FXUS62 KRAH 221147
AFDRAH
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC
647 AM EST TUE DEC 22 2009
.SYNOPSIS...
HIGH PRESSURE WILL EXTEND INTO NC THROUGH THURSDAY. A COLD FRONT
WILL CROSS THE REGION CHRISTMAS DAY.
&&
.NEAR TERM /THROUGH TONIGHT/...
AS OF 245 AM TUESDAY...
PRE DAWN TEMP/DEWPOINT SPREADS WERE MAINLY AOB 4 DEGREES ACROSS THE
COASTAL PLAIN....SANDHILLS AND SECTIONS OF THE PIEDMONT. WITH
CONTINUED CALM AND MOSTLY CLEAR CONDITIONS...WILL LIKELY SEE AREAS
OF GROUND FOG DEVELOP. OVER THE FAR NORTHERN-NW PIEDMONT...PERIODS
OF CLOUDINESS HAS RESULTED IN WARMER TEMPS RESULTING IN DEWPOINT
SPREADS AOA 5 DEGREES. ANY FOG THAT DEVELOPS IN THIS REGION WILL BE
ISOLATED. NOT EXPECTING DENSE FOG NOR WIDESPREAD VISIBILITIES BELOW
3 MILES...SO WILL NOT MENTION PATCHY FOG IN GRIDS/FORECAST AT THIS
TIME.
OTHERWISE MINOR SURFACE HIGH (1025MB) WILL DRIFT ACROSS CENTRAL NC
TODAY. THIS WILL RESULT IN RELATIVELY LIGHT WINDS. ALOFT...WEAK
PERTURBATION WILL CROSS OVERHEAD IN WLY FLOW ALOFT. THIS FEATURE MAY
CAUSE PATCHES OF MID/HIGH LEVEL CLOUDINESS...BULK OF WHICH SHOULD BE
TRANSLUCENT. LOW LEVEL THICKNESSES (AROUND 1310M) WITH FULL TO
PARTIAL SUN SUGGEST MAX TEMPS 2-3 DEGREES WARMER THAN MOS GUIDANCE.
MOS HAS BEEN RUNNING A FEW DEGREES TOO COOL...ESPECIALLY AT RDU AND
FAY...THE PAST FEW DAYS. BASED ON THIS TREND AND EXPECTED
FULL/PARTIAL SUN CONDITIONS...WILL SHADE MAX TEMPS NEAR OR A COUPLE
OF DEGREES ABOVE THE WARM SIDE OF GUIDANCE.
TONIGHT...A STRONG LOW PRESSURE SYSTEM WILL DEEPEN OVER THE DESERT
SW. MEANWHILE A DEEP UPPER LOW OVER NEW ENGLAND WILL DRIFT E-SE. THE
ABOVE TWO FEATURE WILL GRADUALLY AMPLIFY A S/W RIDGE OVER THE OH
VALLEY INTO THE UPPER MS VALLEY. THIS OCCURRENCE WILL INDUCE A
SURFACE HIGH OVER SOUTH CENTRAL CANADA TO BUILD SE WITH SURFACE
RIDGE EXTENDING INTO CENTRAL NC BY 12Z WEDNESDAY. BUILDING SURFACE
RIDGE WILL RESULT IN A NEAR SURFACE BASED INVERSION BY 12Z WED.
MOISTURE TRAPPED BELOW THIS FEATURE MAY CAUSE AREAS OF FOG OR LOW
STRATUS TO DEVELOP. MODEL RH PROFILES NOT AS MOIST AS 24 HOURS AGO
WHILE GFS PROFILE APPEARS TOO MOIST ALOFT CONSIDERING PRESENCE OF
S/W RIDGE TO OUR NW. PLAN TO CONTINUE FORECAST TREND OF MOSTLY CLEAR
SKIES IN THE NW PIEDMONT BECOMING PARTLY CLOUDY. WILL REFRAIN FROM
MENTIONING FOG AT THIS TIME. DUE TO INCREASING CLOUDINESS IN THE NW
PIEDMONT...WILL BUMP UP MIN TEMPS A FEW DEGREES FROM EARLIER
FORECAST. EXPECT COOLEST MIN TEMPS IN THE NE WHERE SKIES SHOULD
REMAIN MOSTLY CLEAR THROUGH THE NIGHT.
&&
.SHORT TERM /WEDNESDAY THROUGH THURSDAY/...
AS OF 315 AM TUESDAY...
