Flash Flood Warning 167, NWS Baltimore/Washington

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC
1057 AM EDT FRI SEP 23 2011

.SYNOPSIS...

A WEAK COLD FRONT OVER WEST VIRGINIA WILL APPROACH THE AREA TODAY...AND
MOVE INTO THE MID-ATLANTIC REGION TONIGHT AND EARLY SATURDAY. THE
FRONT WILL STALL OUT NEAR THE MID-ATLANTIC COAST OVER THE
WEEKEND. MEANWHILE...AN UPPER LOW WILL REMAIN OVER THE OHIO VALLEY
THROUGH EARLY NEXT WEEK.

&&

.NEAR TERM /THROUGH TONIGHT/...

MDT RAFL HAS MOVED INTO THE I-95 CORRIDOR...THE BEGINNING OF 8-10
HRS OF CONTINUOUS RAFL. ON RGNL RDR/STLT THE THING THAT CONCERNS ME
MOST IS THE COMMA SHAPE OF THE RDR ECHO XTNDG FM NW NC TO NE GA
THAT IS MOVING NNE...AND WL LKLY BE MOVG INTO THE RIC-DC RGN FM
MID AFTN INTO THE EVE HRS. FLSH FLD WTCH WAS EXPANDED SLTLY TO
INCLUDE SPOTSY/FREDICKSBURG.

PRVS DSCN...

A SOAKING RAIN IN STORE FOR TDA...WITH HEAVY RAIN LIKELY EAST OF
THE BLUE RIDGE. THE CUTOFF LOW OVER THE WRN GREAT LAKES WILL
STRENGTHEN WHILE NOT MOVING MUCH TDA. DEEP SLY FLOW WILL ALSO
STRENGTHEN TDA AS THE GRADIENT TIGHTENS BETWEEN THE DEEPENING LOW
AND BUILDING RIDGE OVER THE WRN ATLANTIC. THIS SLY FLOW WILL
DRAWING TROPICAL MOISTURE /PWATS OF 1.75-2.25 INCHES/ UP THE ERN
SEABOARD.

THE COMBINATION OF A STRENGTHENING SLY LLVL JET AND UPPER-LVL JET
STREAK OVER THE GREAT LAKES WILL FURTHER ENHANCE LIFT ACROSS THE
REGION. WITH THE TROPICAL AIRMASS IN PLACE...SHOWERS AND ISOLATED
STORMS WILL BECOME EFFICIENT RAINFALL PRODUCERS. THERE IS STILL
SOME CONCERNING MODEL DIFFERENCES /THIS CLOSE TO THE EVENT/ WITH
REGARDS TO AMOUNTS AND PLACEMENT OF THE AXIS OF HEAVY
RAINFALL...BUT THE DIFFERENCES HAVE DECREASED SOMEWHAT WITH THE
LATEST RUNS. THE MODEL CONSENSUS FOR THE AXIS OF HEAVY RAINFALL IS
NEAR THE I-95 CORRIDOR...WITH W-E VARIABILITY OF THIS AXIS RANGING
FROM JUST EAST OF THE BLUE RIDGE TO THE CHSPK BAY. RAINFALL TOTALS
OF 2-3 INCHES WITH ISOLATED AMOUNTS OF 4-5 INCHES ARE POSSIBLE
WITHIN THIS BAND...WHICH IS ENOUGH TO PRODUCE FLASH FLOODING.

TSTM POTENTIAL WILL BE LIMITED BY CLOUD COVER AND WEAK MID-LEVEL
LAPSE RATES. HOWEVER...IF ANY BREAKS IN THE CLOUDS
DEVELOP...THEN POCKETS OF STRONGER INSTABILITY WOULD SUPPORT A
NON-NEGLIGIBLE WIND THREAT WITH THE GIVEN SHEAR PROFILES.

