FXUS62 KRAH 042043 CCA
AFDRAH
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC
343 PM EST FRI DEC 04 2009
.SYNOPSIS...
WEAK SURFACE RIDGING WILL EXTEND OVER THE AREA THROUGH TONIGHT.
BENEATH A VERY ENERGETIC SOUTHWEST FLOW ALOFT... LOW PRESSURE WILL
CONSOLIDATE FROM THE NORTHERN GULF OF MEXICO TO OFFSHORE THE MID
ATLANTIC COAST SATURDAY AND SATURDAY NIGHT. HIGH PRESSURE WILL
RETURN FOR THE DAY SUNDAY.
&&
.NEAR TERM /THROUGH SATURDAY NIGHT/...
AS OF 340 PM FRIDAY...
A POLAR VORTEX OVER ONTARIO WILL MAINTAIN A VERY ENERGETIC
SOUTHWESTERLY FLOW ALOFT OVER THE EAST COAST THROUGH THROUGH
SATURDAY EVENING... BY WHICH TIME A VIGOROUS NEGATIVELY-TILTED S/W
TROUGH ALOFT WILL HAVE CROSSED OUR REGION AND CLEARED OUT CLOUDS AND
PRECIPITATION. MEANWHILE... A SERIES OF SURFACE WAVES -- ONE OVER
THE WESTERN GOM AND OTHERS THAT FLANK THE FL PENINSULA (THE LATTER
TWO OF WHICH SHOULD BECOME DOMINANT) WILL TRACK UP THE SOUTHEAST AND
MID ATLANTIC COASTS. AN H85 WAVE IS ALSO FORECAST TO CONSOLIDATE
NORTHEASTWARD FROM NEAR BIRMINGHAM AL AT 00Z THIS EVENING TO WESTERN
NC BY 12Z SATURDAY.
TONIGHT: THE LOW LEVEL (H85-925) FLOW WILL ACCORDINGLY BACK TO
SOUTHEAST/EASTERLY AND TRANSPORT ATLANTIC MOISTURE INLAND AND
PROMOTE THE DEVELOPMENT OF RAIN ACROSS EASTERN AND EVENTUALLY
CENTRAL NC OVERNIGHT. TEMPERATURES SHOULD ONLY SLOWLY FALL OWING TO
WIDESPREAD CLOUD COVER... BUT THEN DIP VIA EVAPORATIONAL COOLING TO
OVERNIGHT LOWS OF ABOUT 40 DEGREES NW TO UPPER 40S SOUTHEAST.
SATURDAY: THE EVOLVING PRECIPITATION SHIELD -- INCLUDING SNOW OVER
SOUTHEASTERN TX AND DRIVEN PRIMARILY BY MID LEVEL DEFORMATION/
FRONTOGENESIS -- IS EXPECTED TO MERGE WITH THE LOW LEVEL-FORCED RAIN
(BY THE AFOREMENTIONED H85 WAVE) OVER CENTRAL NC BY SATURDAY
MORNING. WHILE THE HEAVIEST PRECIPITATION WILL REMAIN IN CLOSER
PROXIMITY TO THE OFFSHORE PASSING SURFACE LOW... THE PROGRESSIVE
DEFORMATION BAND SHOULD MAINTAIN MAINLY LIGHT RAIN OVER PORTIONS OF
CENTRAL NC THROUGHOUT THE DAY. THE MODELS CONTINUE TO SHOW A QUICK
PLUNGE OF PARTIAL THICKNESSES FROM 18Z AND 00Z... TO VALUES THAT
WOULD SUPPORT A BRIEF MIXING WITH OR CHANGING OVER TO SNOW FROM THE
TRIAD TO ROXBORO AND HENDERSON AREAS THEN PERHAPS AS FAR EAST AS THE
TRIANGLE (MAINLY CHAPEL HILL AND DURHAM) TO LOUISBURG TO ROANOKE
RAPIDS BETWEEN 4 PM AND 8 PM. WHILE STORM TOTAL LIQUID EQUIVALENT
AMOUNTS COULD RANGE FROM ONE QUARTER TO THREE QUARTERS OF AN INCH
(HELD DOWN BY THE RELATIVELY QUICK MOVEMENT OF THE STORM BUT MADE
RESPECTABLE BY THE DEGREE OF FORCING FROM THE VERY COLD AND DYNAMIC
UPPER TROUGH)... ALL BUT PERHAPS SEVERAL HUNDREDTHS OF THAT SHOULD
FALL BEFORE THE ATMOSPHERE COOLS SUFFICIENTLY TO SUPPORT SNOW. THAT
SCENARIO WOULD SUGGEST THAT ANY SNOW WOULD MOSTLY MELT ON CONTACT
DUE TO A PRECEDING WET AND RELATIVELY WARM GROUND (RUNWAY
TEMPERATURES IN THE MID 40S PER RECENT REPORT FROM GSO AIRPORT).
