Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 041840 AFDLWX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC 240 PM EDT FRI SEP 4 2015 .SYNOPSIS...
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A BACKDOOR COLD FRONT WILL SINK SOUTH ACROSS THE WATERS WATERS TONIGHT. HIGH PRESSURE WILL BUILD OVERHEAD DURING THE LABOR DAY WEEKEND BEFORE MOVING OFF THE COAST DURING THE MIDDLE PORTION OF NEXT WEEK.
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&& .NEAR TERM /THROUGH SATURDAY/...
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UPPER-LEVEL LOW PRESSURE WILL WEAKEN OVER THE GREAT LAKES THROUGH SATURDAY WHILE HIGH PRESSURE BUILDS OVER NEW ENGLAND. A BACKDOOR COLD FRONT WILL APPROACH THE AREA FROM THE NORTH AND EAST LATE THIS AFTERNOON BEFORE STALLING OUT OVER THE AREA TONIGHT. THE FLOW WILL CONTINUE TO TURN TO THE EAST LATE THIS AFTERNOON AS HIGH PRESSURE BUILDS OVER NEW ENGLAND. HOWEVER...THE TEMPERATURE AND DEWPOINT GRADIENT SHOULD REMAIN WELL OFF TO OUR NORTH AND EAST. THEREFORE...MORE HOT AND HUMID CONDITIONS ARE EXPECTED WITH MAX TEMPS IN THE UPPER 80S AND LOWER 90S ACROSS MOST LOCATIONS. THE HEAT AND HUMIDITY HAS LED TO MORE INSTABILITY. LATEST MESOANALYSIS SHOWS OVER 2KJ/KG OF MLCAPE THIS AFTERNOON. FORCING MECHANISMS WILL BE WEAK...WITH LITTLE CONVERGENCE ALONG THE BACKDOOR BOUNDARY AND TERRAIN CIRCULATION OVER THE MOUNTAINS. HOWEVER...THE CAPPING INVERSION HAS BEEN WEAK ENOUGH CAUSING SCATTERED THUNDERSTORMS TO DEVELOP. THE MOST WIDESPREAD ACTIVITY SHOULD BE NEAR AND WEST OF INTERSTATE 95 WHERE INSTABILITY IS HIGHEST. MORE STABLE AIR BEHIND THE BACKDOOR BOUNDARY WILL CAUSE ANY THUNDERSTORMS TO QUICKLY DISSIPATE EARLY THIS EVENING NEAR WASHINGTON AND BALTIMORE. HOWEVER...A FEW SHOWERS AND THUNDERSTORMS MAY CONTINUE THROUGH MOST OF THE EVENING NEAR AND WEST OF THE BLUE RIDGE MOUNTAINS. THE BACKDOOR BOUNDARY WILL STALL OUT NEAR THE POTOMAC HIGHLANDS INTO CENTRAL VIRGINIA AND SOUTHERN MARYLAND LATE TONIGHT. A FEW SHOWERS ARE POSSIBLE NEAR THESE AREAS...BUT PRECIPITATION AMOUNTS SHOULD BE LIGHT DUE TO LIMITED INSTABILITY. ELSEWHERE...DRY CONDITIONS WILL PREVAIL OVERNIGHT BUT LOW CLOUDS ARE EXPECTED AS MOISTURE GETS TRAPPED UNDERNEATH THE SUBSIDENCE INVERSION. HIGH PRESSURE WILL CONTINUE TO WEDGE FARTHER SOUTH AND WEST INTO OUR AREA FOR SATURDAY. AN EASTERLY FLOW WILL ALLOW FOR COOLER CONDITIONS COMPARED TO RECENT DAYS AND IT WILL TURN OUT NOTICEABLY LESS HUMID ACROSS NORTHERN MARYLAND INTO THE WASHINGTON METROPOLITAN AREA WHERE SOME DRIER AIR WILL ADVECT INTO THE REGION. FARTHER SOUTH AND WEST...CLOSE TO THE BOUNDARY A SHOWER OR TWO CANNOT BE RULED OUT...BUT MOST OF THE TIME SHOULD BE DRY. LOW CLOUDS WILL BE AROUND EARLY SATURDAY MORNING AS MOISTURE REMAINS TRAPPED UNDERNEATH THE SUBSIDENCE INVERSION...BUT CLOUDS SHOULD BURN OFF LATER IN THE MORNING INTO THE EARLY AFTERNOON HOURS.
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&& .SHORT TERM /SATURDAY NIGHT THROUGH MONDAY/...
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HIGH PRESSURE WILL CONTINUE TO SLOWLY BUILD SOUTH OVERHEAD SATURDAY NIGHT THROUGH MONDAY. MOST AREAS WILL REMAIN DRY DURING THIS TIME...BUT THERE MAY BE SOME LOW CLOUDS AND FOG DURING THE OVERNIGHT AND EARLY MORNING HOURS EACH DAY. THERE MAY BE ENOUGH INSTABILITY FOR A SHOWER OR TWO OVER THE POTOMAC HIGHLANDS AND EXTREME SOUTHERN MARYLAND DURING THE AFTERNOON AND EVENING HOURS EACH DAY...BUT EVEN ACROSS THESE AREAS MOST OF THE TIME WILL TURN OUT DRY. MAX TEMPS WILL BE NEAR CLIMO FOR SUNDAY...BUT ABOVE NORMAL CONDITIONS WILL RETURN FOR MONDAY DUE TO MORE SUBSIDENCE WITH THE HIGH DIRECTLY OVERHEAD.
