Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 040004 AFDLWX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC 804 PM EDT FRI JUL 3 2015 .SYNOPSIS... LOW PRESSURE WILL ADVANCE EAST-NORTHEAST UP THE OHIO VALLEY TONIGHT. A SECONDARY LOW WILL DEVELOP OVER VIRGINIA AND HEAD EAST-NORTHEAST INTO THE ATLANTIC OCEAN ON SATURDAY. HIGH PRESSURE WILL BUILD ACROSS THE AREA ON SUNDAY. ANOTHER LOW PRESSURE WILL MOVE INTO THE AREA MONDAY AND LINGER IN THE AREA FOR A FEW DAYS. && .NEAR TERM /THROUGH SATURDAY/... A BOUNDARY REMAINS STALLED OUT NEAR THE APPALACHIAN MOUNTAINS INTO CENTRAL VIRGINIA. SHORTWAVE ENERGY IN THE SOUTHWEST FLOW ALOFT IS MOVING INTO THE AREA THIS EVENING...AND THIS WILL CAUSE SURFACE LOW PRESSURE TO DEVELOP ALONG THE BOUNDARY OVERNIGHT. WARM AND MOIST AIR WILL OVERRUN THE SURFACE COOLER AIR IN PLACE...CAUSING PLENTY OF CLOUDS ALONG WITH AN INCREASE IN SHOWER ACTIVITY. THERE MAY BE A PERIOD OF HEAVIER RAIN AFTER MIDNIGHT AS FORCING ALONG THE 850MB BOUNDARY INCREASES DUE TO DEVELOPING LOW PRESSURE ALONG THE SURFACE BOUNDARY. THE BEST CHANCE FOR THE HEAVIER RAINFALL TO OCCUR WILL BE IN THE POTOMAC HIGHLANDS/SHENANDOAH VALLEY TOWARD NORTHERN MARYLAND AND THE WESTERN SUBURBS OF WASHINGTON DC. A STABLE ATMOSPHERE SHOULD PREVENT TORRENTIAL RAINS FROM DEVELOPING...THEREFORE ANY FLOOD THREAT WILL BE LIMITED AT BEST. THE SURFACE LOW WILL EMERGE OVER THE DELMARVA PENINSULA SATURDAY MORNING. THE SURFACE BOUNDARY WILL REMAIN NEARLY STATIONARY EXTENDING THROUGH CENTRAL VIRGINIA INTO THE APPALACHIAN MOUNTAINS. SHOWERS WILL BE WIDESPREAD SATURDAY MORNING ALONG AND NORTH OF THE BOUNDARY...BUT COVERAGE OF PRECIPITATION SHOULD DISSIPATE FROM SOUTHWEST TO NORTHEAST LATER IN THE MORNING AS THE LOW PULLS FARTHER AWAY FROM THE AREA AND FORCING DISSIPATES IN THE LOWER- LEVELS OF THE ATMOSPHERE. HOWEVER...ADDITIONAL SHOWERS AND A FEW THUNDERSTORMS MAY POP UP WITH THE HEATING OF THE DAY MAINLY FOR LOCATIONS ALONG AND SOUTH/WEST OF THE BOUNDARY. THEREFORE...THE BEST CHANCE FOR THUNDERSTORMS SATURDAY AFTERNOON WILL BE ACROSS THE SHENANDOAH VALLEY OF VIRGINIA INTO THE POTOMAC HIGHLANDS AND VIRGINIA PIEDMONT. FARTHER NORTH AND EAST ACROSS THE WASHINGTON AND BALTIMORE METROPOLITAN AREAS...MUCH OF THE TIME MAY TURN OUT DRY. MAX TEMPS WILL RANGE FROM MID TO UPPER 70S ACROSS NORTHEASTERN MARYLAND WHERE CLOUDS AND A COOL NORTHEAST WIND WILL PERSIST TO THE LOWER AND MIDDLE 80S ACROSS CENTRAL VIRGINIA WHERE WARMER AIR IS EXPECTED SOUTH AND WEST OF THE BOUNDARY. && .SHORT TERM /SATURDAY NIGHT THROUGH SUNDAY NIGHT/... LOW PRESSURE CONTINUES TO MOVE AWAY FROM THE AREA SATURDAY NIGHT...BUT UPPER TROUGH MAY STILL KICK OFF A FEW SHOWERS. OTHERWISE...CONDITIONS SHOULD GENERALLY BE IMPROVING...BUT IF THERE IS CLEARING LATE SOME PATCHY