000 FXUS61 KPHI 041527 AFDPHI AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE MOUNT HOLLY NJ 1125 AM EDT SAT JUL 4 2009 .SYNOPSIS... WEAK HIGH PRESSURE WILL MOVE INTO THE AREA TODAY AS A WEAKENING FRONTAL BOUNDARY STALLS TO OUR SOUTH. LOW PRESSURE WILL DEVELOP OVER THE MISSISSIPPI RIVER VALLEY TONIGHT, APPROACH FROM THE SOUTHWEST ALONG THE WEAK BOUNDARY ON SUNDAY AND PASS BY TO OUR SOUTH SUNDAY NIGHT INTO EARLY MONDAY. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... HAVE UPDATED THE CURRENT ZONE PACKAGE TO INCREASE WINDS ACROSS THE AREA AND MAKE A FEW OTHER RELATIVELY MINOR CHANGES. THE 15Z OBS SUPPORT HIGHER WINDS THAN WERE CURRENTLY FCST, SO UPPED THE SPEEDS A BIT. MODEL PLAN VIEWS AND SOUNDINGS SUGGEST A DRY AND RELATIVELY BREEZY DAY AS THE LATEST, BUT BY NO MEANS LAST, IN A SERIES OF MID LEVEL TROUGH AXES AND ITS ASSOCIATED SURFACE LOW LIFTS AWAY. SOUNDINGS HAVE ALL LOCATIONS EVENTUALLY HITTING THEIR CONVECTIVE TEMPERATURES, WITH THAT HAPPENING FAIRLY EARLY IN THE NORTHWEST AND CONSIDERABLY LATER SOUTHEAST, BUT ALL SOUNDINGS ARE CAPPED. LATER IN THE DAY, SOME MID AND UPPER LEVEL CLOUDS MAY BEGIN TO MOVE IN FROM THE SOUTHWEST AS AN UPPER JET SCOOTS BY IN THE BROAD CYCLONIC FLOW, BUT THERE SHOULD BE A DECENT AMOUNT OF SUNSHINE. GIVEN THE CATABATIC NATURE OF THE SURFACE FLOW, WE LEANED TOWARD THE MILDER STAT GUIDANCE ON TEMPERATURES. SINCE THE SURFACE FLOW TODAY SHOULD BE OFFSHORE, WE DID NOT DRAW TOO MUCH OF A DISTINCTION BETWEEN COASTAL AND INLAND TEMPERATURES. && .SHORT TERM /6 PM THIS EVENING THROUGH MONDAY/... A MID AND UPPER LEVEL IMPULSE WILL MOVE AROUND THE CYCLONIC FLOW AND INTO THE MISSISSIPPI RIVER VALLEY TONIGHT AND THEN TOWARD THE MID-ATLANTIC COAST WHILE GENERATING SURFACE LOW PRESSURE. THERE ARE UPPER LEVEL Q-VECTOR CONVERGENCE AND LOWER LEVEL FRONTOGENESIS WITH THIS FEATURE THAT WOULD BRUSH BY OR OVER OUR SOUTHERN ZONES SUNDAY INTO SUNDAY NIGHT, AND IN ADVANCE OF ALL THIS THERE IS SOME ISENTROPIC LIFT BETWEEN I300 AND I305. THERE PRESENTLY DOES NOT SEEM TO BE A LOT OF ATTENDANT INSTABILITY. IF THERE IS ANY, IT WOULD BE OVER THE FAR SOUTHERN ZONES. THE LATEST ECMWF IS DOWNPLAYING THE EFFECT THAT OUR SOUTHERN ZONES WOULD FEEL AND KEEPING THE BULK OF THE PRECIPITATION SOUTH OF OUR ENTIRE FORECAST AREA. THIS IS IN VERY STARK CONTRAST TO THE GFS, WHICH WANTS A SOLIDLY MODERATE RAIN EVENT OVER THE SOUTHERN ZONES, AND IN SOME CONTRAST TO THE NAM, WHICH IS A TONED DOWN VERSION OF THE GFS. THE GFS LIKELY HAS CONVECTIVE FEEDBACK ISSUES, SO THIS FORECAST IS A BLEND OF THE ECMWF, NAM AND CONTINUITY. FOR WHAT IT IS WORTH, THIS PUTS US ON THE OPTIMISTIC /LOW/ SIDE OF OUR COLLEAGUES AT SURROUNDING OFFICES WITH REGARD TO POPS. WE KEEP ALL BUT THE SOUTHWESTERN ZONES OUT OF ANY POPS TONIGHT AND LIMIT THEM TO LOW CHANCE VALUES. WE BRING MID-RANGE CHANCE POPS TO THE FAR SOUTHERN ZONES SUNDAY INTO SUNDAY NIGHT, WITH LOWER CHANCE POPS OVER THE REST OF THE SOUTHERN HALF OF THE FORECAST AREA. WE KEEP ANY MENTION OF THUNDER OUT FOR NOW. WE LIFT THE SYSTEM AWAY ON MONDAY, ALTHOUGH OF COURSE ANOTHER MID LEVEL TROUGH AXIS IS GOING TO APPROACH FROM THE NORTHWEST ON MONDAY. WE/VE MAINLY KEPT MONDAY DRY FOR NOW. ON TEMPERATURES, AN INCREASE IN CLOUDS AND THE SHORT DURATION