FXUS62 KRAH 081204
AFDRAH
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC
705 AM EST TUE DEC 8 2009
.SYNOPSIS...
A POWERFUL STORM SYSTEM WILL BRING WIDESPREAD RAIN TO THE AREA
BEGINNING LATER TODAY THROUGH TONIGHT...FOLLOWED BY A CHANCE OF
SHOWERS AND AN ISOLATED THUNDERSTORM ON WEDNESDAY. ABOVE NORMAL
TEMPERATURES ARE EXPECTED ON WEDNESDAY... THEN READINGS WILL TREND
TO WELL BELOW NORMAL BY THE END OF THE WEEK. ANOTHER STORM SYSTEM
WILL AFFECT THE REGION OVER THE WEEKEND.
&&
.NEAR TERM /THROUGH TODAY/...
AS OF 140 AM TUESDAY...
FOR TODAY: LATEST SURFACE MAP SHOWS THE SURFACE HIGH CENTERED OVER
WV/OH WITH A SECOND HIGH OVER NORTHERN VA EXTENDING WEAKLY INTO NC
THIS MORNING. THE FORMER HIGH SHIFTS EAST TODAY AND ENGULFS THE
LATTER HIGH WHILE STRENGTHENING AND WEDGING MORE STRONGLY DOWN
THROUGH CENTRAL NC... SETTING THE STAGE FOR THE EXPECTED SHORT-LIVED
BUT PROMINENT UPCOMING DAMMING EVENT. SATELLITE IMAGERY INDICATES
CONSIDERABLE CLOUDS AT MULTIPLE LEVELS JUST UPSTREAM AND AS THESE
ADVECT INTO THE AREA... EXPECT SKIES TO TREND MOSTLY CLOUDY BY
AFTERNOON. MOIST ISENTROPIC UPGLIDE INCREASES AND DEEPENS QUICKLY
THIS AFTERNOON... ACCOMPANIED BY INCREASING UPPER DIVERGENCE WITH
THE 150+ KT JET OVER NM/TX... SO DESPITE THE INITIALLY DRY SURFACE
AIR (DEW POINTS IN THE LOWER-MID 30S)... WILL HOLD ONTO THE EXISTING
FORECAST OF RAIN OVERSPREADING THE AREA THIS AFTERNOON... ALSO IN
LINE WITH THE LATEST MODEL GUIDANCE. INITIAL AMOUNTS THROUGH SUNDOWN
WILL BE ON THE LOW SIDE GIVEN THE INITIAL DRY POCKETS THROUGH THE
COLUMN. THE ENCROACHING THICK CLOUDS AND INCREASING LOW LEVEL COLD
ADVECTION WILL NOT FOSTER MUCH TEMPERATURE RISE TODAY... AND HAVE
NUDGED HIGHS DOWN TO 44-53. -GIH
&&
.SHORT TERM /TONIGHT THROUGH FRIDAY/...
AS OF 300 AM TUESDAY...
FOR TONIGHT THROUGH WEDNESDAY:
THE INTENSE STORM SYSTEM WE'VE BEEN WAITING FOR FINALLY ARRIVES IN
NC. THE PRIMARY SURFACE LOW (STACKED BENEATH THE MID LEVEL VORTEX)
TRACKS UP TOWARD LAKE MICHIGAN WHILE THE SECONDARY LOW DEVELOPS OVER
SOUTHEAST GA AND MOVES RAPIDLY NORTH-NORTHEAST ALONG THE WEDGE
FRONT. WITH VIGOROUS FORCING FOR ASCENT PROVIDED BY STRONG UPPER
DIVERGENCE... INCREASING CYCLONIC MID LEVEL FLOW AND DPVA... DEEP
MOIST UPGLIDE... AND QUICKLY INCREASING LOW LEVEL MASS CONVERGENCE
WITH THE NORTHWEST-MOVING 925 MB WARM FRONT... WIDESPREAD RAIN
TONIGHT IS A SURE BET. WILL STICK WITH CATEGORICAL POPS. BUT THREE
CRITICAL CONCERNS REMAIN: WHERE WILL THE SECONDARY LOW TRACK (WHICH
WILL HAVE RAMIFICATIONS ON TEMPS AND THE SEVERE WEATHER RISK)... HOW
GREAT WILL THE SEVERE WEATHER THREAT BE...AND HOW MUCH RAIN WILL
FALL.
