AFDLWX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC 322 PM EDT THU MAY 12 2011 .SYNOPSIS... HIGH PRESSURE WILL SPAN MUCH OF THE EAST COAST THROUGH FRIDAY. AN UPPER LEVEL LOW WILL MOVE FROM THE OHIO VALLEY TOWARD THE APPALACHIANS THIS WEEKEND...AND REMAIN NEARLY STATIONARY INTO THE MID PORTION OF NEXT WEEK. MULTIPLE UPPER LEVEL DISTURBANCES ROTATING AROUND THE LOW WILL BRING UNSETTLED CONDITIONS TO THE MID ATLANTIC THIS WEEKEND THROUGH THE MIDDLE OF NEXT WEEK. && .NEAR TERM /THROUGH FRIDAY/... SURFACE HIGH PRESSURE WILL REMAIN ALONG THE EASTERN SEABOARD THROUGH 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 THIS AFTERNOON. THE BOUNDARY WILL REMAIN NEARLY STATIONARY THROUGH TONIGHT. WARM AND MOIST AIR AHEAD OF THE FRONT HAS CAUSED MODERATE AMOUNTS OF INSTABILITY ACROSS THE POTOMAC HIGHLANDS. AN UPPER-LEVEL DISTURBANCE HAS COMBINED WITH THE MODERATE INSTABILITY TO TRIGGER SHOWERS AND T-STORMS ACROSS THIS AREA. THERE MAY BE A BREAK DURING THE MID AFTERNOON HOURS BEFORE ANOTHER WAVE TRIGGERS MORE CONVECTION ACROSS THIS AREA LATE THIS AFTERNOON AND EVENING. ANY THUNDERSTORMS THROUGH THIS EVENING WILL BE CAPABLE OF PRODUCING DAMAGING WINDS AND LARGE HAIL DUE TO THE MODERATE INSTABILITY. ISOLATED AREAS OF FLASH FLOODING IS POSSIBLE AS WELL DUE TO THE SLOW MOVEMENT OF THE STORMS. HIGH PRESSURE WILL BRING DRY CONDITIONS LATE THIS AFTERNOON TO MOST OTHER LOCATIONS. HOWEVER...THE UPPER-LEVEL DISTURBANCE MAY TRIGGER A FEW SHOWERS AND THUNDERSTORMS FURTHER EAST INTO THE SHENANDOAH VALLEY THIS EVENING. AN EASTERLY FLOW WILL DEVELOP OVERNIGHT AROUND SURFACE HIGH PRESSURE AS IT SLOWLY MOVES OFF THE COAST. THE COMBINATION OF ISENTROPIC LIFT FROM WARM ADVECTION ALONG WITH INCREASING MOISTURE WILL BRING A CHANCE FOR SHOWERS TOWARD THE WASHINGTON AND BALTIMORE METROPOLITAN AREAS DURING THIS TIME. SURFACE HIGH PRESSURE JUST OFF THE EAST COAST WILL CONTINUE TO BRING AN ONSHORE FLOW TO THE AREA FRIDAY. PLENTY OF CLOUDS ALONG WITH COOLER CONDITIONS ARE EXPECTED ACROSS MOST LOCATIONS. ISOLATED TO SCATTERED SHOWERS CANNOT BE RULED OUT. WARMER CONDITIONS ARE EXPECTED ACROSS CENTRAL VIRGINIA INTO THE POTOMAC AND ALLEGHENY HIGHLANDS ON THE OTHER SIDE OF THE STATIONARY BOUNDARY. MORE WIDESPREAD SHOWERS AND THUNDERSTORMS ARE EXPECTED ACROSS THESE AREAS. THE BEST CHANCE FOR THUNDERSTORMS WILL BE DURING THE AFTERNOON HOURS FRIDAY. INSTABILITY MAY BE MORE LIMITED COMPARED TO TODAY DUE TO RELATIVELY COOLER TEMPS AND A MID-LEVEL RIDGE AXIS OVERHEAD. HOWEVER...THE SLOW MOVEMENT OF STORMS SUGGESTS THAT ISOLATED FLASH FLOODING MAY BE A CONCERN FRIDAY AFTERNOON FOR AREAS THAT RECEIVE