FXUS62 KRAH 020759
AFDRAH
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC
259 AM EST WED DEC 02 2009
.SYNOPSIS...
A STRONG LOW PRESSURE SYSTEM OVER THE GULF COAST REGION WILL MOVE
NORTHEAST THROUGH THE REGION THROUGH TONIGHT...BRINGING WIDESPREAD
RAIN AND GUSTY WINDS TO THE AREA.
&&
.NEAR TERM /THROUGH TONIGHT/...
AS OF 230 AM WEDNESDAY...
THE WEAK 1024 MB HIGH PRESSURE CENTERED JUST OFF THE CAROLINA COAST
WILL CONTINUE TO MOVE FURTHER OFFSHORE LATER THIS MORNING. THE
EAST-NORTHEASTWARD PROGRESSION OF A POTENT SOUTHERN STREAM VORTEX
CURRENTLY OVER CENTRAL TEXAS WILL ALLOW FURTHER DEEPENING OF THE LOW
PRESSURE OVER THE NORTHERN GULF OF MEXICO AS IT LIFTS NE THROUGH THE
LOWER MS AND TN VALLEYS TODAY. STRONG ISENTROPIC LIFT WITHIN THE
STRENGTHENING WARM AIR CONVEYOR BELT OF 50 TO 60KTS AND LIKELY
ENHANCED BY LOW-LEVEL DIABATIC PV MAXIMA FROM GULF COAST
CONVECTION...WILL SPREAD NORTH ACROSS THE AREA TODAY...RESULTING IN
CATEGORICAL POPS ACROSS THE ENTIRE CWA BY 18Z.
WITH MORNING LOWS STARTING OUT IN THE LOWER 40S AND DEWPOINTS IN THE
MID 30S ACROSS THE PIEDMONT OF NORTH CAROLINA...THE ONSET OF RAIN
INTO THE RESIDUAL COOL DRY AIR DOME WILL BRING ABOUT A SHORT-LIVED
IN-SITU DAMMING EVENT ACROSS THE CAROLINAS TODAY. AS PRECIP
INTENSITY INCREASES...PRECIP DRAG SHOULD PULL WARMER AIR ALOFT TO
THE SURFACE...HELPING TO DIMINISH/DISSIPATE THE SHALLOW CAD AIR MASS
FROM SE TO NW. DESPITE THE SHALLOW COOL STABLE DOME...THERE WILL
STILL BE AN IMPRESSIVE TEMPERATURE/DEWPOINT GRADIENT ACROSS THE
FORECAST AREA(DUE TO WIDESPREAD SOLID RAIN)TODAY WITH TEMPERATURES
ACROSS THE NW PIEDMONT EXPECTED TO REMAIN IN THE 40S THROUGH MUCH OF
THE DAYTIME HOURS...WHILE TEMPERATURES RISE TO AROUND 60 ACROSS THE
CENTRAL PIEDMONT...AND INTO THE MID 60S ACROSS THE SANDHILLS AND
SOUTHERN COASTAL PLAIN. IT IS NO SURPRISE TO SEE THAT THE HIGHER
RESOLUTION 12KM NAM DEPICTS THIS CAD EVENT THE BEST...WITH THE GFS
TOO PREMATURELY SCATTERING OUT THE WEDGE AIRMASS TOO QUICKLY.
IT IS WITH THIS THERMAL-MOISTURE-BOUNDARY(TMB) OR WEDGE BOUNDARY THAT
WILL BE OF GREATER CONCERN LATE THIS AFTERNOON AND INTO THE
OVERNIGHT HOURS AS MODELS DEPICT A LOCALIZED UNSTABLE MARITIME REGIME
DEVELOPING ACROSS CENTRAL AND EASTERN NC. THE LATEST 00Z SPC WRF
MODEL DOES INDEED SHOW THIS ATLANTIC MARITIME REGIME SURGING ONSHORE
ACROSS COASTAL SECTIONS BETWEEN 18 TO 00Z...WELL AHEAD OF THE
STRONGER SYNOPTIC SCALE FORCING ASSOCIATED WITH THE NEGATIVE TILTED
UPPER WAVE(DUAL JETS)AND ATTENDANT SURFACE FRONT. THUS THE FIRST
ROUND OF SEVERE WX MAY OCCUR AS EARLY AS MIDDAY AND INTO THE
AFTERNOON ACROSS THE SANDHILLS AND SOUTHERN COASTAL PLAIN IN THE
WARM SECTOR BEHIND THE RETREATING WEDGE FRONT.