FIRST... REGARDING THE LOW TRACK... SINCE ITS RUNS LAST NIGHT THE
NAM HAS HELD ONTO A MORE EASTWARD TRACK OF THE SURFACE LOW EAST OF
I-95... WHEREAS THE GFS/ECMWF/CANADIAN (TO THE DEGREE THAT THESE
MODELS CAN BE ASSESSED GIVEN THEIR LOWER TEMPORAL RESOLUTION) TRACK
IT FARTHER WEST THROUGH THE EASTERN PIEDMONT. THE NAM DOES HAVE A
BIT SLOWER/STRONGER PARENT SURFACE HIGH SHIFTING NORTHEAST OVER THE
NORTHEAST STATES AS COMPARED TO THE OTHERS (POSSIBLY DUE TO ITS MORE
WESTERLY TRACK OF THE PRIMARY LOW OVER LAKE MICHIGAN)... WHICH WOULD
EXPLAIN ITS STRONGER WEDGE OVER THE NC PIEDMONT. AS THE NAM IS ON
THE WESTERN SIDE OF MODEL SOLUTIONS... PREFER A LEAN MORE TOWARD THE
GFS/ECMWF AND A LOW TRACK THROUGH THE WESTERN SANDHILLS AND EASTERN
PIEDMONT TONIGHT. EXPECT INITIALLY NORTHEAST SURFACE WINDS OVER THE
ENTIRE AREA TO SHIFT AROUND TO SOUTH ROUGHLY ALONG/EAST OF HIGHWAY
ONE OVERNIGHT AND PICK UP SPEED TO 10-15 MPH... WHILE THE NORTHWEST
PIEDMONT STAYS IN A COMPARATIVELY LIGHT MAINLY NORTHEAST FLOW.
RESULTANTLY... WILL ALSO RETAIN THE FORECAST OF NEARLY STEADY TEMPS
IN THE EVENING THEN RISING OVERNIGHT PRIMARILY OVER THE EASTERN HALF
OF THE CWA. EXPECT MORNING TEMPS THE LOW SHOULD BE EXITING THE
NORTHEAST PORTION OF THE CWA BY SUNRISE... THEN SURFACE TROUGHING
SETS UP OVER CENTRAL NC AHEAD OF THE COLD FRONT... WHICH DROPS
THROUGH IN THE VERY LATE AFTERNOON THROUGH EARLY EVENING.
SECOND... REGARDING THE SEVERE THREAT... AS PREVIOUS DISCUSSIONS
HAVE DETAILED... THE RAPIDLY STRENGTHENING WIND FIELD TONIGHT IS
MORE THAN SUFFICIENT FOR ORGANIZED CONVECTION AS 925 MB WINDS
INCREASE TO 50 KTS WITH 500 MB WINDS HEADING UP TO 80 KTS OVERNIGHT.
THE LOW LEVEL SHEAR WILL BE MASSIVE RIGHT ALONG THE WEDGE FRONT...
POSING A RISK OF ISOLATED TORNADOES GIVEN ENOUGH FORCING FOR TILTING
AND STRETCHING OF THIS LOW LEVEL VORTICITY. LARGER SCALE LIFT WILL
PERSIST THROUGH THE NIGHT AS THE NOSE OF THE STRENGTHENING (TO
NEARLY 170 KTS) UPPER JET CORE OVER OK/AR EXTENDS OVER THE REGION
WITH MID LEVEL HEIGHT FALLS OF 50-80 M. HOWEVER THE STRONGEST
SYNOPTIC SCALE FORCING HOLDS JUST TO OUR NORTHWEST... AND THE MODELS
CONTINUE TO SHOW A DEEPLY MOIST ADIABATIC PROFILE THROUGH THE MID
LEVELS WITH LIMITED INSTABILITY... AS BOTH THE NAM AND GFS DEPICT A
MUCAPE OF BARELY 200 J/KG CONFINED TO THE COASTAL AREAS. WILL NEED
TO KEEP AN EYE ON ANY DISCRETE CELLS AND/OR ANY SHORT BOWING
SEGMENTS SHOULD THERE BE SOME INSTABILITY... HOWEVER THE RISK OF A
SIGNIFICANT SEVERE WEATHER EVENT IS VERY LOW. AFTER THE SECONDARY
LOW PASSES BY IN THE MORNING... WINDS BECOME UNIDIRECTIONAL THROUGH
THE COLUMN AND WITH MID LEVEL DRYING (AND MINOR WARMING) AND
DESTABILIZATION IN THE LOWEST 1 KM... VERY BRISK AND GUSTY SURFACE
WINDS ARE ANTICIPATED THROUGH WEDNESDAY. WILL RAISE WIND GUSTS A BIT
TO AROUND 35-40 KTS. LATER SHIFTS MAY NEED TO POST A WIND ADVISORY
FOR A PORTION OF CENTRAL NC.
