FXUS62 KRAH 262351
AFDRAH

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC
650 PM EST SAT DEC 26 2009

.SYNOPSIS...
WEAK HIGH PRESSURE WILL BUILD INTO NC FROM THE GULF COAST STATES 
TODAY. A COLD FRONT WILL APPROACH THE CAROLINAS FROM THE WEST SUNDAY 
EVENING...BRINGING COOLER TEMPERATURES TO THE AREA INTO THE UPCOMING 
WEEK.
 
&&

.NEAR TERM /THROUGH SUNDAY/...
AS OF 225 PM SATURDAY...

18Z SFC ANALYSIS SHOWED 1011 MB LOW PRESSURE CENTERED OVER THE 
SOUTHERN CHESAPEAKE BAY. 1003 MB LOW PRESSURE WAS CENTERED OVER 
SOUTHEAST IOWA. 1038 MB HIGH PRESSURE WAS CENTERED OVER NEW 
BRUNSWICK (CANADA)...WITH A SFC RIDGE AXIS EXTENDING SOUTH THROUGH 
NEW ENGLAND. 1022 MB HIGH PRESSURE WAS CENTERED OVER THE DEEP 
SOUTH...WITH A RIDGE AXIS EXTENDING NORTH INTO THE EASTERN TN/OH 
VALLEYS. ALOFT...12Z RAOB ANALYSIS SHOWED A DEEP/EXPANSIVE UPPER LOW 
IN THE MIDWEST...CENTERED OVER SOUTHERN WISCONSIN. SOUTHWEST FLOW 
ALOFT WAS NOTED OVER THE CAROLINAS AND SOUTHEAST STATES. A WEAK 
TROUGH WAS NOTED IN THE LOWER LEVELS (925-850 MB) OVER NC/VA.

THIS EVENING/TONIGHT:
SHALLOW POOL OF MOISTURE ACROSS THE AREA CONTINUES TO ERODE THIS 
AFTERNOON AS LOW LEVEL FLOW BECOMES INCREASINGLY NORTHWESTERLY 
BEHIND THE DEPARTING LOW-LEVEL TROUGH/SFC LOW. EXPECT LOW STRATUS TO 
ERODE AWAY IN THE COASTAL PLAIN BY 21-00Z THIS AFT/EVE. HOWEVER... 
DENSE UPPER LEVEL CLOUD COVER (AOA 18 KFT) SHOULD CONTINUE TO STREAM 
INTO THE AREA VIA THE SUBTROPICAL JET...AND PARTLY TO MOSTLY CLOUDY 
SKIES (DEPENDING ON OPACITY) ARE EXPECTED ACROSS CENTRAL NC THROUGH 
AT LEAST 06Z TONIGHT. THE LATEST GFS/NAM BUFR SOUNDINGS SHOW THE 
UPPER LEVELS DRYING OUT BETWEEN 06-12Z TONIGHT...INDICATING THAT  
UPPER LEVEL CLOUD COVER SHOULD SUBLIMATE AND/OR MOVE OFF TO THE 
EAST. THIS APPEARS TO BE CONSISTENT WITH THE LATEST WV IMAGERY AND 
RUC MODEL DATA...WHICH SHOWS AN ELONGATED SHEAR AXIS (CURRENTLY 
STRETCHING FROM TX NORTHEAST INTO THE OH VALLEY) PUSHING EASTWARD 
TONIGHT AS THE UPPER LOW (CURRENTLY CENTERED OVER CENTRAL WI) BEGINS 
PIVOTING BACK TO THE SOUTH/SE TOWARD NORTHERN IL AND SOUTHERN LAKE 
MICHIGAN. THIS SHOULD HELP PUSH DRIER AIR ALOFT (CURRENTLY NOTED 
UPSTREAM OF THE AREA OVER THE TN AND LOWER MS RIVER VALLEYS) INTO 
THE CAROLINAS FROM WEST-EAST AFTER MIDNIGHT. BASED PRIMARILY ON A 
MOS GUIDANCE BLEND...WITH LIGHT/VARIABLE OR CALM WINDS AND CLEARING 
SKIES AFTER MIDNIGHT...EXPECT LOW TEMPS TO RANGE FROM ~30F IN THE 
N/NW PIEDMONT TO 34-37F IN THE SE COASTAL PLAIN WHERE UPPER LEVEL 
CLOUD COVER WILL HANG AROUND THE LONGEST.

