-- Discussion containing changed information from previous version are highlighted. --
000
FXUS62 KRAH 060845
AFDRAH

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC
345 AM EST THU MAR 6 2014

.SYNOPSIS...A LOW PRESSURE SYSTEM DEVELOPING OVER THE NORTHEAST GULF
OF MEXICO THIS MORNING WILL INTENSIFY AS IT TRACKS NORTHEASTWARD UP
THE SOUTHEAST COAST AND CAROLINAS TONIGHT THROUGH FRIDAY. THIS
SYSTEM WILL MOVE AWAY FROM THE AREA FRIDAY NIGHT... THEN HIGH
PRESSURE WITH DRY AND MILD WEATHER IS EXPECTED OVER THE WEEKEND.

&&

.NEAR TERM /THROUGH TODAY/...
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AS OF 340 AM THURSDAY...

A STRONG 1040MB SURFACE HIGH OVER QUEBEC HAS RIDGES SOUTH INTO THE
CAROLINAS THIS MORNING...WITH DEWPOINTS NOW WELL INTO THE TEENS
ACROSS THE PIEDMONT.  BRIEF CLEARING IS NOW GIVING WAY TO PERIODS OF
THIN CIRRUS THAT WILL THICKEN TODAY AS A POTENT SHORTWAVE CURRENTLY
OVER EASTERN TX PLOWS ACROSS THE GULF COAST STATES.  FOR TODAY...THE
BIGGEST FORECAST CONCERN IS HOW QUICKLY PRECIP WILL SPREAD INTO THE
SOUTHERN CWA.  GIVEN HOW DRY THE AIRMASS IS OVER CENTRAL NC THIS
MORNING AND THE FACT THAT THE STRONG QG FORCING DOESNT ARRIVE UNTIL
TONIGHT...EXPECT PRECIP WILL BE HOLD OFF FOR MOST OF THE DAY.  THE
NAM AND ECMWF APPEAR TO BE HANDLING UP STREAM PRECIP THE BEST...AND
THERE IS A CONSENSUS AMONG THESE MODELS AND THE HI-RES WRF RUNS THAT
PRECIP WONT MOVE INTO THE SOUTHERN PIEDMONT AND SANDHILLS UNTIL
AFTER 18-21Z. HAVE ADJUST POPS DOWN DURING THE EARLY
AFTERNOON...WITH CHANCE TO LIKELY (RAIN) ACROSS THE SOUTH DURING THE
LATE AFTERNOON AND EARLY EVENING.   INCREASING HIGH CLOUDS AND A
CHILLY NORTHEASTERLY WIND OF 10-15MPH WILL CUT DOWN ON
HEATING....WITH A FAIRLY UNIFORM TEMP DISTRIBUTION...IF NOT COOLER
IN THE SOUTH.  40-43.
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&&

.SHORT TERM /TONIGHT THROUGH FRIDAY NIGHT/...
-- Changed Discussion --
AS OF 345 AM THURSDAY...

A POTENT SHORTWAVE CROSSING THE GULF COAST STATES TODAY WILL CLOSE
OFF THIS EVENING AND THEN LIFT NORTHEAST...WITH A DEVELOPING SURFACE
LOW OVER THE GOMEX TO ALSO LIFT NORTHEAST ALONG THE CAROLINA COAST
ON FRIDAY.  STRONG DCVA AND UPPER LEVEL DIVERGENCE...COUPLED WITH AN
40-50 EAST-SOUTHEASTERLY LLJ AND WARM ADVECTION WILL BRING
WIDESPREAD PRECIP ACROSS THE CWA AFTER BY MIDNIGHT TONIGHT. THE NAM
AND SREF SHOW POTENTIAL FOR STRONG BANDING BETWEEN 06Z AND 12Z AS AN
AREA OF INTENSE MID-LEVEL FGEN SETS UP ON THE NORTH-NORTHWESTERN
SIDE OF TEH MID-LEVEL LOW...WHICH TRACKS ALONG THE COAST OR JUST
INLAND.  THE NAM GIVES THE MOST QPF WITH AS MUCH AS 1.5 TO 2 INCHES
ACROSS THE PIEDMONT...HEAVIEST TONIGHT.  THE ECMWF AND GFS ARENT
QUITE AS EXTREME BUT STILL GIVE OVER AN INCH.

