AFDPHI AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE MOUNT HOLLY NJ 740 PM EDT THU MAY 12 2011 .SYNOPSIS... A RIDGE OF HIGH PRESSURE ALONG THE EAST COAST WILL DRIFT EAST THROUGH FRIDAY. AN AREA OF LOW PRESSURE ACROSS THE GREAT PLAINS WILL MOVE EASTWARD TO THE OHIO VALLEY THROUGH SATURDAY THEN TO THE MID ATLANTIC STATES SUNDAY. A FRONTAL BOUNDARY AHEAD OF THIS SYSTEM IS EXPECTED TO MOVE ACROSS THE NORTHERN MID ATLANTIC SATURDAY INTO SUNDAY. A SLOW MOVING LOW PRESSURE SYSTEM THEN SETTLES INTO THE EAST MONDAY THROUGH THE MIDDLE PART OF NEXT WEEK. && .NEAR TERM /THROUGH FRIDAY/... THE PERSISTENT UPPER AIR PATTERN OF A TROUGH ACROSS THE CENTRAL CONUS... RIDGE OVER THE EASTERN MID ATLANTIC...AND CLOSED LOW OFF THE MID ATLANTIC COAST WILL CONTINUE TO DRIFT EAST INTO THE WEEKEND. SFC HIGH PRESSURE WILL PERSIST ALONG THE EAST COAST THROUGH FRIDAY. SOUTHWARD CONVECTIVE ACTIVITY RIDING THE RIDGE IS PROGRESSING EAST WITH THE UPPER AIR PATTERN... THOUGH IS NOT EXPECTED TO AFFECT THE PHI CWA UNTIL FRIDAY EVENING. TONIGHT...AN ONSHORE FLOW...DUE TO THE RIDGE OFF THE COAST...WILL ALLOW LOW CLOUDS TO DEVELOP ACROSS THE NORTHERN MID ATLANTIC...NO PRECIP EXPECTED. MIN TEMPS NEAR NORMAL...LOW TO MID 50S. FRIDAY...MOSTLY CLOUDY CONDITIONS PERSIST THROUGH THE DAY. HOWEVER...SLY FLOW WILL WARM THE LOW LEVELS...THUS EXPECT MAX TEMPS IN THE LOW 70S...ONLY A FEW DEGREES LESS THAN TODAY...DESPITE THE CLOUDS. && .SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY/... LOW PRESSURE...CURRENTLY OVER THE CENTRAL GREAT PLAINS...WILL MOVE EAST FROM THE MID MS VLY TO THE SRN MID ATLANTIC BY SUNDAY. ON ONSHORE FLOW WILL PERSIST...MAKING ANY INSTABILITY ELEVATED. CHANCE POPS INCREASE TO LIKELY THROUGH THE SHORT TERM...THOUGH OCCASIONAL SHOWERS CAN BE EXPECTED. ELEVATED THUNDER A POSSIBILITY ON SATURDAY...THOUGH NOT IN THE GRIDS ATTM. QPF IS UP TO HALF AN INCH...LATE SATURDAY THROUGH SUNDAY. HOWEVER...GIVEN SHOWERY NATURE THE PRECIP TOTALS SHOULD VARY GREATLY ACROSS THE PHI CWA. WARM AIR ADVECTION THROUGH THIS PERIOD...SO SHOULD SUN BREAK OUT MORE THAN ANTICIPATED...TEMPERATURES COULD EASILY BE 10 DEGREES OR SO WARMER THAN CURRENTLY ADVERTISED...PARTICULARLY ON SUNDAY. && .LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/... 