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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KLWX 180759
AFDLWX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC
359 AM EDT SAT MAY 18 2013

.SYNOPSIS...
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A FRONTAL BOUNDARY WILL STALL SOUTH OF THE AREA THIS WEEKEND OVER
THE CENTRAL APPALACHIANS AND SOUTHERN MID-ATLANTIC STATES WHILE
ONSHORE FLOW KEEPS THE FORECAST AREA SEASONABLY COOL. THE FRONTAL
BOUNDARY WILL LIFT NORTH OF THE AREA EARLY IN THE WEEK FOLLOWED BY
HIGH PRESSURE BUILDING OVER THE EASTERN STATES THROUGH MIDWEEK.
-- End Changed Discussion --

&&

.NEAR TERM /THROUGH TONIGHT/...
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FRONTAL BOUNDARY HAS BECOME NEARLY STATIONARY...LOCATED ALONG THE
COASTAL VIRGINIA/NORTH CAROLINA BORDER AND SNAKING ITS WAY BACK
THROUGH/UP THE CENTRAL APPALACHIANS. MEANWHILE...HIGH PRESSURE WAS
LOCATED ALONG THE NEW ENGLAND COAST.

RADAR IMAGERY SHOWED SHOWERS WITH EMBEDDED THUNDERSTORMS ACROSS
CENTRAL AND SOUTHERN VIRGINIA...TO NORTH OF THE BOUNDARY IN A ZONE
OF ISENTROPIC LIFT/ELEVATED INSTABILITY ENHANCED BY PVA FROM A VORT
MAX AT 500 MB. SHOWERS HAVE SNEAKED UP INTO THE CENTRAL VIRGINIA
FOOTHILLS AREA OF THE CWA EARLY THIS MORNING.

THIS FORECAST CYCLE...WHAT IS CERTAIN OVER THE NEXT 24 HOURS IS THAT
EAST-SOUTHEAST FLOW IN THE LOW LEVELS WILL PREVAIL
TODAY/TONIGHT...RESULTING IN CLOUDY SKIES. THIS WILL LEAD TO COOLER
TEMPERATURES TODAY AND GUIDANCE IS IN SOMEWHAT BETTER AGREEMENT THAN
24 HOURS AGO FAVORING THE PREVIOUS COOLER SOLUTION FOR BELOW NORMAL
MAXIMA. BLENDED BIAS CORRECTED MAV/MET WHICH FAVORS UPPER 60S TO
LOWER 70S MAXIMA TODAY...AND THEN MINIMA ABOVE NORMAL TONIGHT IN THE
UPPER 50S TO LOWER 60S.

WHAT IS UNCERTAIN ARE THE DETAILS REGARDING PRECIPITATION. PLENTY OF
DIVERGENCE AMONG THE MODELS IS ALREADY APPARENT WITH THE SHOWERS ON
RADAR NOW...AND WHAT THEIR FATE WILL BE LATER THIS MORNING AND
BEYOND. 03Z HRRR MOVES/FURTHER DEVELOPS THESE SHOWERS TO THE NORTH
THROUGH MORNING...TAKING THEM ALL THE WAY TO THE MASON DIXON LINE BY
LATE MORNING. WRF-ARW/GFS SOLUTIONS GENERALLY KEEP SHOWERS ACROSS
THE SOUTHERN THIRD TO HALF OF THE CWA DURING THE DAY TODAY. THE NAM
AND SREFS ARE SOMEWHERE IN BETWEEN.

INITIALLY...FORECAST IS HEAVILY WEIGHTED TOWARD THE WRF-ARW/GFS
SOLUTIONS /AND RADAR TRENDS/ THIS MORNING THEN BLENDS WITH THE
NAM/SREFS SOLUTIONS FOR THE AFTERNOON AND INTO TONIGHT WITH THE
THINKING THAT LIFT AND THUS SHOWERS EXPAND TO THE NORTH WITH TIME
WITH LIKELY POPS ACROSS THE ENTIRE CWA OVERNIGHT. OBVIOUSLY IF THE
HRRR IS CORRECT THERE WILL BE RAINFALL FURTHER NORTH AND SOONER
THAN WHAT THE FORECAST REFLECTS...AND WILL CONTINUE TO MONITOR
THROUGH THE MORNING BUT FOR NOW THIS HAS BEEN LARGELY DISCOUNTED.

