FXUS62 KRAH 021547
AFDRAH
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC
1045 AM EST WED DEC 02 2009
.SYNOPSIS...
A STRONG LOW PRESSURE SYSTEM OVER THE GULF COAST REGION WILL MOVE
NORTHEAST THROUGH TONIGHT...BRINGING WIDESPREAD RAIN AND GUSTY WINDS
TO OUR AREA. HIGH PRESSURE WILL BUILD INTO THE REGION ON THURSDAY.
&&
.NEAR TERM /THROUGH TONIGHT/...
AS OF 1000 AM WEDNESDAY...
LITTLE CHANGE REQUIRED TO NEAR TERM FORECAST.
RAIN FALLING THROUGH THE COOL DRY AIR DEPOSITED FROM TUESDAY'S AREA
OF HIGH PRESSURE HAS RESULTED IN A INSITU DAMMING EVENT OVER THE
PIEDMONT COUNTIES THIS MORNING. WITHIN THIS AIR MASS TEMPS HOLDING
IN THE MID/UPPER 40S. FARTHER SE INTO THE SOUTHERN COASTAL
PLAIN...LOW LEVEL ELY FLOW OFF THE ATLANTIC HAS PULLED WARMER AIR
INTO THIS SECTION. MID MORNING TEMPS IN THE MID 50S WITH DEWPOINTS
CREEPING UP TO AROUND 50. A MESO-LOW NOTED ON RADAR AND SURFACE
ANALYSIS MOVING N-NE ACROSS COASTAL TROUGH. AS THIS FEATURE CROSSES
THE ILM AREA...WINDS AT FAY...CLINTON AND EVENTUALLY GSB MAY BRIEFLY
BACK TO A NLY DIRECTION...COOLING TEMPS BY A FEW DEGREES.
UPPER AIR AND MESO ANALYSIS DEPICTS WELL DEFINED 925-850MB WARM
FRONT IMMEDIATELY SOUTH OF THE FORECAST AREA. ONE MID LEVEL JET MAX
NOTED EXITING NORTHERN NC WHILE ANOTHER MOVING ONSHORE OF THE
NORTHERN GULF IN VICINITY OF MOB. THIS LATER FEATURE SHOULD ENHANCED
RAINFALL INTENSITY OVER THE WESTERN/SOUTHERN PIEDMONT BY MID DAY OR
EARLY AFTERNOON AS IT LIFTS NE OVER THE NWD MOVING 925/850MB WARM
FRONTS. INCREASE IN RAIN INTENSITY SHOULD ALSO DRAG WARMER AIR ALOFT
TOT EH SURFACE...AIDING TO SCOUR OUT CAD AIR MASS. IF MODERATE TO
LOCALLY HEAVY RAINFALL DOES NOT MATERIALIZE BY 18-19Z IN THIS
REGION...THEN FLASH FLOOD WATCH WILL LIKELY BE CANCELED WITH THE
AFTERNOON FORECAST PACKAGE. IN ADDITION...A RATHER SHARP WEDGE
BOUNDARY MAY EXIST ACROSS THE PIEDMONT...ENHANCING LOW LEVEL
HELICITY. THIS WOULD INCREASE THE THREAT FOR DAMAGING WINDS/ISOLATED
TORNADOES IN THIS PART OF THE FORECAST AREA THIS EVENING. ( WITH THE
AFOREMENTIONED UPPER JET EMERGING OFF THE GULF COAST...AND THE
SYNOPTIC WARM FRONT DRAPED ACROSS GEORGIA...THIS PATTERN FITS THE
CONCEPTUAL MODEL FOR GRAVITY WAVE GENESIS AND PROPAGATION...WITH
CONVECTION AND/OR AGEOSTROPHIC MOTIONS IN THE EXIT REGION OF THE JET
HAVING THE POTENTIAL TO GENERATE MESOSCALE GRAVITY WAVES. THE
QUESTION THEN BECOMES WHETHER OR NOT THE STABILITY NORTH OF THE WARM
FRONT IS SUFFICIENT FOR DUCTING AND PROPAGATION. THESE FEATURES ARE
ALMOST UNFORECASTABLE...BUT GIVEN THE PATTERN AND THEIR POTENTIAL