FOG MAY DEVELOP. LOWS WILL AGAIN BE IN THE 60S. NORTHWARD PROGRESSION OF THE FRONTAL BOUNDARY HOVERING NEAR THE VA/NC BORDER WILL BE MINIMAL ON SUNDAY AS HIGH PRESSURE BUILDS IN AT THE SFC UNDER A DEVELOPING RIDGE ALOFT. LOW PRESSURE RIDING THE FRONT SHOULD EXIT THE AREA BY 12Z SUN...WITH DRY CONDITIONS THEN EXPECTED THE REMAINDER OF THE MORNING. DEPENDING ON WHERE EXACTLY THE FRONT ALIGNS...A STRAY SHOWER OR TSTM COULD MOVE INTO THE SOUTHERN PORTION OF THE CWA...SO WILL JUST CONTINUE WITH THE CHC POPS FOR NOW. BY SUNDAY EVENING...12Z GFS AND ECMWF BOTH INDICATING RIDGE AXIS ALIGNS FAR ENOUGH EAST TO ALLOW PROGRESSION OF A SOUTHERN STREAM SHORTWAVE TO PROGRESS ALONG THE APPALACHIANS. STILL UNSURE HOW FAR NORTH THE PCPN WILL PROGRESS DURING THE NIGHT WITH ANY SFC BOUNDARIES LOOKING TO LINGER FURTHER SOUTH...AND NOT PUSHING NORTH TILL THE BEGINNING OF NEXT WORK WEEK. WARM AND MOIST AIRMASS IN PLACE DURING THE DAY SUNDAY. 850MB TEMPS RIGHT AROUND 13-14C...WITH MOST OF THE AREA PUSHING INTO THE 80S. DEW PTS GENERALLY HOVERING IN THE MID TO UPPER 60S. ENOUGH INSTABILITY SHOULD BE GENERATED TO RESULT IN THUNDER...WHEREVER ANY CONVECTION DOES DEVELOP. HOWEVER...LACK OF SHEAR SHOULD KEEP TSTMS BELOW SVR LVLS. && .LONG TERM /MONDAY THROUGH FRIDAY/... UPPER LEVEL CUT-OFF LOW OPENING UP AND FLOW BECOMING ZONAL OVER MID ATLANTIC REGION AS TROUGH ON NORTHERN TRACK MOVES EASTWARD. NEXT TROUGH TO AFFECT REGION FRIDAY. AREAS OF WEAK TO MODERATE PVA PRESENT THROUGH THE FORECAST PERIOD. SURFACE LOW PRESSURE TROUGH OVER REGION WILL SLOWLY FILL THROUGH TUESDAY NIGHT AND BE REPLACED BY WEAK HIGH PRESSURE RIDGING. DYNAMICS SUPPORT CONVECTIVE DEVELOPMENT DURING THE DAY MONDAY WITH DRYING OVERNIGHT. MORE DEVELOPMENT DUE AS A FRONT FROM THE WEST CROSSES THE REGION WEDNESDAY THROUGH THURSDAY NIGHT. && .AVIATION /00Z SATURDAY THROUGH WEDNESDAY/... AN EASTERLY FLOW WILL CONTINUE TONIGHT. SHOWERS WILL INCREASE IN COVERAGE AND CIGS SHOULD DROP TO MVFR/IFR LEVELS OVERNIGHT. VSBYS WILL REDUCED TO MVFR LEVELS IN SHOWERS AND AREAS OF BR. LOW CLOUDS SHOULD GRADUALLY DISSIPATE SATURDAY MORNING INTO SATURDAY AFTERNOON. HOWEVER...SUBVFR CIGS MAY BE TOUGH TO ERODE DUE TO A NORTHEAST FLOW. A FEW SHOWERS AND THUNDERSTORMS ARE POSSIBLE SATURDAY AFTERNOON/EVENING...ESPECIALLY ACROSS KCHO AND KMRB. AREAS OF FOG ARE POSSIBLE SATURDAY NIGHT. SUBVFR CIGS/VSBYS PSBL INITIALLY SUN MORNING...WITH CONDITIONS IMPROVING TO VFR AT ALL TAF SITES DURING THE DAY. KCHO HAS THE BEST CHC OF SEEING ANY PCPN SUN-SUN NIGHT. WINDS WILL BE LIGHT...LESS THAN 8 KTS. VFR CONDITIONS SHOULD PREVAIL THRU NEXT WEEK. SHOWERS AND THUNDERSTORMS WILL REMAIN A CONCERN. && .MARINE... LOW PRESSURE PASSING TO THE SOUTH WILL INTENSIFY WINDS UP TO LOW- END SCA LEVELS TONIGHT INTO SATURDAY