Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 291853 AFDLWX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC 253 PM EDT WED APR 29 2015 .SYNOPSIS...
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HIGH PRESSURE WILL MOVE OFFSHORE TONIGHT. A WAVE OF LOW PRESSURE WILL DEVELOP OVER THE REGION TONIGHT. THIS LOW WILL REMAIN OVER THE REGION THROUGH THURSDAY. ANOTHER AREA OF LOW PRESSURE WILL MOVE UP THE COAST FRIDAY. HIGH PRESSURE WILL RETURN FOR THE WEEKEND BEFORE MOVING OFFSHORE EARLY NEXT WEEK.
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&& .NEAR TERM /THROUGH THURSDAY/...
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HIGH PRESSURE REMAINS OVER THE MID-ATLANTIC THIS AFTERNOON. FEW TO SCATTERED CUMULUS CLOUDS HAVE DEVELOPED OVER THE AREA. STILL PLENTY OF SUN THOUGH...AND TEMPERATURES HAVE REACHED THE LOW 70S OVER MOST OF THE AREA. LATE AFTERNOON HIGHS SHOULD REACH THE MID 70S OVER THE I-95 CORRIDOR FROM WASHINGTON DC TO THE SOUTH...WITH LOW 70S OVER MOST OF THE REST OF THE CWA. MEANWHILE...TWO AREAS OF LOW PRESSURE HAVE DEVELOPED TO OUR NORTH AND SOUTH...ONE NEAR NYC AND THE SECOND OVER SWRN VA. A TROUGH IS EXPECTED TO DEVELOP BETWEEN THE TWO ALONG THE SPINE OF THE APPALACHIANS. MODELS INDICATE POTENTIAL FOR SHOWERS AND MAYBE AN ISOLATED TSTM TO DEVELOP OVER THE HIGHLANDS LATE THIS AFTERNOON. TONIGHT...THE TWO LOWS ARE EXPECTED TO COMBINE INTO ONE BROAD LOW ACROSS THE MID-ATLANTIC. POTENTIAL FOR SHOWERS WILL INCREASE MAINLY ALONG AND WEST OF THE BLUE RIDGE...WITH THE BEST POTENTIAL ONCE AGAIN OVER THE MOUNTAINS. LOWS WILL BE IN THE UPPER 40S TO LOW 50S...WITH MID 50S POSSIBLE ALONG THE I-95 CORRIDOR. ON THURSDAY...A CLOSED UPPER LOW WILL DROP SOUTH FROM THE GREAT LAKES TO THE OHIO VALLEY. THE LOW OVER THE MID-ATLANTIC WILL DROP TO THE SOUTH INTO SRN VA AND NC...WHILE A COASTAL LOW DEVELOPS EAST OF NC. THESE TWO SYSTEMS ARE NOT EXPECTED TO PHASE IN TIME...BUT SHOWERS ARE EXPECTED TO DEVELOP OVER THE AREA. THERE MAY EVEN BE A FEW RUMBLES OF THUNDER FROM LOW TOPPED CONVECTION...ESPECIALLY ACROSS THE CENTRAL AND WESTERN PORTIONS OF THE CWA CLOSER TO THE UPPER- LEVEL LOW. PRECIPITATION WILL BE MOST WIDESPREAD IN THE AFTERNOON. HIGHEST QPF IS EXPECTED ALONG THE MOUNTAINS. ALTHOUGH MODELS ARENT RESOLVING IT WELL YET...RAIN COULD BE HEAVY AT TIMES ALONG THE EASTERN SLOPES WHERE UPSLOPE WILL BE ENHANCED FROM EASTERLY FLOW. WARMEST TEMPS WILL OCCUR EAST OF THE BLUE RIDGE...WHERE HIGHS WILL BE IN THE UPPER 60S TO LOW 70S. LOW TO MID 60S LIKELY ALONG AND WEST OF THE BLUE RIDGE UNDER MORE PRONOUNCED CLOUD COVER.
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&& .SHORT TERM /THURSDAY NIGHT THROUGH FRIDAY NIGHT/...