OF DARKNESS SHOULD KEEP THE SOUTHERN ZONES FROM RADIATING MUCH. FURTHER NORTH, IT IS JUST THE LATTER THAT WOULD INHIBIT RADIATING AS WINDS ARE EXPECTED TO RELAX THROUGHOUT THE AREA. THERE MAY BE SOME 40S IN THE FAR NORTHWEST. FROM SUNDAY THROUGH MONDAY, WE HAVE LEANED STRONGLY TOWARD THE MET TEMPERATURES. WE HAVE REJECTED THE GFS SOLUTION AND HENCE THE MAV TEMPERATURES. && .LONG TERM /MONDAY NIGHT THROUGH FRIDAY/... THE FIRST PART OF THE EXTENDED PERIOD WILL BE FAIRLY UNSETTLED AS YET ANOTHER CLOSED LOW PRESSURE SYSTEM FROM CANADA WILL BE APPROACHING THE AREA. AT 12Z MON THE UPPER LOW WILL BE OVER ONTARIO AND IT WILL TAKE MUCH OF THE WEEK TO EVENTUALLY TRACK OFF THE NEW ENGLAND COAST. THIS MEANS THAT SHOWERY AFTERNOON/EARLY EVENINGS WITH SOME THUNDER POSSIBLE TOO. THE POPS IN THE PRESENT FCST ARE FOR THE MOST PART IN THE SLIGHT CHC RANGE FOR NOW BUT THEY MAY TREND UPWARD WITH FUTURE FCSTS ONCE THE TIMING OF THE INDIVIDUAL SW/S BECOMES BETTER. TEMPERATURES ACROSS OUR REGION WILL BE A BIT BELOW NORMAL MON-WED WITH LOW TO MID 80S FOR MAX TEMPERATURES. THE LAST PART OF THE EXTENDED PERIOD WILL BE MORE LIKE TYPICAL MIDDLE ATLANTIC SUMMER WEATHER. THE LONGER RANGE GFS/ECMWF MODELS BOTH DEVELOP A RIDGE ACROSS THE AREA WITH INCREASING H8 TEMPS TOO. WE COULD BE LOOKING AT MAX TEMPERATURES NEAR 90 BY THIS TIME NEXT WEEK ACROSS METRO PHILADELPHIA AND THE DELMARVA. THE THU-FRI PERIOD IS EXPECTED TO BE DRY. && .AVIATION /08Z SATURDAY THROUGH WEDNESDAY/... THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY AND SURROUNDING AREAS. VFR CONDITIONS ARE FORECAST FOR TODAY AND TONIGHT. WEAK HIGH PRESSURE WAS BUILDING AND WILL FOLLOW YESTERDAY/S AND LAST NIGHT/S SHORT WAVE, MID LEVEL CLOUDS MOSTLY CLEARED AWAY DURING THE EARLY OVERNIGHT PERIOD. SCATTERED CUMULUS IS EXPECTED TO FORM LATER THIS MORNING AS CONVECTIVE TEMPERATURES ARE REACHED. THEN, HIGHER LEVEL MOISTURE WILL FEED IN AS THE UPPER FLOW BACKS A BIT LATER THIS AFTERNOON AND TONIGHT. WESTERLY SURFACE WINDS, SUSTAINED AROUND 10 KNOTS AFTER THE BOUNDARY LAYER FULLY RECOUPLES BY MID MORNING, MAY GUST UP A BIT IN THE AFTERNOON WITH GOOD VERTICAL MIXING TAKING PLACE. OUTLOOK... VFR CONDITIONS ARE EXPECTED LATER TONIGHT INTO WEDNESDAY, ALTHOUGH THE SYSTEM PASSING BY TO OUR SOUTH MAY OFFER SOME UPWARD VERTICAL MOTION LATER SUNDAY AND SUNDAY EVENING. IF ENOUGH INSTABILITY RESULTS, THIS COULD BE ISOLATED TO SCATTERED CONVECTION, BUT AT THIS TIME THERE DOES NOT SEEM TO BE ENOUGH FORCING FOR THAT. && .MARINE... CONDITIONS LOOK TO REMAIN UNDER SMALL CRAFT ADVISORY CRITERIA THIS WEEKEND, AS WELL AS THIS COMING WEEK. THE SURFACE PRESSURE PATTERN IS WEAK ENOUGH SUCH THAT WINDS WILL REMAIN WELL BELOW 25 KNOTS, AND WAVEWATCH CAN ONLY FORECAST SEAS TO AROUND 3 FEET AT BUOY 44009 AN EARLIER RUN HAD SEAS A BIT MORE ELEVATED FOR BUOY 44025, BUT THE LATEST VERSION IS IN LINE WITH THE MORE SOUTHERN BUOY. THE RIP CURRENT RISK WILL CONTINUE TO BE LOW, DESPITE THE LONG PERIOD ON THE WAVES AND THE NEARING OF THE FULL MOON ON TUESDAY. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...NONE. NJ...NONE. DE...NONE. MD...NONE. MARINE...NONE. && $$ SYNOPSIS...DELISI NEAR TERM...NIERENBERG/DELISI SHORT TERM...DELISI LONG TERM...O'HARA AVIATION... MARINE...