Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000
FXUS62 KRAH 230017
AFDRAH

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC
715 PM EST MON DEC 22 2014

.SYNOPSIS...
A COLD AIR DAMMING RIDGE WILL REMAIN LOCKED IN PLACE
EAST OF THE APPALACHIANS THROUGH TUESDAY. A WARM FRONT WILL LIFT
NORTHWARD THROUGH THE REGION LATE TUESDAY NIGHT AND WEDNESDAY AS A
DEVELOPING LOW PRESSURE SYSTEM STRENGTHENS WEST OF THE MOUNTAINS.

&&

.NEAR TERM /THROUGH TONIGHT/...
AS OF 300 PM MONDAY...

REMAINDER OF THIS AFTERNOON AND THIS EVENING... AT THE
SURFACE...COLD AIR DAMMING AIR MASS FIRMLY IN PLACE DUE TO FAVORABLE
STRENGTH/POSITION OF PARENT HIGH OVER NORTHERN NEW ENGLAND AND
EARLIER RAINFALL. TEMPS HAVE HELD STEADY IN THE 30S ACROSS THE
PIEDMONT WITH LOWER 40S ACROSS THE SE. EXPECT LITTLE CHANGE IN THESE
TEMPS THROUGH TONIGHT.

SPOTTY LIGHT PRECIP WILL INCREASE IN COVERAGE LATE THIS AFTERNOON
AND THIS EVENING AS PERTURBATION IN THE SW FLOW ALOFT LIFTS ACROSS
THE EASTERN CAROLINAS. THIS FEATURE SHOULD INDUCE WEAK OVERRUNNING
OF THE COOL STABLE SURFACE-BASED AIR...RESULTING IN PATCHY LIGHT
RAIN. THROUGH 02Z...EXPECT THE BULK OF THIS PRECIP TO OCCUR MAINLY
OVER THE EASTERN SANDHILLS AND SOUTHERN COASTAL PLAIN.

LATER TONIGHT...WILL LIKELY SEE AN INCREASE IN AREAS OF RAIN OVER
THE PIEDMONT AS ANOTHER PERTURBATION (OVER THE NORTHERN CENTRAL GULF
COAST)ADVANCES NEWD ALONG THE SPINE OF THE APPALACHIANS. THE
EXPECTED TRACK OF THIS SYSTEM SHOULD KEEP MOST OF THE PRECIP
CONFINED TO OUR WESTERN PIEDMONT AND AREAS FARTHER WEST.

TUESDAY...NO MECHANISM NOTED TO SCOUR OUT THE CAD AIR MASS SO EXPECT
A CONTINUATION OF THE EXTENSIVE OVERCAST WITH PATCHES OF LIGHT
RAIN/DRIZZLE. EXPECT TO SEE A NOTEWORTHY INCREASE IN RAIN COVERAGE
AND INTENSITY LATE IN THE DAY AS S/W (NOW CROSSING CENTRAL TX)
ADVANCES EAST TOWARD OUR REGION. THIS FEATURE WILL INCREASE
DIVERGENCE ALOFT AS WELL AS INCREASE ISENTROPIC UPGLIDE IN THE LOWER
LAYERS OF THE ATMOSPHERE. THESE FACTORS IN COMBINATION WITH PRECIP
WATER VALUES WELL ABOVE NORMAL SHOULD RESULT IN AREAS OF MODERATE-
BRIEFLY HEAVY RAINFALL LATE IN THE DAY OR EARLY EVENING ACROSS THE
WESTERN PIEDMONT.

DAYTIME TEMPS WILL REMAIN WELL BELOW NORMAL. USED THE HIGH RES WRF-
ARW FOR HOURLY TEMPS AS GFS-BASED GUIDANCE APPEARS TO BE ERODING THE
CAD (AND ITS LINGERING EFFECTS) TOO QUICKLY. TEMPS BY LATE AFTERNOON
SHOULD VARY FROM THE UPPER 30S/LOWER 40S OVER THE PIEDMONT TO NEAR
50 OVER THE FAR SE.

