Flash Flood Warning 79, NWS Baltimore/Washington

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC
315 PM EDT SAT AUG 31 2013

.SYNOPSIS...
HIGH PRESSURE WILL REMAIN OFFSHORE THIS WEEKEND WHILE A SERIES OF
UPPER DISTURBANCES MOVE THROUGH THE AREA. A COLD FRONT WILL MOVE
THROUGH THE REGION MONDAY NIGHT. HIGH PRESSURE RETURNS MIDWEEK.

&&

.NEAR TERM /THROUGH TONIGHT/...
PLENTY OF SUN TODAY W/ THE SHORTWAVE ONLY NOW ENTERING THE WRN
SECTIONS OF THE CWA. THE LEE TROUGH ASSOCIATED W/ THIS FEATURE NOW
CROSSING THE I-81 CORRIDOR. ISOLATED SPOTS ALONG THIS BOUNDARY
HAVE BEEN AND WILL CONTINUE TO DRUM UP ENOUGH LOW-LEVEL
CONVERGENCE FOR CONVECTIVE INITIATION. BUT EVEN THE MORE ORGANIZED
CELLS DOWN ACROSS THE SRN APLCNS - WELL SOUTH AND WEST OF OUR AREA
HAVING TROUBLE MAINTAINING CONTINUITY AS IT MOVES INTO MORE HUMID
AND UNSTABLE AIR. EVEN LESS LOW-LEVEL SHEAR AND FAVORABLE
CONVECTIVE PARAMETERS ACROSS OUR CWA THIS AFTN...THOUGH TEMPS HAVE
STEADILY CLIMBED INTO THE U80S/L90S.

CAPE VALUES IN THE 2-3K RANGE RESIDING OVER THE NRN VA
PIEDMONT...BUT A STABLE BLOCKAGE ZONE SITS NEAR THE VA TIDEWATER
REGION AND TAPERS OFF BACK INLAND TOWARD THE I-95 CORRIDOR. EVEN
DECENT CONVECTIVE ELEMENTS THAT MAKE IT TOWARD THE I-95 CORRIDOR
LATER THIS EVE WILL LIKELY AND QUICKLY DISSIPATE AS THEY REACH THE
MORE STABLE NEAR-COASTAL ENVIRONMENT.

THE NRN SECTION OF THE LEE TROUGH SLIDING OVER THE NWRN QUADRANT
OF THE CWA IS THE NRN BOOKEND OF THE FEATURE. SCATTERED CELLS WILL
CONTINUE TO DEVELOP OVER THE I-68/70 CORRIDOR AND THE MASON-DIXON
LINE REGION AS IT MOVES DUE EAST. ANOTHER SEMI-ORGANIZED BATCH
OVER THE NRN SHEN VLY - TOWARD HARRISONBURG - WILL CONTINUE ON EWD
AS WELL. IN BETWEEN ONLY ISOLATED SHOWERS AND TSTMS EXPECTED THOUGH
MOST OF THE CWA HAS AN OUTSIDE CHANCE OF A TSTM. THOSE W/ OUTDOOR
ACTIVITIES PLANNED...STAY TUNED TO LOCAL TV/RADIO/NWS FORECASTS AS
AN ISOLATED TSTM MAY APPEAR OUT OF THE OVERHEAD CUMULUS FIELD THIS
AFTN/EVE - W/ ANOTHER ROUND EXPECTED TMRW.

SHOWERS AND STORMS SHOULD DIMINISH AFTER SUNSET WITH THE LOSS OF
HEATING. MOST IF NOT ALL OF THIS ACTIVITY SHOULD END BY THE OVNGT AS
THE SHORTWAVE TROUGH MOVES EAST OF THE CHSPK BAY. PATCHY FOG MAY
DEVELOP OVNGT IN AREAS THAT RECEIVE RAIN TDA AND WHERE SKIES CLEAR.

&&

.SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
PARADE OF SHORTWAVES WILL CONTINUE ON SUNDAY...WITH SFC TROUGHING
EXPECTED TO DEVELOP IN THE LEE OF THE BLUE RIDGE. THIS
COMBINATION...ALONG WITH COPIOUS MOISTURE /PWATS APPROACHING TWO
INCHES/ SHOULD BE SUFFICIENT FOR THE DEVELOPMENT OF NUMEROUS
SHOWERS AND THUNDERSTORMS DURING THE AFTERNOON AND EARLY EVENING.
WENT WITH LIKELY POPS IN MOST AREAS EAST OF THE BLUE RIDGE.
CONVECTION MAY PRODUCE LOCALLY HEAVY RAIN...BUT PROPAGATION OF
STORMS IS LIKELY TO PRECLUDE A WIDESPREAD FLOOD THREAT.