THE WARM UNSTABLE SECTOR WILL CONTINUE TO PENETRATE INLAND INTO THE
PIEDMONT AS THE WEDGE BOUNDARY RETREAT NORTHWARD. MODELS ARE
ACTUALLY IN GOOD AGREEMENT WITH SHOWING RISING TEMPS/DEWPOINTS
DURING THE EVENING AND OVERNIGHT HOURS...WITH THE CALENDAR DAY HIGH
LIKELY OCCURRING JUST BEFORE THE STRUCK OF MIDNIGHT. BUT THE
SIMILARITIES STOP HERE AS THE GFS SIGNALS THAT THE EVENT IS PRETTY
MUCH OVER BY 00Z. HOWEVER...BELIEVE THIS WILL BE JUST THE START OF
ROUND TWO AS THE AS THE STRONGER UPPER DYNAMICS WITH THE NEGATIVE
TILTED TROUGH AND APPROACHING COLD FRONT SPREAD EASTWARD. THE NAM
AND SPC WRF BEST DEPICT THIS SCENARIO WITH A CONVECTIVE LINE
SEGMENT(LEWP)PUSHING EAST ACROSS THE AREA BETWEEN 00 TO 06Z
WITH A CONTINUATION OF INSANE HELICITY/SHEAR VALUES AS 60 TO 70KT
LOW-LEVEL JET EXITS THE AREA AOA 09Z. THE NAM DEVELOPS 400 TO 700
JOULES OF MUCAPE ACROSS THE AREA DURING THAT PERIOD...WITH THE
HIGHEST INSTABILITY ACROSS THE SE. SO WITH THE SECOND ROUND...
THE EVENING/OVERNIGHT SEVERE THREAT COULD EXTEND WESTWARD INTO THE
CENTRAL PIEDMONT.
AS SUCH... SPC HAS THE EASTERN HALF OF CENTRAL NC (MAINLY
ALONG AND EAST OF US HIGHWAY 1) IN A SLIGHT RISK WITH A 30 PERCENT
PROBABILITY FOR SEVERE WINDS AND A 10 PERCENT PROBABILITY OF
TORNADOES.
ALSO..DUE TO RECENT HEAVY RAINFALL OVER THE PAST COUPLE OF WEEKS...
FLASH FLOOD GUIDANCE FOR WESTERN COUNTIES ARE RELATIVELY LOW. WITH
WIDESPREAD RAINFALL AMOUNTS OF 1 TO 2 INCHES AND LOCALLY HIGHER
AMOUNTS POSSIBLE...WILL CONTINUE WITH A FLASH FLOOD WATCH FOR THOSE
AREAS.
IT IS ADVISED THAT ALL RESIDENTS ACROSS CENTRAL NC MAINTAIN A HIGH
AWARENESS AND REMAIN VIGILANT OF WEATHER CONDITIONS AND FORECAST
UPDATES TODAY AND TONIGHT AS CONDITIONS COULD RAPIDLY DETERIORATE.
-BL
.SHORT TERM /THURSDAY THROUGH FRIDAY/...
AS OF 230 AM WEDNESDAY...
THURSDAY AND THURSDAY NIGHT...
THE ACCELERATED DEPARTING LOW PRESSURE SYSTEM IS EXPECTED TO
BE OVER NEW YORK BY 12Z THURSDAY...WITH ONLY A SLIGHT CHANCE OF A
LINGERING SHOWER AS THE SURFACE FRONT PASSES THROUGH THE COASTAL
PLAIN. WITH A DECENT PRESSURE GRADIENT OVER CENTRAL NC IN THE WAKE
OF THE SYSTEM....W-SW WINDS MAY STAY UP IN THE 10-15KT RANGE WITH
GUSTS TO 25KT DURING THE FIRST HALF OF THE DAY BEFORE TURNING TO
NWRLY AND RELAXING. COLD ADVECTION APPEARS TO BE DELAYED
LATE ENOUGH IN THE AFTERNOON THAT THE WESTERLY COMPRESSIONAL
DOWNSLOPE FLOW WILL BRING ABOUT A SEASONABLY MILD
DAY WITH HIGHS IN THE UPPER 50S TO LOWER 60S. MID AND HIGH LEVEL
MOISTURE LOOKS TO OVERSPREAD THE AREA THURSDAY NIGHT IN THE SWLY
FLOW ALOFT AS S/W ENERGY AMPLIFIES OVER THE SOUTHERN ROCKIES.
OVERNIGHT LOWS 35 TO 40. -BL
FRIDAY...
