Flash Flood Warning 40, NWS Baltimore/Washington

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC
449 PM EDT THU JUL 9 2015

.SYNOPSIS...
A COLD FRONT WILL MOVE SOUTHEAST ACROSS THE REGION TONIGHT. THE
FRONT WILL STALL OUT IN NORTH CAROLINA AS HIGH PRESSURE BUILDS IN
OVER OUR AREA FRIDAY. THE FRONT WILL THEN REVERSE AND SLOWLY PUSH
BACK NORTH AS A WARM FRONT EARLY NEXT WEEK.

&&

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
HOISTED A FLASH FLOOD WATCH FOR MUCH OF THE BALTIMORE WASHINGTON
METRO AREAS. MANY OF THESE AREAS HAVE RECEIVED HIGH RAINFALL
TOTALS OVER THE PAST WEEK...LEADING TO VERY LOW 1-HR FFG VALUES.
WHILE SOME SMALL UNCERTAINTY QPF COVERAGE...WITH INCREASING PWATS
AND POTENTIAL FOR STRONG CONVECTION...AWARENESS SHOULD BE
HEIGHTENED FOR THE FLASH FLOOD POTENTIAL.

PREVIOUS DISCUSSION...

POTENT TROUGH MOVING THROUGH LATE THIS AFTERNOON AND EARLY THIS
EVENING ACCOMPANIED BY A SURFACE COLD FRONT AS A WAVE OF LOW
PRESSURE PUSHES EASTWARD TO OUR NORTH. ALREADY A BROKEN LINE OF
THUNDERSTORMS HAS DEVELOPED TO THE WEST AND IS MOVING TOWARDS US.
CAPE IS FAIRLY ROBUST WITH VALUES ABOVE 2K J/KG...SHEAR IN THE
VICINITY OF 30 KNOTS WITH SIGNIFICANT LOW- LEVEL HELICITY...AND A
FAVORABLE JET STREAK POSITION ALOFT. SHOULD HAVE NO TROUBLE
GETTING STORMS INTO THE NORTHERN PORTION OF OUR FORECAST
AREA...WITH DAMAGING WINDS BEING THE PRIMARY THREAT AND AN
ADDITIONAL POSSIBILITY...GIVEN ROBUST LOW LEVEL HELICITY AND
DIRECTIONAL SHEAR...OF AN ISOLATED TORNADO. BEST TIMING FOR STORM
PASSAGE ALONG THE I-95 CORRIDOR IS AROUND 6 PM GIVE OR TAKE AN
HOUR OR TWO. ODDS FAVOR NORTHERN AREAS INCLUDING BALTIMORE...HENCE
THE WATCH NOW IN EFFECT THERE. HOWEVER...SOME RISK EXISTS FURTHER
SOUTH...AND IT IS NOT IMPOSSIBLE THE WATCH COULD GET EXTENDED
FURTHER SOUTH OR THAT WARNINGS MAY BE ISSUED AS FAR SOUTH AS DC
PROPER.

FRONT PASSES ACROSS THE AREA LATE THIS EVENING AND OVERNIGHT WITH
PRECIPITATION ENDING IN MOST AREAS WELL BEFORE MIDNIGHT. UPSLOPE
SHOWERS MAY CONTINUE ALONG THE SPINE OF THE APPALACHIANS THROUGH
DAWN. LOWS WILL REMAIN FAIRLY WARM DESPITE THE FRONTAL PASSAGE.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH SUNDAY/...
FRONT SLIDES SOUTH OF THE AREA ON FRIDAY AND THEN STALLS IN NORTH
CAROLINA. A WAVE OF LOW PRESSURE ASSOCIATED WITH AN UPPER LEVEL
DISTURBANCE IS THEN PROGGED BY SOME GUIDANCE TO RIDE EAST-
SOUTHEAST ALONG THE FRONT ON FRIDAY NIGHT. THIS WOULD RESULT IN
PARTLY SUNNY SKIES FRIDAY...WITH MORE SUN NORTH AND MORE CLOUDS
SOUTH AND WEST...BECOMING CLOUDY FRIDAY NIGHT WITH RAIN
OVERSPREADING PARTS OF THE AREA ESPECIALLY SOUTH AND WEST OF DC.
STILL A LOT OF UNCERTAINTY WITH THIS BUT GUIDANCE SEEMS TO BE
CONVERGING TOWARDS A WETTER SOLUTION. HIGHS FRIDAY WILL BE SEVERAL
DEGREES COOLER THAN TODAY AND HUMIDITY WILL BE LOWER...WHILE LOWS
FRIDAY NIGHT WILL ALSO BE A BIT COOLER THAN TONIGHT.

A SHORTWAVE TROUGH SHOULD BE APPROACHING THE REGION FROM THE OHIO
VALLEY SATURDAY MORNING. MODELS DIFFER WITH THE AMT OF PRECIP
MOVING INTO THE HIGHLANDS BUT THE OVERALL TRACK OF THE SHORTWAVE
TROUGH WILL TAKE SHOWERS ACROSS THE HIGHLANDS...SHENANDOAH VALLEY
AND CENTRAL FOOTHILLS SATURDAY. INSTABILITY IS WEAK AND THEREFORE
STRONG TO SEVERE THUNDERSTORMS ARE NOT EXPECTED AT THIS TIME.

