FXUS62 KRAH 061804
AFDRAH

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

.SYNOPSIS...
COLD HIGH PRESSURE WILL DRIFT ACROSS THE REGION TODAY... AND MOVE 
OFFSHORE TONIGHT. A WEAK COLD FRONT WILL APPROACH THE AREA ON MONDAY 
MORNING... AND CROSS THE REGION ON LATE MONDAY AFTERNOON INTO THE 
EVENING. 

&&

.NEAR TERM /THROUGH TONIGHT/... 
AS OF 949 AM SUNDAY...

OBSERVED 150 METER H5 HEIGHT RISES AT GSO SINCE 00Z LAST NIGHT ARE 
THE HIGHEST IN THE CONUS THIS MORNING... AND THESE WILL CONTINUE TO 
SUPPORT COLD SURFACE HIGH PRESSURE CENTERED OVER THE VIRGINIAS. THE 
COLD WAS NOT RESTRICTED TO THE MID ATLANTIC STATES... HOWEVER... AS 
A COLD LATE FALL PATTERN GRIPPED THE ENTIRE CONUS THIS MORNING. 
SINGLE DIGITS BELOW ZERO TEMPERATURES WERE OBSERVED OVER THE 
ROCKIES... WHILE IT WAS IN THE 30S ALL THE WAY INTO NORTHERN FL AND 
CENTRAL TX. HERE IN CENTRAL NC... 900 AM TEMPERATURES BENEATH A 
MOSTLY SUNNY SKY RANGED FROM AROUND FREEZING IN THE TRIAD TO NEAR 40 
DEGREES AT CLINTON. FORECAST LOW LEVEL THICKNESSES WITH A LIGHT 
NORTHEASTERLY WIND SUGGEST TEMPERATURES SHOULD RISE ANOTHER 10 TO 12 
DEGREES FROM 9 AM READINGS... TO A RANGE OF 45 TO 50 DEGREES THIS 
AFTERNOON. AN EXPANSIVE SHIELD OF CIRRUS OVER THE MS VALLEY AND DEEP 
SOUTH -- IN ADVANCE OF YET ANOTHER IN A SERIES OF DISTURBANCES IN A 
BUSY SOUTHERN STREAM -- WILL CONTINUE TO OVERSPREAD NC AND MAKE FOR 
A PARTLY CLOUDY SKY BY THE END OF THE DAY. .  

THE SURFACE HIGH IS EXPECTED TO SHIFT OFFSHORE TONIGHT... ALLOWING 
FOR THE AIR MASS TO RECOVER A BIT. MOSTLY CLEAR SKIES THIS 
EVENING... SHOULD BECOME PARTLY TO MOSTLY CLOUDY ACROSS THE AREA 
OVERNIGHT (ESPECIALLY ACROSS THE SOUTH)... AS ISENTROPIC UPGLIDE ON 
THE 285K AND 290K SURFACE KICKS IN. GIVEN THE EXPECTED DRY AIR MASS 
IN PLACE ACROSS THE AREA THIS EVENING... EXPECT GENERALLY DRY 
CONDITIONS... WITH ANY POTENTIAL PRECIP FALLING FROM RESULT OF THE 
WAA/ISENTROPIC LIFT FALLING AFTER 12Z MONDAY. THIS INCREASE IN CLOUD 
COVER AND MOISTENING IN THE LOWER LAYERS SHOULD HELP PREVENT TEMPS 
FROM FALLING TOO LOW. THUS... EXPECT LOW TEMPS TONIGHT TO BE IN THE 
UPPER 20S ACROSS THE NORTH TO LOWER 30S ACROSS THE SOUTH.

&&

.SHORT TERM /MONDAY THROUGH WEDNESDAY/...
AS OF 245 AM SUNDAY...

