AFDLWX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC 409 AM EDT THU MAY 12 2011 .SYNOPSIS... HIGH PRESSURE WILL SPAN MUCH OF THE EAST COAST THROUGH FRIDAY. AN UPPER-LEVEL DISTURBANCE WILL MOVE SLOWLY INTO THE OHIO VALLEY FRIDAY AND SATURDAY AND WILL INFLUENCE THE MID-ATLANTIC REGION THROUGH EARLY NEXT WEEK. && .NEAR TERM /THROUGH TONIGHT/... LOW-LEVEL STRATUS FORMED BEFORE MIDNIGHT FOR AREAS WEST OF THE BLUE RIDGE...WITH CLOUDS NOW DEVELOPING ACROSS EASTERN ZONES AS MARINE MOISTURE HELPS TO SATURATE LOW LEVELS. CLOUDS SHOULD ERODE FROM EAST TO WEST AFTER SUNRISE...WITH PARTLY TO MOSTLY SUNNY SKIES EXPECTED BY MIDDAY. SURFACE RIDGE CENTERED OFF DELMARVA WILL CONTINUE TO BE THE DOMINANT WEATHER FEATURE TODAY...BUT AS UPPER RIDGE OVER EASTERN UNITED STATES BEGINS TO WEAKEN AND SHIFT SOUTHWARD...CHANCES WILL INCREASE FOR SHOWERS AND THUNDERSTORMS TO ENCROACH ON WESTERN PORTIONS OF THE CWA THIS EVENING/OVERNIGHT. A MINOR MID- AND UPPER-LEVEL SHORTWAVE TROUGH WILL DIVE SOUTHEAST OUT OF GREAT LAKES TOWARD CENTRAL APPALACHIANS LATE THIS AFTERNOON AND EVENING. AS DIURNAL INSTABILITY PEAKS...COMBINED WITH THE FORCING FROM THE TROUGH...CONVECTION SHOULD DEVELOP NORTH AND WEST OF THE AREA. THIS ACTIVITY SHOULD FOCUS ALONG A STATIONARY BOUNDARY DRAPED FROM OHIO THROUGH WEST VIRGINIA AND INTO NORTH CAROLINA...BUT MAY IMPACT POTOMAC HIGHLANDS. IF THUNDERSTORMS CAN ORGANIZE ENOUGH TO DEVELOP A SIGNIFICANT COLD POOL...THEY MAY BE ABLE TO REACH INTO SHENANDOAH VALLEY OVERNIGHT. WILL WATCH FOR SEVERE POTENTIAL...AS LAPSE RATES ARE STEEP...AND ORGANIZED CONVECTION COULD SUPPORT HAIL AND STRONG WINDS. AS MOISTURE PLUME ADVECTS NORTHWARD INTO REGION AHEAD OF SLOWLY ADVANCING WARM FRONT...CHANCE POPS WARRANTED DURING THE OVERNIGHT ACROSS WESTERN HALF OF CWA. TEMPERATURES TODAY AND TONIGHT VERY SIMILAR TO PREVIOUS 24 HOURS...WITH SLIGHTLY ABOVE-NORMAL MAXIMA AND MINIMA. HIGH TEMPERATURES SHOULD REACH MID TO UPPER 70S...EXCEPT ACROSS MARINE- COOLED LOCATIONS ADJACENT TO CHESAPEAKE BAY. LOW TEMPERATURES TONIGHT SHOULD BE IN MID AND UPPER 50S. && .SHORT TERM /FRIDAY THROUGH FRIDAY NIGHT/... SLOW-MOVG CLOSED UPR LOW OVER CNTRL PLNS WILL MOVE SLOWLY EWD FRI AND PHASE WITH A SECOND WAVE MOVG SEWD FROM THE NRN PLNS. VORTICITY MAXIMA EJECTING FROM MAIN CLOSED LOW WILL HELP ERODE UPR RDG OVER ERN CONUS BY FRI...THE AXIS OF WHICH WILL SHIFT EWD BY FRI EVE. PCPN ON FRI WILL BE CONFINED TO THE WRN ZONES. CHCS WILL SPREAD EWD FRI EVE OWING TO ERODING RDG. TEMPS ON FRI WILL BE TEMPERED BY INCRG CLD CVR AND SELY LOW-LVL FLOW. MAXIMA IN THE LOW 70S XPCD E OF THE MTNS...PERHAPS SLGTLY WARMER IN FAR WRN ZONES. MINIMA WILL BE ELEVATED OWING TO CLD CVR AND ADVCTN OF HIGHER DEWPT AIR. && .LONG TERM /SATURDAY THROUGH WEDNESDAY/... CLOSED LOW WILL CONT ITS SLOW EWD MVMT THRU THE WKEND AND IS XPCD TO BE LOCATED OVER KY BY SAT EVE. LOW-LVL MSTR CONTENT XPCD TO INCR ACRS MID-ATLC RGN AS SFC LOPRES INDUCES SLY FLOW ACRS THE RGN AND AIDS IN NWD MVMT OF WMFNT. PDS OF SHWRS AND TSTMS XPCD