LASTLY... WHILE RAIN WILL BECOME MODERATE AT TIMES... PARTICULARLY
IN AND NEAR THE WEDGE AIR MASS AS THE MOIST AIR ASCENT IS AUGMENTED
BY BEING FORCED UP THE COLD DOME... THE SWIFT MOVEMENT OF THIS
SYSTEM AND THE TRACK OF THE STRONGEST LARGE SCALE DYNAMICS WILL KEEP
RAINFALL AMOUNTS GENERALLY UNDER AN INCH AND A HALF. THIS MAY
EXACERBATE ONGOING ISSUES WITH HIGH WATER LEVELS ON AREA CREEKS AND
RIVERS... AND AREAS OF POOR DRAINAGE AND WET SOIL MAY SEE PROBLEMS
WITH STANDING WATER... BUT SIGNIFICANT FLOODING IS NOT EXPECTED. THE
WIDESPREAD RAIN WILL DIMINISH WEDNESDAY MORNING... THEN THE RISK OF
SCATTERED SHOWERS WILL LINGER UNTIL COLD FRONTAL PASSAGE VERY LATE
IN THE DAY.
FOR WEDNESDAY NIGHT THROUGH FRIDAY: THE COLD FRONT SHOULD CLEAR THE
NC COAST BY 06Z WEDNESDAY NIGHT. CHILLY SURFACE HIGH PRESSURE... THE
ONE CURRENTLY BRINGING SUBZERO TEMPS TO SOUTHERN ALBERTA AND
SASKATCHEWAN AND MT/ND... BUILDS SOUTHEAST ACROSS THE PLAINS AND
EASTWARD OVER THE CAROLINAS THROUGH FRIDAY. FORECAST SOUNDINGS SHOW
A FAIRLY DRY (EXCEPT FOR SMATTERINGS OF HIGH CLOUDINESS - POSSIBLY
OROGRAPHICALLY ENHANCED - DUE TO THE STRONG MID LEVEL FLOW) AND
STABLE COLUMN... THUS EXPECT FAIR TO MOSTLY CLEAR SKIES THROUGH
FRIDAY. THICKNESSES HOLD BELOW NORMAL... THUS THERE IS LITTLE NEED
TO CHANGE THE CURRENT FORECAST OF NEAR-NORMAL TEMPS THURSDAY
DROPPING WELL BELOW NORMAL FOR FRIDAY... WHEN HIGHS MAY STRUGGLE TO
REACH THE LOW-MID 40S. -GIH
&&
.LONG TERM /FRIDAY NIGHT THROUGH MONDAY NIGHT/...
AS OF 315 AM TUESDAY...
THE FORECAST GETS TRICKY FRI NIGHT THROUGH SAT AS A SOUTHERN STREAM
SYSTEM BEGINS TO TRANSLATE EAST FROM THE WESTERN GULF....WITH A
STRONG DRY 1030MB HIGHS OVER CENTRAL NC. A SURFACE LOW IS EXPECTED
TO DEVELOP ALONG A REMNANT BAROCLINIC ZONE AND SHIFT EAST TO THE
CAROLINA COAST BY SAT. AS IT DOES SO...PRECIP WILL SPREAD NORTH
ACROSS CENTRAL NC. AT THE MOMENT...THE GFS IS A FAST OUTLIER...WITH
PRECIP REACHING THE RAH CWA BY EARLY SAT MORNING AND SHIFTING EAST
TO THE COAST BY SAT EVENING. THE ECMWF/CANADIAN HOLD PRECIP OFF
UNTIL SATURDAY DURING THE DAY. GIVEN THE STRENGTH OF THE SURFACE
HIGH EXPECTED TO BE DIRECTLY OVERHEAD ON FRI...AND THE VERY DRY
STATE OF THE BOUNDARY LAYER PRIOR TO PRECIP ONSET...IT
SEEMS POSSIBLE THAT EVEN THOUGH THE SYSTEM MAY MOVE QUITE QUICKLY
INTO THE CWA....ACTUALLY PRECIP AT THE SURFACE COULD LAG. THE
AMOUNT OF LOW LEVEL DRY AIR ALSO COMPLICATES THE FORECAST...AS THE
SURFACE WETBULB 32 DEGREE ISOTHERM REACHES AS FAR SOUTH AS THE
CAROLINA COASTLINE....AND LOW/MID LEVEL THICKNESSES START OUT AT AS
LOW AS 1290/1535M ACROSS THE NORTHERN HALF OF THE CWA AT 06Z ON SAT
MORNING. MID LEVEL WARM ADVECTION IS FORECAST TO DEVELOP A WARM
NOSE BY MIDDAY SAT...WHILE SIMULTANEOUS DRYING ALOFT COULD LIMIT ICE
CRYSTAL GROWTH. THE HEAVIEST QPF IS FORECAST TO MOVE EAST ACROSS
THE SOUTHERN HALF OF THE AREA AS THE COASTAL LOW DEVELOPS. TO SUM
UP...IT APPEARS THERE WILL THE BE POTENTIAL FOR PRECIP TO START OFF
FROZEN IN THE W AND NW IF THE ONSET OF PRECIP IS EARLY ENOUGH.