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A STRONG INVERTED TROUGH WILL PIVOT ACROSS THE BLUE RIDGE MOUNTAINS...SHENANDOAH VALLEY AND POTOMAC HIGHLANDS FROM EAST TO WEST THURSDAY EVENING. MODEL GUIDANCE IS FORECASTING A SUFFICIENT AMOUNT OF CAPE /I.E. 400-800 J/KG/ COMBINED WITH MODERATELY STRONG FORCING ALONG THE INVERTED TROUGH TO CONTINUE INCLUDING A SLIGHT CHANCE OF THUNDERSTORMS OVER THESE AREAS...THOUGH GIVEN THE LACK OF SHEAR THINK THAT LIGHTNING-PRODUCING UPDRAFTS SHOULD REMAIN MORE ISOLATED IN NATURE. ANY HEAVIER SHOWERS OR THUNDERSTORMS THAT DEVELOP WILL BE CAPABLE OF HEAVY DOWNPOURS...AND WILL BE SLOW-MOVERS AS WELL GIVEN VERY LIGHT FLOW IN THE 1000-500 MB LAYER. IN FAVOR OF EFFICIENT RAIN-PRODUCERS IS DEEP SATURATION BUT WORKING AGAINST IT IS A SHALLOW WARM-CLOUD LAYER OF LESS THAN 6500 FEET. IN GENERAL EXPECTING AROUND A SOLID HALF AN INCH OF RAIN OVER THE WESTERN HALF OF THE CWA...ENHANCED BY TERRAIN...WITH LOCALIZED AMOUNTS IN HEAVIER DOWNPOURS OF 1 TO 1.5 INCHES POSSIBLE. SEE THE HYDROLOGY SECTION BELOW FOR MORE DETAILS. RAIN SHUTS OFF FROM NORTHEAST TO SOUTHWEST OVERNIGHT AS THE INVERTED TROUGH PIVOTS AWAY AND GETS ABSORBED INTO A DEVELOPING OFFSHORE LOW PRESSURE SYSTEM. AS HIGH PRESSURE GRADUALLY BUILDS IN AND LOW PRESSURE STRENGTHENS OFFSHORE...NORTHEASTERLY WINDS WILL INCREASE AND GUST 20 TO 25 MPH OVER EASTERN PARTS OF THE CWA. SEE COASTAL FLOODING SECTION BELOW FOR MORE ON POTENTIAL COASTAL FLOODING. GRADUAL CLEARING IS ANTICIPATED FRIDAY NIGHT AS HIGH PRESSURE CONTINUES TO BUILD OVER THE REGION.
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&& .LONG TERM /SATURDAY THROUGH TUESDAY/...
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HIGH PRESSURE WILL BE THE DOMINANT FEATURE THROUGH THE LONG TERM...WITH INCREASING TEMPS EACH DAY AND VERY LOW PRECIP CHANCES. SAT SHOULD START OFF CLOUDY AND DRY...WITH CLEARING EXPECTED FROM W TO E THROUGH THE DAY. SOME LOCATIONS MIGHT NOT CLEAR UNTIL AFTERNOON...WHICH WILL PREVENT TEMPS FROM RISING TOO MUCH (U60S/L70S). WARM UP BEGINS IN EARNEST SUNDAY AND CONTINUES THROUGH EARLY PART OF NEXT WEEK...AS BERMUDA HIGH EXTENDS WESTWARD INTO THE MID-ATLANTIC. HIGH TEMPS BY MONDAY WILL BE IN L80S AND BY TUESDAY M80S...WHICH WILL FEEL LIKE SUMMER. DEWPOINTS WILL ALSO BE INCREASING DURING THIS TIME...WHICH WILL HELP TO PREVENT NIGHTTIME LOWS FROM DROPPING TOO MUCH (U50S/L60S). PRECIP CHANCES DURING THIS PERIOD APPEAR TO BE RATHER SMALL AS DISTURBANCES WILL TEND TO RIDE TO OUR NORTH ALONG THE PERIPHERY OF THE RIDGE.
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&& .AVIATION /18Z WEDNESDAY THROUGH MONDAY/...
