FXUS62 KRAH 050616
AFDRAH
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC
115 AM EST SAT DEC 04 2009
.SYNOPSIS...
A SURFACE LOW PRESSURE CENTER WILL DEVELOP OFF THE SOUTHEAST COAST
OVERNIGHT... AND MOVE QUICKLY NORTHEAST ALONG THE CAROLINA COAST
SATURDAY. CHILLY HIGH PRESSURE WILL BUILD IN FROM THE WEST ON SUNDAY.
&&
.NEAR TERM /THROUGH SATURDAY NIGHT/...
AS OF 955 PM FRIDAY...
FORECAST OF RAIN AS THE PREDOMINATE P-TYPE IS ON TRACK THIS EVENING.
A CHANGE TO SNOW IN THE NW AND FAR N STILL LIKELY ON SATURDAY
AFTERNOON... WITH MINOR GRASSY ACCUMULATIONS POSSIBLE NORTH AND WEST
OF GREENSBORO.
THE 00Z/05 DECEMBER DATA ANALYSIS STRONGLY SUPPORTS THE CURRENT
FORECAST FOR CENTRAL NC. IF ANYTHING... THE DATA SUPPORTS HIGHER QPF
RAINFALL TOTALS NEARING AN INCH OVER OUR EASTERN AND CENTRAL ZONES
FOR THE EVENT. THE 00Z/05 GREENSBORO RAOB INDICATED A WARM BOUNDARY
LAYER WITH THE LOWEST 5K FEET ABOVE 32 DEGREES. HOWEVER... THE MID
LEVELS HAD COOLED RAPIDLY TOWARD 32 DEGREES SINCE 12Z. THE
BLACKSBURG SOUNDING INDICATED COLDER AIR UP ALONG THE BLUE RIDGE
MOUNTAINS WITH THE FREEZING LEVEL AT ABOUT 3K FEET. IN ADDITION...
THE BLACKSBURG SOUNDING IS A SNOW SOUNDING. AS THE MOISTURE
INCREASES OVERNIGHT... EVAPORATIVE COOLING WOULD RAPIDLY TAKE THE
SURFACE TEMPERATURES DOWN TO FREEZING OR BELOW. THIS SOUNDING IS
REPRESENTATIVE OF THE MOUNTAINS... AS THE COLD/DRY AIR TO SUPPORT A
WINTER STORM IS LOCKED UP ALONG AND WEST OF THE BLUE RIDGE. THIS IS
CLEARLY DEPICTED ON THE SURFACE ANALYSIS. A WEAKENING 1025 MB HIGH
WAS CENTERED OVER THE MID MISSISSIPPI VALLEY REGION... WITH RIDGING
EXTENDING NE OVER PA. DEW POINTS ALONG AND WEST OF THE BLUE RIDGE
WERE MOSTLY IN THE TEENS AND 20S. DEW POINTS EAST OF THE MOUNTAINS
WERE IN THE 30S AND 40S. THERE WAS NEUTRAL ADVECTION FROM DCA
SOUTHWARD INTO NC... WITH THE CAA CURRENTLY BLOCKED BY THE
APPALACHIANS. THIS IS IMPORTANT BECAUSE THE LACK OF THE COLD DRY AIR
SOURCE (ON THE EAST SIDE OF THE MOUNTAINS) TO OUR NORTH IS LACKING
WITH THIS EVENT... WHICH DOES NOT SUPPORT ACCUMULATING SNOW EAST OF
THE NW NC FOOTHILLS UNTIL THE COLD AIR FINALLY SURGES THIS WAY VERY
LATE IN THE EVENT.
THE LATEST TEMPERATURE PROFILES AND LOW LEVEL THICKNESSES SUPPORT
ALL RAIN AT GREENSBORO THROUGH 18Z. IT IS AROUND THAT TIME THAT THE
COLD AIR FINALLY BEGINS TO SURGE INTO OUR REGION FROM THE NW-N. THE
COLUMN WILL COOL RAPIDLY FROM THE TOP DOWN... AND THE FORECAST
SOUNDINGS INDICATE A DEEP ISOTHERMAL NEAR FREEZING LEVEL AT
GREENSBORO BETWEEN 18Z-21Z... WITH ONLY THE BOUNDARY LAYER REMAINING
IN THE MID TO UPPER 30S. AN AVERAGE OF THE LOW LEVEL THICKNESS'S
FORECAST AROUND 1310/1540M AT 18Z FOR GSO... SUPPORT A GRADUAL MIX
WITH SNOW IN THE EARLY AFTERNOON OVER THE NW PIEDMONT. THE
THICKNESSES THEN FALL TO 1520/1300M BY 00Z/06... SUPPORTING A CHANGE
TO SNOW BEFORE ENDING. FORECAST SOUNDINGS INDICATE THE NEAR SURFACE
LAYER TO REMAIN AT OR ABOVE 34. THIS COMBINED WITH THE WARM SURFACE
LAYER WILL PRECLUDE ANY ACCUMULATING ON THE ROADS. SOME MINOR
ACCUMULATIONS MAY COLLECT ON GRASSY SURFACE. THE BEST CHANCE OF ANY
ACCUMULATION APPEARS TO BE WEST AND NORTH OF WINSTON-SALEM... WHERE
THE TEMPS MAY FALL TO 32 DURING THE SNOW (IF IT CAN REMAIN HEAVY
ENOUGH FOR SEVERAL HOURS.
