FXUS62 KRAH 150623
AFDRAH
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC
123 AM EST TUE DEC 15 2009
.SYNOPSIS...
AN UPPER LEVEL DISTURBANCE OVER THE DEEP SOUTH WILL TRACK NORTHEAST
ACROSS THE CAROLINAS THROUGH THIS MORNING...AHEAD OF A COLD FRONT
THAT WILL CROSS THE AREA THIS AFTERNOON. COOL HIGH PRESSURE WILL
BUILD OVER THE REGION THROUGH MID TO LATE WEEK.
&&
.NEAR TERM /THROUGH TONIGHT/...
AS OF 935 PM MONDAY...
A MOIST AND MURKY LOW LEVEL AIRMASS REMAINS IN PLACE OVER THE CWA
THIS EVENING...AND VISIBILITIES ACROSS THE AREA QUICKLY FELL AFTER
SUNSET. MOST REPORTING SITES HAVE BOUNCED UP AND DOWN BETWEEN A
QUARTER MILE AND AND 2 TO 3 MILES. THIS IS MOST LIKELY IN RESPONSE
TO A LITTLE MORE SOUTHERLY WIND AND MIXING THAN LAST EVENING WITH
THE APPROACHING COLD FRONT AND SLIGHTLY INCREASING PRESSURE
GRADIENT. THE COMBINATION OF A TRAIN OF WEAK UPPER DISTURBANCES
ORIGINATING ALONG THE GULF COAST AND INCREASING WARM ADVECTION AHEAD
OF THE SYSTEM APPROACHING FROM THE WEST HAS FORCED CONVECTION THAT
DRAPED ACROSS THE SE US...WHICH APPEARS TO BE STRONGLY SUPPRESSING
MOISTURE TRANSPORT AND THE ADVANCEMENT OF THE SYNOPTIC WARM FRONT
TOWARDS THE CAROLINAS....MUCH LIKE THE NAM DEPICTS. ISENTROPIC LIFT
IS EXPECTED TO INCREASE OVERNIGHT...BUT WITH SUCH SHALLOW MOISTURE
WE MAY BE LOOKING AT MORE OF STRATUS AND DRIZZLE FOR MOST OF THE
AREA. HAVE REDUCED POPS TO CHANCE...LEAVING ESSENTIALLY 30 PERCENT
OR LESS DURING THE MORNING HOURS ACROSS THE SE...THINKING THE
TRAILING VORT MAX CURRENTLY OVER THE LA/MS COAST MAY FINALLY BE
ENOUGH TO INCREASE DEEPER MOISTURE CONVERGENCE NORTHWARD TOWARD THE
NC/SC BORDER AS THE THE COLD FRONT BEGINS TO CROSS THE MOUNTAINS.
WILL HOLD OFF ON A DENSE FOG ADVISORY FOR NOW GIVEN THE VARYING
VISIBILITIES...BUT IF MIXING DOESN'T CONTINUE AND THE SURFACE FRONT
STAYS FAR TO OUR SOUTH...AN ADVISORY MAY BE NEEDED LATER. OVERNIGHT
LOWS STILL 43-50. -SMITH
PREVIOUS...
