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FXUS62 KRAH 190816
AFDRAH

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC
316 AM EST SAT JAN 19 2008

.SYNOPSIS...

LOW PRESSURE LOCATED IN THE GULF OF MEXICO WILL APPROACH THE AREA 
FROM THE SOUTHWEST THIS MORNING...THEN MOVE EAST NORTHEAST OFFSHORE
OF THE GEORGIA AND SOUTH CAROLINA COAST. A PERIOD OF SNOW IS
EXPECTED ACROSS THE REGION TODAY WITH SOME ACCUMULATIONS POSSIBLE.
MEANWHILE...A VERY COLD ARCTIC AIR MASS WILL SPREAD ACROSS THROUGH
SUNDAY NIGHT.

&&

.NEAR TERM /THROUGH TODAY/...
AS OF 3AM SATURDAY... DESPITE THE EVENT TIME DRAWING NEAR...THERE
REMAINS CONSIDERABLE UNCERTAINY REGARDING THE EVOLUTION OF WINTER
WEATHER ACROSS THE COUNTY WARNING AREA. FIRST OFF...CURRENT
SURFACE ANALYSIS SHOWS AN AREA OF LOW PRESSURE (AROUND 1011 MB)
OVER THE NORTHWEST GULF OF MEXICO. THIS LOW HAS DEVELOPED IN
RESPONSE TO A EASTWARD MOVING SHORTWAVE CURRENTLY OVER NORTH
CENTRAL MEXICO NEAR THE TX BORDER. THE SURFACE LOW HAS ALSO FORMED
IN THE RIGHT REAR ENTRANCE OF A 170 UPPER JET CENTERED OVER THE TN
VALLEY. STRONG UPPER LEVEL DIVERGENCE AND MID LEVEL FRONTOGENESIS
HAS RESULTED IN WIDESPREAD PRECIPITATION OVER THE SOUTHEAST UNITED
STATES...WITH THE NORTHEAST EDGE OF THE PRECIP ALREADY OCCURRING
OVER CENTRAL GEORGIA AND APPROACHING SOUTH CAROLINA. OVER CENTRAL
NORTH CAROLINA...A BAGGY PRESSURE GRADIENT HAS RESULTED IN LIGHT
WINDS AND SMALL DEWPOINT DEPRESSIONS (TEMPS AND DEWPOINTS UPPER
30S TO LOWER 40S).

THE SURFACE LOW IS EXPECTED TO MOVE EAST NORTHEAST ACROSS THE
NORTHERN GULF OF MEXICO AND INTO THE ATLANTIC WATERS OFFSHORE THE
GA/SC COAST. THE FORECAST SFC LOW TRACK HAS TRENDED FURTHER SOUTH
AND EAST WITH EACH SUCCESSIVE MODEL RUN AND HAS RESULTED IN
GREATER UNCERTAINTY IN PRECIPITATION AMOUNTS FOR CENTRAL NORTH CAROLINA.

BEFORE GETTING INTO WHAT THE MODELS ARE FORECASTING...IT IS VERY
IMPORTANT TO NOTE THAT THEY ARE HAVING AN EXTREMELY HARD TIME
DEPICTING THE NEAR TERM WEATHER...JUST 6 HOURS AFTER MODEL
INITIALIZATION. BOTH THE 00Z GFS/NAM WERE TOO SLOW AND TOO FAR
SOUTH WITH THE NORTHERN EXTENT OF THE PRECIP SHIELD OVER CENTRAL
MS/AL/GA...WITH THE GFS THE CLOSESTS TO REALITY AND THE NAM
SEVERAL HOURS TOO SLOW. 

AS UPPER LEVEL DIVERGENCE AND FRONTOGENETIC FORCING SHIFTS
NORTHEAST...SO WILL THE SHIELD OF PRECIPITATION. GIVEN THE FASTER
TIMING OF THE PRECIP...IT SHOULD BEGIN TO ENTER THE SOUTHWEST
ZONES JUST AFTER DAYBREAK AND QUICKLY SPREAD NORTHEAST THROUGH THE
MORNING. FORECAST SOUNDINGS AND PARTIAL THICKNESSES REVEAL THAT
PRECIP SHOULD BEGIN PRIMARILY AS SNOW IN THE WESTERN PIEDMONT.
FURTHER EAST...PRECIP WILL BEGIN AS RAIN...BUT QUICKLY TRANSITION
TO SNOW IN SUCH PLACES AS RALEIGH...DURHAM...OXFORD...AND SANFORD
BY MIDDAY. THE TRANSITION TO SNOW WILL TAKE PLACE DURING THE
AFTERNOON IN THE COASTAL PLAIN AND SANDHILLS.