TRANQUIL WEATHER REGIME EXPECTED TO CONTINUE THROUGH THURSDAY
WITH SURFACE RIDGE EXTENDING ACROSS THE REGION FROM PARENT HIGH
(1038MB) OVER ONTARIO. S/W RIDGE TRANSLATES EAST THROUGH
PERIOD...KEEPING BULK OF MID/HIGH LEVEL MOISTURE TO OUR WEST. LOW
LEVEL THICKNESS HOVER IN THE LOW 1310S BOTH DAYS. THESE VALUES
COUPLED WITH FULL TO PARTIAL SUN SUPPORT MAX TEMPS IN THE UPPER 40S
TO LOWER 50S BOTH DAYS. THESE VALUES NEAR OR SLIGHTLY WARMER THAN
MOS GUIDANCE. WENT WITH MAX TEMPS ON THE WARM SIDE OF GUIDANCE.
&&
.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
AS OF 325 AM TUESDAY...
MAIN WEATHER CONCERN FOR THE EXTENDED PERIOD REMAINS THE UNSETTLED
WEATHER FOR LATE CHRISTMAS EVE AND CHRISTMAS DAY.
NWP GUIDANCE FAIRLY CONSISTENT SHOWING A DEEP LAYER LOW PRESSURE
SYSTEM OVER THE CENTRAL PLAINS LATE ON THURSDAY. THE MID LEVEL
CLOSED LOW AT 500 MB MOVES FROM THE KS/NE REGION AT 00 UTC ON FRI TO
THE MI/WI AREA BY LATE SAT. A STRONG JET AT 300 MB ROTATES AROUND
THE BASE OF THE LOW ON FRIDAY AS THE SUBTROPICAL JET AT 250 MB
INTENSIFIES OVER THE N GULF OF MEXICO. BY SAT AFTN A STRONG SW JET
AT 250 MB APPROACHING 150 KTS IS ALOFT OVER GA AND THE CAROLINAS.
WHILE THE MAIN SFC LOW ASSOCIATED WITH THE DEEP LAYER LOW ALOFT
REMAINS ACROSS THE MISSOURI VALLEY AND W GREAT LAKES DURING THE
PERIOD...THE ASSOCIATED COLD FRONT MOVES EAST ACROSS THE MISSISSIPPI
RIVER INTO THE DEEP SOUTH ON THU NITE AND FRI. A SECOND AREA OF LOW
PRESSURE IS EXPECTED TO DEVELOP ON THE COLD FRONT ON FRIDAY MORNING
AND THEN MOVE QUICKLY NE ACROSS THE PIEDMONT REGION OF GA INTO
CENTRAL OR EASTERN NC FRIDAY EVENING.
NWP GUIDANCE HAS GRADUALLY SLOWED THE ARRIVAL OF MOISTURE AND PRECIP
ACROSS CENTRAL NC WITH THIS SYSTEM WHICH IS TYPICAL WITH A SLOW
MOVING CLOSED LOW ACROSS THE PLAINS AND GREAT LAKES REGION. A
RESPECTABLE ~1035 MB HIGH PRESSURE SYSTEM IS CENTERED WELL NORTH IN
CANADA NEAR JAMES BAY ON THU EVE. THE HIGH RIDGES S INTO THE
NORTHEAST AND MID ATLANTIC WITH A DRIER AND MORE STABLE AIR MASS IN
THE PIEDMONT REGION OF NC/VA. THE AIR MASS IS NOT TERRIBLY DRY WITH
THE NAM/GFS BOTH INDICATING SFC DEW POINTS IN THE 20S ACROSS W NC/VA
ON FRI AM.
EXPECT TO SEE AN AREA OF PRECIPITATION ACROSS AL/GA INITIALLY DRIVEN
LARGELY BY THE COLD FRONT EXPAND ACROSS GA/SC AND W NC THURS NITE AS
WARM ADVECTION AND ISENTROPIC ASCENT DRIVES LARGE SCALE LIFT. WILL
INCLUDE LOW CHC POPS IN THE W PARTS OF CWA ON THU EVENING WITH HIGH
CHC POPS AFTER MIDNIGHT. MOST OF THE OVERNIGHT WILL BE DRY ACROSS
THE E PIEDMONT AND COASTAL PLAIN AS MOISTURE AND LIFT IS SLOW TO
OVERSPREAD THE AREA. STILL SOME CONCERN ABOUT A BRIEF PERIOD OF
FREEZING RAIN ON FRI MORNING IN THE NW PIEDMONT/TRIAD AREA AS
EVAPORATIONAL COOLING COULD DRIVE SFC TEMPS TO THE WET BULB TEMP
WHICH IS NEAR OR JUST BELOW FREEZING AT 12 UTC ON FRI. FCST
SOUNDINGS SHOW A LARGE WARM NOSE IN THE TRIAD AND ONLY A BRIEF
PERIOD IN WHICH FZRA IS POSSIBLE. GIVEN THE LOW CONFIDENCE IN THE
STRENGTH AND COOLING POTENTIAL OF THE AIR MASS...THE LOCATION AND
MOTION OF THE COLD AIR PARENT HIGH AND THE DELAYED ARRIVAL TIME OF
THE PRECIP WILL OPT TO EXCLUDE MENTION OF FZRA FOR NOW.
