Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 201330 AFDLWX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC 930 AM EDT WED AUG 20 2014 .SYNOPSIS... A STATIONARY BOUNDARY WILL REMAIN OVER THE REGION THROUGH THURSDAY. THE BOUNDARY WILL SLOWLY DROP SOUTH AND WEST AS A COLD FRONT FRIDAY AND HIGH PRESSURE OVER NEW ENGLAND WILL EXTEND INTO OUR AREA FOR THE WEEKEND. HOWEVER...UNSETTLED CONDITIONS MAY PERSIST DURING THIS TIME DUE TO PLENTY OF MOISTURE FROM THE ATLANTIC. && .NEAR TERM /THROUGH TONIGHT/... A SHORTWAVE TROUGH WILL APPROACH THE MID ATLANTIC THIS MORNING. SHOWERS AND THUNDERSTORMS HAVE BEEN ONGOING ACROSS THE OHIO VALLEY AND APPALACHIAN MTNS AND WILL BE MOVING ACROSS THE ALLEGHENY FRONT THIS MORNING. VISIBLE SATELLITE SHOWS MOSTLY SUNNY CONDITIONS WEST OF THE BLUE RIDGE. FURTHER EAST...S-SE WINDS HAVE BANKED CLOUDS AGAINST THE BLUE RIDGE MTNS SOUTH OF THE POTOMAC RIVER RESULTING IN LOW CLOUDS FOR LOCATIONS ADJACENT TO THE MTNS THIS MORNING. THESE CLOUDS MAY CREEP NORTHWARD BEFORE DISSIPATING THIS AFTERNOON. 12Z IAD RAOB HAS ABOUT 500 J/KG OF SBCAPE AND AFTER MODIFYING IT FOR THE AFTERNOON...INSTABILITY IS EXPECTED TO INCREASE TO AROUND 1000 J/KG. INSTABILITY SHOULD INCREASE MORE IF AN AREA RECEIVES MORE HEATING. DEWPTS ARE EXPECTED TO STAY OR REACH THE UPPER 60S TO AROUND 70 BY THE AFTERNOON AND AS MOISTURE MOVES IN FROM THE WEST...PWATS WILL LIKELY INCREASE TO AROUND 1.8 INCHES. SHEAR IS WEAK...AROUND 25KTS AND THUNDERSTORMS SHOULD STAY SUB-SEVERE TODAY. HRRR HAS LINE OF SHOWERS AND THUNDERSTORMS MOVING EASTWARD THROUGH THE MORNING AND REACHING THE BLUE RIDGE BY EARLY AFTERNOON AND THE I-95 CORRIDOR LATE THIS AFTERNOON. ACTIVITY IS MORE ISO-SCT FURTHER SOUTH AND THE BEST CHANCE FOR RAIN WILL BE NORTH OF I-66 AS SHORTWAVE ENERGY MOVES TO THE NORTH. MAIN THREAT IN ACTIVITY WILL BE HEAVY RAIN IN SHOWERS AND THUNDERSTORMS. PREVIOUS DISCUSSION... MAX TEMPS WILL RANGE FROM THE 70S IN THE MOUNTAINS TO THE MIDDLE 80S ACROSS MOST OTHER LOCATIONS. COVERAGE IN CONVECTION SHOULD DIMINISH OVERNIGHT...BUT WITH THE BOUNDARY NEARBY AND SHORTWAVE ENERGY MOVING THROUGH...A FEW SHOWERS CANNOT BE RULED OUT. AREAS OF LOW CLOUDS AND FOG WILL DEVELOP LATE TONIGHT AS WELL. WARM AND HUMID CONDITIONS WILL PERSIST WITH MIN TEMPS RANGING FROM NEAR 60 IN THE ALLEGHENY HIGHLANDS TO THE LOWER 70S IN DOWNTOWN WASHINGTON AND BALTIMORE. && .SHORT TERM /THURSDAY THROUGH THURSDAY NIGHT/... THE UPPER-LEVEL LOW WILL MOVE INTO NEW ENGLAND THURSDAY THROUGH THURSDAY NIGHT. SHORTWAVE ENERGY WILL CONTINUE TO ROTATE THROUGH THE AREA IN A NORTHWEST FLOW ALOFT. THE BOUNDARY WILL REMAIN NEARLY STATIONARY FROM CENTRAL MARYLAND THROUGH CENTRAL VIRGINIA AND THE WARM AND HUMID AIRMASS WILL REMAIN OVERHEAD. MORE SHOWERS AND THUNDERSTORMS ARE EXPECTED WITH COVERAGE BEING SCATTERED TO NUMEROUS...ESPECIALLY DURING THE AFTERNOON AND EVENING HOURS. MAX TEMPS THURSDAY WILL RANGE FROM THE 70S IN THE MOUNTAINS TO THE MID AND UPPER 80S ACROSS MOST OTHER LOCATIONS. MIN