HOWEVER... A FEW LOCATIONS MAINLY ALONG AND WEST OF INTERSTATE 85
COULD PICK UP A QUICK BURST OF A DUSTING (ONE HALF INCH OR LESS)...
PARTICULARLY IF THE CHANGEOVER WERE TO OCCUR AS RAPIDLY AS THAT
OBSERVED OVER EASTERN TX TODAY.
THERE SHOULD ALSO BE A NOTICEABLE AND OCCASIONALLY GUSTY GENERALLY
NORTHERLY BREEZE AS THE SURFACE LOW DEEPENS OFFSHORE... GENERALLY 10
TO 15 MPH WITH GUSTS TO 25 MPH.
SATURDAY NIGHT: CLEARING AND COLDER IN THE WAKE OF THE DEPARTING
STORM SYSTEM... WITH INITIALLY CAA-DRIVEN AND THEN RADIATIONAL-
COOLED OVERNIGHT LOWS IN THE MIDDLE 20S TO AROUND FREEZING...
WARMEST SOUTHEAST.
&&
.SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
AS OF 230 AM FRIDAY...
A RATHER STRONG ~1028MB SURFACE HIGH IS EXPECTED TO SHIFT EAST OVER
NC/VA BY SUNDAY AFTERNOON. GOOD COLD ADVECTION WITH A NW WIND EARLY
ON SUNDAY WILL RELAX BY MIDDAY AS THE HIGH QUICKLY SHIFTS TO THE VA
TIDEWATER REGION. THICKNESSES ONLY CLIMB TO AROUND 1310 BY LATE IN
THE DAY WITH GOOD COLD ADVECTION EARLY. INCREASING HIGH CLOUDS
LATE IN THE DAY MAY OFFSET WARMING POTENTIAL AS COLD ADVECTION
RELAXES. WILL LEAN MORE TOWARDS MET GUIDANCE WITH MID TO UPPER
40S SURFACE HIGH SHIFTS FURTHER OFFSHORE SUNDAY NIGHT...AS AN
INVERTED SURFACE TROF DEVELOPS ALONG THE CAROLINA COAST.
CONCURRENTLY...A WEAK SHORTWAVE MOVING EAT ACROSS THE NORTHERN GREAT
LAKES WILL PULL A WEAK AND MAINLY DRY COLD FRONT TOWARD THE AREA ON
MONDAY. THE DEPARTING SURFACE HIGH WILL LIKELY LEAVE MUCH OF
CENTRAL NC QUITE DRY IN THE LOW LEVELS...AND WHILE THE MID LEVELS
MOISTEN IN RESPONSE TO INCREASING WARM ADVECTION...DONE EXPECT ANY
MEASURABLE PRECIP...ALTHOUGH A FEW AREA OF VERY LIGHT RAIN COULD BE
POSSIBLE ALONG THE EASTERN CWA AS THE COASTAL TROF STRENGTHENS.
WITH ADDED CLOUD COVER... HIGHS MONDAY LIKELY RIGHT AROUND SUNDAY
VALUES OR SLIGHTLY WARMER...DESPITE INCREASING LOW LEVEL THICKNESSES.
&&
.LONG TERM /TUESDAY THROUGH THURSDAY NIGHT/...
AS OF 310 AM FRIDAY...