12Z GFS/ECMWF MAINTAIN THE UPPER LOW ACROSS THE EAST-CENTRAL CONUS INTO MID WEEK. WITH THE PHI CWA PROGGED TO BE ON THE EAST SIDE OF THIS LOW...ACTIVE WEATHER PERSISTS WITH POTENTIAL FOR THUNDERSTORMS...INCLUDING AT NIGHT ACCORDING TO LIFTED INDICES. EXPECT JUST BELOW NORMAL TEMPERATURES AND QUITE A WET PERIOD. BY EARLY NEXT WEEK RAIN MAY BECOME EXCESSIVE...PRODUCING HYDROLOGY ISSUES...THIS WILL HAVE TO BE MONITORED AS THE PATTERN UNFOLDS. && .AVIATION /00Z FRIDAY THROUGH TUESDAY/... THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY AND SURROUNDING AREAS. HE CUMULUS THAT DEVELOPED WITH THE HEATING WILL DISSIPATE WITH THE LOSS OF DAYTIME HEATING. IN ITS WAKE...MAINLY JUST SOME CIRRUS SPILLING SOUTHEAST OVER THE MID LEVEL RIDGE FROM THE CONVECTION ACROSS WESTERN PENNSYLVANIA AND EASTERN OHIO. AS THE MID LEVEL RIDGE COLLAPSES LATER TONIGHT...VFR CEILINGS WILL DEVELOP FROM WEST TO EAST AS THE MID LEVEL CLOUDS OVER WESTERN PENNSYLVANIA MOVE EAST. THE PROBLEM LATER TONIGHT BECOMES THE THREAT FOR MVFR CEILINGS.THE LOWEST CEILINGS ARE CURRENTLY OVER VIRGINIA NEAR THE HIGHER TERRAIN...AS LOW LEVEL MOISTURE BECOMES BANKED UP AGAINST THE MOUNTAINS. STATISTICAL GUIDANCE (THE 1200 UTC NAM MORE THAN THE 1200 UTC GFS) IS POINTING TO AN MVFR/IFR CEILINGS EVENT BETWEEN 0900 UTC AND 1500 UTC FRIDAY FOR MORE OR LESS THE ENTIRE AREA. AS THE LOW LEVEL FLOW BECOMES PRE WARM FRONTAL LATER TONIGHT...MVFR CEILINGS MAKE MORE SENSE FOR KRDG AND KABE...WITH SOME TERRAIN EFFECTS TO HELP WITH LIGHT UPSLOPING. CONFIDENCE IS LOWER CEILINGS DEVELOPING WITH THE LIGHT SOUTHEAST FLOW IS ABOUT AVERAGE. FURTHER SOUTHEAST...IT IS NOT CLEAR THE LOWER CEILINGS WILL AFFECT THE PHILADELPHIA METRO AIRPORTS. FOR NOW...THE MVFR CEILINGS HAVE BEEN LEFT OUT OF THE KPHL FORECAST...AND AMENDMENTS MAY BE NEEDED LATER TONIGHT IF THE CEILINGS DO ACTUALLY DEVELOP. THE CEILING FORECAST FOR KPHL FOR THE TIME PERIOD 0900 UTC THROUGH 1400 UTC FRIDAY IS BELOW AVERAGE. CEILINGS COULD END UP BEING LOWER THAN CURRENTLY FORECAST. THE LOWER LEVEL INVERSION THAT DEVELOPS LATE TONIGHT SHOULD BREAK BY 1500 UTC FRIDAY IN MOST AREAS...AS HEATING HELPS TO EXPAND THE MIXED LAYER. IN FACT...ALL TERMINALS SHOULD BECOME VFR (IF THEY ARE NOT ALREADY) BY 1500 UTC FRIDAY...AND REMAIN THAT WAY THROUGH 0000 UTC SATURDAY. WINDS WILL START SOUTHEAST AHEAD OF THE WARM FRONT TO THE WEST AND SOUTHWEST...BUT SHOULD VEER TO SOUTH IN THE AFTERNOON. RIGHT NOW...IT LOOKS