Flash Flood Warning 55, NWS Baltimore/Washington

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC
325 PM EDT THU SEP 30 2010

.SYNOPSIS...
LOW PRESSURE NORTHWEST OF WASHINGTON DC WILL MOVE ACROSS THE AREA
THIS AFTERNOON AND INTO NEW YORK STATE TONIGHT. DEEP TROPICAL
MOISTURE WILL BE DRAWN INTO THIS SYSTEM...BRINGING HEAVY RAIN TO
AREAS ALONG THE WESTERN SHORE OF THE CHESAPEAKE BAY THIS AFTERNOON
INTO TONIGHT. HIGH PRESSURE WILL BUILD INTO THE REGION DURING THE
WEEKEND. A COLD FRONT MAY PASS THROUGH SUNDAY MORNING.
&&

.NEAR TERM /THROUGH FRIDAY/...
TORNADO WATCH REMAINS IN EFFECT THRU 7 PM FOR NE AND SRN MD. HIGHEST
INSTABILITY...LLVL SHEAR AND HELICITY VALUES ARE EXPECTED TO MOVE NE
TOWARD THE DELMARVA/NJ DURING THE LATE AFTN.

MAIN CONCERN THRU THE NEAR TERM IS THE ONGOING FLOODING
SITUATION...ESPECIALLY TOWARD THE WRN SHORE OF THE CHSPK BAY. RADAR
ESTIMATES AND PRECIP OBS SHOWN RAINFALL TOTALS BETWEEN 6-9 INCHES IN
THIS AREA. MOSAIC RADAR AND IR/WV SAT ANALYSIS SHOW WHILE THE
HEAVIEST PRECIP HAS MOVED NORTH AND EAST OF THE AREA AS AN UPPER DRY
SLOT WORKED IN FROM THE SOUTHWEST...HEAVY PRECIP/CONVECTION IS
RELOADING OFF THE SOUTHEAST COAST ERN CAROLINA. AN INCREDIBLY
MERIDIONAL STREAM OF CARIBBEAN MOISTURE UP THE EAST COAST SHOWS
THAT ERN PARTS OF OUR CWFA ARE NOT OUT OF THE WOODS YET. LATEST
MODEL GUIDANCE CONTINUES TO SHOW ANOTHER WAVE OF LOPRES MOVING UP
THE MID-ATLANTIC COAST THIS EVE INTO THE OVNGT HRS. THERE WILL BE A
SHARP WRN CUTOFF BETWEEN LIGHT PRECIP AND ANOTHER BAND OF HEAVY
PRECIP. THEREFORE...ANY SLIGHT SHIFT BETWEEN THE BACK EDGE OF PRECIP
WILL MAKE A HUGE DIFFERENCE WITH ADDITIONAL PRECIP ACCUMULATIONS
TNGT. CURRENT THINKING IS THE WRN SHORE IS UNDER THE GUN AGAIN WITH
AN ADDITIONAL 1-3 INCHES POSSIBLE WITH THIS ACTIVITY...WHILE AREAS
FARTHER TO THE WEST WILL RECEIVE MUCH LESS.

UPPER TROUGH AXIS WILL PROGRESS THRU THE AREA FRI. NW FLOW
BEHIND THE TROUGH AXIS WILL USHER IN DRIER AND COOLER AIR TO THE
REGION AS HIPRES BUILDS IN FROM THE WEST. PERIODS OF CLDS EXPECTED
FRI WITH ENOUGH LIFT IN THE CYCLONIC FLOW REGIME. HIGH TEMPS FRI
WILL RANGE FROM THE 60S IN THE MTS TO MID 70S ALONG THE COAST.
&&

.SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY/...
LONG-WAVE TROUGH OVER THE ERN US BECOMES REINFORCED THIS WEEKEND AS
SHORTWAVE ENERGY DIGS AROUND THE BACKSIDE OF THE TROUGH. STILL
SIGNIFICANT DIFFERENCES AMONG SWEET OF MODELS REGARDING HOW THE
SYNOPTIC PATTERN EVOLVES ACROSS THE ERN CONUS...AND THUS WHEN THE
UPPER TROUGH AXIS AND ASSOCIATED COLD FRONT SWINGS THRU THE AREA.
THE GFS SOLUTION HAS CONSISTENTLY SHOWN A MORE PROGRESSIVE SOLUTION
WITH THE TROUGH BECOMING SHEARED OUT...ALLOWING FOR THE AXIS TO MOVE
THRU THE AREA SAT NGT. THE 00Z ECMWF/12Z NAM SHOWS THE TRAILING
SHORTWAVE BECOMING CUTOFF OVER THE WRN SIDE OF THE CENTRAL
APPALACHIANS DURING THE WEEKEND. THE PROGRESSIVE SOLUTION WOULD GIVE
CHANCES FOR LIGHT SHOWERS ACROSS THE ENTIRE AREA LATE SAT INTO ERY
SUN WHILE THE CUTOFF SOLUTIONS WOULD KEEP THE AREA DRY...EXCEPT FOR
THE ALLEGHENY FRONT...THIS WEEKEND. DO NOT QUITE BUY THE EXTREME
CUTOFF SOLUTION OCCURRING SO QUICKLY...AND WILL TAKE A BLEND OF BOTH
SOLUTIONS. THE COLD FRONT WILL TRY TO MOVE THRU SAT NGT AND SUN.

