Severe Thunderstorm Warning 172, NWS Raleigh, NC

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC
318 PM EDT FRI JUL 17 2009

.SYNOPSIS...
A COLD FRONT WILL APPROACH FROM THE NORTHWEST THIS EVENING AND CROSS
CENTRAL NORTH CAROLINA OVERNIGHT. WEAK HIGH PRESSURE WILL BUILD INTO
THE AREA SATURDAY... AND DRIFT EAST ON SUNDAY.

&&

.NEAR TERM /THROUGH TONIGHT/...
AS OF 251 PM FRIDAY...

MOIST AND MODERATELY UNSTABLE AIR MASS COVERS ROUGHLY THE EASTERN
HALF OF THE FORECAST AREA WHILE MID LEVEL WINDS HAVE INCREASED IN
RESPONSE TO UPPER LEVEL JET ROTATING THROUGH LOWER OH VALLEY. IN
ADDITION...MID LEVEL TROUGH LIFTING E-NE ACROSS THE SOUTHERN
APPALACHIANS. ALSO...DECENT STRONG LOW LEVEL THETA-E ADVECTION
OCCURRING OVER THE COASTAL PLAIN. THUS...APPEARS BEST PARAMETERS FOR
SEVERE STORMS WILL LIE ALONG AND EAST OF HIGHWAY 1. FARTHER
WEST...LOW LEVEL FLOW HAS VEERED TO A WESTERLY DIRECTION...ADVECTING
SLIGHTLY MORE STABLE AIR INTO THE LOWER LAYERS. STILL CANNOT RULE
OUT AN ISOLATED SEVERE STORM IN THE WESTERN PIEDMONT THROUGH 21Z.

BEST UPPER LEVEL SUPPORT LIFT WELL NE OF CENTRAL NC AFTER 00Z.
THUS...EXPECT MOST STORMS TO DIMINISH AFTER 00Z WITH ANY CONVECTION
ISOLATED AND CONFINED TO FAR EASTERN PERIPHERY BY 02Z. SURFACE FRONT
DOESNT CROSS REGION UNTIL 06Z-12Z. DUE TO LATER FROPA...MIN TEMPS
MAY END UP BEING A FEW DEGREES WARMER DUE TO MIXING.

&&

.SHORT TERM /SATURDAY THROUGH SATURDAY NIGHT/...
AS OF 306 PM...

SURFACE FRONT WILL SLOW OVER THE COASTAL PLAIN BUT DEEP WESTERLY
FLOW FLOW WILL ADVECT DRY...STABLE AIR OVER THE REGION. RESIDUAL
MOISTURE IN THE LOWEST 8000 FT WILL RESULT IN FEW TO SCATTERED CU
INT EH LATE MORNING THROUGH AFTERNOON HOURS BUT MOISTURE DOES NOT
APPEAR DEEP ENOUGH NOR LIFT SUFFICIENT ENOUGH TO GENERATE SHOWERS.
THUS WILL REMOVE SLIGHT CHANCE POPS FROM THE COASTAL PLAIN. MOS
GUIDANCE TEMPS OF MID 80S NW TO 90 SE AGREE WITH LOW LEVEL
THICKNESSES.

SATURDAY NIGHT...WEAK SURFACE HIGH OVERHEAD BY SW FLOW ALOFT MAY
ADVECT ENOUGH MOISTURE TO RESULT IN AREA OF CIRRUS. IF SKIES CLEAR
OFF ENOUGH...CALM WINDS AND RELATIVELY DRY AIR NEAR THE SURFACE WILL
RESULT IN MIN TEMPS 5-7 DEGREES BELOW NORMAL.

&&

.LONG TERM /SUNDAY THROUGH THURSDAY/...
AS OF 100 PM FRIDAY...

WHILE THE NAM AND EVEN THE GFS ARE PERSISTENT WITH KEEPING OVERALL
CONDITIONS DRY SUNDAY A DIGGING UPPER TROUGH APPROACHING FROM THE
WEST AND PRECIPITABLE WATER VALUES OF 1.5 INCHES COULD SUPPORT AN
ISOLATED SHOWER OR THUNDERSTORM MAINLY IN THE EAST AND SOUTHEAST BUT
ACTIVITY IF ANY SHOULD BE VERY LIMITED. WILL KEEP POPS OUT OF THE
FORECAST FOR THURSDAY AFTERNOON BUT WILL KEEP A SLIGHT CHANCE IN
FOR SUNDAY NIGHT INTO MONDAY MORNING AS THE UPPER TROUGH
APPROACHES AND CONDITIONS DESTABILIZE A LITTLE. SEASONABLE HIGHS
IN THE MID 80S STILL LOOK ON TRACK.

SUNDAY NIGHT...THE NAM IS MORE AGGRESSIVE THAN THE GFS AND ECMWF
REGARDING THE QPF AND LIFT SUNDAY NIGHT BUT ALL MODELS INCLUDING
THE ECMWF INCREASE THE RAIN CHANCES AND QPF BY MONDAY. FOR NOW
WILL KEEP JUST SLIGHT CHANCE POPS IN THE FORECAST FOR SUNDAY NIGHT
INCREASING RAIN CHANCES BY MONDAY MORNING. TIMING OF THE NEXT
SYSTEM COULD CAUSE POPS TO INCREASE SUNDAY NIGHT.

