FXUS62 KRAH 080549
AFDRAH
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC
1250 AM EST TUE DEC 8 2009
.SYNOPSIS...
A POWERFUL STORM SYSTEM WILL BRING WIDESPREAD RAIN TO THE AREA
TUESDAY NIGHT...FOLLOWED BY A CHANCE OF SHOWERS AND AN ISOLATED
THUNDERSTORM ON WEDNESDAY. ABOVE NORMAL TEMPERATURES ARE EXPECTED ON
WEDNESDAY...WITH BELOW NORMAL TEMPERATURES EXPECTED BY THE END OF
THE WEEK.
&&
.NEAR TERM /THROUGH TONIGHT/...
AS OF 955 PM MONDAY...
AREAS OF CLOUDINESS ASSOCIATED WITH A WEAK MID LEVEL TROUGH EXITED
THE REGION EARLIER THIS EVENING. THE CLEAR SKIES SKIES AND
LIGHT/NEAR CALM WIND REGIME IN THE MODESTLY DRY AIR HAS ALLOWED
TEMPS TO FALL TO LEVELS NEAR THE FORECAST MINS FOR TONIGHT.
EXPECT TEMPS TO COOL ANOTHER 2-3 DEGREES IN MOST LOCALES THEN
STABILIZE AS MID/HIGH LEVEL CLOUDS INCREASE FROM THE SW AHEAD OF A
S/W LIFTING NE OUT OF THE NW GULF. THUS HAVE LOWERED IN TEMPS A FEW
DEGREES FROM PREVIOUS FORECAST.
&&
.SHORT TERM /TUESDAY THROUGH FRIDAY NIGHT/...
AS OF 255 PM MONDAY...
TUESDAY AND TUESDAY NIGHT: THE RELATIVE DRYNESS OF THE PRESENT
AIRMASS -- CHARACTERIZED BY TD'S GENERALLY IN THE 30S -- WILL BE
MAINTAINED BY THE RENEWED NORTHEASTERLY SURFACE WINDS BY EARLY
TUESDAY... INDUCED BY THE EASTWARD MOVEMENT OF A SURFACE HIGH OVER
THE TN VALLEY TODAY. WHILE THE HIGH SHOULD BE PROGRESSIVE GIVEN THE
EXPECTED RELATIVELY PROGRESSIVE FLOW ALOFT... THE PRECEDING DRYNESS
SHOULD PROVIDE FOR A HYBRID DAMMING EVENT AND TIGHTENING OF BOTH A
WEDGE FRONT AND COASTAL FRONT... AS STRONG ISENTROPIC UPGLIDE
OVERSPREADS CENTRAL NC LATE TUESDAY AND TUESDAY NIGHT. INDEED... THE
UPGLIDE/WAA WILL BE QUITE STRONG AND DEEP DURING THIS TIME... AS A
MOIST 60+ KT LLJ JET DEVELOPS NORTHWARD AND INTERSECTS THE WEDGE
FRONT. THERE MAY BE TWO AXES OF HEAVIER RAINFALL -- PERHAPS ONE INCH
OR MORE -- ONE OVER THE NW PIEDMONT (AS THE PREVIOUS SHIFT NOTED)
AND ANOTHER OVER COASTAL AREAS. THE FORMER OWING TO A DEEPER COLD
DOME THAT SHOULD ACCORDINGLY PROMOTE MORE STRONGLY SLOPED AND DEEPER
ASCENT (AND ALSO IN CLOSER PROXIMITY TO THE BEST UPPER FORCING)...
AND THE LATTER WHERE CONVECTIVE ELEMENTS WOULD ENHANCE RAINFALL IN
THE DEVELOPING WEAK INSTABILITY AXIS EAST OF THE COASTAL FRONT.
HYBRID CAD DAYTIME HIGH TEMPERATURES OF MIDDLE 40S NW TO MIDDLE 50S
EXPECTED... WITH EARLY EVENING OVERNIGHT LOWS NEAR OR ONLY A FEW
DEGREES COOLER. RISING TEMPERATURES EXPECTED OVERNIGHT GIVEN THE
EXPECTED INLAND RETREAT OF THE WEDGE AND COASTAL FRONTS... RANGING
FROM THE MIDDLE 60S EAST TO MIDDLE 40S WEST.
