000
FXUS62 KRAH 070121
AFDRAH

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC
921 PM EDT MON SEP 6 2010

.SYNOPSIS...
A DRY RIDGE OF HIGH PRESSURE WILL EXTEND ACROSS NORTH CAROLINA
THROUGH TUESDAY.  A WEAK COLD FRONT WILL CROSS THE AREA LATE
WEDNESDAY. BEHIND THE FRONT... HIGH PRESSURE WILL BUILD SOUTH INTO
THE AREA FOR THE REMAINDER OF THE WORK WEEK.

&&

.NEAR TERM /THROUGH TUESDAY NIGHT/...
AS OF 920 PM MONDAY...

TONIGHT: ONLY COSMETIC CHANGES REQUIRED TO PREVIOUS FORECAST. MID
LEVEL H5 RIDGING CENTERED OVER NORTHERN AL HAS STRENGTHENED SOME 30
METERS PER 00Z RAOBS... AND WILL CONTINUE TO BUILD EASTWARD
OVERNIGHT IN THE WAKE OF A DEPARTING L/W TROUGH AXIS SITUATED JUST
OFFSHORE THE EAST COAST. MEANWHILE AT THE SURFACE... A 1023 MB RIDGE
JUST OFFSHORE THE TIDEWATER OF VA SHOULD CONTINUE TO REMAIN WELL
ENTRENCHED ACROSS THE CAROLINAS OWING TO THE SUBSIDENCE DOWNSTREAM
OF THE MID LEVEL RIDGING. GENERALLY EASTERLY FLOW AROUND THE SURFACE
RIDGE FOR THE PAST 12-18 HOURS HAS RESULTED IN A MODEST RECOVERY IN
LOW LEVEL MOISTURE ACROSS CENTRAL NC DURING THAT TIME... WITH
DEWPOINTS RANGING AT 9 PM FROM MAINLY UPPER 50S NORTHWEST TO MIDDLE
60S SOUTH. OVERNIGHT LOWS SHOULD NOT BE TOO FAR OFF OF THOSE
OBSERVED DEWPOINT TEMPERATURES... WITH MOST LOCALES EXPECTED TO DIP
TO WITHIN A FEW DEGREES OF 60.

THE INCREASED LOW LEVEL MOISTURE IN CONJUNCTION WITH LIGHT-CALM
WINDS WITHIN THE SURFACE RIDGE AXIS... AND AN INITIALLY MOSTLY CLEAR
SKY (ASIDE FROM MAINLY SCT STRATOCUMULUS TRAPPED AT THE BASE OF A
SUBSIDENCE ATTENDING THE INCOMING RIDGE ALOFT)... MAY PROMOTE THE
DEVELOPMENT OF PATCHES OF STRATUS AND FOG BY DAYBREAK...
PARTICULARLY IN THE HIGHER DEWPOINT AIR OVER THE SOUTHERN AND
SOUTHWESTERN ZONES. THIS TREND WAS ALREADY REFLECTED IN THE GRIDDED
FORECAST.

TUESDAY AND TUESDAY NIGHT: THE AFOREMENTIONED NORTHERN PLAINS
SHORTWAVE AND ITS ASSOCIATED SURFACE LOW WILL DRIVE A COLD FRONT
SOUTHEAST INTO THE OHIO VALLEY REGION ON TUESDAY. THE AIR MASS
SHOULD MODERATE A BUT MORE WITH A DEVELOPING SOUTHWESTERLY FLOW AND
1000 TO 850 MILLIBAR THICKNESSES FORECAST TO RISE INTO THE 1420 TO
1425 METER RANGE. THICKNESS AND 850 MILLIBAR MIXING BOTH YIELD
VALUES CONSISTENT WITH STATISTICAL GUIDANCE...88 TO 92. CONTINUED
WITH DRY FORECAST AS PW RECOVERY IS FORECAST TO BE RATHER WEAK
DURING THE DAY TUESDAY AND MODELS ARE NOT SHOWING MUCH DEVELOPMENT
OF A PREFRONTAL TROUGH WITH HIGH PRESSURE REMAINING DOMINANT.

SHOULD SEE MAINLY CLEAR SKIES TUESDAY NIGHT WITH SOUTHWESTERLY FLOW
AHEAD OF WHAT LOOKS TO BE A MAINLY DRY APPROACHING COLD FRONT.  HAVE
LOWS WARMER THAN GUIDANCE...65 TO 67.

&&

.SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
AS OF 235 PM MONDAY...

