AFDLWX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC 414 AM EDT MON APR 18 2011 .SYNOPSIS... HIGH PRESSURE WILL MOVE EAST OF THE AREA TODAY...WITH LOW PRESSURE PASSING NORTH OF THE REGION TONIGHT. A FRONT WILL STALL NEAR THE MASON DIXON LINE TUESDAY...BEFORE RETREATING NORTH AS A WARM FRONT TUESDAY NIGHT. A COLD FRONT WILL MOVE ACROSS THE MID-ATLANTIC WEDNESDAY AND WEDNESDAY NIGHT...LIKELY BRINGING SHOWERS AND THUNDERSTORMS TO THE AREA. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... TODAY...THE LOW PRESSURE OVER QUEBEC WILL MOVE NORTHEAST ACROSS THE CANADIAN MARITIMES...AS A STATIONARY FRONT PERSIST ACROSS THE OH VLY AND ALONG THE MASON-DIXON LINE AND LOW PRESSURE FROM THE SERN CONUS HEADS TO BERMUDA. SLY FLOW WILL INCREASE TODAY. HOWEVER...THE PERSISTENT STALLED BOUNDARY WILL MAINTAIN CLOUDS AND A NLY COMPONENT TO THE WIND INTO THE AFTERNOON. THEREFORE...QUITE A CONTRAST IN TEMPERATURES WILL DEVELOP. EXPECT MAX TEMPS NEAR 80F FOR THE CNTRL SHEN VLY/N-CNTRL VA/SRN MD...WITH LOW 60S ALONG THE MASON-DIXON. SLY FLOW WILL SPREAD THROUGH THE NRN SHEN VLY AND GREATER DC THROUGH THE AFTERNOON...SO LOW 70S IN THE SRN SUBURBS AND UPR 60S IN THE NRN SUBURBS CAN BE EXPECTED. SHOWERS EXPECTED UP IN PA THIS AFTERNOON...LOW CHANCE OF THEM REACHING SOUTH OF THE MASON-DIXON. TONIGHT...VORT MAX RIDES EAST ALONG THE STALLED BOUNDARY...WITH LIKELY POPS BY LATE TONIGHT FOR THE POTOMAC HIGHLANDS. ELEVATED INSTABILITY WILL ALLOW A THUNDER THREAT WITH ANY ACTIVITY...PUT BEST CHANCES FOR THUNDER IN THE POTOMAC HIGHLANDS...THOUGH 50 KT WLY FLOW WOULD PUSH THE ELEVATED ACTIVITY ACROSS NRN PORTIONS OF THE CWA /WHERE CHANCE POPS ARE IN FOR LATE TONIGHT/. THICKENING CLOUDS ACROSS THE CWA WITH SLY FLOW...WENT MILD WITH MIN TEMPS...TAKING A BLEND OF OPERATIONAL GFS WITH SREF...LOW TO MID 50S. && .SHORT TERM /6 AM THIS MORNING THROUGH TUESDAY NIGHT/... NEARLY STATIONARY FRONT TUESDAY ALONG MASON DIXON LINE WILL CREATE TIGHT THERMAL GRADIENT ACROSS NORTHERN CWA...WHILE SOUTHERN ZONES WILL RISE WELL INTO 70S. WITH MOIST SOUTHERLY FLOW NOISING UP AGAINST SURFACE BOUNDARY AND WEAK UPPER DISTURBANCES RIDING EASTWARD ABOVE IT IN 500 MB FLOW...EXPECT AT LEAST SCATTERED SHOWERS AND THUNDERSTORMS TO DEVELOP BY AFTERNOON. CLOSEST TO THE FRONT AND TRACK OF THE DISTURBANCE...THE POTOMAC HIGHLANDS LOOK TO HAVE A RISK OF SOME STRONG CONVECTION TUESDAY AFTERNOON. TUESDAY NIGHT A DEEPENING LOW PRESSURE LIFTING OUT OF MISSISSIPPI VALLEY INTO THE GREAT LAKES WILL FORCE SURFACE BOUNDARY NORTHWARD...MOVING BULK OF SHOWERS AND THUNDERSTORMS NORTH OF CWA AND LEAVING MID-ATLANTIC IN WARM AIRMASS. && .LONG TERM /WEDNESDAY THROUGH SUNDAY/... STRONG SOUTHERLY FLOW WILL BRING WARM MOIST ARE UP THE EASTERN SEABOARD WEDNESDAY...AS A COLD FRONT DRAGS EAST TOWARD APPALACHIANS. THUNDERSTORMS SHOULD DEVELOP ALONG AND AHEAD OF THIS FRONT WEDNESDAY AFTERNOON AND EVENING...AND CONTINUE INTO THE NIGHT AS THE BOUNDARY CROSSES THE CWA. INSTABILITY AND SHEAR LOOK SIGNIFICANT ENOUGH TO PRODUCE STRONG TO SEVERE CONVECTION ACROSS THE AREA...THROUGH MID-LEVEL INHIBITION MAY BE A LIMITING FACTOR. DAMAGING WINDS APPEARS THE MAIN THREAT...AND WILL HIGHLIGHT THIS POTENTIAL IN BOTH HWO SEGMENTS...AS SPC HAS ENTIRE REGION IN 5 PERCENT RISK FOR DAY 3. HIGH PRESSURE BRIEFLY BUILDS IN