AFDPHI AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE MOUNT HOLLY NJ ISSUED BY NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC 352 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 /19Z THURSDAY THROUGH TUESDAY/... THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY AND SURROUNDING AREAS. FAIR CU EVERYWHERE EXCEPT ALONG THE SHORE BEHIND THE WESTWARD SEA BREEZE FRONT /AT 1930Z...ALONG A LINE FROM KCDW TO KWRI TO JUST EAST OF KMIV/ WILL DISSIPATE THIS EVENING. LIGHT SWLY FLOW EAST OF THE FRONT AND SELY FLOW BEHIND THE FRONT...WILL WILL PUSH WEST THROUGH THE NORTHERN MID ATLANTIC. TONIGHT...VFR TO START. MVFR CONDS IN LOW CLOUDS/PATCHY FOG SHOULD DEVELOP VCNTY KABE AND KRDG NEAR OR AFTER 06Z. VSBY 3-5MI SEEMS TO BE LESS OF A THREAT...WITH LOW CLOUDS DEVELOPING LATE TONIGHT BEHIND THE SEA BREEZE FRONT. MID LEVEL CLOUDS THROUGH THE DAY FRIDAY...WITH PRECIP APPROACHING FROM THE WEST. OUTLOOK... WEEKEND...WIDESPREAD MVFR CIGS AND OCNL IFR CONDS IN SHOWERS. EMBEDDED ISOLATED TSTMS CONFINED MAINLY TO THE SUNDAY PORTION OF THE WEEKEND. LOWEST CIGS/VSBY EXPECTED SAT NIGHT AND SUNDAY MORNING AHEAD OF THE WARM FRONT. E-SE WIND UNDER 20 KTS. CONFIDENCE: ABOVE AVG. MONDAY THRU WEDNESDAY ...CONDS PROBABLY STAY MVFR WITH SHOWERS AND THUNDERSTORMS. ELY/SELY FLOW...UP TO AROUND 15 KT OUTSIDE OF TSTM ACTIVITY. CONFIDENCE AVG. && .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... WATER LEVELS STILL AROUND 1.5 FT ABOVE NORMAL FOR EASTERN SHORE OF THE CHESAPEAKE BAY. MORNING HIGH TIDE WAS THE LESS PREFERRED ONE...WITH HIGH TIDES A COUPLE INCHES LESS THAN THE PREVIOUS. NEXT HIGH TIDE WILL NEED TO BE MONITORED AS IT IS THE MORE PREFERRED ASTRONOMICALLY. PREVIOUS DISCUSSION... MANY MORE OPPORTUNITIES FOR AT LEAST MINOR CF THROUGH THIS COMING WEDNESDAY. BEST CHC FOR EXTENSIVE ADVY THRESHOLD EXCEEDENCE IS SUNDAY EVENING AND NEXT TUESDAY-WEDNESDAY. THERE IS A LOW PROBABILITY OF EXTENSIVE MODERATE COASTAL FLOODING OCCURRING ON THE TUESDAY EVENING THE 17TH HIGH TIDE CYCLE IF THE 00Z/12 ECMWF FCST FOR 00Z/18TH IS ONLY MARGINALLY IN ERROR. PERSISTENT SLY TO SE INFLOW OVER THE NEXT 5 DAYS BUILDS THE SEAS AND WATER LEVELS. IN THE MEANTIME...POCKETS OF MARGINAL MINOR MIGHT OCCUR LATER TODAY /CHES BAY/ OR THIS COMING THURSDAY NIGHT IN UPPER DEL BAY AND CHES BAY BUT NO STMT AT THIS TIME DUE TO MARGINALITY. FULL MOON THIS COMING TUESDAY THE 17TH SO ASTRO TIDES ARE INCREASING DAILY. && .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. && $$ SYNOPSIS...JACKSON/PHI