FOR WEDNESDAY AND WEDNESDAY NIGHT: MID LEVEL VORTEX NOW PUSHING
THROUGH THE DAKOTAS DEEPENS WHILE CROSSING THE UPPER GREAT LAKES
TOWARD FAR SOUTHERN QUEBEC BY DAWN THURSDAY. A CORRESPONDING
SURFACE FRONT PUSHES TO THE SOUTHERN APPALACHIANS BY WEDNESDAY
AFTERNOON THEN DROPS SOUTHEAST THROUGH NORTH CAROLINA WEDNESDAY
NIGHT... BUT WITH INCIPIENT DRY AIR (PRECIPITABLE WATER WELL UNDER
ONE INCH) AND A WESTERLY DOWNSLOPE FLOW AT 850 MILLIBAR PRECEDING
THE FRONT... PRECIPITATION CHANCES APPEAR LOW. WE WILL SEE
PRECIPITABLE WATER VALUES INCREASE BRIEFLY TO NEAR 1.5 INCHES JUST
AHEAD OF THE FRONT... AND THE NAM/GFS DO SHOW A WEAK WAVE DROPPING
SOUTHEAST THROUGH THE STATE WITH THETA-E RIDGING NOSING IN AT 850
MILLIBAR. HOWEVER NO FORCING FOR ASCENT IS EVIDENT ELSEWHERE
THROUGH THE TROPOSPHERE... INCLUDING AT JET LEVEL... AND THE
PRECIPITATION AMOUNTS GENERATED BY THE MODELS ARE SLIM TO NONE.
INSTABILITY IS ALSO LIMITED WITH THE GFS SHOWING SBCAPE PEAKING AT
500 J/KG RIGHT ALONG THE FRONT... WITH A DRY SUBCLOUD LAYER. WILL
KEEP THE FORECAST DRY BUT INCLUDE A PERIOD OF ISOLATED SPRINKLES
LATE AFTERNOON INTO MID EVENING AS LAPSE RATES ALOFT STEEPEN WITH
BROAD MID LEVEL TROUGH PASSAGE. GIVEN THE DRY LOW LEVELS... A FEW
ENHANCED GUSTS ARE POSSIBLE NEAR THESE SHOWERS. THICKNESSES CLIMB
FURTHER WEDNESDAY TO AROUND 15 TO 20 METERS ABOVE NORMAL... AND
WITH A DRY ADIABATIC LOW LEVEL PROFILE... HIGHS OF 89 TO 94 STILL
LOOK FINE. GIVEN THE DELAY IN CLEARING OVERNIGHT ALONG WITH THE
WEAK ADVECTION OF LOWER THICKNESSES AND STILL MILD AIR AT 925 TO
850 MILLIBAR... WILL BUMP LOWS UP JUST A BIT MAINLY ACROSS THE
NORTH TO 64 TO 70.

FOR THURSDAY AND THURSDAY NIGHT: THE FRONT IS EXPECTED TO SETTLE
ALONG THE SOUTHERN BORDER OF NORTH CAROLINA THURSDAY. AN
ELONGATED CANADIAN SOURCE HIGH ATTEMPTS TO BUILD IN BEHIND THE
FRONT FROM THE NORTHWEST... HOWEVER THE COOLER AIR APPEARS TO BE
HINDERED BY THE MOUNTAINS SUCH THAT LOW LEVEL THICKNESSES HOLD
ABOVE NORMAL... AND A WEAK LEE TROUGH DEVELOPS OVER THE PIEDMONT
IN THE AFTERNOON. THE COLUMN DRIES BACK OUT RATHER QUICKLY WITH NO
INSTABILITY NOR LARGE SCALE LIFT... SO DO NOT ANTICIPATE ANY
PRECIPITATION ALONG THE FRONTAL ZONE OR TROUGH ON THURSDAY...
ALTHOUGH WE COULD SEE SOME MID OR HIGH LEVEL CLOUDINESS IN THE
MORNING AS ANOTHER WEAK WAVE ALOFT DROPS THROUGH NORTH CAROLINA.
HIGHS 87 TO 93. LOWS 58 TO 65 WITH A SECONDARY SURGE OF DRIER AIR
DROPPING THROUGH THE STATE OVERNIGHT. -GIH

&&

.LONG TERM /FRIDAY THROUGH MONDAY/...
AS OF 235 PM MONDAY...