FOR MONDAY/MONDAY NIGHT: THE MID LEVEL FLOW INTO NC REMAINS RATHER 
FLAT... HOWEVER A SEEMINGLY MINOR PERTURBATION NOW OVER NORTHERN 
MEXICO PASSES QUICKLY THROUGH THE MIDSOUTH AND SHIFTS OFF THE 
SOUTHEAST COAST MONDAY MORNING. WITH THE CHILLY SURFACE HIGH 
CENTERED OFF THE MIDATLANTIC COAST ALREADY FACILITATING DEVELOPMENT 
OF A BAROCLINIC ZONE OVER THE GULF STREAM... THIS WAVE SHOULD HELP 
LOWER SURFACE PRESSURES ENOUGH FOR A DEFINED INVERTED SURFACE TROUGH 
ALONG THE SOUTHEAST COAST... AND THE NAM/GFS/ECMWF/CANADIAN AGREE 
FAIRLY WELL ON THIS. AN 850 MB THETA-E RIDGE NOSES NORTHWESTWARD 
THROUGH CENTRAL AND EASTERN NC DURING THE DAY AHEAD OF AN 850 MB 
SHORTWAVE MOVING NORTHEAST FROM THE GULF THROUGH NC... AND THIS 
BRINGS A SLUG OF ISENTROPIC UPGLIDE AT 285-295K WHICH SHOULD BE DEEP 
AND STRONG ENOUGH TO KICK OFF LIGHT RAIN OVER THE CENTRAL AND 
EASTERN CWA AS THE INVERTED TROUGH EASES BRIEFLY INLAND. WILL EXPAND 
POPS A BIT FARTHER WEST AND INCREASE COASTAL PLAIN AND SANDHILLS 
POPS TO CHANCE... HOWEVER AMOUNTS SHOULD BE LOW... AROUND A TENTH OF 
AN INCH OR LESS BASED ON THE WEAK MID LEVEL FORCING. THE ASSOCIATED 
CLOUDS AND LOW LEVEL STABILITY DUE TO THE RAIN WILL KEEP TEMPS 
CHILLY... AND EXPECT MUCH OF THE PIEDMONT TO HOLD UNDER 50 DURING 
THE DAY. HAVE NUDGED HIGHS DOWN SLIGHTLY TO 46-55. THE UPGLIDE 
SHIFTS OFFSHORE MONDAY EVENING AS THE 850 MB TROUGH EXITS... AND 
WILL TREND OUT POPS EARLY. WE SHOULD SEE A SHORT PERIOD OF 
CLEARING... BUT THEN THE NAM/GFS SHOW A BATCH OF MID LEVEL MOISTURE 
AHEAD OF A WEAK 700 MB SHORTWAVE PASSING BY OVERNIGHT... SO WILL 
KEEP SKIES PARTLY CLOUDY... WHICH SHOULD KEEP TEMPS FROM DROPPING 
TOO MUCH. LOWS 33-40. 