THE BIG QUESTION MARK IN THE FORECAST IS HOW STRONG AND HOW FAR WEST
A WARM NOSE DEVELOPS WITH THE AFOREMENTIONED EASTERLY LLJ.  THE NAM
MAINTAINS AN EASTERLY FETCH LONGER THAN THE ECMWF AND GFS...AND
ACCORDINGLY HAS THE WARMEST TEMPS (>4C) IN THE 925-850MB LAYER.
THIS MAY BE SLIGHTLY OVERDONE...BUT THERE IS GOOD AGREEMENT BETWEEN
THE ECMWF AND GFS...WHICH ARE PREFERRED THERMALLY...THAT A 1-3C WARM
LAYER WILL DEVELOP WAS FAR WEST AS THE TRIAD AREA.  AT THE
SURFACE...THE AFOREMENTIONED PARENT HIGH IS FORECAST TO SHIFT EAST
THIS EVENING AS SHORTWAVE SWINGS ACROSS THE GREAT LAKES.  THUS...THE
MAIN SOURCE OF COLD/DRY AIR SHOULD BE LOST OVERNIGHT AND THE SURFACE
LOW WILL BE MORE INFLUENTIAL IN MAINTAINING THE NORTHEASTERLY FLOW.

ON THE BROAD SCALE....PARTIAL THICKNESSES SUPPORT AT LEAST FREEZING
RAIN ACROSS THE NORTHWEST PIEDMONT THROUGH 12-15Z FRIDAY...WITH A
QUICKER CHANGEOVER TO RAIN CLOSER TO THE US-1 CORRIDOR AND ALL RAIN
OVER THE COASTAL PLAIN.  WETBULB EFFECTS AND PRECIP RATES WILL
LIKELY LEAD TO MIXING IN OF SLEET OR EVEN SNOW AT THE ONSET...AS
WELL AS WITHIN ANY BAND THAT SETS UP OVER THE NORTHERN AND/OR
NORTHWEST PIEDMONT.  THIS WOULD BE MUCH MORE LIKELY WITH THE COLDER
SCENARIO SHOWN IN THE GFS WHERE A SMALLER WARM NOSE COULD BE
OVERCOME. THE MOST DIFFICULT PART OF THE FORECAST IS TRYING TO
DETERMINE POSSIBLE ICE AMOUNTS.  THE TOP-DOWN APPROACH WITH ABOUT
HALF OF QPF GOING TO ACCRUAL YIELDS AROUND TWO TENTHS OF AN INCH OF
ICE.  HEAVIER RAIN WOULD CUT DOWN ON AMOUNTS DUE TO RUN OFF. THERE
IS ALSO SOME CONCERN ABOUT CLOUD COVER TONIGHT AND HOW THAT MAY HOLD
TEMPS UP AND LIMIT THE AREAL EXTENT OF FREEZING RAIN POTENTIAL.

FROM ALL OF THIS...WE ARE FAIRLY CONFIDENT IN SOME ICE ACCRUAL OVER
THE NORTHWEST AND NORTHERN PIEDMONT AREAS. WE ARE LESS CERTAIN ABOUT
AREAS ALONG US-1...WHICH MAY SEE A LIGHT GLAZE BEFORE CHANGING OVER
TO ALL RAIN.  WILL ISSUE AN ADVISORY FOR
DAVIDSON...FORSYTH..GUILFORD...ALAMANCE...ORANGE AND PERSON COUNTIES
STARTING AT 6PM AND GOING THROUGH NOON ON FRIDAY.