GENERALLY FLAT W-SWERLY FLOW ALOFT WITH SURFACE HIGH PRESSURE
BUILDING EAST TO NEW ENGLAND....WHILE A DEEP UPPER LOW SWINGS SOUTH
THROUGH THE FOUR CORNERS REGION. THIS WILL BRING ANOTHER ROUND OF
UNSETTLED WEATHER TO THE REGION BY LATE TUES INTO WED AS THE UPPER
LOW OPENS UP AND QUICKLY LIFTS NE WITH ITS ASSOCIATED SURFACE LOW
TOWARD THE MIDWEST REGION. MODELS ARE IN FAIRLY GOOD AGREEMENT ON
THE LARGE SCALE EVOLUTION OF THIS EVENT...WITH A SURFACE HIGH OVER
THE MID ATLANTIC STATES TUESDAY...AND WARM ADVECTION/OVERRUNNING
PRECIP KNOCKING ON OUR DOOR BY TUESDAY EVENING. AS THE MAIN LOW
LIFTS TO THE MID WEST STATES...THERE WOULD BE THE POTENTIAL FOR THE
COASTAL THERMAL MOISTURE BOUNDARY TO RETREAT INLAND...DRAWING HIGHER
DEWPOINTS AND INSTABILITY INLAND WITH IT. OF NOTE AT THE
MOMENT...THE 12Z GFS DEVELOPS A SURFACE WAVE ALONG THE TMB OVER THE
COASTAL TUESDAY NIGHT...WHICH COULD AUGMENT RAINFALL AND CONVECTIVE
POTENTIAL IN THE EAST. ON THE OTHERHAND...THE MOST RECENT 12Z ECMWF
HINTS AT A SURFACE WAVE IN THE PIEDMONT. DESPITE MODEL AGREEMENT ON
THE LARGE SCALE PATTERN...GIVEN THESE DIFFERENCES AND SOME OF THE
RECENT TREND TO SPEED UP THE SYSTEMS PROGRESSION...WILL LEAVE POPS
AT CHANCE FOR NOW INSTEAD OF TRYING TO NARROW DOWN A SPECIFIC TIME
FRAME....WITH PRECIP WRAPPING UP LATE WEDNESDAY. HIGH PRESSURE
BUILDS BACK OVER THE AREA ON THURSDAY..WITH INDICATION OF ANOTHER
SYSTEM DEVELOPING ON A SIMILAR STORM TRACK BY THE END OF THE PERIOD.
TEMPS IN THIS TIME FRAME SHOULD GENERALLY BE ABOUT A CATEGORY BE
ABOVE NORMAL PRIOR TO THE PASSAGE OF THE COLD FRONT...WITH THE
EXCEPTION BEING IN THE NW HALF OF THE CWA WITH POTENTIAL CAD
DEVELOPING LATE TUES/WED. BEHIND THE FRONT...MODELS SUGGEST THE
CRUX OF COLD AIR WILL REMAIN NORTH OF CENTRAL NC AND TEMPS WILL FALL
TO AROUND NORMAL IN THE LOWER 50S.
&&
.AVIATION /18Z FRIDAY THROUGH TUESDAY/...
AS OF 105 PM FRIDAY...
A VERY ENERGETIC SOUTHWEST FLOW ALOFT WILL INDUCE SURFACE LOW
PRESSURE DEVELOPMENT FROM THE NORTHERN GULF OF MEXICO TO THE MIDDLE
ATLANTIC COAST THROUGH SATURDAY. ATLANTIC MOISTURE WILL BE WRAPPED
INLAND ON THE NORTHWEST SIDE OF THE DEVELOPING LOW... AND CAUSE
PRESENT VFR CONDITIONS TO BECOME MVFR THEN (L)IFR WITH RAIN AT
CENTRAL NC TERMINALS AROUND 03Z AT FAY... BY 10Z FARTHER NORTHWEST
AT TRIAD TERMINALS... AND SOME TIME IN BETWEEN AT RDU AND RWI.
CONDITIONS SHOULD REMAIN (L)IFR WITH OCCASIONAL RAIN -- MODERATE AT
TIMES -- THROUGH THE 18Z TAF PERIOD.
LOOKING BEYOND 18Z SATURDAY: AS COLD AIR RUSHES INTO THE REGION
THROUGH THE DEPTH OF THE ATMOSPHERE... THE RAIN MAY BRIEFLY MIX WITH
OR CHANGE OVER TO SNOW (WITH ASSOCIATED REDUCED VISIBILITIES) FOR A
FEW HOURS -- MAINLY AT TRIAD TERMINALS -- BEFORE ENDING AND BECOMING
VFR IN THE 22Z TO 03Z/5TH TIME FRAME. THERE WILL BE ANOTHER CHANCE
OF MVFR CONDITIONS AND A LITTLE LIGHT RAIN... ESPECIALLY AT EASTERN
TERMINALS... AHEAD OF A WEAKER STORM SYSTEM ON MONDAY.
&&
.RAH WATCHES/WARNINGS/ADVISORIES...
NONE.
&&
$$
SYNOPSIS...MWS
NEAR TERM...MWS
SHORT TERM...SMITH
LONG TERM...SMITH
AVIATION...MWS
