Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 292008 AFDPHI AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE MOUNT HOLLY NJ 408 PM EDT TUE JUL 29 2014 .SYNOPSIS... WEAK HIGH PRESSURE OVER THE AREA MERGES WITH THE STRONGER WESTERN ATLANTIC HIGH PRESSURE LATE THIS WEEK...FORCING THE OFFSHORE FRONT TO BACK IN TOWARD THE COAST AS A WARM FRONT THIS WEEKEND. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... QUIET WEATHER TONIGHT AS WEAK HIGH PRESSURE BUILDS IN FROM THE WEST. PW VALUES ARE LOW ENOUGH, BELOW ONE INCH, THAT ANY VORTICITY IMPULSES TONIGHT ARE NOT EXPECTED TO CREATE ANY PRECIPITATION. THE MAIN FOCUS FOR TONIGHT WILL BE THE LOW TEMPERATURE FORECAST. MODEL GUIDANCE IS INDICATING LOW TEMPERATURES GETTING WITHIN A COUPLE OF DEGREES OF RECORD LOWS FOR VIRTUALLY ALL SITES. THE BIG QUESTION WILL BE HOW FAST AND HOW MUCH CLEARING TAKES PLACE LATER THIS EVENING INTO THE OVERNIGHT HOURS. IF THE CURRENT CLOUD COVER LASTS LONGER INTO THE NIGHT, WE`LL HAVE LESS TIME FOR GOOD RADIATIONAL COOLING EVEN WITH LIGHT AND VARIABLE/CALM WINDS. IF WE BREAK OUT FASTER, MUCH MORE COOLING WILL TAKE PLACE. FOR NOW, WE STAY CLOSE TO MAV/MET MOS AND MOSGUIDE BLEND. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/... WEAK HIGH PRESSURE CONTINUES TO MOVE ACROSS THE AREA EARLY IN THE DAY WEDNESDAY. HOWEVER, LATE IN THE DAY, A WEAK SURFACE TROUGH MAY DEVELOP ACROSS OUR WESTERN ZONES, AND THERE ARE SOME WEAK VORTICITY IMPULSES THAT MAY SWING THROUGH THE BROAD TROUGH ALOFT AS WELL. THIS, COMBINED WITH SOME MOISTURE MAY HELP ENHANCE THE SHOWER/THUNDERSHOWER POTENTIAL LATE IN THE DAY. THERE IS NOT MUCH INSTABILITY ACROSS OUR AREA, BUT THERE MAY BE JUST ENOUGH FOR AN ISOLATED THUNDERSHOWER TO OCCUR FOR OUR NORTHWESTERN AREAS. THIS BEING SAID, WE EXPECT MOST OF THE FOCUSED ACTIVITY TO REMAIN TO OUR NORTH. 925/850 MB TEMPERATURES INCREASE A DEGREE OR TWO WEDNESDAY, AND THICKNESSES INCREASE EVER SO SLIGHTLY AS WELL. THEREFORE HIGHS ON WEDNESDAY SHOULD BE A FEW DEGREES WARMER THAN TUESDAY. WE USED A MAV/MET MOS, MOSGUIDE, AND MIXED 925MB TEMP BLEND FOR HIGHS ON WEDNESDAY. && .LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/...
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500 MB: THE LARGE TROUGH IN THE EASTERN UNITED STATES WEAKENS LATE THIS WEEK, THEN PROBABLY FLATTENS EARLY NEXT WEEK. TEMPERATURES: CALENDAR DAY AVERAGES SEVERAL DEGREES BELOW NORMAL THURSDAY THEN NEAR NORMAL THEREAFTER POSSIBLY WARMING ABOVE NORMAL EARLY NEXT WEEK. THIS WILL ALLOW JULY TO AVERAGE NEAR OR SLIGHTLY BELOW NORMAL AT MOST LOCATIONS EXCEPT AT LEAST A DEGREE BELOW NORMAL AT KACY. FORECAST BASIS: UNLESS OTHERWISE NOTED THE LONG TERM PERIOD OF THIS FORECAST IS GENERALLY BASED ON A 50 50 BLEND OF THE 12Z/29 NAM/GFS MOS FOR THE PERIOD WEDNESDAY NIGHT - THURSDAY NIGHT... THE 12Z/29 GFS MOS FRIDAY AND FRIDAY NIGHT AND THEN THEREAFTER (SATURDAY ONWARD) THE 1522Z/29 WPC GRIDDED GUIDANCE WHICH AT TIMES IS BLENDED 50 50 WITH THE 12Z/29 MEXMOS. ALL THIS IS EVENTUALLY CHECKED AGAINST THE 12Z/29 ECMWF TO SEE IF WE`RE IN THE BALLPARK WITH ECMWF CYCLICALLY PERSISTENT 18Z 2METER TEMPERATURES. THE PRECIPITATION PROBABILITY GUIDANCE IS CHECKED AGAINST THE 09Z/29 SREF POPS FOR .01 IN 3 HRS THROUGH 00Z FRIDAY AND THEREAFTER THE 12Z/28 GEFS 6HR POPS FOR 0.05 THE DAILIES... WEDNESDAY NIGHT - THURSDAY EVENING...CONTINUING THE PRIOR MID SHIFT FORECAST. MOSTLY