THROUGH TONIGHT AND TUESDAY: THIS IS A RATHER CHALLENGING FORECAST
AS THE MORNING STRATUS HAS BEEN HANGING TOUGH OVER A GOOD CHUNK OF
CENTRAL NC THIS AFTERNOON... LONGER THAN THE 12Z MODEL RUNS
INDICATED. BUT BASED ON OBSERVATIONAL DATA THIS ISN'T ENTIRELY
SURPRISING... CONSIDERING THE STRONG SURFACE-BASED INVERSION (13C IN
THE LOWEST 100 MB THIS MORNING) AND THE RELATIVELY LIGHT SURFACE
WINDS. WITH ISENTROPIC UPGLIDE IN THE 285K-295K LAYER COMMENCING
THIS EVENING AND PLENTY OF LOW LEVEL MOISTURE UPSTREAM BASED ON 12Z
UPPER AIR ANALYSES... ANY BREAKS IN THE CLOUDS OVER CENTRAL NC WILL
FILL BACK IN QUICKLY... RESULTING IN SKIES TRENDING TOWARD CLOUDY BY
LATE EVENING. WITH THE ABUNDANT LOW LEVEL MOISTURE AND GENERALLY
WEAK WINDS PERSISTING... EXPECT AREAS OF FOG TO DEVELOP ONCE AGAIN
OVERNIGHT. MODEL GUIDANCE AS WELL AS UPSTREAM CONDITIONS THIS
MORNING SUPPORT SOME OF THIS FOG BECOMING DENSE WITH VISIBILITIES
AGAIN DROPPING TO 1/4 MILE OR LESS MAINLY OVER THE SOUTHERN HALF OF
THE CWA. WILL NEED TO MONITOR CONDITIONS THIS EVENING AND OVERNIGHT
IN THE EVENT ANOTHER DENSE FOG ADVISORY IS NEEDED. THE 12Z MODEL
RUNS CONTINUE TO SHOW RAIN STREAMING NORTHEASTWARD OVER SC AND INTO
SOUTHERN AND EASTERN NC TONIGHT THROUGH TUESDAY MORNING. SO FAR
CONVECTION HAS FIRED QUICKLY UPSTREAM ACROSS LA/AL/MS. THE GFS
CURRENTLY HAS A GOOD HANDLE ON THIS PRECIP... ALTHOUGH IT IS
DROPPING A FEW HUNDREDTHS OVER THE CAROLINAS WHERE NO PRECIP IS NOW
FALLING... AND IT ALSO BLOWS UP A CONVECTIVELY INDUCED VORTICITY
MAXIMUM OVER SOUTHERN AL/GA WHICH APPEARS SPURIOUS. WHILE LOW LEVEL
MOISTURE TRANSPORT IS DECENT AND THE UPPER DIVERGENCE INCREASES
MARKEDLY IN THE RIGHT ENTRANCE REGION OF THE JET TO OUR NORTHEAST
LATER TONIGHT... FORCING FOR ASCENT IN THE LOW LEVELS... INCLUDING
ISENTROPIC UPGLIDE AND FRONTAL CONVERGENCE... IS VERY WEAK... AND
THE MOISTURE IS SHALLOW OVER CENTRAL NC. BOTH THE NAM AND GFS HAVE
BEEN TRENDING FARTHER SOUTHWARD WITH THE NORTHWEST EDGE OF THE
RAIN.. HOLDING IT MAINLY OVER THE SOUTHEAST CORNER OF THE STATE.
TRYING TO PIN DOWN THE NORTHWARD EXTENT OF PRECIPITATION SHIELDS IN
FLAT PATTERNS LIKE THIS IS ALWAYS DIFFICULT... BUT CONSIDERING THESE
LIMITATIONS WITH THE MOISTURE AND LIFT AS WELL AS THE FACT THAT
STRONG CONVECTION JUST TO OUR SOUTH COULD GREATLY CUT BACK ON PRECIP
OVER CENTRAL NC... HAVE TRIMMED POPS FOR THE EVENT... SHUNTING THEM
MORE TO THE SOUTHEAST. HAVE REMOVED POPS FROM THE NORTHWEST COUNTIES
BUT KEPT A TIGHT GRADIENT TOWARD LIKELY POPS IN THE EXTREME
SOUTHEAST LATE TONIGHT INTO TUESDAY MORNING. AMOUNTS ARE LIKELY TO
BE LOW... UNDER TWO TENTHS OF AN INCH TOTAL IN THE SOUTHERN CWA. THE
SURFACE COLD FRONT NOW JUST NORTHWEST OF THE OHIO VALLEY IS ON TRACK
TO DROP SOUTHEAST THROUGH THE CWA FROM LATE MORNING THROUGH THE
EARLY AFTERNOON. SURFACE WINDS VEER TO WEST THEN NORTHWEST WITH BOTH
TERRAIN AND ISENTROPIC DOWNGLIDE ENSUING... AND WILL END POPS BY
MID-LATE AFTERNOON. REGARDING TEMPS... EXPECT ONLY A SMALL DROP
TONIGHT PARTICULARLY IN THE AREAS WHERE THE STRATUS HAS LINGERED
MUCH OF THE DAY. LOWS 43-51. THE INITIALLY-WARM LOW LEVEL
THICKNESSES BEGIN TO FALL TOMORROW WITH POST FRONTAL COLD AIR
ADVECTION... AND FACTORING IN THE MORNING CLOUDS... HAVE CUT BACK
HIGHS TO 56-64. -GIH
&&
.SHORT TERM /TUESDAY NIGHT THROUGH WEDNESDAY NIGHT/...