COOL...WEAK SURFACE HIGH PRESSURE RIDGE LINGERS OVER THE MID
ATLANTIC ON FRIDAY...WHILE SHORTWAVE ENERGY (AND THE DRIVING FORCE
FOR THE WEEKENDS WEATHER) DIVES SOUTH AROUND THE BACK SIDE OF THE
LONG WAVE TROUGH EXPECTED TO DEVELOP OVER THE EASTERN 2/3 OF THE
COUNTRY. HIGHS FRIDAY A BIT COOLER THAN THURSDAY...ALTHOUGH THE
COLDEST AIR WILL LIKELY HOLD TO OUR WEST UNTIL SATURDAY. HIGHS
SHOULD HOLD IN THE LOW TO MID 50S...ALTHOUGH SOME UPPER 40S QUITE
POSSIBLE...ESPECIALLY AS HIGH CLOUDS INCREASE AHEAD OF THE NEXT
SYSTEM. -BLS
&&
.LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/...
AS OF 245 AM WEDNESDAY...
FRIDAY NIGHT THROUGH SATURDAY NIGHT: A SURFACE LOW WILL DEVELOP OVER
THE EASTERN GULF AS A SHORT WAVE ROUNDS THE BASE OF A BROAD LONGWAVE
TROUGH ENCOMPASSING MUCH OF THE CONUS. THIS SURFACE LOW WILL
EVENTUALLY TRACK OFF THE EAST COAST...RESULTING IN AN INCREASE IN
PRECIP CHANCES BEGINNING FRIDAY NIGHT. MODELS CONTINUE TO SHOW RUN
TO RUN INCONSISTENCY WITH RESPECT TO THE STRENGTH AND LOCATION OF THE
SURFACE LOW. NEVERTHELESS...GIVEN THE SURFACE LOW OFF THE COAST AND
STRONG UPPER SUPPORT (RIGHT ENTRANCE REGION OF A STRONG UPPER JET
CORE OVER THE REGION THROUGH AT LEAST SATURDAY AFTERNOON)...WILL
MAINTAIN CHANCE POPS OVER THE ENTIRE REGION ON SATURDAY. RAIN
CHANCES WILL TAPER OFF SATURDAY NIGHT AS THE LOW MOVES NORTHEAST.
PRELIMINARY SOUNDINGS/THICKNESS VALUES CONTINUE TO INDICATE A
POSSIBLE CHANGE OVER TO SNOW IN THIS WRAP AROUND PRECIP ACROSS THE
FAR NORTHWEST PORTION OF THE CWA. HOWEVER...SATURATION IN THE ICE
NUCLEATION ZONE IS MARGINAL AT BEST WHEN THE COLD AIR IS IN PLACE
AND ALSO THE MODELS TEND TO BRING IN THE COLD AIR TOO FAST.
THEREFORE...BELIEVE THAT THE COLD AIR WILL BE LAGGING THE PRECIP AND
ALL PRECIP WILL REMAIN IN THE LIQUID FORM. AS ALWAYS...THIS SCENARIO
WILL HAVE TO BE MONITORED CLOSELY BY FUTURE SHIFTS. LOWS WILL
GENERALLY BE IN THE 30S ON FRIDAY AND THEN THE UPPER 20S TO LOWER
30S ON SATURDAY AS THICKNESS VALUES FALL SIGNIFICANTLY. HIGHS ON
SATURDAY WILL BE HIGHLY DEPENDENT ON THE PRECIP...BUT ARE GENERALLY
EXPECTED TO BE IN THE 40S AREAWIDE.
SUNDAY THROUGH TUESDAY: A CHILLY SURFACE HIGH WILL DOMINATE THROUGH
THIS MOST OF THIS PERIOD...WITH DRY AND PARTLY CLOUDY CONDITIONS
EXPECTED. SUNDAY MORNING WILL START OFF RATHER CHILLY (LOWS IN THE
MID 20S TO LOW 30S) AS HIGH PRESSURE IS OVERHEAD. HOWEVER...SOME
HIGH CLOUDS AHEAD OF NEXT SYSTEM MAY INHIBIT IDEAL RADIATIONAL
COOLING CONDITIONS. WITH MORE SUNSHINE EXPECTED ON SUNDAY...HIGH
TEMPS WILL BE SLIGHTLY WARMER THAN SATURDAY...GENERALLY IN THE UPPER
40S TO LOW 50S. AS THE HIGH SLOWLY SHIFTS OFF TO THE NORTHEAST
THROUGH EARLY NEXT WEEK...TEMPS WILL SLOWLY REBOUND...WITH LOWS IN
THE MID TO UPPER 30S AND HIGHS INTO THE LOW 50S BY THE EARLY PART
OF THE WEEK.
&&
AVIATION /06Z WEDNESDAY THROUGH SUNDAY/...
AS OF 1245 AM WEDNESDAY...
