Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 271902 AFDRAH AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 300 PM EDT SAT SEP 27 2014 .SYNOPSIS...CANADIAN HIGH PRESSURE WILL RIDGE FROM THE EASTERN GREAT LAKES SOUTHWARD INTO THE MIDDLE ATLANTIC STATES THROUGH SUNDAY. AN AREA OF LOW PRESSURE WILL AFFECT THE CAROLINAS MONDAY AND TUESDAY. && .NEAR TERM /THROUGH TONIGHT/... AS OF 300 PM SATURDAY... A PARENT SURFACE HIGH OVER SOUTHERN NEW ENGLAND WILL EXTEND SOUTH INTO CENTRAL NC OVERNIGHT. THIS SFC HIGH HAS SUPPORT ALOFT FROM A MID LEVEL ANTI-CYCLONE JUST OFF THE NEW ENGLAND COAST. THIS WILL MAINTAIN A LOW LEVEL NE FETCH OFF THE ATLANTIC AND INTO CENTRAL NC. THIS STREAM OF MOISTURE WILL OCCASIONALLY RESULT IN PATCHES OF STRATOCU...MAINLY ACROSS OUR SOUTHERN COUNTIES. THUS EXPECT MOSTLY CLEAR TO PARTLY CLOUDY SKIES TONIGHT. LOW LEVEL DRY AIR IS MORE ENTRENCHED OVER THE NE PIEDMONT-NORTHERN COASTAL PLAIN. EXPECT THIS REGION TO HAVE THE COOLER MIN TEMPS COMPARED TO THE SOUTH-SW WHERE PATCHES OF CLOUDS AND A LIGHT NE SFC WIND MAY OCCUR. MIN TEMPS GENERALLY IN THE MID-UPPER 50S WITH A FEW LOWER 50S ACROSS THE FAR NORTHERN PIEDMONT...AND NEAR 60 ACROSS THE FAR SOUTH. WILL LIKELY SEE MORE AREAS OF FOG LATE TONIGHT/EARLY SUNDAY AS COMPARED TO THIS MORNING BUT DENSE FOG NOT EXPECTED AT THIS TIME. && .SHORT TERM /SUNDAY AND SUNDAY NIGHT/... AS OF 300 PM SATURDAY... SURFACE RIDGE AT THE SURFACE AND ALOFT WEAKEN WITH MID- UPPER LEVEL FLOW BECOMING SW. THIS FLOW ALOFT WILL ADVECT A MOIST AIR MASS CURRENTLY RESIDING OVER THE NORTH CENTRAL GULF INTO OUR REGION. EXPECT SUNDAY MORNING TO START OUT MOSTLY SUNNY THEN TREND TOWARD INCREASING HIGH CLOUDS WEST-TO-EAST AS THE AFTERNOON WEARS ON. MOISTURE IS HIGH BASED SO NOT EXPECTING ANY PRECIP. AFTERNOON THICKNESSES AVERAGE SLIGHTLY ABOVE NORMAL. UNDER A LIGHT WIND REGIME AND DECENT SUNSHINE THROUGH EARLY AFTERNOON...SHOULD SEE MAX TEMPS TOP OUT NEAR 80-LOWER 80S. CAVEAT...IF HIGH CLOUDS ARE THICKER THAN ANTICIPATED...THEN MAX TEMPS WEST OF HIGHWAY 1 COULD END UP BEING 3- 4 DEGREES COOLER THAN FORECAST. SUNDAY NIGHT...CLOUD COVER WILL INCREASE SUNDAY NIGHT AS AN AREA OF LOW PRESSURE LIFTS SLOWLY NE FROM TEH DEEP SOUTH TOWARD THE SOUTHERN APPALACHIANS. THE INCREASING/THICKENING CLOUD COVER WILL RESULT IN OVERNIGHT TEMPS A FEW DEGREES WARMER THAN TONIGHT. MAY SEE A FEW SPRINKLES DEVELOP WELL AFTER MIDNIGHT IN VICINITY OF THE YADKIN RIVER BUT DO NOT EXPECT ANY MEASURABLE RAINFALL TO OCCUR ACROSS THE WESTERN-SOUTHERN PIEDMONT UNTIL DAYBREAK OR LATER AS SUB CLOUD LAYER QUITE DRY. && .LONG TERM /MONDAY THROUGH SATURDAY/... AS OF 300 PM SATURDAY... HOW THE GUIDANCE TREATS MID-LEVEL ENERGY MOVING SOUTH OUT OF CANADA EARLY IN THIS PERIOD IS QUITE VARIABLE. THE ECMWF HAS BEEN CONSISTENT IN DEEPENING