TO THE EAST... THE FORECAST THICKNESSES AND SOUNDINGS INDICATE THAT
THE COLD AIR WILL RAPIDLY CHASE THE PRECIPITATION AS IT IS ENDING.
BUT FORECAST THICKNESSES SUGGEST THAT ONLY THE NORTHERN TIER (NORTH
OF RDU) WOULD BE SUSCEPTIBLE FOR A MIX OR CHANGE TO SNOW IN A NARROW
WINDOW OF TIME BEFORE ENDING. MINOR SLUSHY ACCUMULATIONS FROM
ROXBORO AND HILLSBOROUGH TO WARRENTON WOULD BE POSSIBLE. HOWEVER...
THE WARM WET GROUND (45 DEGREE + TEMPS) AND TIME OF DAY
(AFTERNOON)... AND WARM BOUNDARY LAYER (34-35 DEGREES AT A MINIMUM)
ARGUE AGAINST ANYTHING MORE THAN THAT.
QPF LOOKS MORE IMPRESSIVE WITH EACH NEW MODEL RUN... AND WE NOW HAVE
A HALF INCH TO INCH POTENTIAL THROUGHOUT CENTRAL AND EASTERN NC.
PREVIOUS DISCUSSION...
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 /06Z SATURDAY THROUGH WEDNESDAY/...
AS OF 115 AM FRIDAY...
VFR CONDITIONS IN THE PIEDMONT (INT/GSO/RDU) AND MVFR CONDITIONS IN
THE EAST (RWI/FAY) WILL CONTINUE TO STEADILY WORSEN OVERNIGHT...
DROPPING TO IFR EVERYWHERE (AND POSSIBLY LIFR AT RWI/FAY) BY
DAYBREAK THIS MORNING. AVIATION CONDITIONS WILL REMAIN POOR
IMPROVEMENT TO VFR 02Z-06Z THIS EVENING/TONIGHT.
LOW PRESSURE CENTERS SITTING OVER THE NORTHWEST GULF OF MEXICO AND
OFF THE GA/SC COAST WILL TRACK NORTHEAST AND CONCENTRATE INTO ONE
DISCRETE LOW OFF THE NC COAST TODAY. STRONG LIFT MECHANISMS AND
PLENTY OF AVAILABLE MOISTURE WILL GENERATE WIDESPREAD RAIN THROUGH
TODAY.... ALONG WITH A NORTH BREEZE INCREASING THIS AFTERNOON TO
SUSTAINED 9-13 KTS. THE WORST CONDITIONS IN TERMS OF MORE MODERATE
RAIN AND LOWER VSBYS WILL BE AT RWI/FAY/RDU. IFR/LIFR CONDITIONS ARE
EXPECTED TO DOMINATE THROUGH THE AFTERNOON... THEN AS THE SURFACE
LOW SHIFTS NORTHEAST AWAY FROM THE NC COAST AND SURFACE WINDS BECOME
MORE NORTH-NORTHWEST... COLDER AIR WILL ARRIVE AND THE COLD RAIN MAY
MIX WITH OR CHANGE OVER TO WET SNOW BEFORE ENDING LATE THIS
AFTERNOON THROUGH EARLY EVENING. ANY ACCUMULATION WILL BE VERY LIGHT
ON THE GRASSY SURFACES ONLY... AND THE BEST CHANCES WILL BE NORTH OF
INT/GSO/RDU. SKIES SHOULD BEGIN TO CLEAR AFTER 02Z... FIRST AT
FAY/INT/GSO THEN LAST AT RDU/RWI... WITH DIMINISHING WINDS TO LIGHT
FROM THE NORTH-NORTHWEST.
LOOKING BEYOND TONIGHT: VFR CONDITIONS WILL HOLD AT LEAST INTO
MONDAY. A FAST MOVING AND WEAK DISTURBANCE MAY BRING BRIEF MVFR
CONDITIONS ON MONDAY. VFR WEATHER WILL RETURN FOR MONDAY
NIGHT/TUESDAY... THEN ANOTHER STRONGER STORM SYSTEM MAY BRING
IFR/LIFR CONDITIONS AND MORE COLD RAIN FOR TUESDAY NIGHT/WEDNESDAY.
-GIH
&&
.RAH WATCHES/WARNINGS/ADVISORIES...
NONE.
&&
$$
SYNOPSIS...HARTFIELD
NEAR TERM...MWS/BADGETT
SHORT TERM...SMITH
LONG TERM...SMITH
AVIATION...HARTFIELD