TO AUGMENT PRECIPITATION AND/OR SEVERE WEATHER...WE'LL NEED TO KEEP
THIS IN THE BACK OF OUR MINDS AS THE EVENT UNFOLDS. -SMITH)
STILL APPEARS HIGHEST THREAT FOR SEVERE WEATHER THROUGH 21Z/4 PM
WILL BE IN VICINITY OF THE SOUTHERN COASTAL PLAIN AND EASTERN
SANDHILLS AS COASTAL TROUGH AND 925-850MB WARM FRONT MOVES INTO THIS
REGION. KINEMATICS VERY IMPRESSIVE WITH LOW LEVEL HELICITY VALUES
THROUGH THE ROOF. THUS WILL LIKELY SEE ROTATION WITH THE HEAVIER
SHOWERS AND ISOLATED STORMS THIS AFTERNOON AND EVENING. MAIN
QUESTION STILL REVOLVES AROUND WHETHER NEAR SURFACE LAYER WILL
DESTABILIZE TO ALLOW ROTATION TO EXTEND TO THE SURFACE. THE ANSWER
WILL LIKELY NOT BE KNOWN UNTIL JUST PRIOR TO CONVECTION INITIATION
SO WILL NEED TO MONITOR NEAR SURFACE CONVECTIVE PARAMETERS
THROUGHOUT THE AFTERNOON.
WILL MAINTAIN CATEGORICAL POPS THROUGH THIS EVENING. SHOWERS
EXPECTED TO DIMINISH IN THE WEST PRIOR TO MIDNIGHT WITH THE
PASSAGE OF MID LEVEL TROUGH...AND BY 3 AM IN THE FAR EASTERN FRINGE
OF THE FORECAST AREA.
&&
.SHORT TERM /THURSDAY THROUGH FRIDAY/...
AS OF 230 AM WEDNESDAY...
THURSDAY AND THURSDAY NIGHT...
THE ACCELERATED DEPARTING LOW PRESSURE SYSTEM IS EXPECTED TO
BE OVER NEW YORK BY 12Z THURSDAY...WITH ONLY A SLIGHT CHANCE OF A
LINGERING SHOWER AS THE SURFACE FRONT PASSES THROUGH THE COASTAL
PLAIN. WITH A DECENT PRESSURE GRADIENT OVER CENTRAL NC IN THE WAKE
OF THE SYSTEM....W-SW WINDS MAY STAY UP IN THE 10-15KT RANGE WITH
GUSTS TO 25KT DURING THE FIRST HALF OF THE DAY BEFORE TURNING TO
NWRLY AND RELAXING. COLD ADVECTION APPEARS TO BE DELAYED
LATE ENOUGH IN THE AFTERNOON THAT THE WESTERLY COMPRESSIONAL
DOWNSLOPE FLOW WILL BRING ABOUT A SEASONABLY MILD
DAY WITH HIGHS IN THE UPPER 50S TO LOWER 60S. MID AND HIGH LEVEL
MOISTURE LOOKS TO OVERSPREAD THE AREA THURSDAY NIGHT IN THE SWLY
FLOW ALOFT AS S/W ENERGY AMPLIFIES OVER THE SOUTHERN ROCKIES.
OVERNIGHT LOWS 35 TO 40. -BL
FRIDAY...
COOL...WEAK SURFACE HIGH PRESSURE RIDGE LINGERS OVER THE MID
ATLANTIC ON FRIDAY...WHILE SHORTWAVE ENERGY (AND THE DRIVING FORCE
FOR THE WEEKENDS WEATHER) DIVES SOUTH AROUND THE BACK SIDE OF THE
LONG WAVE TROUGH EXPECTED TO DEVELOP OVER THE EASTERN 2/3 OF THE
COUNTRY. HIGHS FRIDAY A BIT COOLER THAN THURSDAY...ALTHOUGH THE
COLDEST AIR WILL LIKELY HOLD TO OUR WEST UNTIL SATURDAY. HIGHS
SHOULD HOLD IN THE LOW TO MID 50S...ALTHOUGH SOME UPPER 40S QUITE
POSSIBLE...ESPECIALLY AS HIGH CLOUDS INCREASE AHEAD OF THE NEXT
SYSTEM. -BLS
&&
.LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/...