T-STORMS. && .SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY/... UPPER LOW OVER THE OHIO VALLEY WILL MAKE A SLOW EASTWARD PROGRESS TOWARD THE APPALACHIANS OVER THE WEEKEND. MULTIPLE VORT MAXES WILL EJECT FROM THE LOW AND SUPPORT INCREASED CHANCES OF SHOWERS AND THUNDERSTORMS ACROSS THE CWA. TIMING THESE SHORTWAVES BECOMES MORE DIFFICULT THE FURTHER OUT ONE GOES...BUT CONVECTION MAY NOT SIMPLY BE CONFINED TO AFTERNOONS AND EVENINGS. IF THE TIMING IS RIGHT...THERE MAY BE CONVECTION INTO THE NIGHT OR EVEN DURING THE MORNING. GFS SUGGESTS THIS IS THE CASE FOR SATURDAY NIGHT. WITH RELATIVELY HIGH PRECIPITABLE WATER VALUES...AM CONCERNED ABOUT HEAVY RAINFALL WHICH COULD PRESENT AN ISOLATED FLASH FLOODING THREAT. HOWEVER...GIVEN THE PROXIMITY OF THE UPPER TROUGH AND ALSO THE PRESENCE OF A LOW LEVEL BOUNDARY...THERE MAY BE A SEVERE RISK BY SUNDAY AS WELL. THIS WOULD ESPECIALLY BE THE CASE IF ANY LOW CLOUDS IN THE MORNING ARE ABLE TO BREAK UP AND CONTRIBUTE TO INCREASED INSTABILITY. EAST TO SOUTHEAST FLOW MAY LEAD TO LOWER CLOUDS THROUGH THE WEEKEND WHICH IF THEY PERSIST INTO THE AFTERNOON WOULD TEMPER WARMING AND TEMPERATURES. SAME FOR ANY MORNING CONVECTION. OF COURSE IF BREAKS DO OCCUR THEN TEMPERATURES WILL ABLE TO WARM WELL INTO THE 70S. HAVE TAKEN A MIDDLE OF THE ROAD APPROACH WITH MAINLY LOWER 70S FOR HIGH TEMPERATURES THIS WEEKEND. && .LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/... UPPER LOW WILL CONTINUE TO AFFECT THE CWA THROUGH THE MIDDLE OF NEXT WEEK AS IT REMAINS NEARLY STATIONARY OVER THE OHIO VALLEY/APPALACHIANS AREA...PERHAPS EVEN RETROGRADING SOME...BEFORE THE MODELS FINALLY TRY TO EJECT IT UP AND OUT BY THE END OF THE WEEK. THE RESULT WILL BE A CONTINUATION OF UNSETTLED CONDITIONS WITH CHANCES OF SHOWERS AND THUNDERSTORMS THAT MAY NOT NECESSARILY ONLY BE IN THE AFTERNOONS/EVENINGS ALTHOUGH WOULD THINK THERE WOULD BE SUCH AN UPTICK IN CONVECTION DUE TO DIURNAL INSTABILITY INFLUENCES. TIMING INDIVIDUAL VORT MAXES THIS FAR OUT ISNT FEASIBLE AND WILL NEED TO FINE TUNE DETAILS AS WE APPROACH NEXT WEEK. && .AVIATION /19Z THURSDAY THROUGH TUESDAY/... VFR CONDITIONS ARE EXPECTED THROUGH THIS EVENING. THERE IS A CHANCE FOR SHOWERS AND T-STORMS THIS EVENING ACROSS KCHO AND KMRB. HAVE LEFT IT OUT OF THE TAFS FOR NOW DUE TO LOW CONFIDENCE. AN ONSHORE FLOW WILL DEVELOP LATE TONIGHT THROUGH FRIDAY. MVFR CIGS ARE LIKELY DURING THIS TIME WITH POSSIBLE MVFR VSBYS. IFR CIGS/VSBYS CANNOT BE RULED OUT...ESPECIALLY LATE TONIGHT AND FRIDAY MORNING. HAVE ONLY INCLUDED IFR AT KCHO WHERE CONFIDENCE IS A BIT HIGHER. SCATTERED SHOWERS WILL LIKELY IMPACT THE TERMINALS LATE TONIGHT THROUGH FRIDAY. MET MOS AND SREFS GUIDANCE