BEHIND THE SHORTWAVE TROUGH...SUBSIDENCE WILL ALLOW CLEARING LATE
SATURDAY NIGHT AND INTO SUNDAY. TEMPERATURES WILL BE NEAR NORMAL
ACROSS THE REGION SATURDAY AND SUNDAY.

&&

.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
HIGH PRES MOVES OFF THE COAST LATE ON SUNDAY. A TROUGH OF LOW
PRESS APPROACHES ON MONDAY AND FRONTAL BOUNDARY WILL LINGER AROUND
THE ARE THROUGH MID WEEK. THERE IS UNCERTAINTY IN TIMING AND
POSITION OF THE FRONT... BUT WARM AND HUMID CONDITIONS... ALONG
WITH FORCING FROM THE FRONT... WILL ALLOW FOR SHOWERS AND TSTMS TO
DEVELOP IN OUR AREA. TEMPS WILL BE NEAR NORMAL FOR THIS PERIOD.

&&

.AVIATION /21Z THURSDAY THROUGH TUESDAY/...
MAIN CONCERN LATE THIS AFTERNOON AND EARLY THIS EVENING IS STRONG
TO SEVERE THUNDERSTORMS MOVING IN FROM THE WEST. BEST ODDS ARE
NORTHERN TERMINALS...MRB...BWI...AND MTN...WITH DECREASING CHANCES
AS ONE GOES SOUTH TOWARDS DCA AND IAD...AND LEAST CHANCE BY FAR
AT CHO. IFR CIGS AND VIS POSSIBLE IN ANY STORMS ALONG WITH WINDS
IN EXCESS OF 40 KNOTS. OUTSIDE OF STORMS...SOUTHWEST WINDS GUSTING
INTO THE 20 KNOT RANGE ALONG WITH VFR CIGS/VIS ARE EXPECTED TO
PREVAIL...WITH WINDS BECOMING NORTHWEST BEHIND THE FRONT LATER
TONIGHT.

VFR CIGS/VIS ON FRIDAY WITH STILL SOME NORTHWESTERLY WIND GUSTS
UP INTO THE 20 KNOT RANGE POSSIBLE. MVFR CIGS/VIS POSSIBLE
ESPECIALLY SOUTH AND WEST OF DC PROPER ON FRIDAY NIGHT INTO
SATURDAY AS ANOTHER WAVE OF LOW PRESSURE POSSIBLY BRINGS SOME RAIN
TO THE AREA.

VFR CONDITIONS EXPECTED FOR SUN AND SUN NIGHT. SUB-VFR CONDITIONS
PSBL FOR MON DUE TO SHOWERS AND TSTMS.

&&

.MARINE...
BIGGEST CONCERN IS FOR GUSTY THUNDERSTORMS MOVING ACROSS THE
WATERS LATE THIS AFTERNOON AND THIS EVENING. SPECIAL MARINE
WARNINGS FOR WINDS ABOVE 40 KNOTS ARE LIKELY IN NORTHERN
WATERS...WITH A DECREASING CHANCE FURTHER SOUTH. OTHERWISE...WINDS
FROM THE SOUTH SHIFTING TO NORTHWEST AS COLD FRONT PASSES LATER
TONIGHT AND FRIDAY WILL BRING SCA LEVEL GUSTS AND SCA IS IN
EFFECT. WIND SHOULD RELAX FRIDAY NIGHT. SUB-SCA WINDS EXPECTED
BEYOND FRIDAY NIGHT.

&&

.TIDES/COASTAL FLOODING...
LOW PRESSURE WILL PASS BY TO OUR NORTH THIS EVENING. THE FLOW
WILL TURN SOUTHERLY AS THIS SYSTEM PASSES BY TO OUR
NORTH...CAUSING TIDAL ANOMALIES TO INCREASE A BIT. HOWEVER...THE
DURATION OF THE SOUTHERLY FLOW SHOULD BE RELATIVELY SHORT...WITH
WINDS TURNING TO THE NORTHWEST BEHIND A COLD FRONT LATER TONIGHT.
THEREFORE...MINOR TIDAL FLOODING IS NOT EXPECTED WITH THE HIGH
TIDE CYCLE TONIGHT. ANOMALIES WILL DECREASE LATE TONIGHT THROUGH
FRIDAY.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...FLASH FLOOD WATCH UNTIL MIDNIGHT EDT TONIGHT FOR DCZ001.
MD...FLASH FLOOD WATCH UNTIL MIDNIGHT EDT TONIGHT FOR MDZ006-011-
     013-014-504-506>508.
VA...FLASH FLOOD WATCH UNTIL MIDNIGHT EDT TONIGHT FOR VAZ052>054.
WV...NONE.
MARINE...FLASH FLOOD WATCH UNTIL MIDNIGHT EDT TONIGHT FOR ANZ535-538.
     SMALL CRAFT ADVISORY FROM MIDNIGHT TONIGHT TO 4 PM EDT FRIDAY
     FOR ANZ530-531-535-536-538>540-542.
     SMALL CRAFT ADVISORY UNTIL 4 PM EDT FRIDAY FOR ANZ532>534-537-
     541-543.

&&

SYNOPSIS...HAS/RCM
NEAR TERM...ADS/RCM
SHORT TERM...HAS/RCM
LONG TERM...IMR
AVIATION...HAS/IMR/RCM
MARINE...HAS/IMR/RCM
TIDES/COASTAL FLOODING...BAJ

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