AS OF 245 AM WEDNESDAY...
FRIDAY NIGHT THROUGH SATURDAY NIGHT: A SURFACE LOW WILL DEVELOP OVER
THE EASTERN GULF AS A SHORT WAVE ROUNDS THE BASE OF A BROAD LONGWAVE
TROUGH ENCOMPASSING MUCH OF THE CONUS. THIS SURFACE LOW WILL
EVENTUALLY TRACK OFF THE EAST COAST...RESULTING IN AN INCREASE IN
PRECIP CHANCES BEGINNING FRIDAY NIGHT. MODELS CONTINUE TO SHOW RUN
TO RUN INCONSISTENCY WITH RESPECT TO THE STRENGTH AND LOCATION OF
THE SURFACE LOW. NEVERTHELESS...GIVEN THE SURFACE LOW OFF THE COAST
AND STRONG UPPER SUPPORT (RIGHT ENTRANCE REGION OF A STRONG UPPER
JET CORE OVER THE REGION THROUGH AT LEAST SATURDAY AFTERNOON)...WILL
MAINTAIN CHANCE POPS OVER THE ENTIRE REGION ON SATURDAY. RAIN
CHANCES WILL TAPER OFF SATURDAY NIGHT AS THE LOW MOVES NORTHEAST.
PRELIMINARY SOUNDINGS/THICKNESS VALUES CONTINUE TO INDICATE A
POSSIBLE CHANGE OVER TO SNOW IN THIS WRAP AROUND PRECIP ACROSS THE
FAR NORTHWEST PORTION OF THE CWA. HOWEVER...SATURATION IN THE ICE
NUCLEATION ZONE IS MARGINAL AT BEST WHEN THE COLD AIR IS IN PLACE
AND ALSO THE MODELS TEND TO BRING IN THE COLD AIR TOO FAST.
THEREFORE...BELIEVE THAT THE COLD AIR WILL BE LAGGING THE PRECIP AND
ALL PRECIP WILL REMAIN IN THE LIQUID FORM. AS ALWAYS...THIS SCENARIO
WILL HAVE TO BE MONITORED CLOSELY BY FUTURE SHIFTS. LOWS WILL
GENERALLY BE IN THE 30S ON FRIDAY AND THEN THE UPPER 20S TO LOWER
30S ON SATURDAY AS THICKNESS VALUES FALL SIGNIFICANTLY. HIGHS ON
SATURDAY WILL BE HIGHLY DEPENDENT ON THE PRECIP...BUT ARE GENERALLY
EXPECTED TO BE IN THE 40S AREAWIDE.
SUNDAY THROUGH TUESDAY: A CHILLY SURFACE HIGH WILL DOMINATE THROUGH
THIS MOST OF THIS PERIOD...WITH DRY AND PARTLY CLOUDY CONDITIONS
EXPECTED. SUNDAY MORNING WILL START OFF RATHER CHILLY (LOWS IN THE
MID 20S TO LOW 30S) AS HIGH PRESSURE IS OVERHEAD. HOWEVER...SOME
HIGH CLOUDS AHEAD OF NEXT SYSTEM MAY INHIBIT IDEAL RADIATIONAL
COOLING CONDITIONS. WITH MORE SUNSHINE EXPECTED ON SUNDAY...HIGH
TEMPS WILL BE SLIGHTLY WARMER THAN SATURDAY...GENERALLY IN THE UPPER
40S TO LOW 50S. AS THE HIGH SLOWLY SHIFTS OFF TO THE NORTHEAST
THROUGH EARLY NEXT WEEK...TEMPS WILL SLOWLY REBOUND...WITH LOWS IN
THE MID TO UPPER 30S AND HIGHS INTO THE LOW 50S BY THE EARLY PART
OF THE WEEK.
&&
AVIATION /12Z WEDNESDAY THROUGH SUNDAY/...
