Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 260530 AFDRAH AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 130 AM EDT SAT JUL 26 2014 .SYNOPSIS...A WEAK AREA OF HIGH PRESSURE WILL MOVE OFFSHORE OVERNIGHT. A SURFACE TROUGH WILL DEVELOP OVER THE WESTERN PIEDMONT THIS WEEKEND...RESULTING IN A VERY WARM SOUTHWEST FLOW. A STRONG COLD FRONT WILL CROSS THE AREA ON MONDAY. && .NEAR TERM /OVERNIGHT/... AS OF 1010 PM FRIDAY... MAIN FORECAST CHALLENGE FOR THE OVERNIGHT IS THE EXTENT OF FOG TO EXPECT...AS WELL AS COVERAGE OF LOW CLOUDS. LATEST SATELLITE IMAGERY DEPICTS MAINLY CLEAR SKIES ACROSS MOST OF CENTRAL NC AT 02Z. A DECENT DEWPOINT GRADIENT STILL EXISTS WITH THE FAR NORTHERN PIEDMONT REPORTING DEWPOINTS IN THE MID 60S WHILE THE SANDHILLS AND SOUTHERN COASTAL PLAIN STILL EXPERIENCING DEWPOINTS IN THE LOWER 70S. A WEAK RIDGE OF HIGH PRESSURE WILL CONTINUE TO DRIFT OFFSHORE OVERNIGHT. THE BACKING OF THE LOW LEVEL FLOW WILL ADVECT THE WARM MOIST LOW LEVEL AIR MASS BACK INTO PIEDMONT. THIS WARM AIR ADVECTION WILL RESULT IN EITHER THE DEVELOPMENT OF LOW CLOUDS/STRATUS OR AREAS OF FOG OVERNIGHT. SINCE RETURN FLOW A LITTLE STRONGER IN THE PIEDMONT...EXPECT PATCHY LOW CLOUDS TO DEVELOP. ALSO...WEAKENING SHEAR AXIS ASSOCIATED WITH MID MS VALLEY CONVECTION WILL DRIFT INTO OUR AREA BETWEEN 09Z-12Z...POSSIBLY ASSOCIATED WITH PATCHES OF HIGH CLOUDS. MAINLY CLEAR SKIES AND NEAR CALM WIND REGIME EXPECTED TO OCCUR LONGEST IN THE NE...SO THIS REGION WILL HAVE THE BEST POTENTIAL FOR FOG. NOT EXPECTING VISIBILITY TO GO BELOW A HALF MILE AT THIS TIME BUT COULD SEE WIDESPREAD 1-3 MILES. MIN TEMPS IN THE MID 60S FAR NORTH TO NEAR 70 SOUTH APPEAR ON TARGET. && .SHORT TERM /SATURDAY THROUGH MONDAY NIGHT/... AS OF 300 PM FRIDAY... SAT/SAT NIGHT: THE MEAN TROUGH AXIS OVER THE EASTERN US WILL SHIFT OFFSHORE ON SAT...WITH CYCLONIC FLOW ALOFT SHIFTING INTO THE ATLANTIC AND AN INCREASINGLY NW FLOW ALOFT OVER THE CAROLINAS. WITH STRONG INSOLATION AND LINGERING LOW-LEVEL MOISTURE...DIURNAL DESTABILIZATION WILL BE BETTER THAN TODAY WITH MLCAPE VALUES PROGGED AT 500-1500 J/KG. SUBSIDENCE ON THE WESTERN PERIPHERY OF THE OFFSHORE TROUGH SHOULD SUPPRESS CONVECTIVE DEVELOPMENT FOR MOST (IF NOT ALL) OF THE DAY SAT...WITH FORCING OTHERWISE CONFINED TO VERY SHALLOW CONVERGENCE ASSOC/W A PIEDMONT SFC TROUGH. UNLESS POTENT SHORTWAVE ENERGY PROGRESSES INTO THE REGION IN NW FLOW ALOFT DURING THE EVENING HOURS (DOES NOT APPEAR LIKELY AT THIS TIME)...ANTICIPATE LITTLE CHANCE FOR TOWERING CU TO SURVIVE WITHIN AN OTHERWISE DRY/ HOSTILE AIRMASS. HIGHS IN THE LOWER 90S...LOWS IN THE LOWER TO MID 70S. -VINCENT SUN/SUN NIGHT: A SHORTWAVE TROUGH MOVING OUT OF CANADA AND ACROSS THE GREAT LAKES SUNDAY WILL BEGIN THE AMPLIFICATION OF THE LONGWAVE PATTERN FOR NEXT WEEK. EXPECT CONVECTION CONVECTION TO BE ONGOING OVER THE CENTRAL APPALACHIANS EARLY SUNDAY....WHICH SHOULD CONTINUE EAST INTO VA AND MAY BLEED SOUTH ALONG OUTFLOW INTO THE NORTHERN PORTIONS OF THE CWA. THE 12Z GFS BACKED OFF SIGNIFICANTLY ON THE COVERAGE OF CONVECTION OVER VA...SO CHANCE POPS ACROSS THE NORTHERN PIEDMONT AND COASTAL PLAIN ARE STILL A LITTLE UNCERTAIN. THE BIGGER STORY ON SUNDAY WILL BE THE HEAT AND HUMIDITY...AS DEEP MIXING AND 850MB TEMPS IN EXCESS OF 21C WILL LEAD TO HIGHS IN THE MID AND UPPER 90S. THE GFS EVEN SUGGESTS 100 IN A FEW SPOTS. MIXING WILL DROP DEWPOINTS A BIT...ESPECIALLY WEST OF THE PIEDMONT TROUGH...BUT TO THE EAST DEWPOINTS ARE FORECAST TO REMAIN IN THE UPPER 60S TO AROUND 70. THE RESULTING HEAT INDEX VALUES ARE IN THE 100-105 