AS OF 255 PM MONDAY...
LEAD IMPULSE RESPONSIBLE FOR RAIN THREAT ACROSS THE SOUTHERN
COUNTIES TUESDAY WILL EXIT OFF THE SC COAST EARLY TUESDAY EVENING.
SHORT TERM MODELS IN AGREEMENT...DEPICTING LOW LEVEL ISENTROPIC
DOWNGLIDE COMMENCING BY 18Z TUESDAY. WITH SYSTEM EXITING THE REGION
BY 00Z WED AND LOW LEVEL DOWNGLIDE IN PLACE...WILL REMOVE MENTION OF
POPS FOR THE SE TUESDAY EVENING.
IN THE WAKE OF THIS SYSTEM...MODIFIED POLAR HIGH WILL BUILD INTO THE
REGION FROM THE W-NW TUESDAY NIGHT AND WEDNESDAY...AND SETTLE
OVERHEAD WEDNESDAY NIGHT. LOW LEVEL COLD AIR ADVECTION WILL RESULT
IN A BRISK NW WIND TUESDAY NIGHT. DUE TO WELL MIXED AIR MASS...WILL
LIKELY NOT REALIZE FULL COOLING POTENTIAL. USED A BLEND OF MOS
GUIDANCE FOR MIN TEMPS (LOWERS 30S NW TO UPPER 30S SE.)
COOL DRY AIR MASS WILL BE IN PLACE WEDNESDAY AND WEDNESDAY NIGHT.
LOW LEVEL THICKNESSES WITH FULL SUN SUPPORT MAX TEMPS SIMILAR TO THE
MET GUIDANCE. WITH SURFACE RIDGE IN VICINITY...EXPECT NEAR SURFACE
WINDS TO DECOUPLE EARLY. DUE TO DRY AIR MASS (DEWPOINTS WED
AFTERNOON IN THE MID TEENS...POTENTIALLY LOWER)...EXPECT MIN TEMPS
WELL INTO THE 20S WITH NORMALLY COLDER LOCALES LIKELY DROPPING INTO
THE LOWER 20S.
&&
.LONG TERM /THURSDAY THROUGH MONDAY/...
AS OF 300 PM MONDAY...
A BLOCKING UPPER LEVEL FLOW PATTERN WILL CAUSE A DEEP UPPER LOW OVER
THE CANADIAN MARITIMES TO MOVE LITTLE THROUGH THIS PERIOD. A UPPER
RIDGE OUT WEST IS INITIALLY RATHER FLAT BUT GAINS SOME AMPLITUDE BY
THE WEEKEND. THIS WILL AID IN THE DEEPENING OF A S/W AND EVENTUALLY
THE AMPLIFICATION OF A L/W TROUGH OVER THE EASTERN U.S. MODELS
DIFFER ON THE TIMING AND AMPLITUDE...LEADING TO A BELOW AVERAGE
CONFIDENCE FORECAST. TYPICALLY IN THIS TYPE OF PATTERN...EXPECT MAX
TEMPS TO AVERAGE BELOW NORMAL. PRECIP CHANCES ARE HIGHLY DEPENDENT
ON THE TRACK OF SURFACE CYCLONES...MOISTURE DEPTH/AVAILABILITY...AND
THE STRENGTH OF IMPULSES ALOFT.
