FXUS62 KRAH 251206
AFDRAH
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC
706 AM EST FRI DEC 25 2009
.SYNOPSIS...
A STORM SYSTEM TO OUR SOUTHWEST WILL MOVE NORTHEAST AND AFFECT THE
REGION THROUGH CHRISTMAS DAY AND THROUGH THE EVENING... SLOWLY
EXITING TO OUR NORTHEAST SATURDAY MORNING. A DRY HIGH PRESSURE AREA
WILL BUILD IN OVER THE WEEKEND... THEN A SECONDARY COLD FRONT WILL
BRING COOLER TEMPERATURES FOR MONDAY THROUGH TUESDAY.
&&
.NEAR TERM /THROUGH TONIGHT/...
AS OF 640 AM FRIDAY...
FREEZING RAIN ADVISORY REMAINS IN EFFECT THROUGH 9AM THIS MORNING
FOR FORSYTH AND GUILFORD COUNTIES.
QUICK UPDATE TO REMOVE PERSON...ORANGE AND ALAMANCE COUNTIES.
PRECIP HAS BEEN SLOW TO WORK ITS WAY MUCH FURTHER NORTH THAN CLT TO
HKY THIS MORNING. A SLIGHT DEWPOINT RECOVERY HAS PUSHED THE SURFACE
WETBULB FREEZING LINE NORTHWARD THROUGH THE ABOVE COUNTIES. WETBULB
VALUES REMAIN BELOW FREEZING ACROSS FORSYTH AND THE NORTHWESTERN
HALF OF GUILFORD...WHERE PRECIP IS ON THE DOORSTEP. TIMEFRAME FOR
SIGNIFICANT ICE ACCRUAL HAS NARROWED CONSIDERABLY GIVEN THE SLOW
PROGRESSION OF PRECIP...BUT A LIGHT GLAZE WILL STILL BE POSSIBLE.
-SMITH
PREVIOUS DISCUSSION...
PRECIPITATION IS SPREADING NORTH AND EAST THROUGH THE TENNESSEE
VALLEY AND SOUTHERN APPALACHIANS THIS MORNING...IN RESPONSE TO WARM
ADVECTION AHEAD OF A VERTICALLY STACKED LOW PRESSURE SYSTEM ROTATING
NORTH ALONG THE MISSISSIPPI RIVER. THE PRIMARY CONCERN FOR THE
FIRST FEW HOURS OF THE FORECAST IS THE POTENTIAL FOR FREEZING
RAIN..MAINLY ALONG AND NORTH OF THE I-40/85 CORRIDOR. A STEADY FEED
OF DRIER SURFACE AIR FROM THE SURFACE HIGH TO THE NORTH HAS KEPT DEW
POINTS IN THE MID TO UPPER 20S FROM THE TRIAD TO ROXBORO...AND WITH
TEMPS IN THE LOWER 30S...THE SURFACE WETBULB FREEZING LINE HAS BEEN
MEANDER ROUGHLY ALONG THE SAME LINE. ALTHOUGH ONLY A FEW OB SITES
HAVE REPORTED PRECIP SO FAR...EXPECT THE PRECIP SHIELD TO FILL IN
AND BECOME MORE WIDESPREAD BY 12Z ACROSS THE WEST AS BETTER WARM
ADVECTION/ISENTROPIC LIFT ARRIVES. RUC SOUNDINGS SUGGEST THE BEST
OPPORTUNITY FOR FREEZING RAIN WILL BE BEFORE 11Z OR 12Z... AND BASED
ON THE TIMING OF MORE STEADY PRECIP...STILL EXPECTING THAT ANY ICE
ACCRUAL WILL BE ONLY A FEW HUNDREDTHS OF AN INCH AND MOSTLY CONFINED
TO ELEVATED SURFACES. AFTER ~12Z THE COMBINATION OF WARM ADVECTION
AND THE RESPONSE OF THE AMBIENT AIR TO ANY FREEZING WILL CAUSE TEMPS
TO RISE ABOVE FREEZING.
WIDESPREAD RAIN IN THE WEST THIS MORNING WILL LARGELY SHIFT TO THE
EAST BY THE AFTERNOON HOURS. QPF AMOUNTS OF AROUND 1.5" POSSIBLE IN
THE WEST...AIDED BY BETTER ISENTROPIC LIFT...WITH AN AROUND AN INCH
IN THE EAST AS PRECIP BECOMES MORE SHOWERY. ALTHOUGH RIVER HAVE
BEEN RUNNING HIGH AT TIMES DURING THE PAST COUPLE OF WEEKS...AND
MELTING OF ANY LINGERING SNOW WILL ADD TO RUN OFF...NO RIVER
FLOODING IS ANTICIPATED.