RANGE FOR A FEW HOURS AND MAY NECESSITATE A HEAT ADVISORY. A SOUTHWEST WIND WILL CONTINUE OVERNIGHT SUNDAY NIGHT AND LOWS WILL BE MUGGY IN THE MID TO EVEN UPPER 70S. MON/MON NIGHT: A STRONG COLD FRONT APPROACHING FROM THE NORTHWEST WILL CROSS THE CWA ON MONDAY AS ADDITIONAL SHORTWAVE ENERGY DIVES THROUGH THE BACKSIDE OF THE UPPER TROUGH. UNSEASONABLY STRONG 40- 60M HEIGHT FALLS OVERSPREADING WHAT IS EXPECTED TO BE A MOIST AND MODERATELY UNSTABLE AIRMASS WILL AID IN THE DEVELOPMENT OF STORMS ALONG AND AHEAD OF THE FRONT. BASED ON 40-50KT OF 0-6KM BULK SHEAR ASSOCIATED WITH THE DEEP PUSH OF THE STRONG WESTERLIES AND LONG...THERE IS GOOD CONFIDENCE THAT SUPERCELLS WITH A LARGE HAIL AND DAMAGING WIND THREAT WILL BE POSSIBLE. THE BIGGEST QUESTION WILL BE EXACTLY WHERE THE LEE TROUGH SETS UP AND HOW MUCH DEWPOINTS MIX OUT AS WEST-SOUTHWESTERLY LOW-LEVEL FLOW STRENGTHENS. EAST OF THE TROUGH...MOST LIKELY FROM US HWY 1 EAST...MODERATE INSTABILITY IS EXPECTED WITH FAIRLY STEEP MID-LEVEL LAPSE RATES OF -6C/KM TO MAYBE EVEN -7C/KM. FORECAST SOUNDINGS ALSO SHOW RATHER DRY MID- LEVEL AIR...THOUGH THE FORCING AND INSTABILITY SHOULD BE ABLE TO OVERCOME THIS...AND THE DRY AIR WILL LIKELY ENHANCE DAMAGING WIND THREAT. AT THE MOMENT...CONFIDENCE IS STRONG THAT SEVERE WEATHER WILL BE POSSIBLE...BUT THE UNCERTAINTY LIES HOW FAR EAST STORMS WILL DEVELOP AND THE OVERALL COVERAGE. HIGHS MONDAY WILL BE IN THE LOW TO MID 90S. -BLS && .LONG TERM /TUESDAY THROUGH FRIDAY/... AS OF 300 PM FRIDAY... IN THE WAKE OF THE THE FRONT ON MONDAY MEDIUM RANGE ENSEMBLES AND DETERMINISTIC MODELS SHOW AN UNSEASONABLY DEEP UPPER TROUGH DEVELOPING OVER THE EASTERN US BY MID WEEK WITH AN UPPER LOW CENTERED JUST SOUTH OF THE HUDSON BAY. MOST OF THE WEEK APPEARS TO BE STABLE WITH HIGH PRESSURE SETTLING ACROSS THE MID-ATLANTIC STATES AND DEWPOINTS IN THE UPPER 50S AND LOWER 60S. LOW-LEVEL THICKNESSES ARE PROGGED TO DIP BELOW 1390M...WHICH SUGGESTS HIGHS IN THE LOW/MID 90S AND LOWS IN THE LOW/MID 60S... POSSIBLY TOUCHING THE UPPER 50S IN THE COOLEST SPOTS. MORE UNCERTAINTY ARISES LATER IN THE WEEK WITH MODELS SHOWING SOME DEGREE OF FLOW SEPARATION WHERE AN UPPER LOW CUTS OFF AND DRIFTS SOUTH. WHETHER OR NOT THIS OCCURS REMAINS TO BE SEEN...SO AT THIS POINT WILL JUST HAVE A SLIGHT CHANCE POP BY FRIDAY AS MOISTURE STARTS TO INCREASE BACK ACROSS THE AREA. && .AVIATION /06Z SATURDAY THROUGH WEDNESDAY/...
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AS OF 130 AM SATURDAY... 24-HR TAF PERIOD: RESIDUAL LOW-LEVEL MOISTURE AND LIGHT SELY WINDS WILL RESULT IN STRATUS OVERNIGHT AND POCKETS OF FOG OVERNIGHT. CEILINGS OF IFR TO MVFR ARE POSSIBLE WITH GUIDANCE SUGGESTING THE BEST FOG POTENTIAL IN THE EAST...NEAR KFAY AND KRWI...WHERE SFC DEWPOINTS HAVE REMAINED IN THE LOWER 70S. PERHAPS SOME LINGERING MVFR STRATOCUMULUS COULD LINGER INTO THE LATE MORNING BEFORE VFR CONDITIONS RETURN BY EARLY AFTERNOON WITH OVERALL DRY CONDITIONS EXPECTED. LOOKING AHEAD: A COLD FRONT WILL APPROACH FROM THE NW SUNDAY/EARLY NEXT WORK WEEK... BRINGING THE POTENTIAL FOR SHOWERS/STORMS AND SUB- VFR CONDITIONS LATE SUNDAY (IN THE NORTHWEST) INTO MONDAY... PARTICULARLY MONDAY (ESPECIALLY EAST). VFR CONDITIONS ARE EXPECTED TO RETURN BEHIND THE FRONT ON TUESDAY AND CONTINUE THROUGH AT LEAST MID WEEK.
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&& .RAH WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SYNOPSIS...WSS NEAR TERM...WSS SHORT TERM...VINCENT/BLS LONG TERM...BLS AVIATION...CBL

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