MORNING. OTHERWISE...MAIN CONCERN IS LOWERED VISIBILITY IN RAIN SHOWERS TONIGHT INTO SATURDAY. SHOWERS SHOULD DECREASE IN COVERAGE SATURDAY MORNING...BUT A FEW ADDITIONAL SHOWERS AND THUNDERSTORMS MAY POP UP SATURDAY AFTERNOON AND EVENING. THE BEST CHANCE FOR THUNDERSTORMS WILL BE ACROSS THE MIDDLE PORTION OF THE BAY AND LOWER TIDAL POTOMAC RIVER. NORTH TO NORTHEAST WINDS WILL BE BELOW SCA CRITERIA SATURDAY NIGHT WEAK PRESSURE GRADIENT WILL RESULT IN A LIGHT FLOW SUN-SUN NIGHT...KEEPING SUB-SCA CONDITIONS IN PLACE. SLIGHT CHC OF SHOWERS SUN NIGHT...THOUGH WITH MINIMAL IMPACT ON VSBY. SUB-SCA CONDITIONS SHOULD PREVAIL THRU NEXT WEEK. SHOWERS AND THUNDERSTORMS WILL REMAIN A CONCERN. && .HYDROLOGY... SOME GUIDANCE IS SHOWING LOCALLY HEAVY RAINFALL ACROSS THE APPALACHIANS INTO THE SHENANDOAH VALLEY/NORTHERN MARYLAND OVERNIGHT. THOUGHT WAS GIVEN TO THE POTENTIAL OF ISSUING A FLOOD WATCH...BUT DUE TO A STABLE ATMOSPHERE AT THIS TIME...CURRENT FEELING IS THAT PRECIPITATION WILL FALL SLOWLY ENOUGH AND BE LIGHT ENOUGH TO NOT CAUSE SIGNIFICANT FLOOD CONCERNS. WILL CONTINUE TO MONITOR. && .TIDES/COASTAL FLOODING... EASTERLY FLOW AROUND A PASSING LOW PRESSURE SYSTEM WILL HELP ELEVATE WATER LEVELS ALONG TIDAL PORTIONS OF THE POTOMAC AND MARYLAND CHESAPEAKE BAY OVERNIGHT INTO SATURDAY MORNING. THE HIGH TIDE WILL BE THE HIGHER OF THE TWO DURING THIS TIME. THEREFORE...MINOR TIDAL FLOODING IS EXPECTED FOR SENSITIVE AREAS ALONG THE MARYLAND CHESAPEAKE BAY FROM BALTIMORE TO ST MARYS COUNTY AND ALSO ALONG THE UPPER TIDAL POTOMAC RIVER IN WASHINGTON DC AND ALEXANDRIA. THE MINOR FLOODING WILL BE MOST LIKELY AROUND HIGH TIDE SATURDAY MORNING AND CONFINED MAINLY TO SENSITIVE AREAS. ANOMALIES SHOULD DECREASE SATURDAY AFTERNOON AS A NORTHERLY COMPONENT TO THE SURFACE WINDS DEVELOPS BEHIND DEPARTING LOW PRESSURE. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...COASTAL FLOOD ADVISORY FROM 5 AM TO NOON EDT SATURDAY FOR DCZ001. MD...COASTAL FLOOD ADVISORY FROM 5 AM TO NOON EDT SATURDAY FOR MDZ011-014. COASTAL FLOOD ADVISORY FROM 2 AM TO 9 AM EDT SATURDAY FOR MDZ017-018. VA...COASTAL FLOOD ADVISORY FROM 5 AM TO NOON EDT SATURDAY FOR VAZ054. WV...NONE. MARINE...SMALL CRAFT ADVISORY FROM 6 AM TO 2 PM EDT SATURDAY FOR ANZ532- 533-540>542. COASTAL FLOOD ADVISORY FROM 5 AM TO NOON EDT SATURDAY FOR ANZ535-538. COASTAL FLOOD ADVISORY FROM 2 AM TO 9 AM EDT SATURDAY FOR ANZ542. SMALL CRAFT ADVISORY UNTIL 2 PM EDT SATURDAY FOR ANZ534-537- 543. SMALL CRAFT ADVISORY UNTIL 6 AM EDT SATURDAY FOR ANZ536. && $$ SYNOPSIS...SEARS NEAR TERM...BJL/RCM SHORT TERM...SEARS/RCM LONG TERM...CEM AVIATION...BJL/CEM/SEARS/RCM MARINE...BJL/CEM/SEARS/RCM HYDROLOGY...BJL/RCM TIDES/COASTAL FLOODING...BJL

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