APPEARS THAT THE INITIAL PART OF THE LONG TERM IS TRANQUIL WITH A
MODIFIED POLAR HIGH BUILDING INTO AND CROSS THE AREA THU THROUGH
FRI. EXPECT POPS TO BE LESS THAN 20 PERCENT WHILE TEMPS WILL AVERAGE
3-6 DEGREES BELOW NORMAL. THE 00Z ECMWF AND THE 12Z GFS ARE IN
REASONABLE AGREEMENT WITH THE DEVELOPMENT AND EASTWARD MOVEMENT OF
AN AREA OF DISTURBED WEATHER THAT MOVE E-NE FROM NORTHERN GULF
THROUGH THE NORTHERN FL PENINSULA. BASED ON DEEP WLY FLOW ALOFT OVER
OUR REGION...BEST MOISTURE TRANSPORT SHOULD OCCUR WELL SOUTH OF OUR
AREA.
A S/W THAT DIGGS SE INTO THE OH VALLEY SATURDAY MAY ACT TO
STRENGTHEN THE L/W TROUGH OVER THE EASTERN U.S. IN THIS TYPE OF
PATTERN...ENERGY ALOFT MAY AID IN SURFACE CYCLOGENESIS OFFSHORE OF
THE CAROLINAS. THE MODELS DO DEPICT THIS SCENARIO BUT VARY WIDELY ON
PLACEMENT...TIMING
AND STRENGTH OF THE SURFACE LOW LATE SATURDAY THROUGH SUNDAY. GFS
MORE PROGRESSIVE THAN MOST OF THE OTHER MEDIUM RANGE GUIDANCE
(TYPICAL BIAS) WHILE ECMWF SLOWER AND DEEPER.
PREFER TO REMAIN CONSERVATIVE WITH POPS DUE TO MODEL VARIANCE ON
STRENGTH/PLACEMENT AND TIMING OF SURFACE AND UPPER LEVEL FEATURES.
THUS WILL HAVE POPS NO HIGHER THAN 20 PERCENT IN THE EAST-SE
SATURDAY NIGHT AND SUN. MAX TEMPS WILL AVERAGE BELOW NORMAL...MORE
SO MONDAY COMPARED TO SAT-SUN.
&&
.AVIATION /06Z TUESDAY THROUGH SATURDAY/...
AS OF 115 AM TUESDAY...
24-HR TAF PERIOD:
LIFR/IFR CONDITIONS EXPECTED ACROSS THE ENTIRE AREA THROUGH 12Z THIS
MORNING AS A REMNANT WEDGE AIRMASS REMAINS IN PLACE ACROSS CENTRAL
NC. A COLD FRONT CURRENTLY LOCATED WEST OF THE MOUNTAINS SHOULD
PROGRESS EASTWARD THROUGH THE AREA BY 18Z TUE...AND LOW LEVEL FLOW
SHOULD VEER TO THE NORTHWEST BY THE AFTERNOON...USHERING IN A DRIER
AIRMASS FROM THE WEST/NW. AS A RESULT...THE REMNANT WEDGE AIRMASS
SHOULD BEGIN TO SCOUR OUT BY THE EARLY AFTERNOON...WITH IMPROVEMENTS
IN VISBYS/CEILINGS. IT IS DIFFICULT TO DETERMINE EXACTLY WHEN
CONDITIONS WILL BECOME VFR...HOWEVER...BASED ON THE LATEST FCST
SOUNDINGS AND UPSTREAM OBSERVATIONAL DATA...VFR CONDITIONS ARE
ANTICIPATED TO ARRIVE BY 15-18Z IN THE NW PIEDMONT...AND 18-21Z AT
RDU/FAY/RWI.
ONCE THE COLD FRONT CLEARS OUT THE AREA...VFR CONDITIONS ARE
EXPECTED TO HOLD TONIGHT THROUGH THE END OF THE WORK WEEK AS COOL
HIGH PRESSURE BUILDS INTO THE AREA. THE NEXT CHANCE FOR ADVERSE
AVIATION CONDITIONS MAY NOT BE UNTIL THE WEEKEND...WHEN A STORM
SYSTEM EMANATING FROM THE GULF OF MEXICO MAY AFFECT PORTIONS OF THE
SOUTHEAST STATES. -VINCENT
&&
.RAH WATCHES/WARNINGS/ADVISORIES...
NONE.
&&
$$
SYNOPSIS...VINCENT
NEAR TERM...HARTFIELD/SMITH
SHORT TERM...WSS
LONG TERM...WSS
AVIATION...VINCENT