THIS PATTERN IS NOT TOO DISSIMILAR FROM THE RECENT TORNADO NON-EVENT
LAST WEEK... THOUGH WITH SOME IMPORTANT DIFFERENCES. THE PRIMARY
SURFACE LOW AND MID/UPPER LEVEL FORCING FOR ASCENT ARE FORECAST TO
MOVE FROM THE FOUR CORNERS REGION TO THE UPPER GREAT LAKES TUESDAY
AND TUESDAY NIGHT... BUT REMAIN WELL REMOVED FROM TWO SEPARATE WARM
SECTORS FORECAST TO EVOLVE DOWNSTREAM. THE FIRST OF WHICH SHOULD
DEVELOP NORTHEASTWARD FROM THE WESTERN GOM AND LOWER MS VALLEY TO
THE SOUTHEAST (ATTENDANT A LEAD SURFACE WAVE)... WHILE ANOTHER MOVES
ONSHORE THE CAROLINAS AHEAD OF A TERTIARY LOW ALONG THE WEDGE FRONT
TUESDAY NIGHT. WHILE LOW LEVEL SHEAR WILL BE STRONG TO EXTREME...
BOUNDARY LAYER INSTABILITY WILL AGAIN BE LIMITED. IN FACT... THE
LACK OF NEARBY ISALLOBARIC FORCING MAY HOLD THE RICHEST LOW LEVEL
THETA-E -- SUPPORTIVE OF UPDRAFTS ROOTED IN THE BOUNDARY LAYER AND
ATTENDANT THREAT OF TORNADOES AND DMGG WIND -- ALONG IMMEDIATE
COASTAL NC OR OFFSHORE. THUS... THE SEVERE THREAT AT THIS TIME
APPEARS MINIMAL FOR CENTRAL NC.
WEDNESDAY: THE TRAILING EFFECTIVE COLD FRONT -- THE LEADING EDGE OF
THE LINGERING WEDGE AIR MASS -- IS FORECAST TO SWEEP THROUGH THE
CENTRAL AND EASTERN CWA THROUGH EARLY AFTERNOON. RESPECTABLE MID
LEVEL HEIGHT FALLS (~50 METERS/12 HR)... ON THE SOUTHERN PERIPHERY
OF THE PASSING NEGATIVELY-TILTED UPPER TROUGH TO THE NORTHWEST...
WILL MAINTAIN A RISK OF SHOWERS AND THUNDERSTORMS ALONG/AHEAD OF THE
COLD FRONT. UNIFORM SOUTHWESTERLY FLOW MAY LIMIT CONVERGENCE AND
ALSO STRAIGHTEN HODOGRAPHS (UNIDIRECTIONAL SHEAR). DESPITE THE
UNIDIRECTIONAL NATURE OF DEEP LAYER WIND FIELD... THE WESTERN
PERIPHERY/CYCLONIC SHEAR SIDE OF THE STRONG DEPARTING LLJ WILL
MAINTAIN SUFFICIENT LOW LEVEL VERTICAL (SPEED) SHEAR FOR A CONTINUED
LOW PROBABILITY DMGG WIND THREAT (AND TO A LESSER DEGREE TORNADO
THREAT GIVEN THE ABSENCE OF BACKED SURFACE WINDS). THE DEPARTING LLJ
COULD ALSO MAKE FOR QUITE BREEZY TO WINDY CONDITIONS THROUGH MID
AFTERNOON. SUSTAINED WINDS OF 15 TO 25 MPH WITH GUSTS TO NEAR OR IN
EXCESS OF 40 WILL BE POSSIBLE. A WIND ADVISORY WILL BE CONSIDERED BY
LATER SHIFTS... ONCE IT HOPEFULLY BECOMES MORE APPARENT HOW DEEPLY
MIXED THE BOUNDARY LAYER CAN BECOME AHEAD OF AND BEHIND THE
EFFECTIVE COLD FRONT AND ASSOCIATED SHOWERS AND STORMS. HIGH
TEMPERATURES EXPECTED TO RANGE FROM THE LOWER TO MIDDLE 60S NW TO
LOWER TO MIDDLE 70S SE. A SECONDARY COLD FRONT -- THE ONE ATTENDANT
THE AFOREMENTIONED PRIMARY SURFACE LOW DESTINED FOR THE UPPER GREAT
LAKES -- WILL BRING THE NOTICEABLY COOLER AIR TO NC WEDNESDAY
EVENING AND OVERNIGHT.