LATER TODAY...THE CONCERN SHIFTS TO THE POTENTIAL FOR THUNDER ACROSS
THE EASTERN HALF OF THE CWA...AS A SECONDARY/TRIPLE POINT LOW
DEVELOPS ALONG THE APPROACHING COLD FRONT AND RETREATING WEDGE
FRONT. THE INLAND SURGE OF THE COASTAL/WEDGE FRONT WILL ULTIMATELY
DETERMINE THE IMPACT OF POTENTIAL TSTRMS....AS THERE WILL BE
SUFFICIENT DEEP LAYER SHEAR (50-60KTS) AND LOW LEVEL SHEAR AIDED BY
BACKED SURFACE WINDS. LARGE SCALE LIFT WILL BE REMOVED TO THE NW
WITH BEST HEIGHT FALLS OVER THE OHIO VALLEY...AND INSTABILITY
(ESPECIALLY SURFACE BASED) REMAINS QUESTIONABLE. THE BEST MID LEVEL
LAPSE RATES (~5-6 C/KM) PASS OVERHEAD BETWEEN 18Z-00Z...DEPENDING ON
LOCATION AND EVOLUTION OF THE TRIPLE POINT LOW...THIS LOOKS TO BE
THE BEST TIME FRAME FOR DEVELOPMENT. GFS BUFR SOUNDINGS FOR KFAY
SUGGEST TSTORM ACTIVITY COULD BE MORE ROOTED IN THE BOUNDARY
(300-500J/KG OF MLCAPE)...BECOMING MORE ELEVATED (400-500J/KG OF
MUCAPE) NORTH AND WEST AS THE LLJ PASS BY. ALTHOUGH BETTER
INSTABILITY WILL LIKELY REMAIN TO OUR SE TOWARD THE COAST THERE
REMAINS THE POTENTIAL FOR A LOW TOPPED SUPERCELL OR TWO IN THE WARM
SECTOR...WITH A QUASI-LINEAR SEGMENT AS THE COLD FRONT/SURFACE
TROUGH PASSES BY.
HIGHS QUITE TRICKY TODAY...BUT WILL CONTINUE THE PREVIOUS FORECAST
OF LOW TO MID 40S IN THE NW DUE TO THE ERODING WEDGE AIRMASS...WITH
A SHARP GRADIENT AND LOWER 60S IN THE SE. PRECIP SHOULD BE EXITING
THE COASTAL PLAIN BY MIDNIGHT TONIGHT AS THE DRY PUNCH WITH THIS
SYSTEM OVERWHELMS THE AREA AND WINDS TURN TO WESTERLY. A LARGER
THAN NORMAL RANGE OF OVERNIGHT LOWS... AS MODEL SOUNDINGS SHOW
LINGERING STRATUS AND MOISTURE POOLING ALONG A SECONDARY 925MB
TROUGH IN THE EAST..WITH BETTER CLEARING IN THE WEST. LOWS 34-46.
&&
.SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...
AS OF 245 AM FRIDAY...
FOR SATURDAY/SATURDAY NIGHT: MODEL CONSENSUS PLACES THE SURFACE LOW
OVER SOUTHEAST VA AT 12Z SATURDAY... WITH WEAK AND MODIFIED HIGH
PRESSURE BUILDING IN FROM THE SOUTHWEST. FORECAST SOUNDINGS SHOW A
SUBSTANTIAL LOW LEVEL INVERSION DEVELOPING WHICH SERVES TO TRAP
QUITE A BIT OF LOW LEVEL MOISTURE THROUGH THE MORNING. EXPECT AREAS
OF STRATUS TO SLOWLY LIFT AND MIX WITH WEAK SURFACE WINDS AND THE
STABLE LOW LEVELS INHIBITING VERTICAL AND HORIZONTAL DISPERSION. AND
GIVEN THE FAST FLOW IN THE MID-UPPER LEVELS AND MODEL INDICATIONS OF
AMPLE MOISTURE ALOFT STREAMING FROM OLD MEXICO ACROSS THE SOUTHEAST
WITH MOIST UPGLIDE AT 320K-330K... WE SHOULD ALSO SEE A STEADY
STREAM OF HIGH CLOUDS THROUGH THE DAY. ALL IN ALL... SKIES WILL BE
MAINLY PARTLY SUNNY. 1000-850 MB THICKNESSES HOLD ABOVE NORMAL
THROUGH SATURDAY WITH ONLY VERY WEAK SURFACE COLD AIR ADVECTION...