DRY ACROSS THE REGION OUTSIDE THE POSSIBILITY OF ISOLATED INSTABILITY SHOWERS ACROSS THE NORTHERN PORTIONS OF THE CWA THURSDAY AFTERNOON. COLD POOL INTERACTION WITH A PASSING SHORTWAVE MAY BE ENOUGH TO GET AIRMASS SHOWERS TO OCCUR. FRIDAY...TRANSITIONAL DAY WITH THE LOOKS OF AN INVERTED TROUGH SITTING OFF THE ATLANTIC COAST...THE OLD BAROCLINIC ZONE THAT PUSHED THROUGH ON MONDAY. INTERIOR INSTABILITY AND WARM AIR ADVECTION ASSOCIATED WITH SHORT WAVES MOVING NEWD THROUGH THE MID ATLC STATES APPEAR TO MAKE SHOWERS PROBABLE FOR EASTERN PA. IT SHOULD BE UNSTABLE ENOUGH FOR THUNDER THERE DURING THE AFTN. FRIDAY NIGHT - WEEKEND...STILL UNCERTAINTY IN SHOWER TIMING DETAILS WHICH WILL AFFECT MAX TEMPS THIS WEEKEND BUT IT APPEARS THAT THERE HAS TO BE A PERIOD OF SHOWERS...SOME BRIEFLY HEAVY...WITH ISOLATED EMBEDDED TSTMS. THIS IN ASSN WITH A PAIR OF SHORT WAVES MOVING NEWD THROUGH THE TROUGH. AGAIN...TIMING THE PRIMARY PERIOD OF SHOWERS, IN MY OPINION, IS STILL AS OF THIS WRITING...QUITE UNCERTAIN. MAX TEMPS SATURDAY COULD DIFFER 5 DEGREES EITHER WAY OF OUR CURRENT FCST...DEPENDING ON WHETHER ITS RAINING BETWEEN 11AM AND 3 PM. I MAINTAINED OUR PRIOR MID SHIFT FCST TEMPS FOR THIS DAY...NOT USING THE COOLER WPC GUIDANCE, NOR THE MUCH WARMER GFS GUIDANCE. THE ECMWF IS EVEN COOLER THAN THE WPC GUIDANCE. MONDAY - TUESDAY...A SLIGHT DRYING WARMING TREND THOUGH NOT ENTIRELY VOID OF ANY SHOWERS WITH A FEW MORE WEAK SHORTWAVES SHEARING NEWD NEARBY OR OVERHEAD.
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&& .AVIATION /20Z TUESDAY THROUGH SUNDAY/...
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THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY AND SURROUNDING AREAS. VFR CONDITIONS WILL CONTINUE ACROSS THE TAF SITES THROUGH WEDNESDAY. WEST-TO NORTHWEST WINDS THIS AFTERNOON INTO THIS EVENING WILL BECOME LIGHT AND VARIABLE/CALM OVERNIGHT, BEFORE INCREASING OUT OF THE WEST TO SOUTHWEST DURING THE DAY WEDNESDAY. OUTLOOK... WEDNESDAY NIGHT - THURSDAY...MOSTLY VFR. ISOLATED SHOWER OR THUNDERSTORM POSSIBLE MAINLY NORTH OF KABE THU AFTN. FRIDAY - SATURDAY - SUNDAY...MOSTLY VFR CIGS. IFR CONDITIONS IN SHOWERS AND ISOLATED EMBEDDED TSTMS EXPECTED SOMETIME BETWEEN FRIDAY AFTN AND MIDDAY SATURDAY.
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&& .MARINE...
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SUB-ADVISORY CONDITIONS WILL CONTINUE THROUGH WEDNESDAY AS WINDS REMAIN 10-15 KNOTS OR LESS AND SEAS REMAIN 2-3 FEET OR LESS. OUTLOOK... WEDNESDAY NIGHT - SATURDAY...SUB-SCA CONDITIONS EXPECTED. SHOWERS AND TSTMS SHOULD OCCUR SOMETIME LATER FRIDAY INTO SATURDAY. NEXT WEEK: WE ARE MONITORING THE GFS OPERATIONAL RUN DEPICTION (FOR TWO DAYS OF FORECAST CYCLES, THOUGH TODAYS 12Z/29 RUN BACKED OFF) OF A TROPICAL SYSTEM IN THE ATLANTIC BASIN FOR EARLY NEXT WEEK. THE ECMWF OPERATIONAL VERSION HAS BEEN MUCH MORE CONSERVATIVE. WE COULD BE SEEING A 1 FOOT 15 SECOND SWELL ARRIVE ALONG OUR SHORES NEXT MONDAY... MAYBE INCREASE TO A 2 FOOT 13 SECOND SE SWELL NEXT TUESDAY...AGAIN...IF A TROPICAL SYSTEM OCCURS AS OUTLOOKED TODAY.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...NONE. NJ...NONE. DE...NONE. MD...NONE. MARINE...NONE. && $$ SYNOPSIS...DRAG NEAR TERM...ROBERTSON SHORT TERM...ROBERTSON LONG TERM...DRAG 408 AVIATION...DRAG/ROBERTSON 408 MARINE...DRAG/ROBERTSON 408

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