FXUS62 KRAH 222341
AFDRAH
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC
640 PM EST TUE DEC 22 2009
.SYNOPSIS...
HIGH PRESSURE WILL EXTEND ACROSS THE REGION THROUGH
THURSDAY...BRINGING SEASONABLY COOL AND DRY CONDITIONS. A STORM
SYSTEM WILL APPROACH FROM THE SOUTHWEST LATE THURSDAY AND AFFECT THE
AREA CHRISTMAS EVE THROUGH CHRISTMAS DAY.
&&
.NEAR TERM /THROUGH WEDNESDAY NIGHT/...
AS OF 230 PM TUESDAY...
HIGH PRESSURE RIDGE EXTENDING SOUTH FROM THE GREAT LAKES THROUGH
WESTERN NC HAS LEAD TO MOSTLY CLEAR SKIES AND TEMPS GENERALLY IN THE
UPPER 40S TODAY. MOST HIGH CLOUDS HAVE REMAIN WEST OF THE CWA
TODAY...AND WILL REMAIN SO FOR THE BETTER BETTER PART OF THE
EVENING. THE SURFACE HIGH WILL SETTLE OVER THE AREA
TONIGHT...ALLOWING FOR GOOD DECOUPLING AND LIGHT SURFACE WINDS.
MODELS DEPICT A MARGINAL AMOUNT OF LOW LEVEL MOISTURE BELOW THE
INVERSION IN THE WEST...WHICH MAY BE ENOUGH FOR SOME STRATUS ALONG
THE FOOTHILLS. BUT WITH SUCH WEAK BOUNDARY LAYER FLOW...NOT VERY
CONFIDENT THIS WILL SPREAD TO THE NW CWA. THEREFORE HAVE KEPT SKIES
OPTIMISTICALLY LESS CLOUDY...THINKING MAIN CLOUD COVER WILL BE THE
HIGH CLOUDS SPREADING EAST ACROSS THE TENNESSEE VALLEY. LOWS WILL
BE A FEW DEGREES ABOVE GUIDANCE...COOLEST IN THE NE WHERE POTENTIAL
CLOUD COVER IS LEAST LIKELY....24-29.
HIGH PRESSURE RIDGE STRENGTHENING SURFACE HIGH OVER SW ONTARIO WILL
BECOME MORE PRONOUNCED OVER THE EASTERN SEABOARD ON WEDNESDAY. LOW
LEVEL THICKNESSES RISE SOME 10M WHICH WILL ALLOW HIGHS TO MODERATE
A COUPLE MORE DEGREES TO THE UPPER 40S IN THE NW TO LOWER 50S SE.
IF STRATUS DOES DEVELOP IN THE WEST OVERNIGHT...IT SHOULD NOT BE
ENOUGH TO HAVE ANY SIGNIFICANT EFFECT ON HIGHS. WEAK...BUT BETTER
WARM ADVECTION ON WEDNESDAY NIGHT WILL PROMOTE A BETTER CHANCE OF
STRATUS DEVELOPMENT IN THE VICINITY OF THE UPSLOPES...AND THIS
COMBINED WITH THE WARM ADVECTION AND INCREASING PW SHOULD HELP KEEP
LOWS UP SLIGHTLY...ESPECIALLY ACROSS THE SOUTH AND WEST. 27-31
&&
.SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
AS OF 310 PM TUESDAY...
..A THREAT OF SEVERAL HOURS OF LIGHT FREEZING RAIN AT THE ONSET OF
PRECIPITATION OVER THE CLIMATOLOGICAL FAVORED NW PIEDMONT THURSDAY
NIGHT... BEFORE A TRANSITION TO ALL LIQUID BY FRIDAY MORNING...