BUT MUCH OF THE WARMTH IN THIS LAYER IS ABOVE 950 MB. HAVE TRIMMED
BACK HIGHS A BIT TO 51-60. WITH THE 320-330K UPGLIDE DEPARTING
SATURDAY EVENING... EXPECT SKIES TO BECOME MOSTLY CLEAR... ALTHOUGH
THE LIGHT WINDS AND ANY RESIDUAL GROUND MOISTURE COULD LEAD TO
PATCHY FOG DEVELOPMENT LATE SATURDAY NIGHT. LOWS 29-35.
FOR SUNDAY/SUNDAY NIGHT: THE ANOMALOUSLY STRONG VORTEX THAT WILL
HAVE BEEN SITTING AND SPINNING OVER THE MIDWEST FOR TWO FULL DAYS
FINALLY EJECTS EASTWARD ACROSS NRN IN/OH TO PA/NY BY DAYBREAK
MONDAY. THE WEAK SURFACE HIGH COVERS THE SOUTHEAST AND RIDGES UP
INTO NC THROUGH SUNDAY... PRODUCING CONTINUED DRY WEATHER...
ALTHOUGH A WEAK MID LEVEL VORTICITY LOBE SWEEPING THROUGH THE AREA
SHOULD BRING INCREASING MID CLOUDS. LOW LEVEL THICKNESSES DIP
NOMINALLY AND STILL ANTICIPATE SLIGHTLY ABOVE NORMAL HIGHS OF 51-56.
THE WRF/GFS/ECMWF/CANADIAN ARE IN GOOD AGREEMENT ON SECONDARY COLD
FRONTAL PASSAGE SUNDAY NIGHT... MOST LIKELY AROUND OR SHORTLY AFTER
MIDNIGHT... AND ALL ALSO DEPICTS AN UPPER JET CORE ACCELERATING OVER
OR JUST SOUTHEAST OF NC WITH A QUICK SHOT OF LIGHT PRECIP STREAKING
ACROSS SOUTHEAST NC NEAR THE OFFSHORE BAROCLINIC ZONE. THE WRF WITH
ITS GREATER SATURATION THROUGH A DEEPER LAYER AS COMPARED TO THE GFS
SUGGESTS ANY PRECIP MIGHT BE A LIGHT RAIN/SNOW MIX... ALTHOUGH THE
SATURATION JUST BARELY GETS INTO THE CRYSTAL GROWTH ZONE ALOFT. SUCH
RAPID VERTICAL MOTION IS CERTAIN POSSIBLE WITH THE LIKELIHOOD OF
STRONG AGEOSTROPHIC RESPONSE TO THE ACCELERATING JET... BUT SINCE
THE GFS/WRF CONCENTRATE THIS LIFT IN THE MID AND UPPER LEVELS (EVEN
THE WRF HAS A RATHER DRY SUBCLOUD LAYER) WITH THE DEGREE OF MOISTURE
RETURN QUESTIONABLE... WILL LEAVE THE FORECAST DRY FOR NOW BUT HAVE
BEEFED UP CLOUDINESS. LOWS 26-33. -GIH
&&
.LONG TERM /MONDAY THROUGH THURSDAY/...
AS OF 300 AM FRIDAY...
MONDAY THROUGH WEDNESDAY: ARCTIC HIGH PRESSURE BUILDS INTO AND OVER
THE REGION THROUGH MIDWEEK... WITH NORTHWESTERLY MID LEVEL FLOW
FLATTENING TO WESTERLY. WE MAY SEE OROGRAPHIC ENHANCEMENT OF MID AND
HIGH CLOUDS TUESDAY NIGHT INTO WEDNESDAY... OTHERWISE WITH A DRY AND
STABLE COLUMN AND NO MECHANISMS TO GENERATE LIFT... EXPECT DRY
WEATHER AND TEMPS ABOUT A CATEGORY BELOW NORMAL.