WEDNESDAY NIGHT THROUGH FRIDAY NIGHT: TRANQUIL AND MUCH COOLER TO
COLD CONDITIONS EXPECTED THIS PERIOD... AS BROAD SURFACE HIGH
PRESSURE EXPANDS FROM NORTHWESTERN CANADA TO THE MID ATLANTIC
COAST... BENEATH A CONFLUENT AND FAST SOUTHERN AND NORTHERN STREAM
JET CONFIGURATION ALOFT. ANY CLOUD COVER SHOULD LARGELY BE OF THE
MID AND ESPECIALLY HIGH LEVEL VARIETY OWING TO THE PASSAGE OF VERY
LOW AMPLITUDE IMPULSES/EMBEDDED JET STREAKS THROUGH THE CONFLUENT
FLOW ALOFT. TEMPERATURES GENERALLY 45 TO 55 DEGREES THURSDAY...
WITH ~30 KTS OF WNW DOWNSLOPE FLOW PRESENT... AND ONLY IN THE LOWER
TO MIDDLE 40S MOST SPOTS WITH THE HEART OF THE COLD SURFACE HIGH
DIRECTLY OVERHEAD ON FRIDAY. AFTER A WIDE RANGE OF CAA-DRIVEN LOWS
IN THE MIDDLE 30S TO MIDDLE 40S WEDNESDAY NIGHT... THE COLDEST
NIGHTS OF THE SEASON ARE EXPECTED THURSDAY AND PERHAPS AGAIN FRIDAY
NIGHT. LOWER TO MIDDLE 20S SHOULD BE COMMON (WITH SOME UPPER TEENS
POSSIBLE IN THE USUAL COOLER SPOTS) IF THE WELL AGREED UPON AND
CONSISTENT 1275 +/-10 METER LOW LEVEL THICKNESSES VERIFY AT THE END
OF THE WEEK... AND PROVIDED ANY MID/HIGH CLOUD COVER IS RELATIVELY
TRANSPARENT.
&&
.LONG TERM /SATURDAY THROUGH MONDAY/...
AS OF 245 PM MONDAY...
SATURDAY THROUGH SATURDAY NIGHT:
POTENTIALLY INTERESTING SOUTHERN STREAM SYSTEM IS PROGGED TO AFFECT
THE CAROLINAS IN THIS PERIOD. THE GFS/ECMWF/CANADIAN ALL SHOW RATHER
STRONG (1030-1035 MB) HIGH PRESSURE IN PLACE OVER THE CAROLINAS AND
MID-ATLANTIC 00-12Z SAT. AT THIS POINT IN THE EXTENDED FORECAST IT
IS DIFFICULT TO ASCERTAIN THE ORIGINS OF THE AIRMASS EXPECTED TO BE
IN PLACE...HOWEVER...GFS FORECAST SOUNDINGS SHOW DEWPOINTS IN THE
SINGLE DIGITS AND TEMPERATURES IN THE UPPER 20S SATURDAY MORNING...
SUGGESTING AN AIRMASS OF ARCTIC ORIGINS. THE 12Z GFS/ECMWF RUNS SHOW
AN INVERTED TROUGH SHARPENING ALONG THE TX/DEEP SOUTH GULF COAST FRI
NIGHT...EXTENDING ENE/NE TOWARD THE SOUTHEAST COAST...WITH AN AREA
OF LOW PRESSURE DEVELOPING ON A STRONG BAROCLINIC ZONE ALONG OR JUST
OFF THE CAROLINA COAST SAT EVE/NIGHT. THE 12Z GFS/00Z ECMWF
SOLUTIONS WERE VERY SIMILAR W/REGARD TO THEIR HANDLING OF THIS
UPCOMING SYSTEM...HOWEVER...THE LATEST 12Z ECMWF SOLUTION HAS COME
IN WARMER AND A BIT FURTHER WEST WITH THE DEVELOPING SURFACE LOW.