LIKE THE WINDS AT KPHL SHOULD REMAIN BELOW 10 KNOTS FRIDAY AFTERNOON. OUTLOOK... CONDITIONS SHOULD DROP TO MVFR AND IFR AHEAD OF THE WARM FRONT...AND COULD REMAIN THERE FOR MUCH OF THE WEEKEND. WHILE THERE ARE SOME MODEL DIFFERENCES CONCERNING THE PLACEMENT OF FRONTS AND PRESSURE SYSTEM...A PERSISTENT ONSHORE FLOW WILL RESULT IN A LOWERING INVERSION AND DROPPING CEILINGS. A PERIOD OF RAIN SATURDAY NIGHT INTO SUNDAY COULD ALSO RESULT IN IFR VISIBILITIES...WITH SCATTERED THUNDERSTORMS POSSIBLE SUNDAY INTO MONDAY. CONDITIONS COULD IMPROVE A BIT MOVING INTO MIDWEEK...BUT THE POTENTIAL FOR SHOWERS AND THUNDERSTORMS WILL PERSIST. && .MARINE... HAZARDOUS SEAS SCA CONTINUES ON THE ATLC WATERS INTO MIDDAY FRIDAY FOR THE OCEAN ZONES. THE PROBLEM: 4-6 FT E SWELL AT 44025 AND 44065 WITH A 6-8 FT NE SWELL AT 44009 MAKING IT MORE LIKELY THAT THE DEL AND S NJ COASTAL ZONES WILL REMAIN IN THE HAZARDOUS SEAS ADVY REGION THRU AT LEAST MIDDAY FRIDAY. SOME MODEL GUIDANCE CONTS 44009 LINKED SCA HAZARDOUS SEAS 5 FT OR HIGHER AT LEAST INTO THE 16TH OF MAY WHILE 44025 IS MORE OF A MARGINAL 1 FT EITHER SIDE OF 5 FT FOR THE NEXT COUPLE OF DAYS. NO ADVISORY IS NEEDED FOR THE DELAWARE BAY AT THIS TIME AS WINDS ARE EXPECTED TO REMAIN BELOW CRITERIA LEVELS THROUGH SATURDAY MORNING. THIS WEEKEND...SE WINDS INCREASE LATE SATURDAY AND BECOME MORE SLY SOMETIME SUNDAY OCNL NEAR 20 KNOTS. LIKELY THAT A HAZ SEAS ADVY WILL BE NEEDED. FOG OVER THE WATER MIGHT BECOME A MARINE HAZARD. MONDAY...VARIABLE MAINLY S WINDS. LIKELY THAT A HAZ SEAS ADVY WILL BE NEEDED. TUESDAY...ANOTHER PULSE OF SE FLOW OF 20 TO 25 KTS POSSIBLE. LIKELY THAT AN SCA WILL BE NEEDED. && .TIDES/COASTAL FLOODING... TIDES CONTINUE TO RUN BETWEEN ONE AND ONE AND ONE-HALF FOOT ABOVE PREDICTED ON THE CHESAPEAKE BAY AND ON THE DELAWARE RIVER EARLY THIS EVENING. REEDY POINT CRESTED RIGHT AT THE MINOR TIDAL FLOODING BENCHMARK THIS EVENING...AND EXTRAPOLATION AND STORM SURGE GUIDANCE SUGGEST PHILADELPHIA WILL GET CLOSE TO THE BENCHMARK LATER THIS EVENING. BASED ON THIS...A COASTAL FLOOD STATEMENT HAS BEEN ISSUED TO COVER THE SPOTTY MINOR TIDAL FLOODING THREAT FOR THE HIGH TIDE LATER THIS EVENING. HIGH TIDE OCCURS IN PHILADELPHIA AT 1010 PM. ON THE CHESAPEAKE...CAMBRIDGE CONTINUES TO RUN ABOUT ONE AND ONE-HALF FOOT ABOVE PREDICTED EARLY THIS EVENING. IF THIS DEPARTURE WERE TO