GIVEN THAT THE GFS IS AN OUTLIER WITH ITS FAST ONSET...WILL WAIT FOR
FURTHER MODEL RUNS TO ASSESS ANY MENTION OF FROZEN PRECIP.
THE COASTAL SYSTEM IS FORECAST TO MOVE OUT TO SEA BY SUNDAY...WITH
HIGH PRESSURE BUILDING ACROSS THE GREAT LAKES AND THEN SOUTH TOWARD
OUR AREA. TEMPS SHOULD HOLD JUST BELOW NORMAL VALUES WHILE YET
ANOTHER SOUTHERN STREAM SYSTEM GETS GOING ALONG THE GULF COAST BY
THEN END OF THE PERIOD.
&&
.AVIATION /12Z TUESDAY THROUGH FRIDAY/...
AS OF 705 AM TUESDAY...
MID LEVEL CLOUDS HAVE OVERSPREAD CENTRAL NC OVERNIGHT...AHEAD OF A
STORM SYSTEM DEVELOPING OVER THE DEEP SOUTH AND GULF COAST STATES.
AS THE SURFACE LOW ASSOCIATED WITH THIS SYSTEM LIFTS NE INTO THE
MIDWEST TODAY...WIDESPREAD RAIN WILL MOVE INTO NC FROM THE S AND
SW. CIGS WILL ACCORDINGLY LOWER TO MVFR....WITH LIFR/IFR CIGS AND
VSBYS LIKELY OVERNIGHT. IN ADDITION...A VERY STRONG LOW LEVEL JET
WILL TRAVERSE CENTRAL NC OVERNIGHT...WITH SURFACE WINDS SHIFTING
FROM NE TO S-SW AS A WARM FRONT EVENTUALLY LIFTS THROUGH THE AREA
JUST AHEAD OF THE MAIN COLD FRONT. THE LOW LEVEL JET WILL BRING THE
POTENTIAL FOR LOW LEVEL WIND SHEAR....WITH AS MUCH AS 50KTS EXPECTED
AT ABOUT 2K FT...ENHANCED BY THE INITIALLY NERLY SURFACE FLOW. A
FEW THUNDERSTORMS WILL ALSO BE POSSIBLE...MAINLY EAST OF I-95. THE
COLD FRONT IS EXPECTED TO CROSS CENTRAL NC DURING THE LATE MORNING
HOURS ON WEDNESDAY...WITH POTENTIALLY STRONG GUSTY WINDS WITH THE
FRONT.
AS WINDS BECOME WRLY BEHIND THE FRONT...VFR CIGS AND VSBYS WILL
RETURN FOR THE REMAINDER OF THE WORK WEEK. ANOTHER ROUND OF MVFR OR
LOWER CONDITIONS WILL BE POSSIBLE ON SATURDAY AS YET ANOTHER SYSTEM
IS EXPECTED TO DEVELOP ALONG THE GULF COAST AND MOVE INTO THE AREA.
&&
.RAH WATCHES/WARNINGS/ADVISORIES...
NONE.
&&
$$
SYNOPSIS...HARTFIELD
NEAR TERM...HARTFIELD
SHORT TERM...HARTFIELD
LONG TERM...SMITH
AVIATION...SMITH
