Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 180107 AAA AFDLWX AREA FORECAST DISCUSSION...UPDATED NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC 907 PM EDT WED SEP 17 2014 .SYNOPSIS... WEAK HIGH PRESSURE WILL MOVE OFF THE COAST TONIGHT. A LARGER AREA OF HIGH PRESSURE WILL BUILD OVER THE UPPER MIDWEST ON THURSDAY... WHILE AN UPPER TROUGH AXIS PASSES OVERHEAD. THE HIGH WILL DRIFT ACROSS THE MID ATLANTIC REGION TO CLOSE OUT THE WEEK...WITH A COLD FRONT EXPECTED TO CROSS THE AREA LATE SUNDAY INTO MONDAY. && .NEAR TERM /THROUGH THURSDAY/... WHILE ONE HIGH PRESSURE MOVES OFF THE COAST...ANOTHER WILL BEGIN TO BUILD IN FROM THE WEST. THE ONE MOVING OFF THE COAST WILL ACTUALLY BRING SOME WX TO THE AREA ON THU. UNTIL THEN...THE POSITION OF THE HIGH CURRENTLY IS ALLOWING THE EARLIER CLOUD COVER TO DISSIPATE OFF AND CLEAR OUT OVER MUCH OF THE AREA. MILD TEMPS FROM EARLIER THIS AFTN HAVE SLOWLY DROPPED OFF INTO THE 60S IN RECENT HRS...BUT WILL ONLY DROP DOWN INTO THE L-M50S. TONIGHT AND INTO THU...THE ERN AND WRN HALVES OF THE CWA WILL HAVE SOME OPPOSING CONDITIONS. WHILE MOVING TOWARD AND EVENTUALLY OFF THE COAST...THE LIGHT WIND FIELD WILL BEGIN TO BANK RELATIVELY MOIST AIR BACK ONSHORE INTO EARLY THU. THIS WILL KEEP TEMPS MODERATED OVER THE I-95 CORRIDOR/METRO AREAS WHILE THE WEST WILL SEE TEMPS DROPPING BACK INTO THE 40S - OVER THE APLCNS. HEADING INTO THE POST-DAWN HRS ON THU...THE LOW-MID LEVEL STRATUS LAYER WILL REBUILD AGAIN AND HELP TO DEVELOP SOME ISOLATED SHOWERS LATER IN THE DAY. THE NAM IS FAIRLY AGGRESSIVE W/ THE MOISTURE INCREASE - POSSIBLY TOO AGGRESSIVE AS IT COMPLETELY SATURATES THE COLUMN ALONG AND E OF THE BLUE RIDGE AS IF THIS WERE A STRONG CAA/OVERRUNNING SITUATION WERE OCCURRING. THE LIKELY MORE REASONABLE GFS AND TO A LESSER EXTENT/EURO MAKING THE CONVERGENCE ZONE BISECTING THE AREA MODERATELY MOIST W/ BROKEN SKY COVER AND A FEW ISOLATED SHOWERS. THE OPPOSING WIND FIELD OVER THE AREA WILL ALLOW THE I-95 CORRIDOR TO SEE TEMPS POKING BACK INTO THE M-U70S - THOUGH CLOUD COVER DEPENDENT...AND U60S/L70S OVER THE SHEN VLY/MTNS FROM A LIGHT NW WIND. && .SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY/... THE COLD FRONT WILL DROP TO OUR SOUTH THURSDAY NIGHT BEFORE STALLING OUT OVER THE SOUTHEASTERN CONUS FRIDAY THROUGH SATURDAY. HIGH PRESSURE WILL BUILD INTO NEW ENGLAND THURSDAY NIGHT THROUGH FRIDAY BEFORE MOVING OFF THE COAST SATURDAY. THE SURFACE RIDGE AXIS WILL REMAIN OVER THE REGION DURING THIS TIME. THE RIDGE AXIS SHOULD REMAIN CLOSE ENOUGH TO BRING DRY CONDITIONS TO THE AREA DURING THIS TIME BUT CLOUDS MAY HANG AROUND DUE TO AN ONSHORE COMPONENT TO THE LOW-LEVEL FLOW...ESPECIALLY ACROSS THE SHENANDOAH VALLEY INTO THE POTOMAC HIGHLANDS. AN EAST TO NORTHEAST FLOW FRIDAY WILL CAUSE MAX TEMPS TO BE BELOW CLIMO...BUT WINDS SHOULD TURN TOWARD THE SOUTHEAST SATURDAY ALLOWING FOR WARMER CONDITIONS. && .LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/... HIGH PRESSURE WILL REMAIN WEDGED SOUTHWARD BETWEEN THE APPALACHIAN MOUNTAINS AND EAST COAST SATURDAY NIGHT...KEEPING DRY CONDITIONS IN THE REGION. A COLD FRONT WILL MOVE FROM THE EASTERN GREAT LAKES TO THE EAST COAST SUNDAY AND SUNDAY NIGHT. THERE IS A CHANCE FOR SHOWERS ALONG THE FRONT WEST OF THE BLUE RIDGE SUNDAY AND IN ALL AREAS SUNDAY NIGHT. UPPER LEVEL SUPPORT SHOULD PIVOT NORTHEAST INTO EASTERN CANADA BY SUNDAY NIGHT...THE FRONT COULD STALL ACROSS THE DELMARVA PENINSULA AND TIDEWATER OF VIRGINIA BRIEFLY. A REINFORCING