NLY FLOW WILL KEEP COOL CANADIAN AIR IN THE MID-ATLANTIC REGION.
HIGH TEMPS THIS WEEKEND WILL BE IN THE 60S TO NEAR 70 IN THE CITIES
AND ALONG THE COAST...WHICH IS 5-10 DEGREES BELOW NORMAL.
&&

.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
BIG DIFFERENCES REMAIN BETWEEN LONG-RANGE MODEL GUIDANCE. POPULATED
GRIDS WITH HPC...WHICH PREFERS A MORE WEIGHTED OF THE ECMWF. THIS
WOULD KEEP A CUTOFF LOW OVER THE ERN US THRU THE MIDDLE OF NEXT
WEEK...KEEPING LOW CHANCES FOR PRECIP AND SLIGHTLY BELOW NORMAL
TEMPS ACROSS THE AREA. CONFIDENCE IS MUCH LOWER THAN NORMAL FOR THE
EXTENDED PERIOD.
&&

.AVIATION /19Z THURSDAY THROUGH TUESDAY/...
LOPRES MVG THRU VA/MD ATTM. TERMINALS IN A LULL AS A RSLT PCPN-WISE.
HWVR...THE LACK OF PCP HAS FACILITATED INCR WND GUSTS. HV TRIED TO
CAPTURE THAT SOMEWHAT IN THE TAFS...BUT MAY BE UPDTG SHORTLY. IN THE
WAKE OF THE LOW GUSTY SELY WNDS WL SUBSIDE...AND THEN BECOME NWLY
TNGT...LKLY AFTR 00-02Z.

ANTHR AREA OF LOPRES OFF THE SC COAST. THAT WL BRING ANTHR RND OF
LOW VSBYS/CIGS DUE TO PCPN TO THE TERMINALS OVNGT.

THE MRNG PUSH SHUD SEE LINGERING LOW CLDS...BUT CONDS WL IMPV THRU
THE DAY. NOT VERY CERTAIN IF IT/LL BE MVFR OR IFR. FCSTS REFLECT THE
LWR PSBLTY...XCPT FOR METRO DC. THAT CUD GO EITHER WAY.

VFR CONDS THRU AT LEAST FRI NGT...MAYBE INTO SAT. NXT CHC AT FLGT
RESTRICTIONS SUN-MON.

&&

.MARINE...
LOPRES OVER CENTRAL VA WILL QUICKLY MOVE NORTH OF THE AREA THIS EVE.
GALE WRNG EXPANDED TO INCLUDE ALL MARINE ZONES THRU THE REST OF THIS
AFTN. ONCE THE HEAVIEST PRECIP MOVED AWAY FROM THE AREA...INCREASED
MIXING HAS ALLOWED FOR STRONGER WINDS OF 35 TO 40 KT ALOFT TO REACH
THE SFC. GALE WRNG CONTINUES INTO TNGT FOR THE LOWER TIDAL POTOMAC
AND MD CHSPK BAY FROM NORTH BEACH SOUTHWARD...WHERE STRONG SLY FLOW
CHANNELING UP THE BAY WILL INTO THE EVE. AS LOPRES MOVES NORTH OF
THE AREA...WINDS WILL SHIFT TO THE NORTH-NORTHWEST. GALE WRNG WILL
LIKELY NEED TO BE REPLACE BY A SCA BY THE LATE EVE NW WINDS RELAX A
BIT. SCA IN EFFECT FOR MARINE ZONES NOT UNDER A GALE WRNG TNGT AS NW
WINDS WILL GUST 20-30 KT.

SCA IN EFFECT FOR ALL MARINE ZONES ON FRI. NW WINDS GUSTS AROUND
20-30 KT EXPECTED. SCA CONDITIONS MAY CONTINUE INTO FRI NGT FOR
SRN PORTIONS OF THE MD CHSPK BAY.

