AFDPHI AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE MOUNT HOLLY NJ 905 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/... PARTLY CLOUDY CONDITIONS CONTINUE ACROSS THE AREA THIS EVENING. WINDS ARE FOR THE MOST PART E OR SE ATTM. AN AREA OF CI/CS CLOUDS CONTINUES TO SPILL SE AHEAD OF THE UPPER RIDGING JUST TO THE WEST. SOME LO CLOUDS MAY ARRIVE LATER TONIGHT WITH THE CONTINUED ONSHORE FLOW. LOWS TONIGHT WILL BE IN THE LOW TO MID 50S IN MOST AREAS. 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/... THE PATTERN WILL BE WET THROUGH MUCH OF THE PERIOD AS A CLOSED MID LEVEL LOW MEANDERS ACROSS THE OHIO VALLEY FOR THE MUCH OF NEXT WEEK. THE DEEP SOUTHERLY FLOW THAT DEVELOPS AHEAD OF THE CLOSED MID LEVEL LOW WILL BRING MOIST AND MARGINALLY UNSTABLE AIR INTO THE FORECAST AREA SUNDAY NIGHT...AND THIS SHOULD LAST IN SPURTS THROUGH AT LEAST WEDNESDAY. THIS WILL RAISE THE POTENTIAL FOR HEAVY RAIN...WITH SUNDAY NIGHT INTO MONDAY AND TUESDAY INTO TUESDAY NIGHT BEING THE MOST LIKELY TIMES. THE FIRST HEAVY RAIN THREAT LOOKS TO BE SUNDAY NIGHT INTO MONDAY. A SHORT WAVE ROTATING AROUND THE CLOSED LOW WILL BRING PRECIPITABLE WATER VALUES UP TO NEAR 1.25 INCHES IN A MARGINALLY UNSTABLE ENVIRONMENT. THE BEST CHANCE OF CONVECTION SHOULD BE SUNDAY NIGHT...WITH ELEVATED INSTABILITY POSSIBLE FOR THE REMAINDER OF THE NIGHT INTO MONDAY MORNING. RIGHT NOW...IT IS TOUGH TO PIN DOWN THE MOST LIKELY AREA FOR POTENTIAL HEAVY RAIN...BUT IT WOULD APPEAR TO BE NORTH OF PHILADELPHIA...AS THE SURFACE FRONT IS OVERRIDDEN IN THIS AREA. THE AIRMASS WILL BE MARGINALLY UNSTABLE MONDAY AFTERNOON AND EVENING...BUT DEEP CONVECTION APPEARS UNLIKELY. THE NEXT THREAT FOR HEAVY RAIN LOOKS TO BE TUESDAY INTO TUESDAY EVENING. ANOTHER SHORT WAVE ROTATING AROUND THE CLOSED MID LEVEL LOW SHOULD ARRIVE TUESDAY AFTERNOON. THE MOISTURE PLUME WITH THIS SPOKE LOOKS TO HAVE MORE MOISTURE WITH IT (PRECIPITABLE WATER VALUES INCREASING TO NEAR 1.35 INCHES). MID RANGE MODELS ARE SHOWING A DECENT MID LEVEL WIND MAXIMUM WITH THIS SHORT WAVE. WITH MARGINAL INSTABILITY EXPECTED TO BE IN PLACE...THERE IS A LOW PROBABILITY OF DEEP CONVECTION WITH GUSTY WINDS TUESDAY AFTERNOON AND EVENING. HOWEVER...THE GREATER THREAT ONCE AGAIN APPEARS TO BE HEAVY RAIN. WEDNESDAY AND THURSDAY SHOULD STILL HAVE SHOWERS WITH THE SURFACE FRONT STILL NEARBY...BUT RIGHT NOW THE FLOOD THREAT APPEARS TO BE LESS. FOR THE MOST PART...TEMPERATURES SHOULD BE NEAR NORMAL THROUGH THE PERIOD. && .AVIATION /01Z FRIDAY THROUGH TUESDAY/... THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY AND SURROUNDING AREAS. THE 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... OVERALL...THE GRADIENT HAS BEEN SLOWLY WEAKENING THIS EVENING AS THE ELONGATED AREA OF HIGH PRESSURE STRETCHING FROM NORTHERN NEW ENGLAND TO THE MID ATLANTIC STATES CONTINUES TO MOVE EAST. LOCAL FUNNELING OF THE SOUTHEAST FLOW BROUGHT THE HIGHER PLATFORMS IN THE DELAWARE BAY CLOSE TO SMALL CRAFT VALUES LATE THIS AFTERNOON AND THIS EVENING...BUT THIS SHOULD START TO BACK OFF AS WELL. THE MAIN FORECAST PROBLEM FOR TONIGHT (AND PERHAPS MUCH OF FRIDAY) WILL BE THE THE SWELLS (WITH PERIODS NEAR 9 SECONDS) OVER THE OCEAN WATERS. SEAS ARE STILL RUNNING NEAR 7 FEET AT 44009...AND ONLY A SLOW FALL IS EXPECTED TONIGHT INTO FRIDAY. IN FACT...THE SMALL CRAFT ADVISORY FOR THE NORTHERNMOST OCEAN ZONE MAY NEED TO BE EXTENDED AS THE LONGER PERIOD WAVES TEND TO HOLD ONTO ENERGY LONGER...AND COULD REMAIN ABOVE 5 FEET PAST 600 AM FRIDAY. THE FLOW OVER THE WATERS FOR TONIGHT THROUGH FRIDAY NIGHT WILL BE PRE WARM FRONTAL...AS THE FLOW REMAINS SOUTH TO SOUTHEAST. THE GRADIENT SHOULD REMAIN BELOW 20 KNOTS THROUGH THIS PERIOD...AS THE DEPTH OF THE MIXED LAYER REMAINS FAIRLY LOW. IF THERE IS SOME SUNSHINE FRIDAY AFTERNOON...LOCAL FUNNELING COULD BRING A PERIOD OF NEAR SMALL CRAFT SOUTHEAST WINDS TO THE HIGHER OBSERVING PLATFORMS ON DELAWARE BAY. IT WOULD APPEAR THE BEST CHANCE OF SMALL CRAFT WINDS THIS WEEKEND MIGHT BE SATURDAY NIGHT INTO SUNDAY MORNING. AS SURFACE LOW PRESSURE TRIES TO ORGANIZE TO THE SOUTHWEST OF THE WATERS...THE GRADIENT COULD TIGHTEN DURING THIS TIME. THERE IS STILL A FAIR AMOUNT OF SPREAD ON THIS...SO CONFIDENCE IN THE PLACEMENT OF THE LOW IS STILL BELOW AVERAGE. THE DEPTH OF THE MIXED LAYER IS LOW...AND IT WOULD APPEAR AS THOUGH THE 35 KNOT LOW LEVEL JET WILL REMAIN ABOVE THE INVERSION. DURING THIS TIME...IT WOULD APPEAR AS THOUGH 25 KNOT GUSTS ARE POSSIBLE...ESPECIALLY NEARSHORE. IN ADDITION...THE PERSISTENT SOUTHEAST FLOW SHOULD MEAN SMALL CRAFT SEAS FOR THE OCEAN AND PERHAPS EVEN THE LOWER DELAWARE BAY. THE NEXT CHANCE OF SMALL CRAFT WINDS COULD COME TUESDAY...AS THE SOUTHWEST FLOW INCREASES. && .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. && $$