ACTIVITY SHOULD WANE WITH SUNSET SUNDAY NIGHT...HOWEVER...MODELS
KEEP A MOIST AIRMASS IN PLACE AND SHOW A SHORTWAVE PASSING NORTH OF
THE AREA. IT IS CLOSE ENOUGH TO KEEP AT LEAST SMALL POPS IN THE
FORECAST ALL NIGHT. WILL DEFINITELY BE A WARM AND HUMID NIGHT.

LABOR DAY IS CONSIDERED THE UNOFFICIAL END OF SUMMER. UNLIKE
MEMORIAL DAY...WHICH FELT ANYTHING BUT SUMMERLIKE...LABOR DAY SHOULD
BE PLENTY HOT AND HUMID WITH GOOD CHANCES FOR SHOWERS AND STORMS AS
SFC TROUGHING REMAINS IN PLACE...AND A COLD FRONT APPROACHES FROM
THE WEST. NOT QUITE CONFIDENT ENOUGH TO CARRY LIKELY POPS YET. IF
THERE ARE BREAKS IN THE CLOUDS...HIGHS COULD REACH THE LOWER
90S...BUT WILL HOLD GRIDS IN THE UPPER 80S GIVEN EXPECTED CLOUDS.
FROPA IS EXPECTED MONDAY NIGHT OVER MOST...IF NOT ALL...OF THE AREA.

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...
ALTHOUGH LINGERING SHOWERS CANNOT BE RULED OUT ON TUESDAY...I
LIMITED CHANCES TO THE FAR SE AND MARINE ZONES. HIGHS 5 TO 10
DEGREES COOLER THAN MONDAY. THE STRENGTH OF THE COOL AIRMASS BEHIND
THE FRONT REMAINS IN QUESTION...AND WILL ERR ON THE WARMER SIDE FOR
TUESDAY AND WEDNESDAY. A REINFORCING FRONT PUSHES THROUGH...LIKELY
DRY...ON THURSDAY. PRECIP CHANCES MAY RETURN BY NEXT WEEKEND WITH
ONSHORE FLOW FORECAST TO DEVELOP.

&&

.AVIATION /18Z SATURDAY THROUGH THURSDAY/...
VFR CONDITIONS TDA...ALTHOUGH A BRIEF REDUCTION IN VSBY POSSIBLE
LATE THIS AFTN AND EVE AS SCT SHRA/TSRA MOVE THRU. THERE IS MORE
UNCERTAINTY IF THIS ACTIVITY MAKES IT FARTHER EAST TOWARD THE I-95
TERMINALS.

PATCHY FOG MAY DEVELOP AGAIN TNGT...ESPECIALLY AT TERMINALS THAT
RECEIVE APPRECIABLE RAINFALL TDA.

SCTD TO NUMEROUS SHRA/TSRA EXPECTED EACH AFTN/EVENING SUNDAY AND
MONDAY. HEAVY RAIN IN SOME CONVECTION COULD CREATE VSBY RESTRICTIONS
AT TIMES. FROPA ON MONDAY NIGHT SHOULD YIELD GENERALLY VFR
CONDITIONS AFTER THAT THROUGH MIDWEEK.

&&

.MARINE...
SLY CHANNELING HAS PRODUCED 20 KT GUSTS IN THE CHSPK BAY. WITH
HIPRES PARKED OFFSHORE...THE PRESSURE GRADIENT WILL REMAIN STRONG
ENOUGH TO YIELD A CONTINUATION OF THE SCA FOR THE CHSPK BAY AND
LOWER TIDAL POTOMAC INTO TNGT.

GUSTY WINDS WILL GRADUALLY SUBSIDE SUNDAY MORNING AS SLY CHANNELING
LESSENS AND WINDS TURN OUT OF THE SOUTHWEST. SCATTERED TO NUMEROUS
SHOWERS AND STORMS ARE EXPECTED SUNDAY AND MONDAY...MAINLY IN THE
AFTERNOON AND EVENING. A FRONTAL PASSAGE MONDAY NIGHT OR TUESDAY
MORNING WILL TURN WINDS OUT OF THE NORTH...BUT THE SYSTEM IS NOT
PARTICULARLY STRONG SO WINDS SHOULD REMAIN GENERALLY LIGHT.

&&

.TIDES/COASTAL FLOODING...
POSITIVE ANOMALIES CONTINUE 1-1.25 FT BUT A GRADUAL DECLINE HAS
BROUGHT THE TRENDS TOWARD JUST BELOW MINOR THRESHOLD IMPACTS AT
HIGH TIDE - UNLESS ANOMALIES INCREASE AGAIN. WILL KEEP THE
POTENTIAL FOR ANOTHER ROUND OF MINOR COASTAL FLOODING W/ TNGT'S
HIGH TIDE CYCLE.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...NONE.
MD...NONE.
VA...NONE.
WV...NONE.
MARINE...SMALL CRAFT ADVISORY UNTIL 11 AM EDT SUNDAY FOR ANZ530>534-537-
     539>541-543.

&&

UPDATE...GMS
PREV DISCUSSION...JRK/JE

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