UPPER-LEVEL LOW PRESSURE AS A STRONG WAVE MOVES SOUTH OVER THE GREAT LAKES EARLY IN THE WORK WEEK...THEN NOTING A CLOSED LOW SLOWLY PROGRESSING EAST ACROSS THE MID-ATLANTIC STATES. MOST OTHER GUIDANCE SHOWS A MUCH WEAKER TROUGH UNDER BROAD UPPER RIDGING. DESPITE THESE DIFFERENCES...THE OVERALL TREND HAS BEEN FOR A FARTHER SOUTH AND SLIGHTLY SLOWER TRACK OF SURFACE LOW PRESSURE SOUTH OF CENTRAL NORTH CAROLINA EARLY IN THE LONG-TERM PERIOD...AND SLIGHTLY LESS QPF THOUGH STILL HIGH CHANCES FOR PRECIPITATION. TOWARD THE END OF THE LONG-TERM PERIOD...A DEEPENING UPPER TROUGH SHOULD RESULT IN CHANCES FOR SHOWERS BY OVERNIGHT FRIDAY CONTINUING INTO SATURDAY. FORCING ALOFT WITH THIS NEXT SYSTEM COULD BE SUFFICIENT FOR AT LEAST ISOLATED THUNDER GIVEN THE STRENGTH OF THE TROUGH AND THE COOL THICKNESSES BEHIND THE SURFACE FRONT. FOR THE PERIOD MONDAY THROUGH TUESDAY...RELATIVE HUMIDITY FIELDS AND BUFR SOUNDINGS SHOW INCREASING MOISTURE TAKING PLACE THROUGH THE DAY MONDAY AND INTO MONDAY EVENING SPREADING WEST-TO-EAST ACROSS CENTRAL NORTH CAROLINA. 850MB THETA-E VALUES ARE NOTICEABLY LOW TO START THE DAY MONDAY...BUT SLOWLY INCREASE AS ISENTROPIC LIFT ON THE 310K SURFACE TRANSLATES WEST-TO-EAST IN CENTRAL NORTH CAROLINA DURING THE DAY ALONG WITH 700MB VORTICITY. BASICALLY EAST OF U.S. 1...THE NAM AND GFS ARE ESSENTIALLY DRY UNTIL AFTERNOON. WEST OF THERE TO THE YADKIN RIVER...THINK THE DAY WILL START OUT WITH PATCHY LIGHT RAIN OR SPRINKLES...WITH LIGHT RAIN BECOMING GRADUALLY MORE NOTICEABLE AS THE OVERALL AIR MOISTENS UNDER SUFFICIENT LIFT. SOMETIMES THESE TOP- DOWN PROCESSES CAN TAKE TIME...AND AS THE GUIDANCE HAS TRENDED FARTHER SOUTH WITH THE TRACK OF THE SURFACE LOW WONDER IF THE PROCESS MAY TAKE LONGER THAN GUIDANCE INDICATES. STILL...BY SUNSET MONDAY...LIKELY THAT MANY AREAS WEST OF INTERSTATE 95 SHOULD MEASURE...WITH JUST A CHANCE OF RAIN FROM ABOUT KIXA TO KCTZ. HIGHS ON THE LOWER END OF GUIDANCE...UPPER 60S TRIAD TO UPPER 70S EAST OF INTERSTATE 95. MONDAY NIGHT AND TUESDAY...SURFACE LOW PRESSURE GRADUALLY MOVES NORTH ALONG THE CAROLINA COAST...AND K INDICES STAY SOLIDLY HIGH. 850MB THETA-E VALUES ARE BEST SOUTH OF U.S. 64...AND FOR THIS PERIOD THIS IS WHERE THE HIGHER QPF WILL BE NOTED. A MID-LEVEL TROUGH REMAINS IN PLACE...AND EVEN IF THE BETTER LIFT STARTS TO SHIFT EAST CLOSER TO THE COASTAL PLAIN ON TUESDAY AS THE NAM DEPICTS...UNDER A NORTHEAST FLOW WITH NAM AND GFS BUFR SOUNDINGS SHOWING AMPLE LOW- LEVEL MOISTURE FOR MUCH IF NOT ALL OF THE DAY TUESDAY...CLOUDS AND PATCHY LIGHT RAIN AND/OR SPRINKLES SHOULD BE A THREAT THROUGHOUT THE DAY EVEN TOWARD THE YADKIN RIVER AND NORTHWEST TOWARD KINT. LIKELY CHANCES THROUGHOUT CENTRAL NORTH CAROLINA MONDAY NIGHT WHICH AT SOME POINT FOR AT LEAST SOME AREAS SHOULD BECOME CATEGORICAL...THEN