SUNDAY:
THE DEEP UPPER LOW THAT HAS BEEN STALLED OVER THE MIDWEST WILL MOVE 
EASTWARD INTO MICHIGAN DURING THE DAY SUNDAY...PUSHING A SURFACE 
TROUGH/COLD FRONT EASTWARD TOWARD THE CAROLINAS. EXPECT WEAK WARM 
ADVECTION DURING THE AFTERNOON HOURS AS SOUTHERLY FLOW DEVELOPS IN 
ADVANCE OF THE APPROACHING FRONT. FORECAST THICKNESSES WOULD SUGGEST 
HIGHS RANGING FROM THE LOWER 50S IN THE NW PIEDMONT TO THE MID/UPPER 
50S ELSEWHERE...AND MOS GUIDANCE AGREES FAIRLY WELL WITH THIS RANGE. 
ALTHOUGH THE LOWER/MID LEVELS WILL BE MOISTENING DURING THE DAY 
SUNDAY IN ASSOC/W WEAK WARM ADVECTION...FCST SOUNDINGS AND MODEL RH 
CROSS SECTIONS ARE FAR FROM SATURATION...AND EXPECT MOSTLY SUNNY OR 
SUNNY SKIES...WITH A LIGHT SOUTHERLY BREEZE DURING THE AFTERNOON 
HOURS. -VINCENT

&&

.SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY NIGHT/...
AS OF 250 PM SATURDAY...

SUNDAY NIGHT...MID LEVEL SHEAR AXIS ASSOCIATED WITH A DEEP UPPER LOW 
OVER THE EASTERN GREAT LAKES WILL USHER A SURFACE FRONT ACROSS THE 
FORECAST AREA. MOISTURE TOO LIMITED FOR ANYTHING MORE THAN A BAND OF 
CLOUDS BETWEEN 3000-5000FT. LOW LEVEL MIXING ASSOCIATED WITH THE 
FRONTAL PASSAGE WITH PREVENT AIR MAS FROM REALIZING FULL COOLING 
POTENTIAL. MIN TEMPS LOWER 30S NW TO MID 30S SE.

MONDAY THROUGH TUESDAY NIGHT...EXPECT TRANQUIL WEATHER AS UPPER 
LEVEL TROUGH GRADUALLY LIFT NE...ALLOWING S/W RIDGE TO TRANSLATE 
EWD. AT THE SURFACE...SURFACE HIGH OVER THE SOUTHERN PLAINS WILL 
GRADUALLY BUILD EAST AND RESIDE OVER THE REGION BY TUESDAY NIGHT. 
850MB THERMAL LIES NW OF THE REGION MONDAY...THEN LIES OVERHEAD BY 
TUESDAY. PROLONG CAA WILL RESULT IN MAX TEMPS SEVERAL DEGREES BELOW 
NORMAL. LOW LEVEL THICKNESS AVERAGE ABOUT 30M BELOW NORMAL. ALLOWING 
FOR SOME SLIGHT MODIFICATIONS SUPPORT MAX TEMPS IN THE LOW AND MID 
40S. MIN TEMPS IN THE 20S COMMON WITH THE COLDEST NIGHT BEING 
TUESDAY NIGHT INTO WEDNESDAY MORNING DUE TO PRESENCE OF SURFACE 
RIDGE IN OUR VICINITY.

&&

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...
AS OF 222 PM EST SATURDAY...

WEDNESDAY...S/W RIDGE AND ATTENDANT SURFACE RIDGE OVER THE FORECAST 
AREA WEDNESDAY MORNING. CONFLUENCE ALOFT WILL ANCHOR SURFACE RIDGE 
ALONG THE ATLANTIC SEABOARD. MEANWHILE DEEPENING MID/UPPER LEVEL 
TROUGH MOVING INTO THE SOUTHERN PLAINS WILL INDUCE CYCLOGENESIS OVER 
THE NW GULF. AS THIS FEATURE LIFTS E-NE LATE WEDNESDAY INTO 
WEDNESDAY NIGHT...LOW LEVEL ISENTROPIC LIFT WILL INCREASE. THIS 
ACTION WILL MOISTEN LOWER LAYERS OF THE ATMOSPHERE LEADING TO THE 
OUTBREAK OF LIGHT PRECIPITATION. AT THIS TIME MAJORITY OF THE PRECIP 
WILL LIKELY BE LIQUID BUT POTENTIAL FOR PATCHES OF FREEZING RAIN IN 
THE NW PIEDMONT. A LITTLE EARLY TO MENTION THIS PROBABILITY IN THE 
FORECAST AS LATER ONSET OF PRECIP (SIMILAR TO 00Z ECMWF) WOULD 
DIMINISH THREAT OF WINTRY PRECIP.