PARTIAL THICKNESSES RISE ABOVE CRITICAL THRESHOLDS BY MID-MORNING
FRIDAY...AND ANY SHALLOW SUB-FREEZING LAYER NEAR THE SURFACE SHOULD
BE GONE WITH THE LOSS OF THE PARENT HIGH. DYNAMIC COOLING WITH THE
PASSING UP TROUGH MAY ALLOW FOR A SNOW/RAIN MIX AS THE DEFORMATION
BAND MOVES WEST TO EAST ON FRIDAY (MAINLY OVER THE WEST)...BUT WITH
SURFACE TEMPS ABOVE FREEZING...THERE SHOULDNT BE ANY ADDITIONAL
IMPACT.  MODELS ARE SLIGHTLY SLOWER MOVING PRECIP OUT OF THE
AREA....SO WILL END POPS IN THE 00Z-06Z TIME FRAME.  WILL ALSO KEEPS
TEMPS LOWER THAN GUIDANCE FRIDAY AFTERNOON...WITH HIGHS ONLY IN THE
UPPER 30S/LOWER 40S.  SOME POTENTIAL FOR BLACK ICE FRIDAY NIGHT AS
LOWS DIP BACK DOWN TO AROUND FREEZING IN THE WEST.  LOWS 32-36.
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&&

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
AS OF 125 AM THURSDAY...

FOR SAT THROUGH SUN NIGHT: THE SHORTWAVE TROUGH WILL BE SHIFTING
OFFSHORE BY DAYBREAK SATURDAY... THEN WEAK BUT WARM SURFACE HIGH
PRESSURE WILL BUILD EASTWARD OVER THE GULF AND SOUTHEAST STATES...
BRINGING DRY AND STABLE LOW-MID LEVELS THROUGH SAT NIGHT. A SPLIT
FLOW PERSISTS ACROSS NOAM... HIGHLIGHTED BY A SHORTWAVE TROUGH
CROSSING THE ST LAWRENCE VALLEY/NORTHEAST/MARITIMES SUNDAY WHILE THE
POSITIVE-TILTED SOUTHERN STREAM TROUGH HANGS BACK AND SLOWLY SHEARS
ACROSS AR/OK/TX INTO NRN MEXICO. THE NORTHERN STREAM WAVE PASSAGE
PUSHES A SURFACE BACKDOOR FRONTAL ZONE SOUTHWARD INTO VA... BUT THE
GFS/ECMWF AGREE THAT THE COOLEST AIR SHOULD STAY NORTH OF OUR
BORDER... SO EXPECT JUST MINOR COOLING ON SUNDAY DESPITE A SURFACE
WIND SHIFT TO NORTHWESTERLY WITH WEAK SURFACE TROUGH PASSAGE. EXPECT
HIGH CLOUDS AND A FEW MID CLOUDS LATE SAT THROUGH SAT NIGHT WITH THE
APPROACH OF THE SHEARING TROUGH AND SURFACE FRONT... BUT NO
PRECIP... AND A RETURN TO PLENTIFUL SUNSHINE ON SUNDAY AND CLEAR
SKIES SUNDAY NIGHT. NEAR-NORMAL THICKNESSES BOTH DAYS SUPPORT HIGHS
IN THE LOWER-MID 60S. LOWS 37-43 SAT NIGHT AND 35-40 SUNDAY NIGHT.

FOR MON THROUGH WED: THE SOUTHERN STREAM TROUGH OVER NW MEXICO OVER
THE WEEKEND WILL DRIFT SLOWLY OUT OVER THE NW GULF EARLY IN THE
WEEK... WHILE THE LARGELY FLAT AND WNW MID LEVEL FLOW OVER MUCH OF
THE CONUS WILL KEEP OUR WEATHER RATHER TRANQUIL THROUGH TUE WITH
ONLY WEAK SURFACE TROUGHING AND A DRY DOWNSLOPE MAINLY WESTERLY LOW
LEVEL FLOW. EXPECT FAIR TO PARTLY CLOUDY SKIES MON-TUE WITH TEMPS
TRENDING ABOVE NORMAL AS THICKNESSES RISE. WEATHER BECOMES MORE
ACTIVE TUE NIGHT/WED AS THE GULF LOW TRACKS EASTWARD WHILE FURTHER
WEST... A TROUGH OVER THE ROCKIES SHIFTS SE INTO THE PLAINS... WHILE
A BROAD POLAR STREAM LONGWAVE TROUGH DEEPENS OVER QUEBEC/ONTARIO AND
THE GREAT LAKES REGION... PUSHING A SURFACE COLD FRONT SOUTHWARD
INTO NC. THE ECMWF TRENDS TOWARD PHASING OF THE TWO STREAMS AS IT
DIGS THIS TROUGH DEEPLY OVER THE SOUTHERN PLAINS... ABSORBING THE
GULF TROUGH AND RESULTING IN A STRENGTHENING LOW APPROACHING NC FROM
THE SW... WHILE THE GFS DAMPENS THE GULF TROUGH OVER FL WITH A
FLATTER/SHEARING TROUGH OUT WEST. DESPITE THESE MAJOR LONGWAVE
PATTERN DIFFERENCES... BOTH MODELS PUSH THE FRONT SOUTH OF NC WITH
INCREASED OVERRUNNING OVER NC... SO EXPECT A TREND TOWARD COOLER
TEMPS TUE NIGHT/WED AND INCREASING CHANCES FOR RAIN. -GIH