&&

.SHORT TERM /TUESDAY NIGHT THROUGH WEDNESDAY NIGHT/...
AS OF 300 PM MONDAY...

A POWERFUL 175KT PACIFIC JET DIGGING THROUGH THE WESTERN US WILL
HELP TO CARVE OUT A HIGH AMPLITUDE TROUGH OVER THE CENTRAL US...WITH
AN EMBEDDED SOUTHERN STREAM CLOSED LOW. AN ALMOST EQUALLY POWERFUL
JET WILL EVOLVE OVER THE EASTERN US ON THE LEADING SIDE OF THE UPPER
TROUGH WHICH WILL BE THE IMPETUS FOR A WET AND UNSETTLED PERIOD
TUESDAY NIGHT THROUGH CHRISTMAS EVE AS THE UPPER TROUGH AND
ATTENDANT COLD FRONT MOVES SLOWLY EAST-NORTHEASTWARD THROUGH THE
REGION EARLY CHRISTMAS DAY.

IN ADVANCE OF THE HIGH AMPLITUDE LONGWAVE TROUGH AND ATTENDANT SFC
COLD FRONT...DEEP INFLUX OF PACIFIC MOISTURE...SUPPLEMENTED WITH A
LOW-LEVEL CARIBBEAN FEED...WILL ADVECT ANOMALOUSLY HIGH PWATS OF 1.5-
1.7" INTO THE AREA TUESDAY NIGHT-WEDNESDAY(WITHIN THE 99TH
PERCENTILE WITH THE HIGHEST OBSERVED DEC PWAT DATING BACK TO 1948 IS
1.59"-12/21/1991). THIS STRONG MOISTURE FLUX CONVERGENCE ALONG THE
NORTHWARD RETREATING WARM FRONT...AND ENHANCED LOCALLY OVER THE MORE
STEEPLY SLOPED PORTION OF THE LINGERING COLD DOME WILL RESULT IN A
WIDESPREAD SOAKING RAIN OF 1-2"...HEAVY AT TIMES TUESDAY NIGHT AND
INTO CHRISTMAS EVE MORNING. ONCE THE WARM FRONT LIFTS NORTHWARD INTO
THE VIRGINIA...THERE WILL LIKELY BE A LULL IN PRECIP DURING THE DAY
WITH BREEZY CONDITIONS DEVELOPING IN THE WARM SECTOR. THEN BY LATE
AFTERNOON AND INTO THE EVENING(BETWEEN 00Z-06Z THURSDAY)....THE EAST-WEST
PASSAGE OF THE COLD FRONT AND ACCOMPANYING BAND OF SHOWERS AND POSSIBLY
A FEW THUNDERSTORMS WILL BRING ONE LAST ROUND OF RAIN INTO THE AREA.

SEVERE POTENTIAL: EVALUATING SEVERE POTENTIAL IN A LOW CAPE-HIGH
SHEAR ENVIRONMENT IS ALWAYS A CHALLENGE...EVEN AT CLOSER TIME SCALES
OF 6-12 HOURS. THE LULL IN PRECIP DURING MUCH OF THE DAYTIME
HOURS...MAY ALLOW FOR SOME MODEST HEATING...ESPECIALLY FROM THE US 1
CORRIDOR EASTWARD...WHICH COULD LEAD TO BETTER DESTABILIZATION THAN
WHAT MODELS CURRENTLY SHOW(100-200J/KG MUCAPE). GIVEN STRONG SHEAR
ALONE...CANNOT RULE OUT AN ISOLATED SEVERE...BUT WEAK LAPSE RATES
ALOFT OF 5.0-5.5 C/KM ALONG WITH THE MAIN DYNAMICS PASSING OVER THE
GREAT LAKES WILL LIMIT A MORE ROBUST SEVERE THREAT. ADDITIONALLY...
IF WE SEE WIDESPREAD RAINFALL AMOUNTS NEAR THE HIGHER SIDE OF THE
EXPECTED QPF OF 2.0"...MINOR FLOODING WILL BE POSSIBLE ACROSS
PORTIONS OF THE NEUSE AND TAR. SEE HYDROLOGICAL DISCUSSION BELOW FOR
ADDITIONAL DETAILS.