PUSH OF UPPER LEVEL ENERGY MONDAY SHOULD PUSH THE FRONT OFFSHORE BY LATE MONDAY. HIGH PRESSURE WILL GRADUALLY BUILD INTO THE REGION MONDAY NIGHT THROUGH WEDNESDAY TO ALLOW FOR THE NEXT PUSH OF COLDER AIR. && .AVIATION /00Z THURSDAY THROUGH MONDAY/... VFR CONDITIONS AND LIGHT WINDS ARE EXPECTED INTO THU. SOME PATCHY VALLEY BR IS POSSIBLE LATE TONIGHT/EARLY THU...THEN A BKN MID STRATUS DECK AND SOME STRAY SHOWERS POSSIBLE TMRW AFTN. HIGH PRESSURE WILL RETURN FOR FRIDAY AND SATURDAY...BRINGING MORE VFR CONDITIONS. VFR CONDITIONS SATURDAY NIGHT THROUGH SUNDAY WITH LIGHT AND VARIABLE WINDS. && .MARINE... HIGH PRESSURE WILL SLOWLY BUILD TOWARD THE COAST TONIGHT...INTO THURSDAY. AN ISOLATED SHOWER IS POSSIBLE THURSDAY AFTERNOON. HIGH PRESSURE WILL BUILD TOWARD THE WATERS THURSDAY NIGHT AND FRIDAY. A PRESSURE SURGE WILL CAUSE NORTH-NORTHEAST WINDS TO INCREASE. A SMALL CRAFT ADVISORY IS IN EFFECT FOR THE MARYLAND CHESAPEAKE BAY AND LOWER TIDAL POTOMAC RIVER LATE THURSDAY NIGHT THROUGH FRIDAY MORNING. THE FLOW WILL TURN TOWARD THE SOUTHEAST LATER FRIDAY THROUGH SATURDAY. WINDS MAY BE CLOSE TO SCA CRITERIA AT TIMES...BUT GUSTS HAVE BEEN CAPPED AT 15 KNOTS FOR NOW SINCE THE GRADIENT IS EXPECTED TO WEAKEN A BIT. NO MARINE HAZARDS SATURDAY NIGHT. SMALL CRAFT ADVISORIES ARE POSSIBLE SUNDAY WITH AN AREA OF LOW PRESSURE NEAR THE TIDEWATER OF VIRGINIA AND THE MAIN STEM CHESAPEAKE BAY. && .TIDES/COASTAL FLOODING... TIDAL ANOMALIES RUNNING CLOSE TO A FT ABV ASTRO NORMS ATTM AT NEARLY ALL SITES EXCEPT IN THE UPPER POTOMAC. HOWEVER...SUSPECT THOSE DEPARTURES WILL INCREASE AS WELL WITH THE INCOMING TIDE /THEYRE AT LOW TIDE NOW/. BASED ON THESE VALUES...THE COASTAL FLOOD ADVISORY LOOKS GOOD FOR ST MARYS...CALVERT...AND ANNE ARUNDEL COUNTIES. WOULD NEED DEPARTURES OF 1.1 TO 1.3 FT FOR CBOFS GUIDANCE TO VERIFY. DONT THINK WE WL GET THERE...AND ONCE AGAIN SHADED FORECASTS UNDER THAT GUIDANCE. BASED ON THAT LOGIC...BALTIMORE CITY WILL COME REAL CLOSE TO MINOR CRITERIA. MOST LIKELY SCENARIO SEEMS TO BE A PEAK JUST UNDER THRESHOLD. WL OPT TO NOT ISSUE AN ADVISORY...BUT CONFIDENCE NOT THE GREATEST. IF TRENDS PROVE OTHERWISE...THERE MAY NEED TO BE A SHORT-FUSED ADVY. DONT THINK HARFORD WILL RISE ABOVE CAUTION STAGE DUE TO THEIR HIGHER THRESHOLDS. IN THE POTOMAC...HEC-RAS GUIDANCE SUGGESTS A PEAK OF 4 FT AS WASD2... WHICH WOULD BE A 1 FT ANOMALY. DO NOT BELIEVE WE WILL BE HIGHER THAN THAT. THUS...THE ADVISORY WONT BE EXPANDED UP THE POTOMAC FOR THIS CYCLE. A WEAK COLD FRONT WILL PASS THROUGH THE WATERS THURSDAY. HIGH PRESSURE WILL BUILD OVER NEW ENGLAND BEHIND THIS FRONT. AN EASTERLY COMPONENT TO THE WIND IS EXPECTED TO CONTINUE DURING THIS TIME AND IT WILL LIKELY LAST THROUGH THE WEEKEND. THEREFORE...MINOR TIDAL FLOODING IS POSSIBLE NEAR TIMES OF HIGH TIDE THROUGH THE WEEKEND. THE BEST CHANCE FOR MINOR TIDAL FLOODING WILL BE DURING THE LATE EVENING AND MORNING HIGH TIDE CYCLE WHICH IS THE HIGHER OF THE TWO. WE WOULD NEED 2 FT DEPARTURES FOR MDT FLOODING TO BE REALIZED. DO NOT FORSEE THAT HAPPENING. IF IT DID...ANNAPOLIS AND ST GEORGE ISLAND WOULD BE THE MOST SUSCEPTIBLE. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...NONE. MD...COASTAL FLOOD ADVISORY UNTIL 4 AM EDT THURSDAY FOR MDZ014-017- 018. VA...NONE. WV...NONE. MARINE...SMALL CRAFT ADVISORY FROM 2 AM TO NOON EDT FRIDAY FOR ANZ530>534-537-539>541-543. && $$ UPDATE...GMS PREV DISCUSSION...BJL/KLW TIDES/COASTAL FLOODING...HTS/BJL

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