AS OF 630 AM WEDNESDAY...
LIGHT TO MODERATE RAIN HAS ALREADY BEGUN THIS MORNING ACROSS MUCH OF
THE CWA AS LOW PRESSURE ACROSS THE GULF COAST MOVES NORTHEAST. A
VERY STRONG LOW LEVEL JET WILL BE OVERHEAD FOR MUCH OF THE TAF
PERIOD...WHICH WILL RESULT IN LOW LEVEL WIND SHEAR...WITH SURFACE
FLOW OUT OF THE E-NE BETWEEN 10-12 KNOTS AND THE FLOW BETWEEN
1000-2000FT OUT OF THE SE BETWEEN 35-45 KNOTS. THE LOW LEVEL WIND
SHEAR WILL PERSIST THROUGH AT LEAST 18Z WEDNESDAY AT ALL TAF
SITES...LIKELY THROUGH 00Z WEDNESDAY AT KINT AND KGSO AND POSSIBLY
KRDU. BETWEEN 18Z AND 00Z...WINDS WILL LIKELY BECOME SOUTHEASTERLY
AND GUSTY AT KRWI AND KFAY...POSSIBLY KRDU...AS THE STRONG
SOUTHEASTERLY FLOW IN THE LOW LAYERS ERODES THE SHALLOW STABLE AIR
AT THE SURFACE. THERE STILL IS SOME UNCERTAINTY WITH REGARDS TO HOW
FAST THIS MIXING OUT WILL OCCUR ON WEDNESDAY AFTERNOON. IF THE
WARMER AIR SURGES WESTWARD FASTER...DUE TO THE STRONG SE
WINDS...CIGS COULD LIFT TO MVFR AND GUSTY WINDS OF UP TO 30 MPH WILL
BE POSSIBLE PRIOR TO 00Z THURSDAY.
RAIN WILL BECOME WIDESPREAD ACROSS CENTRAL NC OVER THE NEXT FEW
HOURS AS VERY MOIST AND WARM AIR IS LIFTED OVER THE SHALLOW STABLE
LAYER NEAR THE SURFACE. THE STRONG LOW PRESSURE SYSTEM WILL LIFT
RAPIDLY NE ACROSS THE TN AND OH VALLEYS THROUGH WEDNESDAY NIGHT.
THIS SYSTEM WILL CONTINUE TO SPREAD MOIST AND SLIGHTLY UNSTABLE AIR
INTO CENTRAL NC WED NIGHT. THE PRECIP WILL BECOME MORE SHOWERY IN
NATURE DURING THE AFTERNOON HOURS. THEN...A BAND OF HEAVY SHOWERS
AND ISOLATED STRONG/SEVERE THUNDERSTORMS ASSOCIATED WITH THE SURFACE
FRONT...LOOKS TO CROSS THE REGION BETWEEN 00-06Z. WIND GUSTS WITHIN
THIS LINE MAY EXCEED 60 MPH.
THE LOW PRESSURE SYSTEM WILL EXIT THE REGION EARLY THU. STRONG
SUBSIDENCE ON THE BACK SIDE OF THE LOW WILL INDUCE RAPID CLEARING
FROM SOUTHWEST-TO-NORTHEAST TOWARD DAYBREAK THU. VFR CONDITIONS WILL
RETURN THURSDAY AND PERSIST AT LEAST FRIDAY NIGHT. A DEVELOPING AREA
OF LOW PRESSURE OFFSHORE SATURDAY WILL SPREAD MVFR CEILINGS INTO THE
VICINITY OF KRDU AND KRWI. OTHERWISE VFR EXPECTED SATURDAY INTO
SUNDAY.
&&
.RAH WATCHES/WARNINGS/ADVISORIES...
FLASH FLOOD WATCH FROM WEDNESDAY AFTERNOON THROUGH WEDNESDAY EVENING
FOR NCZ021>023-038-039-073-074-083-084.
&&
$$
SYNOPSIS...WSS
NEAR TERM...WSS/SMITH
SHORT TERM...BL/SMITH
LONG TERM...KRR
AVIATION...KRR