COOL WEDGE WILL BE PRESENT ACROSS MUCH OF THE CWA TO PRECLUDE
THUNDER. HOWEVER GUIDANCE SUGGESTS SOME INSTABILITY ACROSS THE
SOUTHWEST CWA THIS AFTERNOON/EARLY EVENING CLOSER TO THE SNAKING
FRONTAL BOUNDARY. THEREFORE...CONTINUE TO INCLUDE CHANCE OF THUNDER
IN THIS AREA AND ANY CONVECTION MAY BE EFFICIENT RAINFALL PRODUCERS
FOR LOCALLY HEAVY RAINFALL.
-- End Changed Discussion --

&&

.SHORT TERM /SUNDAY THROUGH SUNDAY NIGHT/...
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FRONTAL BOUNDARY WILL REMAIN STALLED ACROSS SRN VA ON SUN WHILE SFC
RIDGE FROM HIPRES OFF THE NEW ENGLAND COAST WILL EXTEND WWD INTO NJ
AND PA. SFC PATTERN WILL YIELD A CONTINUED ONSHORE FLOW ACROSS THE
CWA ON SUN. REMNANT MID-LVL SHORTWAVE TROUGH WILL MEANDER OVER THE
CENTRAL APPALACHIANS AS IT REMAINS EMBEDDED WITHIN WEAK STEERING
PATTERN OF THE ERN CONUS RIDGE. FLOW VEERS WITH HEIGHT WITH SLY FLOW
AT H8-7 PROMOTING ISENTROPIC LIFT. CLOUDS AND PERIODS OF SHOWERS
EXPECTED ON SUN. THE HEAVIEST QPF AND CAT POPS RESIDE OVER CENTRAL
VA AND LWR SRN MD...WHERE LIFT MAY BECOME ENHANCED NEAR THE BOUNDARY
AND NORTH OF THE MID-LVL TROUGH. MOST OF THE CWA WILL BE WEDGED IN
STABLE MARINE AIRMASS ALTHOUGH LOCATIONS IN CENTRAL VA MAY TAP INTO
SOME INSTABILITY CLOSER TO THE BOUNDARY. WILL NEED TO MONITOR THE
CONDITIONAL RISK FOR FLOODING OVER THESE AREAS IF CONVECTION
DEVELOPS. WPC HAS HIGHLIGHTED CENTRAL VA WITH A SLIGHT RISK FOR
EXCESSIVE RAINFALL ON SUN.

CLOUDS AND ONSHORE FLOW WILL LIMIT DIURNAL HEATING/COOLING SUN-SUN
NGT. HIGHS NEAR 70F ON SUN AND LOWS IN THE 60S SUN NGT WILL BE
COMMON.
-- End Changed Discussion --

&&

.LONG TERM /MONDAY THROUGH FRIDAY/...
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THE MID-ATLANTIC REGION WILL BE SITUATED ALONG WRN PERIPHERY OF AN
UPPER RIDGE THROUGH MIDWEEK. PERSISTENT AND DEEP RETURN FLOW AROUND
THE RIDGE WILL CONTINUE TO SUPPLY WARM/MOIST AIR FOR OUR AREA. WITH
THE PREVIOUSLY STALLED FRONTAL BOUNDARY LIFTING NORTH OF THE AREA ON
MON...INSTABILITY AND MOISTURE WILL BE GREATER IN WARM SECTOR. DAILY
CHANCES FOR SHOWERS AND TSTMS WILL BE HIGHEST DURING THE PEAK
HEATING HRS. DURING THE FIRST HALF OF THE WORK WEEK...MON EXPECTED
TO HAVE THE BEST COVERAGE OF PRECIP WITH THE LINGERING MID-LVL
SHORTWAVE TROUGH NEARBY. POPS TUE AND WED WERE CONFINED TO THE
HIGHER TERRAIN WITH OROGRAPHIC LIFT THE PRIMARY DRIVING MECHANISM
FOR DIURNAL CONVECTION. MAX TEMPS NEAR 80F ON MON AND MU80S TUE/WED.

