AFDLWX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC 946 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/... SURFACE HIGH PRESSURE WILL REMAIN ALONG THE EASTERN SEABOARD TONIGHT WHILE AN UPPER-LEVEL LOW TRACKS INTO THE CENTRAL PLAINS. ANOTHER UPPER-LEVEL LOW WILL REMAIN CUTOFF FROM THE JETSTREAM OVER THE ATLANTIC WHILE AN UPPER-LEVEL RIDGE AXIS BETWEEN THESE SYSTEMS REMAINS JUST TO OUR WEST. A STATIONARY BOUNDARY IS LOCATED OVER THE POTOMAC HIGHLANDS AND CENTRAL VIRGINIA. THE BOUNDARY WILL REMAIN NEARLY STATIONARY THROUGH TONIGHT. WARM AND MOIST AIR AHEAD OF THE FRONT WILL MOVE INTO THE POTOMAC HIGHLANDS TODAY CAUSING MODERATE AMOUNTS OF INSTABILITY DURING THE AFTERNOON AND EVENING HOURS. AN UPPER-LEVEL DISTURBANCE WILL ROTATE AROUND THE RIDGE AXIS MOVING THROUGH THE POTOMAC HIGHLANDS LATE THIS AFTERNOON AND EVENING. THE COMBINATION OF MODERATE INSTABILITY AND LIFT FROM THE UPPER-LEVEL DISTURBANCE ALONG WITH THE BOUNDARY WILL CAUSE SHOWERS AND THUNDERSTORMS TO DEVELOP. SOME THUNDERSTORMS THIS AFTERNOON AND EVENING WILL BE CAPABLE OF PRODUCING DAMAGING WINDS AND LARGE HAIL DUE TO THE MODERATE INSTABILITY. FURTHER EAST ACROSS THE REST OF THE CWA...DRY AND SEASONABLE CONDITIONS ARE EXPECTED THIS AFTERNOON. AN ONSHORE FLOW ON THE COOL SIDE OF THE FRONT WILL CAUSE INCREASING AMOUNTS OF MOISTURE TONIGHT. THE MOISTURE WILL COMBINE WITH ISENTROPIC LIFT DUE TO WARM ADVECTION FOR POSSIBLE SHOWERS. TEMPERATURES THIS AFTERNOON 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 /14Z THURSDAY THROUGH MONDAY/... VFR CONDITIONS ARE EXPECTED THROUGH THIS AFTERNOON. 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. CIGS AND VSBYS WILL LIKELY BE REDUCED TO MVFR LEVELS AND POSSIBLE IFR LEVELS LATE TONIGHT INTO FRIDAY MORNING DUE TO AN ONSHORE FLOW. ISOLATED TO SCATTERED SHOWERS ARE POSSIBLE TONIGHT THROUGH FRIDAY. T-STORMS ARE POSSIBLE FRIDAY AFTERNOON ACROSS THE WESTERN TERMINALS. 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... ONSHORE FLOW WILL CONTINUE THROUGH TODAY. MINOR FLOODING IS EXPECTED ACROSS MOST AREAS NEAR TIMES OF HIGH TIDE THIS AFTERNOON. WILL CONTINUE WITH THE COASTAL FLOOD WARNING FOR ALEXANDRIA AND ARLINGTON WHERE MODERATE FLOODING IS POSSIBLE NEAR HIGH TIDE THIS AFTERNOON. A COASTAL FLOOD ADVISORY WILL LIKELY BE WARRANTED TONIGHT INTO FRIDAY FOR MOST OF THE AREA. WILL CONTINU TO MONITOR THIS AFTERNOON. 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. && $$ LASORSA/SBK/KRAMAR