Severe Thunderstorm Warning 11, NWS Blacksburg, VA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BLACKSBURG VA
313 PM EDT MON APR 20 2009

.SYNOPSIS...
STRONG LOW OVER THE GREAT LAKES WILL SEND A COLD FRONT THROUGH THE
AREA THROUGH EARLY EVENING. THIS SLOW MOVING LOW IS GOING TO
MEANDER AROUND THE GREAT LAKES THROUGH TUESDAY...WITH HIGH
PRESSURE FINALLY SLIDING IN TOWARD THURSDAY.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
AT 15 UTC WARM FRONT WAS LIFTING NORTH OF LYH-FVX...WITH COOL
STABLE AIR TO THE NORTH...AND A WARM MORE UNSTABLE AIRMASS TO THE
SOUTH. SKIES ARE CLEARING SOUTH OF THE WARM FRONT AND AHEAD OF THE
APPROACHING COLD FRONT. SURFACE DEWPOINTS ARE CLIMBING...U50 TO
L60 READINGS ADVECTING NORTH FROM THE CAROLINAS. TEMPERATURES IN
THE WARM SECTOR ARE CLIMING INTO THE 70S.

AS UPPER SHORT WAVE TROUGH PHASES WITH SURFACE FRONTS THIS
AFTERNOON...ANTICIPATE INCREASED COVERAGE OF SHOWERS AND
THUNDERSTORMS. LOW LEVEL CONVERGENCE IS INCREASING ALONG AND EAST
OF THE BLUE RIDGE...WITH THE MOST UNSTABLE AIR...LI M3 TO M6...FROM
SOUTHSIDE VA INTO EASTERN VA...AND SUPPORTED BY CAPES OF ABOUT
1000 J/KG.

LOOKING AT OUR LOCAL RADAR (18 UTC) IT WOULD APPEAR THIS LOW
LEVEL CONVERGENCE ZONE IS ALREADY BECOMING ESTABLISHED...ASSOCIATED
WITH BKN BAND OF SHOWERS ALONG THE BLUE RIDGE PARKWAY. OTHER LINES
OF CU AND TCU IN THE WARM SECTOR ARE SHOWING BETTER DEVELOPMENT
AND WILL LIKELY SHOW FURTHER VERTICAL DEVELOPMENT AS WE TRANSITION
THROUGH THE AFTERNOON.

THERE IS A SLIGHT RISK OF SEVERE WEATHER TODAY. THE ENVIRONMENT
WILL BE MODERATELY UNSTABLE SUPPORTED BY A DEEP LAYER OF SPEED
SHEAR. LOOK OUT FOR STRONG DAMAGING WINDS/HAIL. A MESOSCALE
DISCUSSION WAS ISSUED BY THE STORM PREDICITION CENTER AT 18 UTC
FOR OUR FAR EASTERN CWA...DANVILLE AND LYNCHBURG EASTWARD...
CONCERNING THE SEVERE THUNDERSTORM THREAT. THE CONCERN IS FOR THE
POTENTIAL DEVELOPMENT OF SUPERCELLS WITHIN THE WARM SECTOR. IN
OTHER WORDS...TORNADOES ARE ALSO POSSIBLE...THUS WARRANTING THE
ISSUANCE OF A TORNADO WATCH...#178.

THE FRONT IS EXPECTED TO CLEAR THE CWA BY 00 UTC WITH
ANY SIGNIFICANT ACTIVITY EAST OF THE CWA BY 800 PM. UPSLOPE
SHOWERS WILL PERSIST IN THE MOUNTAINS...BUT SHOULD ALSO WANE
SHORTLY AFTER MIDNIGHT IF NOT BEFORE AS THERE IS SHORT WAVE
RIDGING OVER THE MTNS BY 06Z AND LOW LEVEL FLOW BACKS TO THE WEST-
SOUTHWEST. HAVE STAYED ON THE WARM SIDE OF GUIDANCE AS IT LOOKS
LIKE THE MUCH COOLER AIRMASS DOES NOT MOVE IN UNTIL TUESDAY.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH THURSDAY NIGHT/...
GOING WITH A BLEND OF THE 00Z ECMWF/12Z GFS THIS PERIOD. LOOKING
AT UPPER AND SFC LOW TO MEANDER AROUND THE GREAT LAKES REGION
THROUGH WEDNESDAY BEFORE LIFTING EASTWARD WED NIGHT. THE COOL
POCKET WILL MEANDER OVERHEAD TUESDAY AND AGAIN WEDNESDAY WITH
SHOWERS EXPECTED...ESPECIALLY OVER THE NRN/WRN CWA. AS THE COLD
POOL MOVES ACROSS TUESDAY AFTERNOON...SOME THUNDERSTORMS COULD
DEVELOP EAST OF THE BLUE RIDGE...AND COULD PRODUCE SOME GUSTY
WINDS AND SMALL HAIL AS THE LOW LVLS WILL BE DRYING OUT ON
WESTERLY FLOW.