&&

.AVIATION /06Z THURSDAY THROUGH MONDAY/...
AS OF 1235 AM THURSDAY...

VFR CONDITIONS WILL PREDOMINATE THROUGH MUCH OF THE OVERNIGHT HOURS.
THERE IS A CHANCE THAT MVFR CEILINGS WILL ENCROACH UPON KRWI AROUND
DAYBREAK AND LINGER THROUGH MOST OF THE AY. ELSEWHERE VFR CONDITIONS
WILL RULE THROUGH DAYBREAK. SFC NE WINDS WILL GRADUALLY INCREASE TO
AROUND 10KTS WITH GUSTS NEAR 20KTS. NE WINDS WILL INCREASE A BIT
MORE AFTER DAYBREAK WITH SUSTAINED WINDS 12-15KTS AND GUSTS AROUND
25 KTS.

LATER THIS MORNING...ELY FLOW JUST ABOVE THE SURFACE IN RESPONSE TO
AN AREA OF LOW PRESSURE THAT WILL TRACK ALONG THE SE U.S. COAST WILL
ADVECT MOISTURE INTO OUR REGION. THIS WILL RESULT IN THE
LOWERING/THICKENING OF A STRATUS DECK WITH MVFR CEILINGS SPREADING
SOUTH TO NORTH THIS AFTERNOON THROUGH EARLY EVENING. EXPECT MVFR
CONDITIONS AT ALL TERMINALS SITES BY SUNSET.

AREAS OF PRECIPITATION WILL OVERSPREAD CENTRAL NC FROM THE SOUTH
THIS AFTERNOON AND GRADUALLY SPREAD INTO THE NORTHERN PIEDMONT BY
THIS EVENING. THE PRECIP MAY START IN KFAY AS A RAIN/SLEET MIXTURE
BEFORE TRANSITIONING TO ALL RAIN. THE SAME WILL OCCUR AT KRWI BUT
ABOUT 3-4 HOURS LATER. PRECIP IN THE KRDU VICINITY WILL LIKELY START
OUT AS RAIN/SLEET THEN TRANSITION TO FREEZING RAIN FOR SEVERAL
HOURS TONIGHT... THEN TRANSITION TO RAIN/FREEZING RAIN AFTER
DAYBREAK FRIDAY. THE LONGEST PERIOD OF WINTRY PRECIP WILL LIKELY
OCCUR IN THE TRIAD WHERE A OF SLEET/SNOW AND FREEZING RAIN IS
EXPECTED. THIS WINTRY MIX SHOULD COMMENCE IN THE TRIAD LATE TONIGHT
AND CONTINUE INTO FRIDAY MORNING. SOME MEASURABLE SNOW/SLEET AND
ICING IS A GOOD PROBABILITY IN THIS REGION.

AVIATION CONDITIONS WILL BEGIN TO IMPROVE WEST-TO-EAST LATE FRIDAY
AND MORE SO FRIDAY NIGHT AS AN AREA OF LOW PRESSURE WILL PULL AWAY
FROM THE COAST. VFR CONDITIONS SHOULD RETURN LATE FRIDAY NIGHT INTO
SATURDAY MORNING AND PERSIST INTO EARLY NEXT WEEK.

&&

.RAH WATCHES/WARNINGS/ADVISORIES...
NONE.

&&

$$

SYNOPSIS...HARTFIELD
NEAR TERM...SMITH
SHORT TERM...SMITH
LONG TERM...HARTFIELD
AVIATION...WSS