TEMPERATURES VERY CHALLENGING WITH HIGH BUST POTENTIAL...DEPENDENT
ON EVOLUTION/TIMING/LOCATION OF THE RETREATING WEDGE FRONT INTO THE
CLIMATOLOGICALLY FAVORED AREA ACROSS THE FAR NW PIEDMONT TUESDAY
NIGHT AND INTO THE DAY ON WEDNESDAY. SUCH A STRONG CAD EVENT WOULD
STRONGLY SUGGEST FAVORING THE COOLER NAM/MET GUIDANCE THROUGH EARLY
WEDNESDAY WITH THE WELL KNOWN MODEL BIAS OF ERODING THE CAD TOO
QUICKLY. LOWS TUESDAY NIGHT WILL LIKELY OCCUR EARLY IN THE NIGHT
WITH RISING TEMPERATURES THEREAFTER IN ALL BUT THE FAR NW PIEDMONT
THROUGH DAYBREAK. A LARGE RANGE IN TEMPERATURES ARE EXPECTED ON
WEDNESDAY WITH THE POTENTIAL FOR THE SHALLOW WEDGE AIRMASS TO HOLD
ON ACROSS THE EXTREME NW CORNER...KEEPING TEMPS IN THE 50S. THE
REMAINDER OF THE FORECASTER AREA WILL BE IN THE  WARM SECTOR WITH
HIGHS IN THE UPPER 60S ACROSS THE CENTRAL PIEDMONT...AND LOWER 70S
ACROSS THE SANDHILLS AND COASTAL PLAIN. CLEARING AND COOLING
WEDNESDAY NIGHT IN THE WAKE OF THE COLD FRONT LOOKS A BIT DELAYED
THAN WHAT MODELS WERE SHOWING A DAY OR TWO AGO. NOT AS COOL AS WE
ORIGINALLY THOUGHT. LOWS IN THE MID TO UPPER 40S NW TO MID 50S SE.

&&

.LONG TERM /THURSDAY THROUGH MONDAY/...
AS OF 300 PM MONDAY...

DEEPENING SURFACE LOW LIFTING NEWD INTO SE CANADA WILL RESULT IN
BREEZY CONDITIONS ON CHRISTMAS DAY. OTHERWISE DRY WITH SEASONABLY
MILD TEMPERATURES CONTINUING INTO FRIDAY AS HIGH PRESSURE BUILDS IN
FROM THE NORTHERN GULF.

MODEL SPREAD BEGINS TO INCREASE DRAMATICALLY LATE FRIDAY THROUGH THE
WEEKEND...WITH THE EC MUCH DEEPER WITH THE NEXT MID/UPPER LEVEL
TROUGH...RESULTING IN YET ANOTHER WET AND UNSETTLED PERIOD LATE AS
IT TRACKS AN AREA OF LOW PRESSURE ACROSS THE SOUTHERN/CENTRAL
APPALACHIANS LATE SATURDAY AND INTO SUNDAY. THE GFS REMAINS MUCH
FLATTER WITH THE UPPER TROUGH...PRODUCING LITTLE TO NO PRECIP OVER
THE CAROLINAS. WILL INDICATE SLIGHT CHANCE POPS UNTIL BETTER
AGREEMENT IS REACHED. COOLER TO SLIGHTLY BELOW NORMAL TEMPERATURES
ARE EXPECTED BEHIND THE FRONT EARLY NEXT WEEK.

&&

.AVIATION /00Z TUESDAY THROUGH SATURDAY/...
AS OF 715 PM MONDAY...

AVIATION CONDITIONS WILL REMAIN POOR WELL ON THROUGH TONIGHT AND
TUESDAY...WITH PERIODS OF RAIN/DRIZZLE AND A LIGHT NORTHEASTERLY
WIND ALONG WITH THE ALREADY LOW CLOUDS AND LIMITED VSBYS.   THERE IS
HIGH CONFIDENCE OF LIFR CONDITIONS THROUGH THE NIGHT AND THERE MAY
ALSO BE SOME VLIFR CONDITIONS RESULTING FROM FOG BETWEEN 06Z AND 12Z.