THE SHOWER ACTIVITY WILL DIMINISH IN THE EAST TUESDAY NIGHT AS ONE
THE WESTERLY FLOW DEEPENS AND LOSS OF HEATING TAKES PLACE.
HOWEVER...NW TO WEST FLOW AND ANOTHER SERIES OF DISTURBANCES WILL
KEEP COLDER AIR FLOWING IN AND SHOWERS CONTINUING IN THE MTNS
TUESDAY NIGHT. ATTM...SNOW SHOWERS WILL BE CONFINED TO HIGHER
ELEVATIONS AND WITH WARM GROUND TEMPERATURES...NOT EXPECTING MUCH
ACCUMULATION...EXCEPT ONE INCH OR SO IN WRN GREENBRIER BY 12Z WED.

FINAL SHORWAVE PASSES THROUGH WEDNESDAY AND WILL SEE SHOWERS
SCATTERED ACROSS THE FAR NW/WRN MTNS...WITH ISOLATED COVERAGE TO
THE BLUE RIDGE.

HIGH PRESSURE NOSES IN WED NIGHT-THU WITH DRY WX RETURNING TO THE
MTNS. AIRMASS REMAINS COOLER THAN NORMAL TUE-WED...WITH MODERATING
TEMPS ON THURSDAY BACK TO NEAR SEASONAL NORMS.

&&

.LONG TERM /FRIDAY THROUGH MONDAY/...
FORECAST THIS PERIOD WILL FOLLOW THE ECMWF MODEL...AS THE TROUGH
OVER THE EAST IS REPLACED BY A RIDGE...AND WE ARE EXPECTING MAINLY
DRY AND WARM WEATHER FOR THE WEEKEND INTO MONDAY. THE ONLY WEATHER
FEATURE THAT COULD CREATE AN ISSUE WOULD BE A BACKDOOR FRONT
ARRIVING IN THE LATE WEEKEND-MONDAY TIME FRAME. TIMING BY THE
MODELS KEEPS THIS FEATURE BRUSHING THROUGH BY LATE MONDAY AND OUR
AREA REMAINING DRY...AS THE UPPER RIDGE STAYS OVER THE TN
VALLEY/GULF COAST.

&&

.AVIATION /19Z MONDAY THROUGH SATURDAY/...
SURFACE COLD FRONT WILL MOVE ACROSS ALL TAF SITES BETWEEN 18Z-24Z
THIS AFTERNOON/EVENING. THREAT EXISTS FOR THUNDERSTORMS ALONG AND
EAST OF THE BLUE RIDGE. THERE IS THE POTENTIAL FOR SEVERE
THUNDERSTORMS AT BOTH LYNCHBURG AND DANVILLE...THREAT ENDING AT
SUNSET. ASIDE FROM THE CONVECTIVE THREAT...MVFR CONDITIONS WILL
PERSIST INTO THE OVERNIGHT DUE TO LOW CIGS. FOG IS ALSO EXPECTED
TO FORM EAST OF THE MOUNTAINS WITH POTENTIAL FOR IFR CONDITIONS
EARLY TUESDAY MORNING.

BEHIND THE FRONT ON TUESDAY AND WEDNESDAY A DEEP UPPER TROF AND
RETURN TO UPSLOPE FLOW IS LIKELY TO BRING MVFR CONDITIONS ACROSS
THE MOUNTAINS...MAINLY EFFECTING THE SOUTHEAST WEST VIRGINIA
SITES. SKIES SHOULD CLEAR EAST OF THE MOUNTAINS WITH WITH MAINLY
VFR THERE. HIGH PRESSURE BUILDS IN FROM THE SOUTHWEST THURSDAY
RESULTING IN A RETURN TO MAINLY VFR FOR WEEKS END.

&&

.RNK WATCHES/WARNINGS/ADVISORIES...
VA...NONE.
NC...NONE.
WV...NONE.

&&

SYNOPSIS...WP
NEAR TERM...PM
SHORT TERM...WP
LONG TERM...WP
AVIATION...PM

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