THE SLOW OUTLIER NAM/WRF HAS BEEN DISCOUNTED IN FAVOR OF THE
CONSENSUS OF OTHER NWP GUIDANCE... WHICH SPREADS ISENTROPICALLY-
DRIVEN PRECIPITATION ACROSS MOST OF CENTRAL NC (EXCEPT THE COASTAL
PLAIN) THURSDAY NIGHT. A DEEP OCCLUDED LOW IS FORECAST TO BECOME
NEARLY STATIONARY OVER THE MID MS VALLEY THURSDAY NIGHT AND
FRIDAY... WHILE A SEPARATE SURFACE FRONTAL WAVE EVOLVES FROM THE
TRIPLE POINT OF THE TRAILING PRIOR- COLD FRONT EASTWARD ALONG AN
EXPECTED WEDGE BOUNDARY FROM CENTRAL GA THROUGH THE CENTRAL AND
EASTERN CAROLINAS. MEANWHILE... A ~1035 MB SURFACE HIGH CENTERED
OVER EAST CENTRAL CANADA WILL EXTEND SOUTHWARD INTO NC... BUT
GRADUALLY SUBMIT TO INCREASING LOW LEVEL SOUTHEASTERLY FLOW AND
RETREAT INTO THE WESTERN PORTION OF THE STATE BY FRIDAY
AFTERNOON.
THURSDAY: A SURFACE LOW OVER THE CENTRAL PLAINS WILL BEGIN TO
LIFT NORTH TO THE UPPER MIDWEST...WHILE ITS ASSOCIATED CLOUD
COVER...PRECIP...AND COLD FRONT KICK EAST. ALL MODEL GUIDANCE
KEEPS NC DRY THROUGH 00Z THURSDAY...WITH THE CLOUD COVER THE ONLY
SENSIBLE WEATHER IMPACT. HIGHS ON THURSDAY POTENTIALLY COOLER THAN
WEDNESDAY...DESPITE SIMILAR LOW LEVEL THICKNESSES....DUE TO
INCREASING MID AND HIGH CLOUDS LATE AND AN ESTABLISHED
NORTHEASTERLY LOW LEVEL FLOW. HIGHS 45-52.
THURSDAY NIGHT (CHRISTMAS EVE): THE AFOREMENTIONED SURFACE RIDGE
SHOULD BE PROGRESSIVE (OWING TO A PROGRESSIVE SUPPORTING RIDGE
ALOFT)... AND NOT PARTICULARLY DRY (WITH DEWPOINTS MAINLY IN THE
MIDDLE 20S PRIOR TO ONSET). HOWEVER... THE INITIAL STRENGTH AND
PLACEMENT OF THE RIDGE AND MARGINALLY DRY SURFACE DEWPOINTS SHOULD
SUPPRESS THE WET BULB FREEZING LINE AND CRITICAL LOW LEVEL
THICKNESS VALUES SUFFICIENTLY SOUTHWARD FOR A SEVERAL HOUR WINDOW
OF FREEZING PRECIPITATION OVER THE CLIMATOLOGICAL FAVORED NW
PIEDMONT. GIVEN EXPECTED LIQUID EQUIVALENT AMOUNTS OF NEAR ONE
QUARTER INCH ALONG AND WEST OF THE YADKIN RIVER TO A TRACE OR LESS
OVER THE COASTAL PLAIN -- DRIVEN BY INITIALLY SHALLOW (WARM)
ISENTROPIC LIFT UNTIL THE UPGLIDE DEEPENS AND ACCORDINGLY EXTENDS
INTO BERGERON- SUPPORTIVE HEIGHTS/TEMPERATURES BY FRIDAY MORNING
-- THERE IS A POTENTIAL FOR A LIGHT GLAZE PRODUCED BY THE FIRST
TENTH OF AN INCH OF PRECIPITATION... BEFORE A TRANSITION TO ALL
LIQUID BY AROUND DAYBREAK. A WINTER WEATHER ADVISORY MAY BE NEEDED
FOR THE NW PIEDMONT COUNTIES. LOWS FROM A DEGREE OR TWO BELOW
FREEZING NORTHWEST... TO AROUND 40 DEGREES OR SO SE... WITH A
GRADUAL RISE IN TEMPERATURES EXPECTED OVERNIGHT.
FRIDAY AND FRIDAY NIGHT (CHRISTMAS): EXPECT THE DEEP ISENTROPIC
LIFT TO PROVIDE FOR A CATEGORICAL CHANCE OF SHOWERS ON FRIDAY.