THE SHORTWAVE TROUGH RESPONSIBLE FOR TDA/S HEAVY RAIN WILL EXIT THE
AREA DURING THE MID EVE HRS. ANY LEFTOVER SHOWERS SHOULD DECREASE IN
COVERAGE/INTENSITY AS SYNOPTIC LIFT AND INSTABILITY WANES. THE
BOUNDARY LAYER WILL REMAIN MOIST...SUPPORTING THE DEVELOPMENT OF LOW
CLOUDS AND PATCHY FOG LATE TNGT.

&&

.SHORT TERM /SATURDAY THROUGH SUNDAY/...

UPPER LOW REMAINS CENTERED OVER THE MIDWEST NEAR CHICAGO OVER THE
WEEKEND. AT THE SURFACE...FRONTAL BOUNDARY MOVES ACROSS THE
CHESAPEAKE BAY REGION EARLY SATURDAY...THEN STALLS OUT ALONG THE
MID ATLANTIC COAST THROUGH THE WEEKEND. CWA REMAINS IN A JUICY AIR
MASS OVER THE WEEKEND...WITH HIGH PRECIPITABLE WATERS. WITH CLOSE
PROXIMITY TO FRONTAL BOUNDARY AND SHORTWAVES TO THE EAST OF THE
UPPER LOW...UNSETTLED CONDITIONS WILL REMAIN. POPS WILL BE HIGHEST
ACROSS THE EASTERN PORTION OF THE CWA.

FOR TEMPERATURES...CONTINUED WITH THE IDEA OF BELOW MOS HIGHS AND
NEAR/ABOVE MOS LOWS.

&&

.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...

MODELS INDICATE THAT THE UPPER LOW OVER THE MIDWEST WILL BEGIN TO
LIFT NORTHEAST TOWARD THE GREAT LAKES EARLY NEXT WEEK. THIS WOULD
DRAG A NEW COLD FRONT THROUGH THE CWA LATE MONDAY NIGHT INTO
TUESDAY. UNTIL THAT HAPPENS...THERE WILL CONTINUE TO BE A CHANCE OF
SHOWERS IN THE FORECAST. HIGH PRESSURE AND DRIER AIR WILL MOVE INTO
THE AREA DURING THE MID WEEK IF THE UPPER LOW/COLD FRONT MOVE AS
EXPECTED DURING THE EARLY PART OF THE WEEK.

&&

.AVIATION /15Z FRIDAY THROUGH TUESDAY/...

IFR CONDITIONS WILL CONTINUE AT ALL TAF SITES DUE TO LOW CLOUDS
AND FOG. MODERATE TO HEAVY RAIN WILL REDUCE VSBYS FROM THE MIDDAY
HRS THRU PART OF THE EVE...ESPECIALLY AT DCA/BWI/MTN.

BULK OF THE RAINFALL SHOULD BEGIN TO EXIT THE AREA FROM SW TO NE
DURING THE EVE. IN WAKE OF THE SHOWERS...CIGS/VSBYS WILL LIKELY
LOWER TO IFR LVLS AGAIN LATE THIS EVE AND OVNGT AS THE LLVLS REMAIN
SATURATED AND WINDS BECOME CALM. IF BREAKS IN THE CLOUDS DEVELOP
LATE TNGT...THEN LIFR VSBYS MAY OCCUR.

IFR CONDITIONS ARE LIKELY OVERNIGHT INTO SATURDAY MORNING. PER
SREFS/NAM...IMPROVEMENT TO AT LEAST MVFR MAY OCCUR SATURDAY
AFTERNOON BEFORE CONDITIONS DEGRADE AGAIN SATURDAY NIGHT. LATEST GFS
GUIDANCE KEEPS CIGS IN IFR THROUGH SUNDAY NIGHT. WHILE THIS SOLUTION
CANT BE RULED OUT...WILL LEAN TOWARD THE SREFS/NAM SOLUTIONS ATTM.
FRONTAL BOUNDARY WILL RESIDE ALONG THE MID ATLANTIC COAST...WITH
SHORTWAVES ALSO PROMOTING SHOWERS THROUGH THE WEEKEND. COLD FRONT IS
FORECAST TO MOVE THROUGH FROM THE WEST LATE MONDAY NIGHT INTO
TUESDAY.