THURSDAY THROUGH FRIDAY...MODELS DIVERGE LEADING TO A LOW CONFIDENCE 
FORECAST. LATEST GFS CONTINUES TREND OF PLACING EMPHASIS ON THE LEAD 
VORT MAX WHICH RESULTS IN MORE SUBSTANTIAL SURFACE CYCLOGENESIS BY 
12Z THU. THE 00Z ECMWF SUGGEST THAT THE TRAILING VORT MAX WILL BE 
THE KEY PLAYER...PLACING MORE OF THE ENERGY WELL TO OUR SW IN THE 
NORTHERN GULF. WHILE THE GFS DEPICTS THE AIR MASS DRYING OUT BY 00Z 
FRI...THE EURO IS JUST BEGINNING TO CRANK UP THE PRECIP OVER CENTRAL 
NC. LATEST GEM COINCIDE MORE WITH THE 12Z GFS.  BY FRIDAY...MOST 
GUIDANCE SUGGEST CENTRAL NC IN BETWEEN THE SYSTEM OFFSHORE OF THE VA 
CAPE...AND THE UPPER LEVEL LOW OVER THE LOWER OH VALLEY. BY 00Z 
SATURDAY...MODELS COME BACK INTO SLIGHTLY BETTER AGREEMENT WITH THE 
IDEA OF CYCLOGENESIS OFF THE NORTHERN MID ATLANTIC COAST THOUGH THE 
00Z ECMWF ABOUT 6-12 HOURS FASTER THAN THE 12Z GFS. CIRCULATION 
AROUND THE LOW LEVEL CYCLONE WILL AID TO USHER COLDER AIR INTO 
CENTRAL NC FRIDAY NIGHT INTO SATURDAY. IF MID LAYERS SATURATE 
ENOUGH...POTENTIAL WINTRY PRECIP ACROSS THE REGION FRIDAY NIGHT INTO 
SATURDAY.

STILL A LOT OF TIME TO ADJUST THIS FORECAST. FOR NOW WILL KEEP POPS 
IN THE CHANCE/SLIGHT CHANCE CATEGORIES THU INTO SATURDAY AND KEEP 
P-TYPE LIQUID FOR NOW. WILL ADJUST MAX TEMPS DOWN A FEW DEGREES FROM 
HPC/GFS MOS GUIDANCE FRIDAY AND SATURDAY.

&&

.AVIATION /00Z SUNDAY THROUGH TUESDAY/...
AS OF 640 PM SATURDAY...

24-HR TAF PERIOD:
HIGH PRESSURE OVER THE DEEP SOUTH WILL CONTINUE TO BUILD INTO 
CENTRAL NORTH CAROLINA OVERNIGHT... PROVIDING MOST OF THE AREA WITH 
VFR CONDITIONS WITH BROKEN TO OVERCAST CIGS AT 15 KFT OR HIGHER. THE 
ONE EXCEPTION MAY BE AT KRWI... WHERE THE DEWPOINT DEPRESSION IS 
ALREADY ONE DEGREE FAHRENHEIT. THUS... EXPECT THAT WE WILL SEE SOME 
MVFR/IFR VISBYS THERE (KRWI)... WITH POSSIBLY SOME LIFR VISBYS JUST 
PRIOR TO DAYBREAK.

ANY SUB VFR CONDITIONS THAT DEVELOP WILL QUICKLY BECOME VFR AFTER 
SUNRISE ON SUNDAY MORNING... AS MIXING BEGINS AND HIGH PRESSURE 
EXTENDS INTO THE AREA. WINDS FROM MID SUNDAY MORNING ONWARD... WILL 
GENERALLY BE OUT OF THE SOUTHWEST TO WEST-SOUTHWEST BETWEEN 4 TO 8 
KTS... WITH THE SURFACE HIGH EXTENDING INTO THE AREA.

LOOKING AHEAD...VFR CONDITIONS ARE EXPECTED TO PREDOMINATE FROM 
SUNDAY EVENING THROUGH WEDNESDAY...WITH THE NEXT CHANCE FOR ADVERSE 
AVIATION CONDITIONS HOLDING OFF UNTIL LATE IN THE WEEK.

&&

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

&&

$$

SYNOPSIS...VINCENT
NEAR TERM...VINCENT
SHORT TERM...WSS
LONG TERM...WSS
AVIATION...BSD