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VFR CONDITIONS EXPECTED AT THE TERMINALS THROUGH TONIGHT. ONLY POSSIBLE EXCEPTION IS AT KCHO...WHERE THERE IS A CHANCE OF SHOWERS LATE TONIGHT. LIGHT WINDS EXPECTED. RAIN/SHOWERS ARE EXPECTED TO DEVELOP ACROSS THE REGION THURSDAY. NOTED PREVAILING MVFR FOR RAIN DURING MUCH OF THE MORNING AND AFTERNOON...ALTHOUGH ACTIVITY MAY BE MORE SPORADIC IN NATURE. REFINEMENTS LIKELY IN FUTURE UPDATES. NELY FLOW AROUND 10 KT THU EVENING INCREASING TO 10-15 KT THU NIGHT INTO FRI. SUB-VFR LIKELY IN RA/LOW CIGS THU EVE...GRADUALLY IMPROVING TO VFR FRI. VFR W/ NLY FLOW 10 KT FRI NIGHT. VFR CONDS SAT THROUGH SUN WITH HIGH PRESSURE OVER AREA.
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&& .MARINE...
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SOUTHERLY FLOW OVER THE WATERS THIS AFTN. A FEW GUSTS TO 15KT...BUT MOSTLY WINDS WILL BE 1OKT OR LESS. LIGHT WINDS EXPECTED TONIGHT WITH BROAD LOW PRESSURE DEVELOPING OVER THE WATERS. PERIODS OF RAIN EXPECTED THURSDAY WITH LOW PRESSURE MIGRATING INLAND. WINDS OVER SOUTHERN WATERS WILL LIKELY BE FROM THE SOUTH...TRANSITIONING TO AN EAST FLOW OVER THE NRN BAY. NO ADVISORIES EXPECTED DURING THE DAY. AS LOW PRESSURE STRENGTHENS OFFSHORE AND HIGH PRESSURE BUILDS TOWARD THE REGION THURSDAY NIGHT INTO FRIDAY...NORTHEASTERLY WINDS WILL INCREASE TO 10 TO 20 KT WITH GUSTS IN THE MID-20S LIKELY ACROSS MUCH OF THE WATERS. SCA HAS BEEN ISSUED FOR THU NIGHT FOR ALL BUT UPPER TIDAL POTOMAC RIVER WHERE HIGHEST CONFIDENCE IS. THIS WILL LIKELY NEED TO BE EXTENDED THROUGH MUCH OF FRIDAY. WINDS DIMINISH AS HIGH PRESSURE STARTS TO NOSE OVERHEAD FRIDAY NIGHT. HIGH PRESSURE OVER AREA OVER THE WEEKEND...WITH NO MARINE CONCERNS EXPECTED THROUGH SUN.
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&& .HYDROLOGY...
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ISOLATED HEAVIER SHOWERS OR THUNDERSTORMS THURSDAY AND THURSDAY NIGHT COULD BE SLOW-MOVING AND RESULT IN 1 TO 1.5 INCHES OF RAINFALL OVER LOCALIZED AREAS...MAINLY FROM NEAR THE BLUE RIDGE MOUNTAINS ACROSS THE SHENANDOAH VALLEY TO THE POTOMAC HIGHLANDS. 6-HOUR FLASH FLOOD GUIDANCE ACROSS A HANDFUL OF BASINS IN THESE AREAS IS BETWEEN 1.25 AND 1.5 INCHES. A FEW HEAVIER DOWNPOURS COULD DEVELOP OVER THESE MORE VULNERABLE BASINS. THERE COULD BE AT LEAST MINOR NUISANCE FLOODING AS A RESULT. THERE IS NOT ENOUGH CONFIDENCE FOR A FLOOD WATCH OR HWO MENTION AT THIS TIME GIVEN THE ISOLATED NATURE OF THE THREAT.
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&& .TIDES/COASTAL FLOODING...
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STRENGTHENING NORTHEASTERLY FLOW WILL RAISE TIDAL ANOMALIES AND MAY LEAD TO AREAS OF MINOR COASTAL FLOODING DURING THE LATE THURSDAY NIGHT/EARLY FRIDAY MORNING HIGH TIDE CYCLE.
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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 8 PM THURSDAY TO 6 AM EDT FRIDAY FOR ANZ530>534-536>543.
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&& $$ SYNOPSIS...KCS NEAR TERM...KCS SHORT TERM...DFH LONG TERM...MSE AVIATION...KCS/MSE/DFH MARINE...KCS/MSE/DFH HYDROLOGY...KCS/DFH TIDES/COASTAL FLOODING...DFH

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