Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 260208
AFDRAH

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC
1010 PM EDT FRI JUL 25 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 /00Z SATURDAY THROUGH WEDNESDAY/...
AS OF 815 PM FRIDAY...

24-HR TAF PERIOD: VFR CONDITIONS ARE GENERALLY EXPECTED FOR THE
REMAINDER OF THE EVENING WITH ONLY A VERY LOW PROBABILITY OF SOME
MVFR CIGS AT KFAY THROUGH 02Z. LOW LEVEL FLOW TONIGHT IS EXPECTED TO
BECOME MORE SOUTHWESTERLY... HELPING TO FAVOR LESS IN THE WAY OF
WIDESPREAD SUB-VFR CONDITIONS AREAWIDE. STILL GIVEN THE AMOUNT OF
LOW LEVEL MOISTURE FROM RECENT RAINFALL AND EASTERLY TO SOUTHERLY
FLOW TODAY... THINK WE CLOUD SEE A LEAST SOME PATCHY IFR-MVFR CIGS
AND VISBYS ACROSS THE AREA. GUIDANCES CONTINUES TO HINT AT THE BEST
CHANCE FOR PREVAILING SUB-VFR CONDITIONS IN THE PRE-DAWN HOURS
ACROSS THE WESTERN PIEDMONT (KGSO/KINT). THUS... CONFIDENCE IS
HIGHEST AT KGSO AND KINT FOR IFR CONDITIONS IN THE 08-14Z TIME
FRAME... WITH PERHAPS SOME LINGERING MVFR STRATOCUMULUS INTO THE
LATE MORNING. EXPECT VFR CONDITIONS WILL RETURN BY EARLY AFTERNOON
AT THE LATEST AREAWIDE... WITH ONLY A SMALL CHANCE OF
AFTERNOON/EVENING SHOWERS AND STORMS.

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.

&&

.RAH WATCHES/WARNINGS/ADVISORIES...
NONE.

&&

$$

SYNOPSIS...WSS
NEAR TERM...WSS
SHORT TERM...VINCENT/BLS
LONG TERM...BLS
AVIATION...BSD



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