THURSDAY...BEFORE WARM FRONT FRIDAY AND SUBSEQUENT COLD FRONT SATURDAY BRING ANOTHER CHANCE OF PRECIPITATION TO THE REGION. && .AVIATION /08Z MONDAY THROUGH FRIDAY/... VFR CONDS TODAY AS SLY FLOW INCREASES THROUGH THE DAY /THOUGH A STATIONARY FRONT WILL PERSIST NEAR THE MASON-DIXON LINE/...GUSTS TO AROUND 20 KT THIS AFTERNOON. ELEVATED INSTABILITY LEADS TO A THUNDER THREAT GENERALLY NORTHWEST OF DC LATE TONIGHT AS A DISTURBANCE MOVES EAST ALONG THE FRONT. VFR CONDITIONS EXPECTED TO PREVAIL TUESDAY INTO EARLY WEDNESDAY...THOUGH WITH A FRONT STATIONARY NORTH OF THE TERMINALS...AT LEAST ISOLATED CONVECTION CANT BE RULED OUT. A COLD FRONT CROSSES ON WEDNESDAY AFTERNOON AND NIGHT...LIKELY BRINGING SHOWERS AND THUNDERSTORMS WITH IT. HIGH PRESSURE ARRIVES ON THURSDAY. && .MARINE... 15 KT SWLY FLOW PERSISTS ACROSS THE TIDAL POTOMAC AND SRN MD BAY. FLOW BECOMES SLY THROUGH THE DAY...INCREASING THROUGH THE AFTERNOON. REVISED SCA FOR ALL WATERS TO BEGIN AT 2PM AND END AT 7PM...EXCEPT FOR THE MAIN STEM OF THE BAY AND ERN TRIBUTARIES WITH SLY CHANNELING EXPECTED THROUGH THE NIGHT. A STALLED BOUNDARY NORTH OF BALTIMORE MAY KEEP SLY GUSTS BELOW SCA CRIT NORTH OF ABOUT SANDY POINT. SMALL CRAFT ADVISORY CONTINUED INTO TUESDAY FOR PORTIONS OF THE WATERS...AS SOUTHERLY FLOW WILL CONTINUE. WITH LOW LEVEL FLOW INCREASING FURTHER TUESDAY NIGHT THROUGH WEDNESDAY...A SMALL CRAFT ADVISORY IS EXPECTED TO BE NECESSARY. A COLD FRONT WILL BRING SHOWERS AND THUNDERSTORMS TO THE WATERS LATE WEDNESDAY...FOLLOWED BY NORTHWEST WINDS. THESE WINDS MAY GUST ABOVE SMALL CRAFT ADVISORY THRESHOLDS WEDNESDAY NIGHT INTO THURSDAY. && .HYDROLOGY... MODERATE RIVER FLOODING IS ONGOING FOR PORTIONS OF THE SHENANDOAH AND POTOMAC BASINS. MINOR FLOODING IS ALSO ONGOING IN PORTIONS OF THE RAPPAHANNOCK BASIN. PLEASE SEE LATEST FLSLWX OR THE AHPS PAGE ON WEATHER.GOV FOR SPECIFICS AT PARTICULAR LOCATIONS. && .TIDES/COASTAL FLOODING... FRESHWATER INPUT FROM THE POTOMAC WILL CONTINUE COASTAL FLOODING FOR THE UPPERMOST TIDAL POTOMAC /DC...ARLINGTON AND ALEXANDRIA/. WISC AVE MODERATE FLOOD THRESHOLD OF 7FT EXPECTED TO BE TOPPED AROUND BOTH HIGH TIDES TODAY...THUS A WARNING FOR DC. THE ONLY FLOOD REFERENCE ON THE VA SIDE IS IN DOWNTOWN ALEXANDRIA WHICH RECEIVES LESS INFLUENCE FROM POTOMAC RIVER FLOODING...SO AN ADVISORY IS IN EFFECT FOR ARLINGTON/ALEXANDRIA THROUGH THE NEXT TWO HIGH TIDE CYCLES. TIDAL DEPARTURES AROUND ONE FOOT ON THE WRN SHORE OF THE BAY...ALONG WITH A FULL MOON...WILL RESULT IN MINOR COASTAL FLOODING FOR PARTICULARLY SENSITIVE LOCATIONS AROUND HIGH TIDE. A COASTAL FLOOD ADVISORY FOR ANNE ARUNDEL...BALTIMORE...AND HARFORD COUNTIES IS OUT UNTIL NOON /END TIME BASED ON HIGH TIDE AT HAVRE DE GRACE OF 10:33AM/. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...COASTAL FLOOD WARNING UNTIL MIDNIGHT EDT TONIGHT FOR DCZ001. MD...COASTAL FLOOD ADVISORY UNTIL NOON EDT TODAY FOR MDZ007-011-014. VA...COASTAL FLOOD ADVISORY UNTIL MIDNIGHT EDT TONIGHT FOR VAZ054. WV...NONE. MARINE...SMALL CRAFT ADVISORY FROM 2 PM THIS AFTERNOON TO 8 PM EDT TUESDAY FOR ANZ531>534-537-539>541-543. SMALL CRAFT ADVISORY FROM 2 PM THIS AFTERNOON TO 7 PM EDT THIS EVENING FOR ANZ530-535-536-538-542. SMALL CRAFT ADVISORY FROM NOON TO 8 PM EDT TUESDAY FOR ANZ535- 536-542. && $$ SYNOPSIS...BAJ/SBK