Severe Thunderstorm Warning 201, NWS Wakefield, VA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE WAKEFIELD VA
420 PM EDT SUN JUL 12 2009

.SYNOPSIS...
A WEAKENING COLD FRONT DROPS SOUTH OF THE VIRGINIA...NORTH
CAROLINA BORDER TONIGHT. THE FRONT WILL STALL OVER THE CAROLINAS
THROUGH TUESDAY...THEN DISSIPATE WHILE WEAK HIGH PRESSURE BUILDS
OVER THE MID ATLANTIC REGION. ANOTHER COLD FRONT APPROACHES FROM
THE WEST BY LATE WEDNESDAY...THEN WILL SLOWLY MOVE ACROSS THE MID
ATLANTIC REGION THURSDAY AND FRIDAY.

&&

.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
LATEST SFC ANALYSIS SHOWS COLD FRONT ORIENTED WSW-ENE ACROSS
VA/LWR MD/DE. MULTICELLULAR STORMS HAVE FIRED ALONG AND S OF THE
FRONT...WHERE MODERATE INSTBY EXITS (MUCAPES 2000-2500 J/KG... SFC
BASED LIS -3 TO -5C). MORE IMPRESSIVE IS THE DEGREE OF SHEAR...
WITH 45-50 KT WNW 500 MB FLOW NOTED. CONSIDERING THE TIME OF YEAR...
WHERE MORE BARATROPIC AIRMASSES ARE EXPECTED OVER THE SRN MID
ATLC REGION...THIS AMOUNT OF SHEAR IS FAIRLY SIGNIFICANT. 0-6 KM
BULK SHEAR VALUES RANGE FROM 45-50 KTS OVER NRN PTNS OF THE REGION...
35-40 SOUTH. BALANCE OF INSTBY/SHEAR IS SUCH TO SUPPORT ORGANIZED
STRONG TO ISOLD SEVERE CELLS...WITH SOME SUPERCELLS ANTICIPATED AS
WELL (ESP OVER ERN VA AND LWR MD/VA ERN SHORE...GIVEN THE LLVL
EASTERLY COMPONENT VIA SEA/BAY BREEZE).

OTHERWISE...FRONT VERY SLOWLY PUSHES TO SOUTHERN ZONES TONIGHT
INTO ERY MON...AND HV ALLOWED FOR SLGT SHRA CHC OVERNIGHT AS THIS
HAPPENS. SUSPICIOUS OF GFS QPF FORECAST OVER NORTHERN ZONES GIVEN
AFOREMENTIONED CONVECTIVE FEEDBACK ISSUES UPSTREAM...AND AM
INCLINED TO AGAIN SIDE WITH DRIER NAM/SREF SOLN HERE. STAYED
CLOSER TO WARMER ENVELOPE OF MOS GUIDANCE GIVEN SIGNIFICANT SKY
CVR OVER SOUTHERN ZNS. WL LKLY SEE A FEW STRAY LOWS IN THE UPR 50S
ACROSS INTERIOR EASTERN SHORE...BUT GENERALLY LOOKING FOR EARLY
MORNING LOWS MONDAY IN THE 60S NORTH...TO NR 70 SE.

&&

.SHORT TERM /6 AM MONDAY MORNING THROUGH WEDNESDAY/...
GFS/NAM/SREF MODELS CONTINUE SHOW WEAK AREA OF LOW PRESSURE
DEVELOPING ALONG THE STALLED FRONT LATER MON. BASED ON THE TRENDS...
ESPECIALLY WITH THE ENSEMBLES...HAVE BUMPED UP POPS OVER FAR SRN
VA AND ESP ACROSS NERN NC MONDAY AS LLVL MSTR/CONVERGENCE EXPANDS
BACK NORTH OF THE SFC FRONT.

AGAIN THOUGH...ANTICIPATE POPS INCREASING MORE APPRECIABLY MONDAY
NIGHT INTO TUE AS SHORTWAVE ENERGY MOVES THROUGH. FOR NOW HAVE
BUMPED UP POPS BTWN 40-50% OVER THE FAR S/SERN ZONES...20-30% CENTRAL...
AND 10-15% OVER NRN PTNS MON NIGHT-TUE. HIGHS BOTH MON AND TUE AGAIN
MAINLY IN MID/UPR 80S...WITH AROUND 90/LWR 90S EXPECTED ALONG/W OF
I-95 WITH THE WARMER TEMPS ALOFT.

