AFDLWX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC 1105 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 /THROUGH TONIGHT/... WEAK FRONTAL BOUNDARY HAS SETTLED NEAR THE MASON DIXON LINE THIS MORNING. MID LEVEL CLOUDS CONTINUE TO INCREASE ACROSS THE CWFA. MADE A FEW TWEAKS TO THE WIND/POP GRIDS FOR THE AFTERNOON AND EVENING. OTHERWISE NO MAJOR CHANGES. WAVE OF LOW PRESSURE/SHORTWAVE WILL MOVE EAST ALONG THE BOUNDARY LATE THIS AFTERNOON AND EVENING. HAVE INCREASED POPS RIGHT ALONG THE MASON DIXON LINE FOR THESE FEATURES. 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 /TUESDAY 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 /15Z MONDAY THROUGH FRIDAY/... SKY COVER WILL REMAIN BROKEN ACROSS AREA AIRPORTS THROUGH THE REST OF THE DAY WITH SKIES BECOMING OVERCAST LATE TONIGHT. EXPECT CEILINGS TO REMAIN AT OR ABOVE THE 10000 FOOT LEVEL THROUGHOUT THE TAF VALID PERIOD. SOUTH WIND WILL INCREASE TO 10 TO 15 KT WITH HIGHER GUSTS UP TO 20 KT POSSIBLE BY THE EARLY AFTERNOON. INCREASING WIND TREND WILL BEGIN NEAR CHARLOTTESVILLE AND TRANSITION NORTHWARD TOWARDS BALTIMORE BY MID AFTERNOON. OVERNIGHT...WINDS WILL DIMINISH TO 5 TO 10 KT AND PRIMARILY REMAIN SOUTHERLY. 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... WIND SPEEDS BELOW SCA THRESHOLDS EXPECTED FOR THE REMAINDER OF THE MORNING. LATER THIS AFTERNOON...GUSTS MAY APPROACH 10-22 KT. LIGHT AND VARIABLE WINDS THIS MORNING EXPECTED TO SHIFT TO THE SOUTH BY THE AFTERNOON. 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... THE MONOCACY RIVER HAS GONE BELOW FLOOD STAGE THIS MORNING AND THE FLOOD WARNING HAS BEEN DISCONTINUED. RIVER FLOODING CONTINUES ON PORTIONS OF THE SHENANDOAH AND POTOMAC RIVERS. MILLVILLE AND FRONT ROYAL HAVE CRESTED...BUT REMAIN ABOVE FLOOD STAGE AT THIS TIME. RIVER LEVELS ARE EXPECTED TO GO BELOW FLOOD STAGE THIS AFTERNOON/EVENING. AT LITTLE FALLS AND POINT OF ROCKS...THE RIVER IS EXPECTED TO CREST THIS AFTERNOON/EVENING. HOWEVER...WATER LEVELS WILL REMAIN AOA FLOOD STAGE THROUGH EARLY TUESDAY. && .TIDES/COASTAL FLOODING... WATER LEVELS HAVE DECREASED ALONG THE WESTERN SHORE OF THE CHESAPEAKE BAY THIS MORNING. HOWEVER ACROSS THE UPPER TIDAL POTOMAC...WATER LEVELS CONTINUE TO RISE IN THE WASHINGTON HARBOR AND IN ALEXANDRIA. FRESH WATER RUN OFF WILL CONTINUE TO CONTRIBUTE TO ELEVATED WATER LEVELS THROUGH EARLY TUESDAY. WINDS WILL SHIFT AROUND TO THE SOUTH THIS AFTERNOON...AND REMAIN SOUTHERLY THROUGH WEDNESDAY AHEAD OF A COLD FRONT. THE PROLONGED SOUTHERLY FLOW COULD HELP KEEP TIDAL ANOMALIES ABOVE NORMAL ACROSS THE WESTERN SHORE AND THE TIDAL POTOMAC THROUGH WEDNESDAY. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...COASTAL FLOOD WARNING UNTIL MIDNIGHT EDT TONIGHT FOR DCZ001. MD...NONE. VA...COASTAL FLOOD WARNING UNTIL MIDNIGHT EDT TONIGHT FOR VAZ054. WV...NONE. MARINE...SMALL CRAFT ADVISORY UNTIL 8 PM EDT TUESDAY FOR ANZ531>534-537- 539>541-543. SMALL CRAFT ADVISORY UNTIL 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. && $$ UPDATE...LISTEMAA/LARSEN