TEMPS THURSDAY NIGHT WILL RANGE FROM THE LOWER 60S IN THE ALLEGHENY HIGHLANDS TO THE LOWER 70S IN DOWNTOWN WASHINGTON AND BALTIMORE. PATCHY LOW CLOUDS AND FOG ARE EXPECTED OVERNIGHT AS MOISTURE GETS TRAPPED UNDERNEATH THE NOCTURNAL INVERSION. && .LONG TERM /FRIDAY THROUGH TUESDAY/... H5 PTTN MUCH MORE AMPLIFIED/ACTIVE THAN NORMAL FOR LT AUG...BUT THEN AGAIN THIS HASNT BEEN A TYPICAL SUMMER. A SUBTROPICAL RDG WL BE ANCHORED OVER THE MS VLY THRU MUCH OF THE PD. THAT LEAVES CWFA IN NW FLOW ALOFT...AND TEMPS COOLER THAN CLIMO. WUD LOVE TO BE AS DEFINITIVE W/ POPS. HWVR...SUBSIDENCE WL BE CONFINED TO OVER NEW ENGLAND. WHILE AMS LKLY WONT BE OUTRIGHT CAPPED...THERE ARENT SPECIFIC DISCERNABLE FORCING MECHANISMS EITHER...BUT THERE WL BE THE JETSTREAM ABV AREA AT HIER LVLS. THAT LEAVES SENSIBLE WX SUSCEPTIBLE TO MID-UPA DYNAMICS SUCH AS S/WV IMPULSES EMBEDDED W/IN MEAN LYR FLOW. IT APPEARS AS THOUGH THERE WL BE SUCH A DISTURBANCE FRI NGT INTO SAT. INSTBY MEAGER...AND BECOME INCREASINGLY LMTD AS SFC FLOW BECOMES ELY. BYD THAT...EVEN BROAD DETAILS BECOME PROBLEMATIC. BY ELY NXT WK... RDGG SHUD RETROGRADE...PROVIDING A BETTER BET AT SOME DRY CONDS IN THE CSTL PLAIN. STILL CANT RULE OUT A SHRA IN THE MTNS...BUT THE CHCS FAR LESS. && .AVIATION /13Z WEDNESDAY THROUGH SUNDAY/... MVFR CONDITIONS WILL CONTINUE AT CHO THIS MORNING. SCT MVFR CONDITIONS ARE LIKELY AT IAD-DCA AS LOW CLOUDS CONTINUE TO MOVE NORTHWARD AGAINST THE MTNS. A FEW SHOWERS AND THUNDERSTORMS ARE POSSIBLE THIS AFTERNOON AND EVENING. MORE LOW CLOUDS AND FOG WILL DEVELOP LATE TONIGHT INTO THURSDAY MORNING. SHOWERS AND THUNDERSTORMS ARE POSSIBLE THURSDAY AND THURSDAY NIGHT...ESPECIALLY DURING THE AFTERNOON AND EVENING HOURS. FLGT RESTRICTIONS PROBABLE W/IN SHRA/TSRA THRU THE WKND. HWVR... AREAL CVRG A WILD CARD. FRI NGT-SAT LOOK TO BE THE WETTEST PART OF THE OUTLOOK ATTM. && .MARINE... A BOUNDARY WILL REMAIN NEAR THE WATERS THROUGH THURSDAY NIGHT...BUT THE PRESSURE GRADIENT SHOULD BE WEAK ENOUGH TO PREVENT SCA CRITERIA WINDS DURING THIS TIME. A FEW SHOWERS AND THUNDERSTORMS ARE POSSIBLE...ESPECIALLY LATE THIS AFTERNOON THROUGH THIS EVENING...AND AGAIN THURSDAY AFTERNOON AND THURSDAY NIGHT. ATTM NO ADVYS XPCTD THRU THIS WKND. HWVR UNSTTLD CONDS WL PREVAIL... AND CANNOT PRECLUDE A FEW MARINE WRNGS INVOF TSRA. && .TIDES/COASTAL FLOODING... AN ONSHORE FLOW WILL CONTINUE THROUGH TONIGHT...BUT IT WILL REMAIN WEAK. THE FLOW MAY TURN MORE TOWARD THE SOUTHEAST WHICH WOULD CAUSE WATER LEVELS TO BE SLIGHTLY HIGHER THAN THEY HAVE BEEN RECENTLY. THIS WILL CAUSE WATER LEVELS TO BE CLOSE TO MINOR FLOODING THRESHOLDS DURING THE HIGH TIDE CYCLE LATE TONIGHT INTO EARLY THURSDAY MORNING. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...NONE. MD...NONE. VA...NONE. WV...NONE. MARINE...NONE. && $$ SYNOPSIS...BJL NEAR TERM...BJL/HAS SHORT TERM...BJL LONG TERM...HTS AVIATION...BJL/HAS/HTS MARINE...BJL/HAS/HTS TIDES/COASTAL FLOODING...BJL

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