A BIT HOTTER ON WED THE CONTINUED DOWNSLOPE (SW-WSW) LLVL
FLOW...ALONG WITH 850 MB THERMAL RIDGE NUDGING INTO THE FCST AREA
FROM THE WEST. 850 MB TEMPS CLIMBING TO NEAR +18 TO +20C BY LATE
AFTN WOULD SUPPORT HIGHS APPROACHING THE MID 90S OVER WRN PTNS OF
THE FCST AREA (WHERE IT WILL BE WARMEST ALOFT)...UPPER 80S/LWR 90S
ELSEWHERE. IN TERMS OF RAIN CHANCES...IT APPEARS THERE WILL BE
LITTLE IN THE WAY OF SYNOPTIC TRIGGERS...AT LEAST DURING THE DAY
ON WED...WITH THE NEXT FRONT STILL W OF THE FCST AREA. HOWEVER...
GIVEN THE ANTICIPATED FVRBL LLVL INSTBY (THERMODYNAMICS)...COULD
CERTAINLY SEE ACTIVITY DEVELOPING ALONG THE LEE SIDE TROUGH DURING
THE PEAK HEATING HRS OF THE AFTN/EVENING. AS A RESULT HAVE
MAINTAINED AN ISOLATED OR 20% POP AREA-WIDE...WITH BETTER CHANCES
ANTICIPATED WED NIGHT AND ON THU WITH THE PASSAGE OF THE FRONT.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/...
AKQ CWA WILL BE NEAR THE SRN EDGE OF A FAST W/NW FLOW DURING THE
EXTENDED PERIOD. WENT CLOSE TO HPC GUIDANCE AND EURO MODEL. FAST
MOVING LOW PRESSURE SYSTEMS ALONG THE CANADIAN BORDER WILL SEND COLD
FRONTS INTO OUR LATITUDES WHICH WILL TEND TO STALL ROUGHLY NEAR THE
VA/NC BORDER.

HIGH PRESSURE WED WILL MOVE OFF THE COAST AND ALLOW A COLD FRONT
FROM THE UPPER MIDWEST TO MOVE INTO THE AREA THU AND FRI. EURO
SEEMS TO BRING THE FRONT IN A LITTLE QUICKER BUT WENT CLOSE TO HPC
TIMING. EURO SURFACE WINDS INDICATE A RATHER SLOW TRANSITION TO A
CLEAN FRONTAL PASSAGE ACRS VA.

LEESIDE TROUGH MAY GENERATE SCT TS THU. ACTIVITY WILL INCR FRI AND
SAT MAINLY ACRS SRN PORTIONS AS FRONT SETS UP OV SRN PORTIONS. THE
MAJORITY OF THE TS WILL BE DURING THE AFTN AND EARLY EVNG. FINALLY
EURO INDICATES LOW PRESSURE DVLPMT OFF THE COAST AND INCR POPS OV
ERN PORTIONS SUN AND SUN NIGHT.

MAX TEMPERATURES PEAK OUT INTO U80S TO M90S ON THU AHEAD OF THE
FRONT. HIGHS U80S TO N90 FRI AS THE FRONT BEGINS TO CROSS THE AREA.
BEHIND THE FRONT...HIGHS ARE FCST TO COOL INTO THE L TO M80S SAT AND
SUN BEHIND THE FRONT. MIN TEMPS WILL MAINLY RUN IN THE U60S TO
L70S...LWRG TO L-M60S WELL INLAND OVER THE WEEKEND.

&&

.AVIATION /20Z SUNDAY THROUGH FRIDAY/...
TAFS A BIT MORE TRICKY LATE THIS AFTN AND EARLY EVENING AS SHWRS
AND TSTMS TRIGGER ALONG A SLOW MOVING FRONTAL BNDRY. HV DECIDED TO
BEST DEPICT THIS IN THE TAFS BY USING A 3-4 HR TEMPO GROUP...FROM
ROUGHLY 19-23 Z FOR NRN TERMINALS...AND FROM 20-24Z FOR SRN ONES.
(WITH KECG POSSIBLY BEING AFFECTED A LITTLE LATER IN THE EVENING.)
GENERALLY KEPT ALL TERMINALS AT MFVR IN THESE TEMPO GROUPS...
THOUGH IT IS POSSIBLE FOR LOWER CATEGORIES IN ANY OF THE HEAVIER
SHWRS.

OTHER THAN THE POSSIBLE CONVECTION THIS AFTN/EARLY EVENING...
GENLY/PREVAILING VFR CONDS NEXT SVRL DAYS. THIS WKNG FNTL BNDRY
WILL SLIDE ACRS THE RGN LTR THIS EVENING...AND THEN REMAIN JUST
SOUTH OF THE FORECAST AREA THRU TUE...BEFORE RETURNING NWRD AS A
WARM FRONT BY MIDWEEK.

&&

.MARINE...
A FRONTAL BNDRY WILL DROP S OF THE WATERS OVERNIGHT TONIGHT
ALLOWING SCATTERED SHOWERS AND THUNDERSTORMS TO DEVELOP. DO NOT
ANTICIPATE ANY LONG FUSED (SMALL CRAFT) HEADLINES...BUT CERTAINLY
CAN EXPECT SPECIAL MARINE WARNINGS AND MARINE STATEMENTS WITH ANY
OF THE STRONGER THUNDERSTORMS.

WEAK SFC HI PRES MVS OVR THE WTRS LATE TNGT THROUGH
MIDWEEK...RESULTING IN VARYING WND DRCTN...AND AVG SPDS AOB 10 KT.
SSW WNDS BY WED AFTN INTO THU AHD OF NEXT CDFNT...W/ SPDS GRDLY
INCRSG (BUT AT THIS TIME...XPCTD TO RMN BLO SCA MOST PLCS).

&&

.AKQ WATCHES/WARNINGS/ADVISORIES...
MD...NONE.
NC...NONE.
VA...NONE.
MARINE...NONE.

&&

SYNOPSIS...BKH
NEAR TERM...BKH
SHORT TERM...BKH/MAM
LONG TERM...LSA
AVIATION...SMF
MARINE...SMF

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