UNSETTLED WEATHER PATTERN SHOULD BE IN PLACE FOR EARLY NEXT WEEK
AS THE UPPER TROUGH DIGS SOUTH AND EVEN BECOMES NEGATIVELY TILTED
ON THE NAM AND ECMWF BY LATE MONDAY. THE UPPER LEVELS WILL BE
VERY SUPPORTIVE AS THE REGION FALLS INTO THE RIGHT ENTRANCE REGION
OF A NORTH TO SOUTH ORIENTED 85+KT 250MB JET. THIS WILL PROVIDE
ENHANCE UPPER DIVERGENCE. ALTHOUGH THERE ARE EXACT TIMING AND
STRENGTH DIFFERENCES BETWEEN THE GFS AND THE ECMWF IN THE MID-
LEVEL SHORTWAVES WE CAN EXPECT SCATTERED SHOWERS AND THUNDERSTORMS
MONDAY THROUGH WEDNESDAY NIGHT. A WEAK COASTAL TROUGH OR FRONTAL
ZONE APPEARS TO MOVE INLAND AND SHOULD PROVIDE A FOCUS FOR SHOWERS
AND STORMS WITH SUPPORT FROM THE UPPER TROUGH JUST TO THE WEST.
SHORTWAVES ALOFT...PRESENCE OF THE FRONT...AND A PERSISTENT FETCH
OFF OF THE ATLANTIC ALL RESULT IN CHANCES FOR SHOWERS AND
THUNDERSTORMS MONDAY INTO WEDNESDAY AND MOST LOCATIONS ACROSS THE
AREA SHOULD RECEIVE PERIODIC RAINFALL. THE ECMWF REMAINS STRONGER
THAN THE GFS WITH A NEGATIVELY-TILTED UPPER TROUGH MOVING THROUGH
THE MID ATLANTIC LATE MONDAY INTO TUESDAY...AND IF THAT
VERIFIES...IT IS QUITE POSSIBLE CHANCES FOR SHOWERS AND
THUNDERSTORMS WILL NEED TO BE RAISED INTO AT LEAST THE LIKELY
CATEGORY FOR A PERIOD OR TWO FROM MONDAY AFTERNOON THROUGH
TUESDAY.

CONTRAST BETWEEN THE GFS AND ECMWF INCREASES DURING THE
LATTER PART OF THIS PERIOD. WHILE THE GFS BUILDS HEIGHTS WEST FROM
THE ATLANTIC ACROSS THE CAROLINAS DURING THE MIDDLE OF THE
WEEK...THE ECMWF SUPPORTS MORE OF A PATTERN INDICATIVE OF RECENT
DAYS WITH A RIDGE IN THE WEST AND A TROUGH ALOFT CLOSE TO THE
ATLANTIC SEABOARD. THE RESULT BEING CONSISTENCY WITH THE CURRENT
EXTENDED FORECAST SHOWING GENERALLY HIGHER CHANCES FOR SHOWERS
WEDNESDAY AND THURSDAY EAST VERSUS WEST. BASED ON EXPECTED CLOUD
COVER AND MOISTURE...HIGH TEMPERATURES SHOULD BE AT OR SLIGHTLY
BELOW NORMAL... WITH LOWS AT OR SLIGHTLY ABOVE NORMAL.

&&

.AVIATION /18Z FRIDAY THROUGH WEDNESDAY/...
AS OF 312 PM FRIDAY...

SCATTERED CONVECTION WILL THREATEN KRDU...KRWI AND KFAY...ENDING
FROM WEST TO EAST AFTER 22Z. BREEZY SW WINDS THIS AFTERNOON WILL
DIMINISH WITH LOSS OF INSOLATION. SURFACE FRONT WILL DRIFT EAST
ACROSS REGION BETWEEN 04Z-11Z. WINDS WILL VEER TO W-NW AFTER FROPA
BUT WINDS SPEEDS GENERALLY LESS THAN 6 KTS.

WEAK HIGH PRESSURE WILL BUILD INTO REGION SATURDAY AND SETTLE
OVERHEAD SATURDAY NIGHT. RESIDUAL MOISTURE BELOW 800- FT WILL RESULT
IN SCATTERED CU BETWEEN 14Z-23Z SATURDAY. HOWEVER...MOISTURE DOESNT
APPEAR DEEP ENOUGH TO SUPPORT DEVELOPMENT OF SHOWERS.

VFR CONDITIONS EXPECTED TO PERSIST THROUGH SUNDAY NIGHT. PERIODS OF
MVFR CONDITIONS MAY OCCUR MONDAY THROUGH WEDNESDAY...MAINLY DUE TO
CEILINGS AND FOG IN THE EARLY MORNING...AND SCATTERED CONVECTION IN
THE AFTERNOON AND EARLY EVENING.

&&

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

&&

SYNOPSIS...WSS
NEAR TERM...WSS
SHORT TERM...WSS
LONG TERM...JO
AVIATION...WSS

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