FXUS62 KRAH 061957
AFDRAH

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC
257 PM EST SUN DEC 6 2009

.SYNOPSIS...
ONE COOL HIGH PRESSURE RIDGE WILL DRIFT OFF THE MID ATLANTIC COAST 
TONIGHT... ONLY TO BE REPLACED BY ANOTHER MONDAY NIGHT AND TUESDAY. 
MOISTURE WILL INCREASE LATETUESDAY THROUGH WEDNESDAY AHEAD OF A 
POWERFUL STORM SYSTEM TO THE WEST.  

&&

.NEAR TERM /THROUGH MONDAY/... 
AS OF 245 PM SUNDAY...

TONIGHT: AN EXPANSIVE SHIELD OF CIRRUS -- IN ADVANCE OF YET ANOTHER 
PAIR OF DISTURBANCES IN A BUSY SOUTHERN STREAM -- HAS OVERSPREAD NC 
THIS AFTERNOON. UPSTREAM ALTOCUMULUS WILL ALSO SPREAD EAST 
NORTHEASTWARD OVERNIGHT... AND PROVIDE FOR CONSIDERABLE MID AND 
ESPECIALLY HIGH CLOUDINESS. THE CLOUDS MAY HINDER OTHERWISE PRETTY 
GOOD RADIATIONAL COOLING CONDITIONS GIVEN THE PROXIMITY OF THE DRY 
SURFACE RIDGE CENTERED OVER VA. WILL FORECAST LOWS IN OF AROUND 29 
TO 35 DEGREES... BUT THESE COULD BE 2-4 DEGREES TOO HIGH IF THE 
CLOUDS HAVE LESS IMPACT THAN EXPECTED OR DEPART JUST PRIOR TO 
DAYBREAK.  

MONDAY: THE NWP MODELS TO VARYING DEGREES HAVE DEPICTED THE ARRIVAL 
FROM SC OF ISENTROPIC LIFT/WAA IN THE LOWEST ~5 THOUSAND FT (285-295 
K LAYER) FROM JUST BEFORE SUNRISE OVER THE SANDHILLS TO ALONG AND 
EAST OF I-95 DURING THE DAY MONDAY. THE ISENTROPIC LIFT IS 
MANIFESTED AS THE AGEOSTROPHIC LOW LEVEL RETURN BRANCH OF THE 
THERMALLY INDIRECT CIRCULATION WITHIN A ~125 KT JET EXIT REGION AT 
200-150 MB FORECAST TO NOSE ACROSS THE CAROLINAS BETWEEN 09Z AND 
21Z. GIVEN THE SHALLOWNESS OF THE SATURATION... WHICH IS EXPECTED TO 
REMAIN CONFINED BELOW 6-7 THOUSAND FT AND AT TEMPERATURES ABOVE 
0C... DO NOT EXPECT MUCH IF ANY MEASURABLE RAIN. THE CLOUD COVER 
SHOULD... HOWEVER... HOLD TEMPERATURES DOWN... PARTICULARLY IN THE 
EAST. HIGHS GENERALLY UPPER 40S TO LOWER 50S... WARMEST SOUTHWEST 
AND COOLEST NORTHWEST AND EAST. 

&&

.SHORT TERM /MONDAY NIGHT THROUGH TUESDAY/... 
AS OF 255 PM SUNDAY...

ONCE THE ISENTROPIC LIFT ATOP THE FIRST SURFACE RIDGE THAT WAS OVER 
THE VIRGINIAS TODAY PIVOTS OFFSHORE BY MONDAY EVENING ... ANOTHER 
SURFACE HIGH (~1025 MB) WILL FOLLOW AND RIDGE SOUTHWARD INTO NC 
MONDAY NIGHT THROUGH TUESDAY. STRONG ISENTROPIC LIFT -- AHEAD OF A 
POWERFUL STORM SYSTEM EXPECTED TO LIFT NORTHEASTWARD FROM THE FOUR 
CORNERS MONDAY NIGHT TO THE GREAT LAKES BY WEDNESDAY -- WILL RESULT 
IN AN INCREASING CHANCE OF RAIN FROM THE SOUTHWEST TUESDAY 
AFTERNOON... RANGING FROM SLIGHT CHANCE NORTHEAST TO LIKELY WEST.   