FOR TUESDAY: HIGH PRESSURE BUILDS OVER THE MIDATLANTIC REGION... 
WEAKLY RIDGING SOUTHWARD INTO NC. THIS HIGH IS AN OFFSHOOT OF A 
LARGER POLAR HIGH NOW OVER WESTERN CANADA WITH DEW POINTS OF ZERO TO 
-20F. BASED ON THE WEAKNESS AND MODIFICATION OF THIS OFFSHOOT 
HIGH... IT SHOULD NOT POSSESS AIR QUITE THIS DRY/COLD... BUT IT DOES 
STRENGTHEN A BIT TO NEAR 1027 MB AS IT BECOMES CENTERED OVER THE ST 
LAWRENCE VALLEY AND STARTS TO TAP INTO THIS COLDER AIR... AND THIS 
WILL HAVE RAMIFICATIONS ON THE PIEDMONT AIR MASS ONCE PRECIP STARTS. 
MEANWHILE... THE MID LEVEL VORTEX NOW OVER SOUTHERN OR AND NORTHERN 
CA EJECTS EASTWARD ACROSS THE ROCKIES INTO THE PLAINS TUESDAY... AND 
THE RESULTING BROAD CYCLONIC FLOW HELPS TO PULL ABUNDANT DEEP 
MOISTURE... NOW OVER MEXICO AND OFF ITS PACIFIC COAST AND OVER THE 
NORTHWEST GULF (PW OVER 1.5 IN.)... RAPIDLY NORTHEASTWARD. AN 850 MB 
WARM FRONT APPROACHES FROM THE SOUTH LATE TUESDAY... BUT DOESN'T 
QUITE GET INTO THE CWA UNTIL AROUND 00Z. THE INCREASE IN 925 MB 
MOISTURE TRANSPORT IS SIMILARLY DELAYED UNTIL AFTER DARK... WHEN THE 
UPPER DIVERGENCE -- MAXIMIZED BENEATH THE IDEALLY JUXTAPOSED RIGHT 
ENTRANCE REGION OF THE JET OFF NEW ENGLAND AND LEFT EXIT REGION OF 
THE JET STRETCHING FROM AZ/NM TO AR/TN -- ALSO RAMPS UP QUICKLY OVER 
NC. WILL INCLUDE A SMALL POP OVER THE SOUTHWEST CWA LATE IN THE 
DAY... AFTER 2 PM. BUT MID AND HIGH CLOUDS SHOULD BE PLENTIFUL 
AREAWIDE WITH LOWER CLOUDS ARRIVING IN THE AFTERNOON. FACTORING IN 
THICKNESSES AND CLOUDS... EXPECT HIGHS OF 47-54... WITH THE 50S ONCE 
AGAIN CONFINED TO THE SOUTHEAST CWA. 