AS THE SFC LOW DEVELOPS AND MOVES NE ALONG THE I95 CORRIDOR...THE
LOW LEVELS E OF THE SFC LOW ACROSS THE COASTAL PLAIN ARE EXPECTED TO
DESTABILIZE LATE FRI AND FRIDAY EVENING. THE ECMWF AND CANADIAN
BRING THE SFC LOW FURTHER W ACROSS THE W PIEDMONT AND TRIAD AREA
WHICH IS UNLIKELY GIVEN THE LINGERING CAD AIRMASS AND PAST
EXPERIENCE WHICH SUGGEST THESE SYSTEM TRACK FURTHER E OF THE MTNS.
REGARDLESS SOME POTENTIAL FOR CONVECTION EXISTS ACROSS THE E COASTAL
PLAIN FOR A SHORT DURATION FRIDAY EVENING. GIVEN THE SFC LOW AND A
~50 KT LOW LEVEL JET WITH 30-40 KTS OF DEEP LAYER SHEAR SOME
POTENTIAL EXISTS FOR SEVERE WEATHER IF SUFFICIENT INSTABILITY
MATERIALIZES. THE LATEST GFS PROVIDES NEGATIVE LI VALUES LAPSE RATES
IN THE 850-500 MB LAYER OF 6-7 DEG. WHILE MOST OF THE INSTABILITY IS
CONFINED TO THE MID LEVELS NEAR 500MB WILL KEEP THE MENTION OF
ISOLATED THUNDER IN THE FCST WHILE TRIMMING ITS AREAL AND TEMPORAL
EXTENT. WITH THE RESIDUAL WEDGE AIR MASS ACROSS THE NW PIEDMONT
HIGHS WILL VARY WIDELY ACROSS THE AREA ON FRI... FROM THE MID 40S
NORTHWEST TO AROUND 60 SOUTHEAST... AFTER MORNING LOWS OF 32-42.
FOR SATURDAY THROUGH MONDAY...EXPECT THE FRONT TO EXIT TO THE EARLY
SATURDAY WITH A GRADUAL TAPERING OF POPS FROM SOUTHWEST TO
NORTHEAST. SURFACE HIGH PRESSURE SLOWLY BUILDS IN FROM THE WEST OVER
THE WEEKEND. WITH A GENERALLY DRY AND STABLE ATMOSPHERE. PARTLY
CLOUDY TO MOSTLY CLEAR SKIES AND BELOW NORMAL TEMPS CAN BE EXPECTED
LATE SATURDAY THROUGH MONDAY. -BLAES
&&
.AVIATION /12Z TUESDAY THROUGH SATURDAY/...
AS OF 645 AM TUESDAY...
A RIDGE OF HIGH PRESSURE AT THE SURFACE WILL EXTEND ACROSS CENTRAL
NC THROUGH THURSDAY. MEANWHILE....A SERIES OF WEAK PERTURBATIONS
ALOFT WILL CROSS OVERHEAD. PATCHES OF MID/HIGH LEVEL CLOUDINESS WILL
PRECEDE EACH DISTURBANCE.
THURSDAY...LOW LEVEL MOISTURE BENEATH A STRONG LOW LEVEL SUBSIDENCE
INVERSION MAY RESULT IN THE FORMATION OF STRATUS BY 12Z
THU...CREATING LOW END MVFR CEILINGS. DUE TO LIMITED MIXING
POTENTIAL...STRATUS MAY LINGER INTO THE AFTERNOON HOURS. MAY SEE
INCREASING AND THICKENING MID LEVEL CLOUDS LATE THU AS A FRONTAL
SYSTEM ASSOCIATED WITH A STRONG LOW PRESSURE SYSTEM OVER THE MIDDLE
PART OF THE COUNTRY APPROACHES FROM THE WEST. POTENTIAL EXIST LATE
THU NIGHT INTO EARLY FRI FOR LOW LEVEL WIND SHEAR CONDITIONS AS LLJ
PROGGED TO INCREASE TO 30-40KTS BY CHRISTMAS MORNING.
MAY SEE MVFR CEILINGS AND VISIBILITY FRIDAY INTO FRIDAY NIGHT WITH
PASSAGE OF FRONTAL SYSTEM. EXPECT A RETURN TO VFR CONDITIONS
SATURDAY.
&&
.RAH WATCHES/WARNINGS/ADVISORIES...
NONE.
&&
$$
SYNOPSIS...WSS
NEAR TERM...WSS
SHORT TERM...WSS
LONG TERM...BLAES
AVIATION...WSS