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/... 
AS OF 245 PM...

TUESDAY NIGHT THROUGH WEDNESDAY NIGHT

SEVERE THREAT TUESDAY NIGHT IF ANY... LIKELY KEYING ON WARM FRONT 
PASSAGE AFTER MIDNIGHT TUESDAY NIGHT AND ROUGHLY ALONG AND EAST OF A 
WADESBORO... DURHAM... HENDERSON LINE. HOWEVER CONFIDENCE IN AN 
EVENT IS VERY LOW AT THIS TIME AND WILL OMIT FROM OUTLOOK PRODUCTS 
AT THIS TIME. 

WINDS OF 50 TO 60 KNOTS BELOW 5000 FEET EXPECTED AS ABOVE MENTIONED 
WARM FRONT LIFTS NORTH FROM SOUTH CAROLINA. STRONG HELICITY IN THE 
VICINITY OF THE FRONT WOULD MAKE FOR A RISK OF TORNADOES WHILE
MOST RESIDENTS ARE IN BED... IF SUFFICIENT INSTABILITY WERE
PRESENT. WITH RAINS STABILIZING THE LOWEST LAYERS CURRENT THINKING
IS ANY CONVECTION WOULD BE ELEVATED AND UNABLE TO PUNCH THROUGH TO
THE SURFACE... AND THIS THINKING WILL BE FOLLOWED AT THIS TIME.
THE MARGIN FOR ERROR IS THIN AND THE SEVERE THREAT WILL OBVIOUSLY
CONTINUE TO BE EVALUATED AS THE EVENT NEARS.

RAIN LOOKS UNAVOIDABLE TUESDAY NIGHT INTO WEDNESDAY MORNING WITH 
STRONG OVERRUNNING MOIST FLOW DEVELOPING TUESDAY NIGHT AS A STRONG
SURFACE LOW CLOSELY ASSOCIATED WITH AN UPPER TROUGH LIFTS
NORTHEAST TOWARD LAKE MICHIGAN. EVENT RAINFALL TOTALS IN EXCESS
OF ONE INCH FORECAST AT THIS TIME. SOME DRYING OF THE LOW LEVEL
AIR MASS BEGINS BEHIND THE WARM FRONT AND AHEAD OF THE COLD FRONT
TO THE WEST AND RAINFALL AMOUNTS AFTER SUNRISE WEDNESDAY PERHAPS
NO MORE THAN ONE QUARTER INCH...PERHAPS MUCH LESS... WITH THE
COLD FRONT ITSELF RELATIVELY DRY.

WEDNESDAY COULD BE QUITE BREEZY ONCE TEMPERATURES WARM AND STRONG 
WINDS ALOFT MIX TO THE SURFACE. CURRENT FORECAST GUSTS TO 35 MPH 
WEDNESDAY ACCEPTED. 

ASSUMING THE LOW LEVEL DRYING OCCURS... WEDNESDAY NIGHT SHOULD BE 
DRY... THOUGH IF THERE IS ANY RISK FOR MEASURABLE RAIN IT WOULD BE 
EARLY AND EAST OF INTERSTATE 95. MOST OF THE NIGHT WILL BE PARTLY 
CLOUDY TO MOSTLY CLEAR.

THE PARADE OF FRONTS WILL MAKE FOR A ROLLER COASTER RIDE FOR 
TEMPERATURES. TEMPERATURES IN THE MID 40S TO LOW 50S TUESDAY EVENING 
WILL RISE INTO THE 60S IN THE SOUTHEAST TOWARD SUNRISE WITH THE 
PASSAGE OF THE WARM FRONT. TEMPERATURES CONTINUE TO WARM WEDNESDAY 
AHEAD OF THE COLD FRONT IN GUSTY SOUTHWEST WINDS... WITH SOME 70S 
IN THE EAST... AND THE NORTHWEST PIEDMONT PERHAPS TO 60 DEGREES
AHEAD OF THE COLD FRONT. TEMPERATURES QUICKLY DIVE IN COLD
ADVECTION BEHIND THE FRONT... REACHING THE MID 30S NORTHWEST...
AND MID 40S SOUTHEAST BY SUNRISE THURSDAY.