WHILE MARITIME TROPICAL LOW LEVEL THETA-E AND ASSOCIATED
SUFFICIENT SURFACE BASED INSTABILITY SHOULD REMAIN OFFSHORE GIVEN
THE EXPECTED TRACK OF THE TRIPLE POINT -- THROUGH THE EASTERN
PIEDMONT OR COASTAL PLAIN -- THERE IS SOME CONCERN FOR ELEVATED
CONVECTION PRIOR TO TRIPLE POINT PASSAGE. IN FACT... THE PATTERN
WILL BE ONE SUPPORTIVE OF POTENTIAL (CONVECTIVE) INSTABILITY...
OWING TO THE OVERRUNNING OF THE MID LEVEL DRY SLOT ATOP UNSTABLE
PARCELS ORIGINATING IN SOUTHEASTERLY FLOW AROUND H85... THEN
LIFTED AND REALIZED BY WEAK HEIGHT FALLS ALOFT AND THE EXIT
REGIONS OF A PHASED POLAR AND SUBTROPICAL JET OVER THE REGION.
WILL MAINTAIN ISOLATED THUNDER OVER ROUGHLY THE EASTERN HALF OF
THE CWA (ALONG THE AXIS OF THE SOUTHEASTERLY H85 JET. HIGHS IN THE
MID 40S OVER THE RESIDUAL LY-WEDGED NW PIEDMONT TO AROUND 60 EAST OF
THE WEDGE FRONT WARM SECTORED SE.
DRYING... COOLING... AND CLEARING FRIDAY NIGHT... AS THE SYSTEM
PIVOTS NORTHEASTWARD AND AWAY FROM CENTRAL NC. LOWS IN THE MIDDLE
30S WEST TO MIDDLE 40S EAST.
&&
.LONG TERM /SATURDAY THROUGH MONDAY/...
AS OF 315 PM TUESDAY...
SATURDAY THROUGH TUESDAY: THIS PERIOD WILL BE DOMINATED BY A POLAR
VORTEX OVER THE GREAT LAKES/NEW ENGLAND. ANY PRECIPITATION IS
EXPECTED TO REMAIN CONFINED TO UPSLOPE REGIONS IN THE CENTRAL
APPALACHIANS... THOUGH WE WILL NEED TO MONITOR THE MAGNITUDE OF S/W
TROUGHS THAT MAY PIVOT AROUND THE BASE OF THE VORTEX. HIGHS AND LOWS
IN THE 40S AND 20S RESPECTIVELY... ABOUT A CATEGORY BELOW NORMAL.
&&
.AVIATION /00Z WEDNESDAY THROUGH SUNDAY/...
AS OF 630 PM TUESDAY...
VFR CONDITIONS WILL CONTINUE THROUGH THE 24 TAF PERIOD.
WEAK SURFACE RIDGE OVER THE REGION WILL BECOME REINFORCED BY
COLDER HIGH PRESSURE TO THE NORTH ON THURSDAY. THE ACCOMPANYING
DRY AIR MASS WILL ENSURE VFR CONDITIONS THROUGH WEDNESDAY.
OTHERWISE...HIGH CLOUDS WILL STREAM INTO THE AREA IN THE PROGRESSIVE
ZONAL FLOW ALOFT.
MAY SEE STRATUS DEVELOP THURSDAY MORNING AS 925MB EASTERLY FLOW JUST
ABOVE THE SURFACE WILL OVERSPREAD SOME ATLANTIC MARITIME AIR INTO
THE STATE. A STORM SYSTEM WILL APPROACH NC FROM THE WEST-SOUTHWEST
AND BRING MVFR/IFR CONDITIONS THURSDAY NIGHT AND INTO FRIDAY.
THERE IS ALSO THE POTENTIAL FOR LOW LEVEL WIND SHEAR CONDITIONS
THURSDAY NIGHT/FRIDAY MORNING FROM A 40 TO 45KT SELY LLJ . VFR
CONDITIONS ARE EXPECTED TO RETURN ON SATURDAY AS THE STORM SYSTEM
EXITS THE AREA.
&&
.RAH WATCHES/WARNINGS/ADVISORIES...
NONE.
&&
$$
SYNOPSIS...LP
NEAR TERM...SMITH
SHORT TERM...SMITH/MWS
LONG TERM...MWS
AVIATION...CBL
