000
FXUS61 KLWX 121628
AFDLWX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC
1128 AM EST FRI MAR 12 2010

.SYNOPSIS...
LOW PRESSURE OVER THE GREAT PLAINS REGION WILL SLOWLY MOVE TOWARD
THE AREA THROUGH SATURDAY. A ROUND OF HEAVY RAIN IS EXPECTED
FRIDAY NIGHT AND SATURDAY AS A SECOND AREA OF LOW PRESSURE
DEVELOPS OVER THE SOUTHEAST COAST TODAY AND TRACKS ACROSS THE
AREA. LOW PRESSURE WILL LINGER ACROSS THE REGION THROUGH EARLY
NEXT WEEK BEFORE HIGH PRESSURE BUILDS INTO THE REGION TUESDAY.

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
PERIODS OF RAIN WILL CONTINUE THRUOUT THE DAY AS INCREASING
ONSHORE FLOW AHEAD OF A DEVELOPING LOW OVER THE SOUTHEAST COAST
WILL TRANSPORT ATLANTIC MOISTURE INTO THE AREA. THE BEST CHANCE TO
SEE MORE FREQUENT BREAKS IN THE RAIN WILL BE ACROSS THE WRN
SLOPES OF THE ALLEGHENY FRONT AND POTOMAC HIGHLANDS DUE TO DRIER
ENTRAINING AIR AND DOWNSLOPING FLOW.

PRECIPITATION AMOUNTS AROUND ONE HALF INCH ARE EXPECTED DURING THE DAY
WHICH COULD CAUSE LOCALIZED FLOODING IN SMALL STREAMS AND CREEKS AS
WELL AS THE URBAN AREAS. HOWEVER...THE MORE WIDESPREAD FLOODING AND
MAIN STEM RIVER FLOODING ARE NOT EXPECTED DURING THE DAY.

OVC SKIES WILL PREVENT TEMPS FROM REACHING TO YESTERDAY/S READING.
HIGHS WILL GENERALLY BE IN THE 50S WITH AREAS IN THE NRN PIEDMONT
APPROACHING 60F.

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/...
SECOND AND MORE SIGNIFICANT AREA OF LIFT/RAIN MOVES BACK INTO THE
CWA TONIGHT...AND CONTINUES INTO SATURDAY. RAINFALL MAY BE HEAVY
OVERNIGHT TONIGHT INTO SATURDAY...WITH WIDESPREAD 2 TO 4 INCHES AND
LOCALLY HIGHER AMOUNTS. SEE HYDROLOGY SECTION FOR DETAILS REGARDING
FLOOD POTENTIAL.

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
MODELS INDICATE THAT AXIS OF HEAVY RAINFALL SHIFTS NORTH SATURDAY
EVENING...BUT LOW PRESSURE OVER THE MID ATLANTIC IS SLOW TO
DEPART...THUS RAINFALL IS LIKELY TO CONTINUE INTO AT LEAST SUNDAY
AND POSSIBLY EVEN MONDAY AS LOW PRESSURE GRADUALLY MOVES OFF AND
AWAY FROM THE MID ATLANTIC COAST EARLY NEXT WEEK.

HIGH PRESSURE BUILDS IN DURING THE MIDDLE OF THE WEEK...PROVIDING A
DRY FORECAST AND A RETURN TO MOSTLY CLEAR SKIES. TEMPERATURES ARE
FORECAST TO BE AT OR SLIGHTLY ABOVE CLIMO.

.AVIATION /16Z FRIDAY THROUGH TUESDAY/...
MSTLY IFR CIGS/VSBYS WILL CONTINUE THRUOUT THE DAY. HOWEVER...VSBYS
WILL OCCASIONALLY IMPROVE TO MVFR LVLS DURING THE DAY. PERIODS OF
LIGHT RAIN SHOULD PERSIST THRUT THE DAY. ELY FLOW WILL INCREASE
THRUT THE DAY WITH GUSTS TO 15-20 KT THIS AFTN.

HAVE HIGH CONFIDENCE IN IFR/LIFR CIGS PERSISTING THROUGH SATURDAY
NIGHT...WITH IFR VSBYS IN TIMES OF MODERATE TO HEAVY RAINFALL.
LATEST GUIDANCE SUGGESTS IFR/LIFR CIGS MAY CONTINUE ON SUNDAY WITH
RAINFALL CONTINUING AS LOW PRESSURE IS SLOW TO DEPART.

GUSTY EAST WINDS ARE EXPECTED FRIDAY NIGHT INTO SATURDAY...THEN
WINDS SHIFT TO THE WEST BY SUNDAY AS LOW PRESSURE TREKS TOWARD THE
MID ATLANTIC COAST.

.MARINE...
STRENGTHENING ELY FLOW WILL LEAD TO SCA CONDITIONS THIS MRNG AND
AFTN. 20-25 KT GUSTS ARE EXPECTED DURING THE AFTN.

EASTERLY JET STRENGTHENS FRIDAY NIGHT...ENOUGH WHERE WE HAVE
REASONABLY GOOD CONFIDENCE IN A PERIOD OF GALE FORCE WINDS OVER THE
BAY/LOWER POTOMAC. GUIDANCE SUGGESTS GALE FORCE WINDS WILL DIMINISH
SUNDAY...WITH SOLID SCA CONDITIONS REMAINING. USUALLY DO NOT FAVOR
RAMP UP/RAMP DOWN SCENARIOS WITH HEADLINES...HOWEVER SINCE GUIDANCE
IS FAIRLY AGREEABLE WITH GALES FRIDAY NIGHT AND SCA WINDS
SATURDAY...HAVE RAMPED DOWN TO SMALL CRAFT ADVISORY ON SATURDAY
/WHICH WOULD BE IN EFFECT ANYWAY FOR THE UPPER POTOMAC NOT IN THE
GALES FRIDAY NIGHT/.