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&& .LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
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WEAK UPPER LEVEL TROUGHING AFFECTING MID ATLANTIC THROUGH EARLY HALF OF PERIOD AND AGAIN LATE FRIDAY. MOISTURE ASSOCIATED WITH SURFACE RIDGING WILL WORK ITS WAY INTO SHENANDOAH VALLEY/POTOMAC HIGHLANDS BY MID WEEK. WEAK PVA...AFTERNOON HEATING AND ORTHOGRAPHIC LIFT MAY SUPPORT FOR MARGINAL CONVECTION DURING EARLY HALF OF PERIOD. ADDITIONAL DYNAMICS WHEN COLD FRONT CROSSES REGION WILL INCREASE CHANCES OF CONVECTION DURING LATTER HALF OF PERIOD. USED GFS20 TO TWEAK TIMING.
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&& .AVIATION /18Z FRIDAY THROUGH WEDNESDAY/...
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SCATTERED THUNDERSTORMS ARE EXPECTED THIS AFTERNOON AND EVENING. THUNDERSTORMS WILL BE CAPABLE OF PRODUCING LOCALLY GUSTY WINDS AND BRIEF PERIODS OF IFR CIGS/VSBYS IN HEAVY RAINFALL. THE BEST CHANCE FOR THUNDERSTORMS WILL BE ACROSS THE WESTERN TERMINALS...KIAD...KMRB AND KCHO. AN EASTERLY FLOW IS EXPECTED OVERNIGHT THROUGH SUNDAY. LOW CLOUDS ARE EXPECTED OVERNIGHT INTO SATURDAY MORNING...AND POSSIBLY OVERNIGHT SUNDAY INTO MONDAY MORNING. THERE IS A BETTER CHANCE FOR FOG LATE SUNDAY NIGHT INTO MONDAY MORNING AS WELL. THE HIGH SHOULD MOVE OFF THE COAST DURING THE MIDDLE PORTION OF NEXT WEEK...BUT IT SHOULD BE CLOSE ENOUGH FOR VFR CONDITIONS MOST OF THE TIME. PATCHY FOG IS POSSIBLE DURING THE EARLY MORNING HOURS.
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&& .MARINE...
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SCATTERED THUNDERSTORMS ARE EXPECTED THIS AFTERNOON AND EARLY EVENING. THUNDERSTORMS WILL BE CAPABLE OF PRODUCING GUSTY WINDS. THE BEST CHANCE FOR THUNDERSTORMS WILL BE ACROSS THE NORTHERN PORTION OF THE CHESAPEAKE BAY AND THE NORTHERN PORTION OF THE TIDAL POTOMAC RIVER. AN EASTERLY FLOW HAS DEVELOPED THIS AFTERNOON. A PRESSURE SURGE IS EXPECTED BEHIND A BACKDOOR COLD FRONT LATE THIS AFTERNOON THROUGH THIS EVENING. A SMALL CRAFT ADVISORY IS IN EFFECT FOR THE WATERS. THE SMALL CRAFT ADVISORY CONTINUES THROUGH 2 PM SATURDAY FOR THE MARYLAND CHESAPEAKE BAY AND LOWER TIDAL POTOMAC RIVER...AND IT MAY NEED TO BE EXTENDED THROUGH SATURDAY EVENING FOR LOCATIONS NEAR DRUM POINT TO SMITH POINT. HIGH PRESSURE WILL BUILD OVER THE WATERS SATURDAY NIGHT THROUGH MONDAY. NO MARINE HAZARDS ARE EXPECTED DURING THIS TIME. THE HIGH WILL MOVE OFF THE COAST DURING THE MIDDLE PORTION OF NEXT WEEK.
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&& .TIDES/COASTAL FLOODING...
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TIDAL ANOMALIES ARE AROUND ONE-HALF FOOT ABOVE NORMAL THIS AFTERNOON. AN EASTERLY FLOW WILL STRENGTHEN THIS EVENING AND THIS WILL CAUSE TIDAL ANOMALIES TO INCREASE. CONFIDENCE AT THIS TIME IS TOO LOW FOR AN ADVISORY SINCE CURRENT WATER LEVELS ARE BELOW THE CBOFS FORECASTED LEVELS...AND THE CBOFS IS THE ONLY GUIDANCE FORECASTING MINOR FLOODING WITH THE TIDE CYCLE THIS EVENING. IT WILL BE CLOSE...ESPECIALLY NEAR ANNAPOLIS AND ST GEORGE CREEK NEAR HIGH TIDE THIS EVENING. TIDAL ANOMALIES SHOULD BE HIGH ENOUGH FOR MINOR FLOODING DURING THE HIGHER OF THE TWO HIGH TIDE CYCLES SATURDAY NIGHT.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...NONE. MD...NONE. VA...NONE. WV...NONE. MARINE...SMALL CRAFT ADVISORY UNTIL 2 PM EDT SATURDAY FOR ANZ530>534- 537>543. SMALL CRAFT ADVISORY FROM 8 PM THIS EVENING TO 2 AM EDT SATURDAY FOR ANZ535. SMALL CRAFT ADVISORY FROM 8 PM THIS EVENING TO 2 PM EDT SATURDAY FOR ANZ536.
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&& $$ SYNOPSIS...BJL NEAR TERM...BJL SHORT TERM...BJL LONG TERM...CEM AVIATION...BJL/CEM MARINE...BJL/CEM TIDES/COASTAL FLOODING...BJL

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