THURSDAY THROUGH SUNDAY

SURFACE HIGH PRESSURE OVER THE MIDWEST THURSDAY WILL SLOWLY ADVANCE 
EAST AND CONTROL NORTH CAROLINA WEATHER INTO FRIDAY. THE ECMWF AND 
GFS ARE IN ROUGH AGREEMENT IN DEVELOPING A SURFACE WAVE SATURDAY IN 
THE NORTHERN GULF OF MEXICO AND DRIVING IT EAST INTO THE ATLANTIC. 
HOWEVER THE 00Z RUN OF THE ECMWF IS MUCH SLOWER WITH THE SURFACE LOW 
STILL SOUTH OF WILMINGTON AT SUNRISE SUNDAY. THE 12Z ECMWF HUGS A
SURFACE LOW TO THE NORTH CAROLINA COAST EARLY SUNDAY MORNING.
CONFIDENCE IN THE SURFACE FEATURES IN THE EXPECTED FAST FLOW
PATTERN IS LOW... BUT 48 HOURS OF CHANCES FOR MEASURABLE RAIN
UNLIKELY. WILL FOLLOW THE FASTER GFS AND HAVE BEST RAIN CHANCES
SATURDAY... BUT WITH LOW CONFIDENCE... AND LEAVE SUNDAY DRY.

AFTERNOON HIGHS MOSTLY IN THE 50S DURING THIS PERIOD... WITH SOME 
UPPER 40S IN THE NORTH. THURSDAY NIGHT LOOKS TO BE THE COLDEST OF 
THIS PERIOD AT 25 TO 30... WITH THE SURFACE HIGH NEAR AND THE AIR
MASS DRY. LOWS IN THE 30S FRIDAY AND SATURDAY NIGHTS.

&&

.AVIATION /20Z SUNDAY THROUGH FRIDAY/...
AS OF 100 PM SUNDAY...

CONSIDERABLE HIGH CLOUDS -- BUT VFR CONDITIONS UNDER THE INFLUENCE 
OF COLD AND DRY HIGH PRESSURE -- WILL PERSIST THROUGH MOST OF 
TONIGHT. AN MVFR CLOUD BASE-BEARING MOIST SOUTH TO SOUTHWESTERLY 
FLOW JUST ABOVE THE SURFACE WILL SPREAD NORTHWARD INTO PORTIONS OF 
CENTRAL AND EASTERN NC FROM AROUND SUNRISE THROUGH EARLY AFTERNOON 
MONDAY. THUS... VARYING CLOUD LAYERS BETWEEN 1500 AND 8000 FT ARE 
EXPECTED... LOWEST AND MOST LIKELY BROKEN TO OVERCAST ALONG AND EAST 
OF THE INTERSTATE 95 CORRIDOR... INCLUDING AT FAY AND RWI GENERALLY 
DURING THE LAST 6 HOURS OF THE 18Z TAF PERIOD. LIGHT RAIN OR DRIZZLE 
ARE ALSO POSSIBLE IN THESE AREAS... BUT NO ASSOCIATED VISIBILITY 
RESTRICTIONS ARE EXPECTED AT THIS TIME GIVEN THE LIGHT NATURE AND 
LOW PROBABILITY OF ANY PRECIPITATION.   

AFTER A RETURN TO AREA WIDE VFR CONDITIONS FOR MONDAY NIGHT AND MOST 
OF TUESDAY... EXPECT LIFR/IFR CONDITIONS AND A RAIN TO ONCE AGAIN 
DEVELOP TUESDAY EVENING THROUGH WEDNESDAY... IN ASSOCIATION WITH 
STRONG LIFT-INDUCING ISENTROPIC UPGLIDE AHEAD OF A POWERFUL MS 
VALLEY STORM SYSTEM.

&&

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

&&

$$

NEAR TERM...MWS
SHORT TERM...MWS
AVIATION...MWS