SCA CONDITIONS MAY CONTINUE INTO FRI AND FRI NGT...ESPECIALLY ACROSS
THE MAIN CHANNEL OF THE CHSPK BAY...WHERE NLY CHANNELING WOULD
ENHANCE THE FLOW. HIPRES BUILDS TOWARD THE WATERS FRI INTO THE
WEEKEND...RESULTING IN LIGHT FLOW.
&&

.HYDROLOGY...
FLASH FLOOD WATCH CONTS. CURRENT FCSTD EVOLUTION FOCUSES ON PCPN
ENHANCEMENT IN TROPICAL FEEDER BAND. PCPN EXTENDS BACK THROUGH THE
EASTERN CAROLINAS. CNTYS ADJACENT TOT HE BAY MOST IMPACTED. WL
CONSIDER REVISIONS SHORTLY...AS DRY SLOT HAS WORKED INTO CWFA.
FIRST CANDIDATE FOR CANX WL BE W OF BLURDG.

RIVER FLOOD WARNINGS IN EFFECT FOR SENECA CREEK AT DAWSONVILLE...
FAIRVIEW...MARTINSBURG...POINT OF ROCKS...FREDERICK...AND
LEESBURG. AWAITING A NEW ROUND OF GDNC...AND WL BE MAKING
DECISIONS ON MAINSTEMS SHORTLY. ALL THIS WATER WL BE MIKING IT
INTO THE RIVERS...AND CRESTS XPCTD FRI.
&&

.TIDES/COASTAL FLOODING...
WATER LVLS HV BEEN STEADILY INCREASING THRU THE DAY...BARELY
RECOGNIZING LOW ASTRONOMICAL TIDE. DEPARTURES NOW 2-3 FT ABV NRML...
THE HIEST OF WHICH WL BE IN THE UPR REACHES OF THE CHSPK BAY. LOPRES
JUST SW OF AREA ATTM...RESPONSIBLE FOR A WND SURGE...WHICH WL
SUPPORT INCR WATER LVLS. HWVR...THAT LOW WL BE N OF WATERS BY EVE...
PERMITTING WNDS TO SWING ARND TO THE NW. THE TIMING WL BE COINCIDENT
W/ HIGH TIDE...AND THE PM CYCLE WL BE THE HIER OF THE TWO. ANY
DECREASE DUE TO BLOWOUT CONDS MAY BE OFFSET BY THE HIER ASTRONOMICAL
SETUP. HENCE...WL BE XTNDD ADVY/WRNG WHERE THEY EXIST TO COVER THE
DURATION OF THE NIGHTTIME TIDE CYCLE. WL NEED TO REASSESS TNGT.
&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...FLASH FLOOD WATCH THROUGH LATE TONIGHT FOR DCZ001.
     COASTAL FLOOD ADVISORY UNTIL 3 AM EDT FRIDAY FOR DCZ001.
MD...FLASH FLOOD WATCH THROUGH LATE TONIGHT FOR MDZ004>007-009>011-
     013-014-016>018.
     COASTAL FLOOD ADVISORY UNTIL 3 AM EDT FRIDAY FOR MDZ013-016.
     COASTAL FLOOD WARNING UNTIL 5 AM EDT FRIDAY FOR MDZ007-011-014-
     017-018.
     FLASH FLOOD WATCH UNTIL 9 PM EDT THIS EVENING FOR MDZ003-502.
VA...FLASH FLOOD WATCH THROUGH LATE TONIGHT FOR VAZ036>040-042-
     050>057-501-502.
     COASTAL FLOOD ADVISORY UNTIL 3 AM EDT FRIDAY FOR VAZ052>055-
     057.
     FLASH FLOOD WATCH UNTIL 9 PM EDT THIS EVENING FOR VAZ021-
     025>031.
WV...FLASH FLOOD WATCH UNTIL 9 PM EDT THIS EVENING FOR WVZ050>055-
     502-504.
MARINE...SMALL CRAFT ADVISORY UNTIL 6 PM EDT FRIDAY FOR ANZ530>532-535-
     536-538>540-542.
     GALE WARNING UNTIL 6 PM EDT THIS EVENING FOR ANZ530>532-535-
     536-538>540-542.
     SMALL CRAFT ADVISORY FROM 6 AM TO 6 PM EDT FRIDAY FOR ANZ533-
     534-537-541-543.
     GALE WARNING UNTIL 6 AM EDT FRIDAY FOR ANZ533-534-537-541-543.

&&

SYNOPSIS...HTS
NEAR TERM...KLEIN
SHORT TERM...KLEIN
LONG TERM...KLEIN
AVIATION...HTS
MARINE...KLEIN
HYDROLOGY...HTS
TIDES/COASTAL FLOODING...HTS

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