&&

.MARINE...

SLY FLOW WILL INCREASE TDA...BUT REMAIN BELOW SCA LVLS THRU TNGT.
MAIN IMPACT ON THE WATERS LATER THIS MRNG THRU MID EVE WILL BE LOW
VISIBILITY DURING PERIODS OF HEAVY RAINFALL. ISO TSTMS WILL ALSO BE
POSSIBLE...ESPECIALLY ACROSS SRN MARINE ZONES.

FRONTAL BOUNDARY STALLS ALONG THE MID ATLANTIC COAST OVER THE
WEEKEND. SHOWERS WILL PERIODICALLY REDUCE VSBYS TO 1 TO 3
NM...ALTHOUGH WINDS ARE FORECAST TO GENERALLY REMAIN AOB 10 KT DUE
TO WEAK GRADIENT. COLD FRONT MOVES THROUGH FROM THE WEST BY TUESDAY.

&&

.TIDES/COASTAL FLOODING...

WATER LEVELS MAY APPROACH MINOR COASTAL FLOODING THRESHOLDS BY
THE HIGH TIDE CYCLE LATE TNGT. LATER SHIFTS MAY NEED TO ISSUE A
COASTAL FLOOD ADVISORY.

WITH LOPRES NEAR THE BAY AND PERSISTENT SE FLOW...THE THREAT OF
MINOR COASTAL FLOODING WILL CONTINUE INTO SAT AND SUN.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...FLASH FLOOD WATCH UNTIL 10 PM EDT THIS EVENING FOR DCZ001.
MD...FLASH FLOOD WATCH UNTIL 10 PM EDT THIS EVENING FOR MDZ004>007-
     009>011-013-014-016>018.
VA...FLASH FLOOD WATCH UNTIL 10 PM EDT THIS EVENING FOR VAZ042-
     052>057.
WV...NONE.
MARINE...NONE.

&&

PRODUCTS...WOODY!
PRVS...JRK/BPP

HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC
923 AM EDT FRI SEP 23 2011

ANZ530>543-DCZ001-MDZ006-007-011-013-014-016>018-VAZ052>057-
241030-
CHESAPEAKE BAY NORTH OF POOLES ISLAND MD-
CHESAPEAKE BAY FROM POOLES ISLAND TO SANDY POINT MD-
CHESAPEAKE BAY FROM SANDY POINT TO NORTH BEACH MD-
CHESAPEAKE BAY FROM NORTH BEACH TO DRUM POINT MD-
CHESAPEAKE BAY FROM DRUM POINT MD TO SMITH POINT VA-
TIDAL POTOMAC FROM KEY BRIDGE TO INDIAN HEAD MD-
TIDAL POTOMAC FROM INDIAN HEAD TO COBB ISLAND MD-
TIDAL POTOMAC FROM COBB ISLAND MD TO SMITH POINT VA-
PATAPSCO RIVER INCLUDING BALTIMORE HARBOR-
CHESTER RIVER TO QUEENSTOWN MD-EASTERN BAY-
CHOPTANK RIVER TO CAMBRIDGE MD AND THE LITTLE CHOPTANK RIVER-
PATUXENT RIVER TO BROOMES ISLAND MD-
TANGIER SOUND AND THE INLAND WATERS SURROUNDING BLOODSWORTH
ISLAND-DISTRICT OF COLUMBIA-NORTHERN BALTIMORE-HARFORD-
SOUTHERN BALTIMORE-PRINCE GEORGES-ANNE ARUNDEL-CHARLES-ST. MARYS-
CALVERT-PRINCE WILLIAM/MANASSAS/MANASSAS PARK-FAIRFAX-
ARLINGTON/FALLS CHURCH/ALEXANDRIA-STAFFORD-SPOTSYLVANIA-
KING GEORGE-
923 AM EDT FRI SEP 23 2011

THIS HAZARDOUS WEATHER OUTLOOK IS FOR THE MARYLAND PORTION OF THE
CHESAPEAKE BAY...TIDAL POTOMAC RIVER...AND ADJACENT COUNTIES IN
CENTRAL MARYLAND AND NORTHERN VIRGINIA AS WELL AS THE DISTRICT OF
COLUMBIA.