CONTINUE INTO THE EARLY MORNING HIGH TIDE CYCLE (HIGH TIDE OCCURS AT CAMBRIDGE AT 1244 AM FRIDAY)...THE HIGH TIDE WOULD COME IN CLOSE TO THE MINOR TIDAL FLOODING BENCHMARK. THE CBOFS GUIDANCE SUGGESTS THAT CAMBRIDGE WILL EXCEED THE BENCHMARK...BUT HAS BEEN RUNNING HIGH OF LATE. IF TRENDS CONTINUES...A COASTAL FLOOD STATEMENT COULD BE ISSUED LATER THIS EVENING. LOOKING AHEAD...THE APPROACHING FULL MOON (THE MOON IS FULL ON THE 17TH) AND ON THE ONSHORE FLOW WILL PROVIDE A FEW OPPORTUNITIES FOR MINOR TIDAL FLOODING...BUT FOR NOW MODERATE TIDAL FLOODING LOOKS UNLIKELY. THE MDL EXTRATROPICAL STORM SURGE GUIDANCE SUGGESTS THAT THE MORNING HIGH TIDE CYCLE ON THE DELAWARE RIVER COULD COME CLOSE TO THE MINOR TIDAL FLOODING BENCHMARK. HIGH TIDE OCCURS AT 753 AM FRIDAY AT REEDY POINT AND 1032 AM FRIDAY AT PHILADELPHIA. THE BENCHMARK COULD ALSO BE APPROACHED DURING THE FRIDAY EVENING HIGH TIDE CYCLE (WHICH IS THE HIGHER OF THE TWO TIDES FRIDAY BY ABOUT ONE-HALF FOOT). THE TIDE CYCLE WITH THE BEST CHANCE FOR MINOR TIDAL FLOODING ON THE DELAWARE RIVER COULD THE LATE SATURDAY EVENING TIDE. THE COMBINATION OF HIGHER ASTRONOMICAL TIDES AND THE PERSISTENT SOUTHEAST FLOW COULD HELP BOTTLE UP WATER IN THE UPPER DELAWARE BAY AND THE DELAWARE RIVER FOR THIS TIDE CYCLE. HOWEVER...AT THIS TIME IT APPEARS AS THOUGH THE THREAT FOR MODERATE TIDAL FLOODING IS LOW. CBOFS SUGGESTS THAT THE FRIDAY AFTERNOON TIDE COULD APPROACH THE MINOR TIDAL FLOODING BENCHMARK ON THE CHESAPEAKE...ESPECIALLY AT CAMBRIDGE. SINCE THE GUIDANCE HAS BEEN RUNNING HIGH AT BOTH CAMBRIDGE AND TOLCHESTER BEACH...THIS HIGH TIDE CYCLE DOES NOT LOOK LIKE IT WILL REACH THE BENCHMARK. THE GREATER THREAT FOR MINOR TIDAL FLOODING ON THE CHESAPEAKE MAY BE THE EARLY MORNING HIGH TIDE CYCLE. CBOFS SHOWS BOTH LOCATIONS REACH 4 FEET ABOVE MLLW DURING THIS TIDE CYCLE. FINALLY...THE INCREASING ASTRONOMICAL TIDES AND PERSISTENT ONSHORE FLOOD COULD RESULT IN MINOR TIDAL FLOODING ON THE OCEAN FRONT DURING THE SATURDAY AFTERNOON AND SUNDAY AFTERNOON HIGH TIDE CYCLES. THIS WOULD BE SUPPORTED BY THE MDL STORM SURGE GUIDANCE. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...NONE. NJ...NONE. DE...NONE. MD...NONE. MARINE...SMALL CRAFT ADVISORY FOR HAZARDOUS SEAS UNTIL 2 PM EDT FRIDAY FOR ANZ451>455. SMALL CRAFT ADVISORY FOR HAZARDOUS SEAS UNTIL 6 AM EDT FRIDAY FOR ANZ450. && $$