PART OF THE PROBLEM APPEARS TO BE THAT UPPER LEVEL FORCING WILL BE
IN THE FORM OF RELATIVELY WEAK SOUTHERN STREAM IMPULSES TRAVERSING
THE SOUTHERN CONUS...A DIFFICULT UPPER LEVEL PATTERN TO ACCURATELY
PREDICT...PARTICULARLY AT THIS RANGE IN THE FORECAST. AS A RESULT...
DESPITE RELATIVELY GOOD AGREEMENT ON THE OVERALL PATTERN AMONG THE
LONG TERM MODELS...THERE IS STILL A CONSIDERABLE AMOUNT OF
UNCERTAINTY W/REGARD TO THE SENSIBLE WEATHER THAT CENTRAL NC WILL
SEE...SINCE THAT WILL ULTIMATELY DEPEND ON THE AMOUNT OF COLD/DRY
AIR IN PLACE AT THE ONSET OF PRECIPITATION AND THE *EXACT*
TRACK/STRENGTH OF THE DEVELOPING SURFACE LOW. AT THIS TIME WILL KEEP
POPS ALL LIQUID AND IN THE CHANCE RANGE (40%)...WITH THE BEST
CHANCES FOR PRECIPITATION SATURDAY NIGHT. EXPECT BELOW NORMAL TEMPS
ON SAT...ALTHOUGH EXACT VALUES WILL HIGHLY DEPEND ON THE FACTORS
MENTIONED ABOVE. WILL FCST HIGHS IN THE LOWER/MID 40S...ALTHOUGH
THEY COULD BE MUCH LOWER. WILL KEEP LOW TEMPS SAT NIGHT ABOVE
FREEZING IN LOWER/MID 30S. TOO EARLY TO MENTION ANY P-TYPE ISSUES IN
THE ACTUAL FORECAST...HOWEVER...IT'S SOMETHING WE MAY HAVE TO
CONTEND WITH IN THE COMING DAYS.
SUNDAY THROUGH MONDAY:
HIGH PRESSURE IS EXPECTED TO BUILD BACK INTO THE AREA IN THE WAKE OF
THE DEPARTING SOUTHERN STREAM SYSTEM...AND PARTLY CLOUDY SKIES WITH
NEAR NORMAL TEMPS ARE EXPECTED AT THIS TIME. -VINCENT
&&
.AVIATION /06Z TUESDAY THROUGH FRIDAY/...
AS OF 1250 AM TUESDAY...
VFR CONDITIONS ARE EXPECTED TO PREVAIL THROUGH THIS MORNING...WITH
ONLY MID/UPPER CLOUDS STREAMING OVERHEAD. WINDS WILL BECOME E-NE AT
8-12 KT BY 15-18Z LATER THIS AFTERNOON....WHILE CEILINGS GRADUALLY
BEGIN TO LOWER AS A POTENT STORM SYSTEM MOVES NORTHEAST TOWARDS THE
CAROLINAS. THIS SYSTEM WILL AFFECT CENTRAL NORTH CAROLINA BY TUE
AFTERNOON/EVENING AND WILL LAST THROUGH THE DAY WEDNESDAY. LIFR/IFR
CEILINGS AND VISBYS AND LOCALLY HEAVY RAINFALL WILL BE MOST
PREDOMINATE BETWEEN 00Z AND 12Z. A VERY STRONG LOW LEVEL JET WILL
BRING THE POTENTIAL FOR LOW LEVEL WIND SHEAR BY THE END OF THE VALID
TAF PERIOD...LASTING INTO THE DAY ON WEDNESDAY.
A STRONG COLD FRONT WILL THEN SWEEP ACROSS THE AREA ON
WEDNESDAY...AND VFR CONDITIONS ARE EXPECTED TO RETURN BY 12Z
THURSDAY AND THEN CONTINUE THROUGH FRIDAY.
&&
.RAH WATCHES/WARNINGS/ADVISORIES...
NONE.
&&
$$
SYNOPSIS...VINCENT
NEAR TERM...WSS
SHORT TERM...MWS
LONG TERM...VINCENT
AVIATION...KRR/SMITH