THERE WILL BE NO CHANGE IN THE AIRMASS OVER NC ON TUESDAY...SO ONLY
MINIMAL IMPROVEMENT IN CONDITIONS IS EXPECTED.  CEILINGS MAY LIFT A
LITTLE DURING THE AFTERNOON ALLOWING IFR CONDITIONS TO DEVELOP...BUT
CONFIDENCE IN THIS IS LOW GIVEN HOW LITTLE CONDITIONS CHANGED
EARLIER TODAY.  PERIODS OF LIGHT RAIN AND DRIZZLE WILL CONTINUE.


OUTLOOK... AN UPPER LEVEL TROUGH AND ASSOCIATED COLD FRONT WILL
APPROACH THE REGION TUESDAY NIGHT AND WEDNESDAY...LEADING TO AN
INCREASED CHANCE OF SHOWERS...ALONG WITH GUSTY WINDS AND A FEW
ISOLATED THUNDERSTORMS (MAINLY OVER EASTERN NC).  THUS...AVIATION
CONDITIONS WILL REMAIN POOR THROUGH AT LEAST WEDNESDAY....BEFORE A
COLD FRONT CROSSES THE AREA LATE WEDNESDAY NIGHT AND EARLY THURSDAY.
BEHIND THE FRONT...BLUSTERY WESTERLY WINDS WILL OCCUR CHRISTMAS DAY
WITH GUSTS 30-35KTS POSSIBLE. OTHERWISE VFR CONDITIONS EXPECTED TO
RETURN CHRISTMAS DAY AND PERSIST THROUGH SATURDAY.

&&

.HYDROLOGY...
AS OF 230 PM MONDAY...

ANTECEDENT RAINFALL OVER THE PAST 24 HOURS RANGED FROM A MINIMA OF
ABOUT A TENTH OF AN INCH IN THE NORTHWEST PIEDMONT (THE UPPER
YADKIN/PEE DEE AND UPPER HAW RIVER BASINS) TO A STRIPE OF 1 INCH
PLUS ACROSS THE SOUTHEAST (THE CENTRAL NEUSE AND CENTRAL CAPE FEAR
BASINS). WILL SEE ONLY MINOR RISES ON THE MAINSTEM RIVERS IN
RESPONSE...BUT THIS IS THE 3RD FAIRLY SIGNIFICANT RAINFALL EVENT
(>0.5 INCHES) OVER THE AREA SINCE 12/10. RELATIVE SOIL MOISTURE
PERCENTAGES IN THE 0-200 CM COLUMN HAVE BEEN INCREASING...WITH
HIGHER PERCENTILES NEARER THE SURFACE...SO QUICKER RUNOFF IS
EXPECTED FROM OUR UPCOMING RAIN EVENT.

CURRENT QUANTITATIVE PRECIP FORECASTS FROM THE GEFS AND NAEFS
ENSEMBLES ARE IN LOCKSTEP AGREEMENT AT PRESENT...WITH HEAVIEST RAIN
(~1.3-1.5 INCHES) FROM TOMORROW NIGHT THROUGH CHRISTMAS MORNING.
RAINFALL AMOUNTS IN THIS RANGE COULD POTENTIALLY CAUSE SOME MINOR
FLOODING ON THE NEUSE RIVER AND TAR RIVER LATE ON CHRISTMAS
DAY...BUT IT WOULD BE LOW IMPACT WITH THOSE RIVERS BARELY REACHING
MINOR FLOOD STAGE. THE UPSHOT...RIVER FLOODING WILL ONLY BE AN ISSUE
IF RAINFALL FORECASTS BEGIN TRENDING HIGHER...INTO THE 2 INCH PLUS
RANGE.

&&

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

&&

$$
SYNOPSIS...VINCENT
NEAR TERM...WSS
SHORT TERM...CBL
LONG TERM...CBL
AVIATION...BLS
HYDROLOGY...MLM



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