SOUTHEAST FLOW JUST OFF THE SURFACE WILL INCREASE OVERNIGHT AS THE
PRESSURE PATTERN TIGHTENS BETWEEN A HIGH PRESSURE THAT IS MOVING
OFFSHORE AND AN ABNORMALLY STRONG LOW PRESSURE THAT WILL BE LIFTING
NE ACROSS THE LOWER MS VALLEY. THIS SE FLOW WILL ADVECT ATLANTIC
MOISTURE INTO CENTRAL NC AFTER 09Z...RESULTING IN WIDESPREAD
IFR/LIFR CEILING BY 12Z WITH PATCHES OF LIGHT RAIN OCCURRING OVER
THE SANDHILLS AND THE SOUTHERN/WESTERN PIEDMONT AROUND DAYBREAK. LOW
LEVEL WIND SHEAR WILL EXIST AS SURFACE FLOW WILL BE E-NE BETWEEN 10
TO 12KTS WHILE THE FLOW BETWEEN 1000-2000FT WILL BE SE 35-45KTS. THE
LOW LEVEL WIND SHEAR WILL PERSIST THROUGH 18Z WEDNESDAY AT ALL TAF
SITES...LIKELY THROUGH 00Z WEDNESDAY AT KINT AND KGSO AND POSSIBLY
KRDU. WINDS WILL LIKELY BECOME SOUTHEASTERLY AND GUSTY AT KRWI AND
KFAY...POSSIBLY KRDU...BETWEEN 18Z TO 00Z...AS THE STRONG
SOUTHEASTERLY FLOW IN THE LOW LAYERS ERODES THE SHALLOW STABLE AIR
AT THE SURFACE. THERE STILL IS A BIT OF UNCERTAINTY WITH REGARDS TO
HOW FAST THIS WILL HAPPEN WEDNESDAY AFTERNOON... IF THE WARMER AIR
SURGES WESTWARD FASTER (STRONG SE WINDS)... CIGS MAY LIFT TO MVFR
AND GUSTY WINDS OF UP TO 30 TO 35 MPH WILL BE POSSIBLE PRIOR TO 00Z
THURSDAY.
RAIN WILL BECOME WIDESPREAD ACROSS THE SE HALF OF CENTRAL NC
WEDNESDAY MORNING AND OVER THE ENTIRE AREA DURING THE AFTERNOON AS
VERY MOIST AND WARM AIR IS LIFTED OVER THE SHALLOW STABLE LAYER NEAR
THE SURFACE. RAIN MAY BE HEAVY AT TIMES IN VICINITY OF KINT AND KGSO
IN THE AFTERNOON DUE TO THE UPSLOPE COMPONENT OF THE LOW LEVEL WIND.
THE STRONG LOW PRESSURE SYSTEM WILL LIFT RAPIDLY NE ACROSS THE TN
AND OH VALLEYS WEDNESDAY NIGHT. THIS SYSTEM WILL CONTINUE TO SPREAD
MOIST AND SLIGHTLY UNSTABLE AIR INTO CENTRAL NC WED NIGHT. NEAR
SURFACE WINDS WILL BECOME SOUTHERLY AND INCREASE TO AROUND 20 MPH
WITH GUSTS UP TO 35 MPH PROBABLE. THE PRECIP TYPE WILL BECOME MORE
SHOWERY WITH A BAND OF HEAVY SHOWERS AND ISOLATED STRONG/SEVERE
THUNDERSTORMS CROSSING THE REGION BETWEEN 7 PM AND 4 AM. WIND GUSTS
WITHIN THIS LINE MAY EXCEED 60 MPH.
THE LOW PRESSURE SYSTEM WILL EXIT THE REGION EARLY THU. STRONG
SUBSIDENCE ON THE BACK SIDE OF THE LOW WILL INDUCE RAPID CLEARING
FROM SOUTHWEST-TO-NORTHEAST TOWARD DAYBREAK THU. VFR CONDITIONS WILL
RETURN THU WITH BLUSTERY WESTERLY WINDS LIKELY THU MORNING.
VFR CONDITIONS WILL PERSIST THU NIGHT THROUGH FRI NIGHT. A
DEVELOPING AREA OF LOW PRESSURE OFFSHORE SATURDAY WILL SPREAD MVFR
CEILINGS INTO THE VICINITY OF KRDU AND KRWI. OTHERWISE VFR EXPECTED
SATURDAY INTO SUNDAY.
&&
.RAH WATCHES/WARNINGS/ADVISORIES...
FLASH FLOOD WATCH FROM WEDNESDAY AFTERNOON THROUGH WEDNESDAY EVENING
FOR NCZ021>023-038-039-073-074-083-084.
&&
$$
SYNOPSIS...BL
NEAR TERM...BL
SHORT TERM...BL/SMITH
LONG TERM...KRR
AVIATION...KRR/BSD