.DAY ONE...TODAY AND TONIGHT

A FLASH FLOOD WATCH IS IN EFFECT FOR THE OUTLOOK AREA THROUGH
THIS EVENING. HEAVY RAINFALL COULD CAUSE FLASH FLOODING. PLEASE
REFER TO THE LATEST HAZARD MESSAGE /FFALWX/ FOR FURTHER DETAILS.

SOUTHERLY FLOW MAY CAUSE MINOR TIDAL FLOODING DURING HIGH TIDE
BEGINNING THIS EVENING ALONG THE WESTERN SHORE OF THE MARYLAND
CHESAPEAKE BAY AND TIDAL POTOMAC RIVER.

.DAYS TWO THROUGH SEVEN...SATURDAY THROUGH THURSDAY

MINOR COASTAL FLOODING WILL REMAIN POSSIBLE DURING TIMES OF HIGH
TIDE OVER THE WEEKEND ALONG PORTIONS OF THE TIDAL POTOMAC RIVER
AND WESTERN SHORE OF THE MARYLAND CHESAPEAKE BAY.

.SPOTTER INFORMATION STATEMENT...

DUE TO THE ISOLATED NATURE OF THE SEVERE WEATHER THREAT...SPOTTER
ACTIVATION IS NOT EXPECTED AT THIS TIME. PLEASE RELAY ANY REPORTS
OF HEAVY RAINFALL AND FLASH FLOODING TO THE NATIONAL WEATHER
SERVICE.

MDZ003>005-009-010-501-502-VAZ025>031-036>040-042-050-051-501>504-
WVZ050>053-055-501>506-241030-
WASHINGTON-FREDERICK MD-CARROLL-MONTGOMERY-HOWARD-
EXTREME WESTERN ALLEGANY-CENTRAL AND EASTERN ALLEGANY-AUGUSTA-
ROCKINGHAM-SHENANDOAH-FREDERICK VA-PAGE-WARREN-CLARKE-NELSON-
ALBEMARLE-GREENE-MADISON-RAPPAHANNOCK-LOUDOUN-ORANGE-CULPEPER-
NORTHERN FAUQUIER-SOUTHERN FAUQUIER-WESTERN HIGHLAND-
EASTERN HIGHLAND-HAMPSHIRE-MORGAN-BERKELEY-JEFFERSON-HARDY-
WESTERN GRANT-EASTERN GRANT-WESTERN MINERAL-EASTERN MINERAL-
WESTERN PENDLETON-EASTERN PENDLETON-
923 AM EDT FRI SEP 23 2011

THIS HAZARDOUS WEATHER OUTLOOK IS FOR EASTERN WEST VIRGINIA...
NORTHERN AND CENTRAL VIRGINIA...AND CENTRAL AND WESTERN MARYLAND
WITH THE EXCEPTION OF GARRETT COUNTY.

.DAY ONE...TODAY AND TONIGHT

A FLASH FLOOD WATCH IS IN EFFECT THROUGH THIS EVENING FOR PORTIONS
OF NORTHERN VIRGINIA. HEAVY RAINFALL COULD CAUSE FLASH FLOODING.
PLEASE REFER TO THE LATEST HAZARD MESSAGE /FFALWX/ FOR FURTHER
DETAILS.

ISOLATED THUNDERSTORMS MAY ALSO PRODUCE LOCALLY DAMAGING WIND
GUSTS THIS AFTERNOON.

.DAYS TWO THROUGH SEVEN...SATURDAY THROUGH THURSDAY

.SPOTTER INFORMATION STATEMENT...

DUE TO THE ISOLATED NATURE OF THE SEVERE WEATHER THREAT...SPOTTER
ACTIVATION IS NOT EXPECTED AT THIS TIME. PLEASE RELAY ANY REPORTS
OF HEAVY RAINFALL AND FLASH FLOODING TO THE NATIONAL WEATHER
SERVICE.

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