHEAD EARLY WED... THEN SHIFT OFF THE COAST LATE WED INTO THU (GFS IS FASTER THAN THE ECMWF WITH THIS) WHILE A WARM FRONT MOVES NORTHWARD THROUGH NC. ENERGY EMANATING OUT OF THE BASE OF DEEP TROUGHING JUST OFF THE WEST COAST WILL CROSS THE DESERT SOUTHWEST THEN MOVE INTO OUR AREA LATE THU INTO FRI. WITH A SOUTHERLY LOW LEVEL FLOW ON THE BACK SIDE OF THE EXITING HIGH... WE SHOULD SEE INCREASING PW AND RETURNING CHANCES FOR SHOWERS LATE THU INTO FRI... HOWEVER THE MOISTURE INFLUX STILL DOESN`T LOOK GREAT WITH LIGHT AND DIFFLUENT LOW LEVEL FLOW... SO WILL KEEP POPS BELOW CLIMATOLOGY. TEMPS SHOULD HOLD NEAR OR JUST BELOW NORMAL THROUGH THE PERIOD. -GIH && .AVIATION /18Z FRIDAY THROUGH WEDNESDAY/... AS OF 1250 PM FRIDAY... LINGERING MVFR CEILINGS WILL LIFT AND THIN...WITH PREDOMINANT VFR CEILINGS THIS EVENING AND EARLY TONIGHT. MOIST AIRMASS IN PLACE IN THE LOW LEVELS WILL PROVIDE FUEL FOR SHOWER DEVELOPMENT AS AN APPROACHING UPPER WAVE MOVES ACROSS THE AREA OVERNIGHT. EXPECT SCATTERED SHOWERS TO DEVELOP...AND BECOME MORE NUMEROUS ACROSS THE SOUTH (FAY) AREA. CEILINGS WILL FALL EVERYWHERE AS WE COOL DOWN...HOWEVER...WITH WIDESPREAD IFR TO POSSIBLY SOME LIFR CEILINGS AFTER 06Z. SIMILARLY...EXPECT CLOUDINESS TO HINDER DENSER FOG DEVELOPMENT...BUT WIDESPREAD MVFR VISIBILITY IS EXPECTED AFTER MIDNIGHT...WITH PATCHY IFR/LIFR TOWARDS MORNING. CONDITIONS WILL IMPROVE SLOWLY...WITH MVFR CEILINGS AND VFR VISIBILITY BY LATE MORNING (14-15Z)...WITH VFR CONDITIONS AFTER 18Z. DRIER AIR WILL EDGE IN BRIEFLY FROM THE NORTH SATURDAY NIGHT...WITH POSSIBLE MVFR FOG AND VISIBILITIES LATE SATURDAY NIGHT. A FRONT LIFTING NORTH WILL PRODUCE WIDESPREAD HEAVY SHOWERS AND SOME STRONG THUNDERSTORMS ON SUNDAY INTO SUNDAY NIGHT...WITH FAIRLY WIDESPREAD IFR CONDITIONS AND GUSTY WIND IN SHOWERS AND STORMS SUNDAY AFTERNOON THROUGH SUNDAY NIGHT. SHOWERS AND STORMS WILL BE MORE SCATTERED ON MONDAY AS A COLD FRONT MOVES THROUGH...WITH PREDOMINANT VFR CONDITIONS TUESDAY AND WEDNESDAY. && .RAH WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SYNOPSIS...WSS NEAR TERM...WSS/MLM SHORT TERM...WSS LONG TERM...HARTFIELD AVIATION...MLM

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