LIKELY CHANCES IN THE COASTAL PLAIN MAINLY FOR TUESDAY...CHANCES WEST OF THERE IN DEFERENCE TO THE DRIER...NEW ECMWF. LOWS UPPER 50S FAR NORTHWEST TO MID 60S SOUTHEAST...AND IF THE CLOUDS AND NORTHEAST FLOW REALLY LOCK IN THE CLOUDS TUESDAY...THE MID 60S TO LOWER 70S HIGHS MAY BE TOO WARM. AVERAGE QPF CURRENTLY NO HIGHER THAN 1.5 INCHES...SO CURRENTLY THINK FLOOD POTENTIAL IS LOW...PARTICULARLY WITH THE GUIDANCE TRENDS GENERALLY LOWER ON QPF RUN TO RUN. FOR THE PERIOD TUESDAY NIGHT THROUGH SATURDAY...THE ECMWF REMAINS CONSISTENT IN DRYING CONDITIONS MORE THAN THE GFS TUESDAY NIGHT... ALTHOUGH IT HAD TRENDED A LITTLE WETTER WITH MINOR QPF FOR WEDNESDAY SIMILAR TO THE GFS. THE GFS MAY BE OVERDONE WITH ITS QPF...BUT OVERNIGHT TUESDAY INTO WEDNESDAY...AS THE SURFACE RIDGE NOTICEABLY RIDGES SOUTH...THERE COULD BE ENOUGH LOW-LEVEL MOISTURE LINGERING TO RESULT IN CLOUDS AND PATCHY LIGHT RAIN INTO THE DAY WEDNESDAY AS THE SURFACE LOW DEPARTS. COARSE SOUNDINGS FROM BOTH THE GFS AND ECMWF START TO DRY THE LOW LEVELS BY WEDNESDAY NIGHT...SO FROM THERE WILL FORECAST DRY CONDITIONS THROUGH THE DAY FRIDAY. BY FRIDAY NIGHT... CHANCES FOR SHOWERS AND POSSIBLY A THUNDERSTORM RETURN...DRYING WEST- TO-EAST LATE FRIDAY NIGHT THROUGH SATURDAY MORNING AND ALL OF CENTRAL NORTH CAROLINA HAVING A DRY FORECAST CURRENTLY BY SATURDAY EVENING. TEMPERATURES GRADUALLY WARMING INTO THURSDAY AND FRIDAY... THEN BY LATE SATURDAY NIGHT LOWS COULD BE WELL DOWN INTO THE 50S OVER THE NORTHWEST PIEDMONT. && .AVIATION /18Z SATURDAY THROUGH THURSDAY/... AS OF 100 PM SATURDAY... A PATCH OF STRATOCU HAS DEVELOPED IN VICINITY OF KGSB AND KFAY...ALONG A WEAKENING 925-850MB TROUGH. THIS STRATOCU DECK WILL CAUSE A FEW HOURS OF MVFR CEILINGS EARLY THIS AFTERNOON NEAR GSB AND KFAY BEFORE DRIER AIR ALOFT SCATTERS THE CLOUDS. OTHERWISE AN AREA OF HIGH PRESSURE NEAR THE SURFACE AND ALOFT WILL SUPPORT VFR CONDITIONS OVER MUCH OF CENTRAL NC THROUGH SUNDAY NIGHT. THE EXCEPTION WILL BE A BRIEF PERIOD LATE TONIGHT/EARLY SUNDAY MORNING OF MVFR/IFR VISIBILITY DUE TO FOG OVER SECTIONS OF THE PIEDMONT...SANDHILLS AND COASTAL PLAIN. THE OCCURRENCE OF THIS IS MORE LIKELY IN VICINITY OF KRWI AND KFAY. AN APPROACHING LOW PRESSURE SYSTEM ON MONDAY WILL ADVECT A MOIST AND SLIGHTLY UNSTABLE AIR MASS INTO CENTRAL NC. THIS WILL LEAD TO DETERIORATING AVIATION CONDITIONS WEST-TO-EAST ACROSS CENTRAL NC DURING THE DAY MONDAY WITH AN EXTENDED PERIOD OF SUB-VFR CONDITIONS DUE TO CEILINGS AND RAIN MONDAY NIGHT THROUGH WEDNESDAY. A GRADUAL IMPROVEMENT IN AVIATION WEATHER CONDITIONS EXPECTED BY THURSDAY. && .RAH WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SYNOPSIS...WSS NEAR TERM...WSS SHORT TERM...WSS LONG TERM...DJF AVIATION...WSS

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