INDICATE HIGH PROBABILITIES OF IFR CONDITIONS OVER MUCH OF THE WEEKEND FOR TERMINALS. GFS MOS IS A LITTLE MORE ON THE CONSERVATIVE SIDE BUT ALSO SUGGESTS IFR CONDITIONS DURING THE OVERNIGHT/MORNING PERIODS GENERALLY FROM CEILINGS. SYNOPTICALLY THIS IS REASONABLE GIVEN LOW LEVEL EASTERLY FLOW AND POTENTIAL ADVECTION OF STRATUS...AND HAVE INCREASING CONFIDENCE IN OVERNIGHT/MORNING IFR CIGS FROM STRATUS. CONFIDENCE IS LOWER OUTSIDE OF THIS TIME WINDOW...ALTHOUGH CANT BE VIEWED AS NIL. IN ADDITION...THERE WILL BE PERIODIC SHOWERS AND THUNDERSTORMS AND VISIBILITIES MAY BE REDUCED AT TIMES IN THIS ACTIVITY. BOTTOM LINE IS A POTENTIALLY ACTIVE WEEKEND WITH FLIGHT RESTRICTIONS POSSIBLE FROM BOTH LOW CLOUDS AND FROM THUNDERSTORMS. WILL HAVE TO FINE TUNE THE DETAILS AS WE GET CLOSER. UPPER LOW RESPONSIBLE FOR WEATHER THIS WEEKEND WILL CONTINUE TO BRING UNSETTLED CONDITIONS THROUGH THE MIDDLE OF NEXT WEEK. && .MARINE... HIGH PRESSURE WILL SLOWLY MOVE OFF THE EAST COAST TONIGHT THROUGH FRIDAY. AN ONSHORE FLOW WILL DEVELOP DURING THIS TIME. HAVE KEPT WIND GUSTS CAPPED AROUND 15 KNOTS FOR NOW DUE TO THE RELATIVELY COOLER WATERS AND WEAKER PRESSURE GRADIENT. UPPER LOW APPROACHES FROM THE WEST THIS WEEKEND...BRINGING INCREASED CHANCES OF SHOWERS AND THUNDERSTORMS. EAST/SOUTHEAST FLOW WILL HAVE TO BE WATCHED AS IT COULD FLIRT WITH SMALL CRAFT ADVISORY CONDITIONS ESPECIALLY ON SATURDAY. EASTERLY FLOW LIKELY LASTS THROUGH THE MIDDLE OF NEXT WEEK...AS LOW PRESSURE BECOMES NEARLY STATIONARY WEST OF THE WATERS. && .TIDES/COASTAL FLOODING... THE ONSHORE FLOW THAT HAS BROUGHT MINOR COASTAL FLOODING TO MOST LOCATIONS WILL PERSIST THROUGH FRIDAY. A COASTAL FLOOD ADVISORY HAS BEEN EXTENDED THROUGH THE HIGH TIDE CYCLE TONIGHT INTO EARLY FRIDAY MORNING. A COASTAL FLOOD WARNING REMAINS IN EFFECT FOR ALEXANDRIA AND ARLINGTON WHERE TIDAL ANOMALIES WILL CAUSE WATER LEVELS TO REACH MODERATE FLOODING THRESHOLDS. COASTAL FLOOD HEADLINES WILL LIKELY BE WARRANTED DURING THE DAY FRIDAY AS WELL. POSITIVE TIDAL ANOMALIES WILL CONTINUE THROUGH THE WEEKEND AND PERHAPS INTO EARLY NEXT WEEK. THEREFORE...THE RISK OF TIDAL FLOODING WILL CONTINUE THROUGH THEN. WILL HAVE BE CONCERNED ABOUT THE POTENTIAL FOR MODERATE TIDAL FLOOD STAGES BEING REACHED BY THE WEEKEND AS WELL. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...COASTAL FLOOD ADVISORY UNTIL 7 AM EDT FRIDAY FOR DCZ001. MD...COASTAL FLOOD ADVISORY UNTIL 7 AM EDT FRIDAY FOR MDZ007-011- 013-014-016>018. VA...COASTAL FLOOD ADVISORY UNTIL 7 AM EDT FRIDAY FOR VAZ052-053- 055-057. COASTAL FLOOD WARNING UNTIL 7 AM EDT FRIDAY FOR VAZ054. WV...NONE. MARINE...NONE. && $$ SYNOPSIS...BJL/BPP