FOR TUESDAY NIGHT/WEDNESDAY: YET ANOTHER SIGNIFICANT STORM SYSTEM 
AFFECTS THE REGION. STRONG FORCING FOR ASCENT THROUGH THE COLUMN... 
INDUCED BY UPPER DIVERGENCE / MID LEVEL DPVA / DEEP MOIST UPGLIDE... 
ALONG WITH INCOMING PW NEARLY 300% OF NORMAL MAKES PRECIP IN NC A 
NEAR CERTAINTY. THE NAM/GFS ARE IN REMARKABLY GOOD AGREEMENT ON AN 
INTENSE PRIMARY SURFACE LOW TRACKING UP THROUGH IL/IN TUESDAY NIGHT 
WHILE IN NC... THE COASTAL FRONT SURGES INLAND INTO THE EASTERN 
PIEDMONT WHERE A SECONDARY SURFACE LOW FORMS ALONG THE EDGE OF 
DAMMING WEDGE THAT APPEARS LIKELY TO SET UP IN THE CLIMATOLOGICALLY 
FAVORED AREA OF THE NORTHWEST PIEDMONT BACK INTO THE FOOTHILLS. THE 
ECMWF/CANADIAN ARE LESS DEFINED WITH A SECONDARY LOW ALONG THE WEDGE 
FRONT BUT SHOW A SIMILAR PATTERN... PUSHING CONFIDENCE IN THIS 
SCENARIO ABOVE NORMAL. WILL CONTINUE CATEGORICAL POPS CWA-WIDE 
TUESDAY NIGHT THROUGH WEDNESDAY MORNING. RAINFALL AMOUNTS ARE TOUGH 
TO PINPOINT AT THIS RANGE BUT THE HIGH MOISTURE... DEEP WARM LAYER 
OVER EASTERN NC... AND STRONG FORCING SUGGEST AN INCH OR TWO OF RAIN 
IS POSSIBLE. FORTUNATELY... THE CHANCE FOR A LITTLE ICING IN THE 
NORTHWEST PIEDMONT AT ONSET IS LOOKING VERY LOW BASED ON TRIAD 
FORECAST SOUNDINGS SHOWING ABOVE-FREEZING AIR AT THE SURFACE. THE 
POTENTIAL FOR A SIGNIFICANT SEVERE WEATHER EPISODE REMAINS... WITH 
VERY STRONG KINEMATICS EXPECTED... HOWEVER AS WAS THE CASE WITH THIS 
PAST WEDNESDAY NIGHT... THE LOW INSTABILITY MAY BE THE LIMITING 
FACTOR HERE. MODELS AGREE QUITE WELL ON AN INCOMING 50-60 KT 850 MB 
JET NOSING INTO CENTRAL NC FROM THE SOUTHWEST OVERNIGHT... AND DEEP 
LAYER SHEAR NEAR 70 KTS IS MORE THAN ENOUGH FOR ORGANIZED 
CONVECTION... PLUS THE UPPER DIVERGENCE STRENGTHENS AS THE NOSE OF 
THE 120+ KT JET TO OUR WEST ARRIVES. BUT FORECAST SOUNDINGS SHOW 
RELATIVELY STABLE AIR BELOW 700 MB EVEN IN THE WARM SECTOR... AND 
EVEN ELEVATED CONVECTION LOOKS UNLIKELY. BUT GIVEN THIS PATTERN WE 
COULD STILL SEE BOWING SEGMENTS PRODUCING STRONG DAMAGING WINDS... 
AS WELL AS ISOLATED TORNADOES ALONG THE WEDGE FRONT WHERE LOW LEVEL 
VORTICITY WILL BE ENHANCED. EXPECT TEMPS TO HOLD STEADY IN THE 
NORTHWEST PIEDMONT OVERNIGHT AND RISE SLOWLY ELSEWHERE. ON 
WEDNESDAY... ONCE THE LOW LEVEL JET HEADS TO OUR EAST... THE MID 
LEVEL TROUGH AXIS MOVES TO OUR NORTHEAST... AND THE UPPER JET CORE 
SHIFTS NORTH... LIFT WILL DIMINISH AS THE MID AND UPPER LEVEL DRY 
AIR PUNCHES FROM THE WEST-SOUTHWEST. WILL TAPER OFF POPS THROUGH THE 
AFTERNOON. THE COLD FRONT IS NOT EXPECTED TO PASS THROUGH CENTRAL NC 
UNTIL AFTER SUNDOWN... SO ONCE THE AREA SEES A LITTLE SUN IN THE 
AFTERNOON WITH THICKNESSES STILL HOLDING WELL ABOVE NORMAL AT 
1360-1375 M AHEAD OF THE FRONT... TEMPS SHOULD EASILY WARM INTO THE 
MID 50S TO UPPER 60S. -GIH

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SATURDAY/...
AS OF 255 AM SUNDAY...

LITTLE CHANGE TO EXISTING FORECAST. A CHILLY SURFACE HIGH BUILDS IN 
FROM THE WEST-SOUTHWEST WEDNESDAY NIGHT THROUGH FRIDAY... YIELDING 
PARTLY TO MOSTLY SUNNY SKIES... WITH A STABLE/DRY COLUMN AND LITTLE 
MORE THAN HIGH THIN CLOUDS... AND BELOW NORMAL TEMPS FOLLOWING THE 
LOW LEVEL THICKNESSES. THE GFS/ECMWF AGREE ON BROADLY CYCLONIC FLOW 
COVERING THE CONUS INTO THE WEEKEND. A MID LEVEL PERTURBATION 
APPROACHES FROM THE SOUTHWEST FRIDAY NIGHT/SATURDAY... WITH THE GFS 
REMAINING MUCH FASTER BRINGING THIS FEATURE IN AS COMPARED TO THE 
ECMWF. WILL HOLD ONTO CHANCE POPS SPREADING IN FROM THE SOUTHWEST 
LATE FRIDAY NIGHT INTO SATURDAY... AND WAIT FOR BETTER MODEL 
AGREEMENT ON TIMING BEFORE INCREASING POPS. -GIH

&&

.AVIATION /18Z 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.

&&

$$

SYNOPSIS...BSD
NEAR TERM...MWS/BSD
SHORT TERM...HARTFIELD
LONG TERM...HARTFIELD
AVIATION...MWS