WEDNESDAY NIGHT/THURSDAY: IT APPEARS THAT WE TRANSITION BACK TO A
SOUTHERN STREAM DOMINANT PATTERN WITH BROAD TROUGHING COMING INTO
THE FAR SOUTHWEST STATES AND NORTHERN MEXICO INTO THE SOUTHERN
PLAINS. THE 00Z/25 ECMWF SHOWS THIS LEADING TO SURFACE CYCLOGENESIS
OVER THE NORTHWEST GULF AND CENTRAL GULF COAST WITH INCREASING
OVERRUNNING OF THE SOUTH-SOUTHWEST 850 MB FLOW ATOP THE WEDGING
SURFACE HIGH CENTERED TO OUR NORTH. THE LATEST GFS IS NOT TOO
DISSIMILAR... SHOWING A SECOND LOW DEVELOPING OFF THE CAROLINA COAST
THURSDAY WITH A 160 KT JET JUST TO OUR NORTH. WILL INCLUDE JUST
SLIGHT CHANCE POPS THURSDAY GIVEN THE GOOD MODEL AGREEMENT. EXPECT
SOME MODERATION OF TEMPS BACK TOWARD NEAR NORMAL VALUES... ALTHOUGH
ANY APPRECIABLE PRECIP INTO THE SURFACE STABLE AIR MAY MEAN BELOW
NORMAL HIGHS THURSDAY OVER THE PIEDMONT. -GIH
&&
.AVIATION /12Z FRIDAY THROUGH TUESDAY/...
AS OF 110 AM FRIDAY...
A NARROW...BUT SIGNIFICANT AREA OF PRECIP ASSOCIATED WITH A WELL
DEVELOPED STORM SYSTEM OVER THE MISSISSIPPI RIVER VALLEY REGION IS
BEGINNING TO WORK ITS WAY INTO SW NC THIS MORNING. CEILINGS WHICH
ARE ALREADY IN THE MVFR RANGE OVER THE WESTERN HALF OF CENTRAL
NC...WILL CONTINUE TO LOWER TO IFR/LIFR THIS MORNING AS THE PRECIP
ARRIVES. BECAUSE TEMPS ARE AROUND FREEZING AND DEWPOINTS ARE IN
THE UPPER 20S...THE PRECIP MAY START OUT AS LIGHT FREEZING RAIN AT
KINT AND KGSO BEFORE 14Z. STRONG WARM ADVECTION WILL CAUSE TEMPS
TO RISE QUICKLY ABOVE FREEZING...TURNING ALL PRECIP TO RAIN FOR
THE REMAINDER OF THE DAY. PERIODS OF MODERATE TO HEAVY RAIN WILL
BE POSSIBLE THROUGH MIDDAY...WITH THUNDERSTORMS POSSIBLE FURTHER
SOUTH AND EAST TOWARDS KFAY BETWEEN 18Z AND 00Z. CONFIDENCE IS
SOMEWHAT LOW ON HOW FAR INLAND THUNDERSTORMS WILL DEVELOP...AND
THUS NO MORE THAN CB HAS BEEN MENTIONED IN THE CURRENT TAF
PACKAGE. WINDS WILL ALSO BE AN ISSUE ACROSS CENTRAL NC TODAY. AT
KINT/KGSO AND EVEN KRDU...STRONG WINDS ABOVE A LOW LEVEL STABLE
LAYER MAY CAUSE LOW LEVEL WIND SHEAR ISSUES...WHILE STRONG GUSTY
WINDS WILL BE POSSIBLE WITH THUNDERSTORMS OR HEAVIER SHOWERS.
ONCE THE PRECIP DEPARTS TO THE EAST BY MIDNIGHT TONIGHT...WINDS WILL
VEER AROUND TO MORE WEST-SOUTHWESTERLY AND WIND WILL RELAX. LOW
STRATUS MAY BE SLOW TO SCATTER AS DRIER AIR LOOKS LIKE IT WILL BE
SLOW TO PUSH ACROSS THE AREA. IN ADDITION...THERE MAY BE SOME PATCHY
FOG ACROSS THE AREA AS WINDS DIE OFF. THUS...MVFR/IFR CEILINGS AND
VISIBILITIES MAY LINGER THROUGH SATURDAY MORNING.
SATURDAY THROUGH MONDAY:
VFR CONDITIONS ARE EXPECTED TO RETURN BY LATER SATURDAY MORNING AND
LAST THROUGH MONDAY AS HIGH PRESSURE BUILDS TOWARD THE CAROLINAS FROM
THE SOUTHWEST.
&&
.RAH WATCHES/WARNINGS/ADVISORIES...
FREEZING RAIN ADVISORY UNTIL 9 AM EST THIS MORNING FOR NCZ021-
022.
&&
$$
SYNOPSIS...HARTFIELD
NEAR TERM...SMITH
SHORT TERM...HARTFIELD
LONG TERM...HARTFIELD
AVIATION...SMITH