THE BIG QUESTION WILL BE HOW SNOW ACCUMULATION TO EXPECT. WITH THE
MORE SUPRESSED TRACK OF THE LOW...MODEL QPF HAS TRENDED DOWNWARD
ACROSS THE NW AND NORTH PIEDMONT...EVEN IN SOME OF THE COUNTIES
CURRENTLY UNDER A WINTER STORM WARNING. FURTHER SOUTH ACROSS THE
SOUTHERN COASTAL PLAIN THERE WILL BE MORE QPF TO COME BY BUT A
LATER TRANSITION TO SNOW.

OUR PRIMARY LIFTING MECHANISMS WILL BE STRONG DIVERGENCE AS WE
COME UNDER THE RIGHT ENTRANCE REGION OF A 160+ KT UPPER JET. THE
SECOND WILL BE MID LEVEL FRONTOGENESIS. AT THE CURRENT
TIME...THERE ARE TWO AREAS OF FRONTOGENSIS NOTED BACK TO THE WEST.
THE MAIN ZONE IN ASSOCIATION WITH THE HEAVY PRECIP OVER THE
COASTAL AREAS OF THE NORTHERN GULF. THERE IS ALSO A SECONDARY AREA
OF 700 MB FRONTOGENSIS OVER NORTH CENTRAL MS AND AL WHICH CAN BE
SEEN BY SLIGHTLY ENHANCED RADAR RETURNS. THE MAIN ZONE WILL SHIFT
ACROSS THE SOUTHEAST COASTAL PLAIN TODAY...THUS THE HIGHER QPF
THERE. THERE ARE ALSO VARIOUS SIGNS OF A SECONDARY AREA (IN
ASSOCIATION WITH THE FURTHER NORTH PRECIP) THAT MAY RESULT IN A
ZONE OF ENHANCED SNOWFALL FURTHER NORTH. MODEL GUIDANCE IS NOT
CONSISTENT IN THE PLACEMENT OF THIS FEATURE AND WE MAY NOT WHERE
THE HEAVIER BANDS SET UP FOR SEVERAL MORE HOURS. THIS COULD MAKE
UP FOR THE LOWER AMOUNTS OF PRECIP ADVERTISED BY THE MODELS.

CONSIDERING THE POOR MODEL PERFORMANCE IN HANDLING THE CURRENT
PRECIP PATTERN AND THE UNCERTAINTY IN ANY SNOW BANDING...WILL LEAVE
ADVISORIES AND WARNINGS STAND AS IS. GRANTED...THIS LOOKS TO BE
VERY BORDERLINE BETWEEN ADVISORY AND WARNING STATUS...WHICH MAKES
IT HARDER TO MAKE A DETERMINATION OF WHERE TO START AND STOP THE
ADVISORY/WARNINGS. THE ONE AREA OF CONCERN IS ACROSS THE SOUTHEAST
COASTAL PLAIN. IF THE COLD AIR MAKES IT EARLIER...WARNING CRITERIA
OF 3" MAY BE MET. THIS WILL HAVE TO BE CLOSELY MONITORED.

IN SUMMARY...EXPECT GENERALLY 1-2" IN THE ADVISORY AREA...WITH
MORE IN THE ORDER OF 2" OR SO IN THE SOUTHEAST COASTAL PLAIN. THE
WARNING AREA LOOKS TO RECEIVE BETWEEN 2-3" BUT THERE COULD BE
ISOLATED HIGHER TOTALS WITH ANY BANDING.

&&

.SHORT TERM /TONIGHT THROUGH 6 PM SUNDAY/...
LIGHT SNOW WILL CONTINUE THIS EVENING ACROSS THE EASTERN HALF OF
THE AREA...QUICKLY PUSHING EAST AS THE COASTAL STORM RAPIDLY MOVES
FURTHER OFFSHORE. MOST PRECIP SHOULD BE GONE BY 06Z. AFTER
THAT...ARCITC HIGH PRESSURE BEGINS TO BUILD IN AND OUR
TEMPERATURES WILL QUICKLY DROP OVERNIGHT. LOWS IN THE LOW TO MID
20S EXPECTED. THIS COULD PRESENT A PROBLEM ON ROADWAYS WHICH WILL
BE PRONE TO ICING OVER.

SUNDAY LOOKS TO BE VERY COLD. LOW LEVEL THICKNESSES DO NOT CRACK
1250M. GUIDANCE APPEARS TO WARM AT THIS POINT SO WILL GO UNDER
GUIDANCE A FEW DEGREES. LOW TO MID 30S EXPECTED FOR HIGHS.