SAT-SUN WITH FAVORABLE DIFFLUENT RGN OF UPR LOW FCSTD TO MOVE ATOP WARM SECTOR. SFC-BASED INSTBY IN GREAT QUESTION GIVEN XPCD CLD CVR AND DP SLY FLOW...SO GFS DEPICTION OF UNCAPPED ENVIRONMENT WITH MDT CAPE ON SUN AFTN IS VERY SUSPECT. NEVERTHELESS...IF SUCH AN ENVIRONMENT DVLPS...SVR POTENTIAL EXISTS. MOST LKLY OUTCOME WILL BE TSTMS PRODUCING HVY RAIN. THUS...FLASH FLOOD THREAT HAS BEEN PLAYED UP IN HWO. SFC LOPRES MOVES NEWD MON AS ONE UPR TROF EJECTS AND DRAWS A CDFNT SEWD INTO THE FCST AREA. A SECOND VORTICITY MAXIMUM WILL MOVE INTO THE CNTRL APLCNS MON AND COULD LEAD TO PDS OF SHWRS UNTIL ITS DEPARTURE MID-WK. TEMPS ON THE WHOLE WILL BE COOLER THAN RECENT DAYS...WITH MAXIMA IN THE UPR 60S-LWR 70S AND MINIMA IN THE MID-UPR 50S THRU THE WKEND. && .AVIATION /07Z THURSDAY THROUGH MONDAY/... SCATTERED TO BROKEN 1200-2000 FT STRATUS EARLY THIS MORNING MAKING FOR A CHALLENGING TAF FORECAST. EXPECT THIS STRATUS TO DISSIPATE AFTER SUNRISE...WITH ONLY CIRRUS THEREAFTER. THIS EVENING AND TONIGHT THERE IS A SLIGHT CHANCE...MAINLY AT KCHO AND KMRB...OF SHOWERS AND THUNDERSTORMS IMPACTING THE TERMINALS. CONFIDENCE QUITE LOW SO NO MENTION IN TAFS AT THE MOMENT. SOUTHEAST WINDS AT 5-10 KNOTS SHOULD PERSIST THROUGH TONIGHT. PCPN CHCS INCR AT ALL TERMINALS FRI NGT-SUN AS CLOSED UPR LOPRES APRCHS FROM THE W. PDS OF SHWRS AND TSTMS XPCD DURG THIS TIME. VSBYS AND CIGS COULD REACH SUB-VFR LVLS IN CNVCTN. FOG ALSO COULD DVLP AT ALL TERMINALS DURG THE LATE EVE THRU THE WKEND GIVEN HIGH BNDRY LYR MSTR CONTENT AND RESTRICTED TEMPS OWING TO CLD CVR. && .MARINE... WINDS BELOW 15 KNOTS THROUGH TONIGHT...AND SHOULD MAINTAIN SOUTHEAST DIRECTION. SELY WINDS INCR THRU THE WKEND...LEADING TO CHANNELING AND POTENTIAL NEED FOR SMALL CRAFT ADZY. MOST LKLY TIMES FOR ADZY WOULD BE FRI EVE AND SAT AFTN-SUN EVE. PCPN CHCS INCR DRASTICALLY SAT-SUN AS CLOSED UPR LOW APRCHS FROM THE W. SHWRS AND TSTMS XPCD DURG THE WKEND. && .TIDES/COASTAL FLOODING... RECENTLY UPGRADED COASTAL FLOOD ADVISORY TO WARNING AT ARLINGTON/ ALEXANDRIA AS OLD TOWN GAGE RIGHT AT MODERATE FLOOD THRESHOLD AS HIGH TIDE APPROACHES. WITH AFTERNOON HIGH TIDE THE LOWER OF THE TWO...LEVELS SHOULD NOT GET MUCH HIGHER...DESPITE GRADUALLY INCREASING ANOMALIES AS SOUTHEAST FLOW CONTINUES. ASIDE FROM ALEXANDRIA...MINOR FLOODING LIKELY ELSEWHERE DURING TIMES OF HIGH TIDE THIS AFTERNOON AND TONIGHT. POSITIVE TIDAL ANOMALIES WILL BE SUSTAINED THRU THE WKEND AS SELY- ELY FLOW IS MAINTAINED ACRS MD CHSPK BAY AND TIDAL POTOMAC RVR. COASTAL FLOODING LKLY WILL CONT THRU THE WKEND AND MAY EVENTUALLY REACH WRNG THRESHOLDS ON A GREATER GEOGRAPHICAL SCALE. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...COASTAL FLOOD ADVISORY UNTIL 6 PM EDT THIS EVENING FOR DCZ001. MD...COASTAL FLOOD ADVISORY UNTIL 6 PM EDT THIS EVENING FOR MDZ007- 011-013-014-016>018. VA...COASTAL FLOOD ADVISORY UNTIL 6 PM EDT THIS EVENING FOR VAZ052- 053-055-057. COASTAL FLOOD WARNING UNTIL 6 PM EDT THIS EVENING FOR VAZ054. WV...NONE. MARINE...NONE. && $$