CHANCES FOR SHOWERS AND STORMS INCREASE LATE WEEK WITH THE APPROACH
OF A MIDWEST TROUGH. WARM/HUMID AIRMASS REMAINS IN PLACE DURING THIS
TIME.
-- End Changed Discussion --

&&

.AVIATION /08Z SATURDAY THROUGH WEDNESDAY/...
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AS MENTIONED IN THE NEAR TERM DISCUSSION...THERE IS SOME UNCERTAINTY
REGARDING ONSET OF PRECIPITATION. HAVE DISCOUNTED RECENT RUNS OF THE
HRRR AND BLENDED OTHER MODELS FOR ONSET TIMING. THERE ALSO IS SOME
UNCERTAINTY REGARDING THE ONSET OF FLIGHT RESTRICTIONS. WITH
EAST-SOUTHEAST FLOW...EXPECT CIGS TO FALL TODAY/TONIGHT. CLOSELY
FOLLOWED A GFS LAMP SOLUTION...WHICH BRINGS IFR CONDITIONS MAINLY
AFTER DARK AND THEN LIFR CONDITIONS AFTER MIDNIGHT. IF PRECIPITATION
DEVELOPS SOONER THAN CURRENTLY FORECAST...THEN LOWER FLIGHT
CONDITIONS MAY ALSO DEVELOP SOONER. IN COOL WEDGE...AM NOT EXPECTING
A CHANCE OF THUNDER THIS AFTERNOON AT MOST TAF SITES EXCEPT FOR CHO
WHERE THERE WILL BE SOME INSTABILITY HOWEVER CONFIDENCE IS TOO LOW
TO MENTION AT CHO AT THIS POINT.

MVFR CIGS SUN WITH ONSHORE FLOW CONTINUING. CIGS LIKELY RETURN TO
IFR SUN NGT. WARM FRONT EVENTUALLY LIFTS NORTH OF THE REGION ON MON.
FLIGHT RULES RETURN TO VFR ERY NEXT WEEK...ALTHOUGH BRIEF
RESTRICTIONS POSSIBLE IN SHRA/TSRA.
-- End Changed Discussion --

&&

.MARINE...
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WITH HIGH PRESSURE MOVING OFF THE NEW ENGLAND COAST AND A FRONTAL
BOUNDARY STALLED TO THE SOUTH OF THE WATERS...EAST-SOUTHEAST WINDS
WILL SET UP TODAY/TONIGHT. WHILE WE EXPECT WINDS TO PREVAIL 10 TO 15
KT...SOME LOCAL ENHANCEMENT IS EXPECTED ALONG THE LOWER TIDAL
POTOMAC RIVER/LOWER MD CHESAPEAKE BAY WHERE A SCA IS IN EFFECT
TODAY. THIS COULD NEED TO BE EXTENDED INTO TONIGHT. IN
ADDITION...SOMETIMES THIS FLOW IS UNDER-FORECAST BY THE MODELS AND
WILL HAVE TO MONITOR THE NEED FOR NORTHERN EXPANSION UP THE CHANNEL
OF THE BAY.

ELY FLOW 10-15 KT ON SUN. WINDS BECOME SLY ONCE A WARM FRONT PASSES
THRU ON MON. SCA MAY BE NEEDED MON FOR THE MARINE ZONES THAT ARE
SUSCEPTIBLE TO SLY-CHANNELING.
-- End Changed Discussion --

&&

.TIDES/COASTAL FLOODING...
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POSITIVE TIDAL ANOMALIES WILL GRADUALLY INCREASE THRUOUT THE
WEEKEND AS ONSHORE FLOW PERSISTS. SLY WINDS DEVELOP ON MON WHEN A
WARM FRONT LIFTS NORTH OF THE AREA. WILL NEED TO MONITOR TIDAL
LVLS THOUGH THE THREAT OF COASTAL FLOODING WILL BE RELATIVELY LOW
JUST PAST THE FIRST QUARTER MOON.
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&&

.LWX WATCHES/WARNINGS/ADVISORIES...
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DC...NONE.
MD...NONE.
VA...NONE.
WV...NONE.
MARINE...SMALL CRAFT ADVISORY FROM 10 AM THIS MORNING TO 8 PM EDT THIS
     EVENING FOR ANZ534-537-543.
-- End Changed Discussion --

&&

$$
SYNOPSIS...JRK
NEAR TERM...BPP
SHORT TERM...JRK
LONG TERM...JRK
AVIATION...BPP/JRK
MARINE...BPP/JRK
TIDES/COASTAL FLOODING...JRK