&&

.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
AS OF 2 AM SATURDAY... COLDEST NIGHT DURING THIS ARCTIC OUTBREAK
TO OCCUR SUNDAY NIGHT/MONDAY MORNING...WITH MODEL CONSENSUS
INDICATING OPTIMAL RADIATIONAL CONDITIONS AS COLD SURFACE HIGH
BUILDS DIRECTLY OVER THE AREA...BEFORE PUSHING OFFSHORE DURING THE
DAY ON MONDAY. CURRENT THICKNESS FORECAST SUGGEST MID TEENS ACROSS
THE NORTH MONDAY MORNING. MODERATION BEGINS MONDAY WITH HIGHS 40
TO 45.

SURFACE TROUGH FORECAST TO MOVE INTO CENTRAL NORTH CAROLINA FROM THE 
NORTHWEST TUESDAY. TIMING AND THICKNESS FORECAST SUGGEST A RAIN 
EVENT. HIGH PRESSURE CONTROLS THE WEATHER INTO TUESDAY NIGHT. THE 
OPERATIONAL GFS IS SUGGESTING ANOTHER EVENT SIMILAR IN NATURE TO 
SATURDAY FOR THURSDAY TWENTY-FOURTH. CONFIDENCE IS EXTREMELY LOW AND 
WILL LEAN MORE TOWARD THE DRIER ECMWF MODEL AT THIS TIME. SOME 
INDICATIONS OF RETURNING MOISTURE FOR A CHANCE FOR RAIN BY FRIDAY... 
BUT THIS IS FAR ENOUGH OUT IN TIME TO ADVISE KEEPING RAIN CHANCES 
LOW.

ONCE SURFACE HIGH PRESSURE STARTS TO SLIP EAST MONDAY...TEMPERATURES 
BEGIN TO MODERATE TOWARD NORMAL FOR THE REMAINDER OF THE PERIOD. 
HIGHS IN THE 40S WITH LOWS 25 TO 35.  

&&

.AVIATION /08Z SATURDAY THROUGH WEDNESDAY/...
AS OF 315 AM SATURDAY... IFR TO LIFR CEILINGS WITH MVFR TO IFR 
VISIBILITIES WILL CONTINUE OVER THE TAF AREA THROUGH MOST OF THE 
DAY. POOR CONDITIONS OVER THE AREA WILL BE FURTHER ENHANCED AS 
PRECIPITATION OVERSPREAD THE REGION. RAIN OR A MIXTURE OF RAIN AND 
SNOW WILL DEVELOP OVER THE AREA BY LATE MORNING. SOUNDING DATA 
INDICATED THAT THE MIXED PRECIPITATION WILL BE BRIEF BEFORE IT 
CHANGES TO ALL SNOW. 

CLOUD BASES WILL LIFT TO ONE TO 3 THOUSAND FEET BETWEEN 00Z AND 03Z 
SUNDAY... AS THE LOW OFF THE CAROLINA COAST TRACKS TO THE NORTHEAST 
PULLING THE PRECIPITATION WITH IT

AS HIGH PRESSURE WITH AN ARCTIC AIRMASS OVERSPREADS THE AREA FROM 
THE WEST EARLY SUNDAY AND DRIER AIR MIXES DOWN TO THE SURFACE... VFR 
CONDITIONS WILL RETURN. 

AS THE HIGH PRESSURE AREA MOVES OFF THE NORTHEAST COAST TUESDAY... A 
MOIST SOUTHWEST FLOW WILL LEAD TO INCREASING MOISTURE AND A LOWERING 
OF CLOUD BASES. MVFR CONDITIONS WITH SCATTERED SHOWERS WILL BE 
POSSIBLE TUESDAY AND TUESDAY NIGHT. HIGH PRESSURE WILL BUILD BACK 
OVER THE AREA WEDNESDAY.

&&

.RAH WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY FROM 4 PM THIS AFTERNOON TO 6 AM EST 
SUNDAY FOR NCZ077-078-085-086-088-089.
WINTER WEATHER ADVISORY FROM NOON TODAY TO MIDNIGHT EST 
TONIGHT FOR NCZ073>075-083-084.
WINTER STORM WARNING FROM NOON TODAY TO MIDNIGHT EST TONIGHT 
FOR NCZ007>010-023>026-040-041-076.
WINTER WEATHER ADVISORY FROM 6 AM THIS MORNING TO 9 PM EST 
THIS EVENING FOR NCZ021-022-038-039.
WINTER STORM WARNING FROM 4 PM THIS AFTERNOON TO 6 AM EST 
SUNDAY FOR NCZ011-027-028-042-043.

&&

$$
SYNOPSIS...JFB
NEAR TERM...JFB
SHORT TERM...RLH
LONG TERM...RLH
AVIATION...RHJ