FOR FRIDAY THROUGH SATURDAY: THE FRONT TO OUR SSW TRIES TO HEAD BACK
NORTHEAST AS A WARM FRONT ON FRIDAY ACCORDING TO THE LATEST
GFS/ECMWF. BUT AS THE MID LEVEL VORTEX SITS AND SPINS OVER NOVA
SCOTIA WITH TROUGHING ENCOMPASSING MOST OF THE EASTERN SEABOARD AND
WESTERN ATLANTIC... THE BLOCKING PATTERN SHOULD HELP KEEP THE
SURFACE RIDGE DOMINANT THROUGH FRIDAY NIGHT. BUT THE ECMWF (AND TO A
LESSER DEGREE THE GFS) INDICATES 850 MILLIBAR NOCTURNAL JETTING
NOSING INTO SOUTHWESTERN NORTH CAROLINA ALONG THE FRONT LATE FRIDAY
INTO FRIDAY NIGHT... WHICH COULD INDUCE ENOUGH FORCING AND LOW LEVEL
MOISTURE POOLING TO KICK OFF SLOW MOVING SHOWERS OR STORMS. THE
GFS/ECMWF BOTH SUGGEST THAT THIS MOISTURE IS ROOTED IN THE REMAINS
OF NOW TROPICAL STORM HERMINE... QUITE REASONABLE CONSIDERING THE
MEAN FLOW PATTERN AND GOOD MODEL AGREEMENT. WILL SPREAD IN CHANCE
POPS FROM SOUTHWEST TO EAST VERY LATE FRIDAY THROUGH FRIDAY NIGHT
INTO SATURDAY MORNING. THE GFS/ECMWF ALSO BOTH SHOW A LOW FORMING
AND TRACKING ALONG THE FRONT JUST TO OUR SOUTH... WHICH RESULTS IN
AN ENHANCED NORTHEAST LOW LEVEL FLOW FRIDAY NIGHT INTO
SATURDAY...POTENTIALLY RESULTING IN A WEAK IN SITU DAMMING EVENT.
WHILE IT IS TOO FAR OFF WITH TOO MUCH UNCERTAINTY TO PUT MUCH FAITH
IN SUCH A SCENARIO... IT WILL BE SOMETHING TO WATCH IN THE COMING
DAYS. FOR NOW WILL TAPER POPS DOWN SATURDAY AFTERNOON BUT KEEP A
GOOD BIT OF CLOUD COVER OVER THE AREA. WILL HAVE HIGHS NEAR
NORMAL... 82 TO 88... BUT CONFIDENCE IS BELOW NORMAL AND IF THE
PRECIPITATION COVERAGE IS GREATER THAN EXPECTED FRIDAY NIGHT AND
SATURDAY...HIGHS COULD BE BELOW NORMAL ON SATURDAY. LOWS IN THE 60S.

FOR SATURDAY NIGHT THROUGH MONDAY: THE BLOCKING VORTEX OVER NOVA
SCOTIA FINALLY WEAKENS AND GETS NUDGED EASTWARD BY A STRONG
SHORT WAVE NOW APPROACHING THE PACIFIC NORTHWEST. THE MID LEVEL
RIDGE BETWEEN THESE TWO FEATURES DEAMPLIFIES SO IT LOOKS LIKELY
THAT BROAD TROUGHING WILL BECOME REESTABLISHED OVER THE EASTERN
UNITED STATES BY MONDAY. THE OPERATIONAL MODELS DIVERGE HOWEVER
WITH THE ECMWF MUCH MORE AMPLIFIED THAN THE GFS... RESULTING IN
THE ECMWF LEANING TOWARD KEEPING THE SURFACE FRONTAL ZONE JUST
SOUTH OF THE CWA WITH COOLER TEMPERATURES AND DRY WEATHER... WHILE
THE GFS ALLOWS THE WARM SECTOR TO SURGE UP INTO THE CAROLINAS WITH
MUCH WARMER TEMPERATURES AND BETTER RAIN CHANCES. THE LESS
AMPLIFIED GFS HAS SUPPORT FROM THE ECMWF ENSEMBLE MEAN AND THUS
WAS PREFERRED. WILL HAVE LOW CHANCES OF AFTERNOON THROUGH EVENING
SHOWERS OR THUNDERSTORMS... WITH TEMPERATURES GENERALLY A
CATEGORY ABOVE NORMAL. -GIH

&&

.AVIATION /23Z MONDAY THROUGH SATURDAY/...
AS OF 745 PM...

VFR CONDITIONS THROUGH MUCH OF THE NIGHT. STRATUS SHOULD BE MORE
WIDESPREAD TOWARD DAWN THAN MONDAY MORNING... ESPECIALLY SOUTH AND
WEST. LIFR CONDITIONS POSSIBLE FROM ABOUT 5 AM TO 10 AM.

CONDITIONS SHOULD MOSTLY REMAIN VFR THROUGH SATURDAY...BUT THE
BEST CHANCE FOR MVFR/IFR CONDITIONS EXISTS WEDNESDAY AFTERNOON INTO
THURSDAY MORNING WITH THE PASSAGE OF A WEAK FRONT. FOG AND/OR LOW
STRATUS IS ALSO POSSIBLE OVERNIGHT TUESDAY NIGHT.

&&

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

&&

$$

NEAR TERM...MWS/SMITH
SHORT TERM...HARTFIELD
LONG TERM...HARTFIELD

NWS RAH Office Area Forecast Discussion