.HYDROLOGY...
FLOOD WATCH IS IN EFFECT FOR ENTIRE CWA THROUGH SATURDAY EVENING.
NEW PITPOT HAS BEEN ISSUED...AND THE FCST FOR THE PTMC IS LOOKING
LK A PRETTY SIG FLD IS COMING. PAW PAW/HANCOCK/WILLIAMSPORT ARE
FCST TO REACH MAJOR FLD STAGE SUNDAY...PT OF ROCKS SUNDAY
NGT...LTL FALLS MONDAY.

FLOW ON CREEKS/STREAMS WILL CONTINUE TO ELEVATE DUE TO SNOW MELT
RUNOFF AND INITIAL BATCH OF RAINFALL THIS MORNING. MOST VULNERABLE
PLACES TODAY LOOK TO BE STREAMS/CREEKS ACROSS WESTERN
MARYLAND/EASTERN WEST VIRGINIA PANHANDLE/VIRGINIA HIGHLANDS.

HEAVIEST RAINFALL IS STILL EXPECTED TONIGHT INTO SATURDAY...WITH
WIDESPREAD 2 TO 4 INCHES AND LOCALLY HIGHER AMOUNTS. THIS RAINFALL
WILL INCREASE THE RISK FOR FLOODING OF STREAMS...CREEKS AND ALSO
URBAN AREAS ACROSS THE CWA TONIGHT INTO SATURDAY.

IN ADDITION...RIVER RESPONSE WILL BECOME MORE PRONOUNCED ON
SATURDAY...AND THIS WILL LIKELY CONTINUE THROUGH THE ENTIRE
WEEKEND AND PERHAPS EARLY NEXT WEEK. SOME OF THE POINTS ON THE
POTOMAC SUCH AS HANCOCK...PAW PAW...POINT OF ROCKS AND LITTLE
FALLS ARE FORECAST TO SURPASS FLOOD STAGE BEGINNING SATURDAY
NIGHT.

.TIDES/COASTAL FLOODING...
CSTL FLD WATCH HAS BEEN ISSUED FOR THE WRN SHORE OF THE CHES BAY
AND THE TIDAL PTMC FOR TNGT AND SAT. CSTL FLD THREAT COULD PERSIST
AT DC/ALEX THRU AT LEAST TUESDAY GIVEN HUGE SURGE OF FRESHWATER TO
COME DOWN THE PTMC.

WATER LVLS SHOULD START TO INCREASE TDA...ESPECIALLY ALONG THE WRN
SHORE OF THE CHES BAY AS ELY FLOW STRENGTHENS OVER THE AREA. LOPRES
DEVELOPING ALONG THE SOUTHEAST COAST WILL MOVE TOWARDS THE
MID-ATLANTIC REGION FRI NGT THRU THE WEEKEND. THE COMBINATION OF
LOPRES INHIBITING WATER FROM DRAINING AT THE MOUTH OF THE POTOMAC
RVR-CHES BAY/ UPSTREAM FRESHWATER CONTRIBUTION IN THE POTOMAC RVR
FROM HEAVY RAINFALL FRI NGT-SAT/ AND INCREASING ASTRONOMICAL FORCES
FROM THE APPROACHING NEW MOON WILL LIKELY LEAD TO MINOR COASTAL
FLOODING WITH THE POTENTIAL FOR MODERATE COASTAL FLOODING SAT AND
SUN.

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...FLOOD WATCH THROUGH SATURDAY EVENING FOR DCZ001.
     COASTAL FLOOD WATCH FROM 6 PM EST THIS EVENING THROUGH
     SATURDAY AFTERNOON FOR DCZ001.
MD...FLOOD WATCH THROUGH SATURDAY EVENING FOR MDZ003007-009011-
     013-014-016018-501-502.
     COASTAL FLOOD WATCH FROM 6 PM EST THIS EVENING THROUGH
     SATURDAY AFTERNOON FOR MDZ007-011-013-014-016018.
VA...FLOOD WATCH THROUGH SATURDAY EVENING FOR VAZ021-025031-
     036040-042-050057-501-502.
     COASTAL FLOOD WATCH FROM 6 PM EST THIS EVENING THROUGH
     SATURDAY AFTERNOON FOR VAZ052055-057.
WV...FLOOD WATCH THROUGH SATURDAY EVENING FOR WVZ050055-501504.
MARINE...SMALL CRAFT ADVISORY UNTIL 7 PM EST THIS EVENING FOR
     ANZ530534-537543.
     GALE WARNING FROM 7 PM THIS EVENING TO 6 PM EST SATURDAY FOR
     ANZ530532-538540.
     SMALL CRAFT ADVISORY FROM 6 AM TO 6 PM EST SATURDAY FOR ANZ533-
     534-537-541543.
     GALE WARNING FROM 7 PM THIS EVENING TO 6 AM EST SATURDAY FOR
     ANZ533-534-537-541543.
     SMALL CRAFT ADVISORY FROM 7 PM THIS EVENING TO 6 PM EST
     SATURDAY FOR ANZ535-536.

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