FXUS61 KPHI 091926 AFDPHI AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE MOUNT HOLLY NJ 326 PM EDT SAT MAY 9 2009 .SYNOPSIS... A COLD FRONT WILL MOVE ACROSS OUR AREA LATER THIS EVENING AS LOW PRESSURE TRACKS INTO NORTHERN NEW ENGLAND OVERNIGHT. HIGH PRESSURE SLIDING SOUTHEASTWARD FROM THE MIDWEST SUNDAY WILL EVENTUALLY SETTLE RIGHT OVER OUR AREA FOR EARLY NEXT WEEK. THIS SURFACE HIGH WILL THEN SLIDE TO OUR EAST ON WEDNESDAY, ALLOWING A WARM FRONT TO LIFT NORTHWARD WEDNESDAY NIGHT. A COLD FRONT IS FORECAST TO APPROACH ON THURSDAY, THEN CROSS OUR REGION THURSDAY NIGHT OR FRIDAY MORNING. HIGH PRESSURE IS SET TO RETURN FRIDAY NIGHT AND SATURDAY. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/... AN 18Z SURFACE ANALYSIS PLACED A SURFACE LOW IN SOUTHWESTERN ONTARIO CANADA WITH A COLD FRONT TRAILING TO THE SOUTHWEST THROUGH THE OHIO VALLEY. AN UPPER AIR ANALYSIS PLACED A SHARP TROUGH IN THE EASTERN GREAT LAKES WITH A TRAILING AXIS DOWN INTO THE APPALACHIANS. THE WATER VAPOR IMAGERY SHOWS THE WELL DEFINED MID LEVEL IMPULSE NOW SLIDING ACROSS THE EASTERN GREAT LAKES WITH A DRY SLOT IN THAT AREA ASSOCIATED WITH A ROBUST MID LEVEL JET MAX. THESE FEATURES WILL CONTINUE TO SLIDE TO THE NORTHEAST THROUGH THE EVENING. THE ASSOCIATED COLD FRONT WILL CROSS OUR AREA LATER THIS EVENING AND ONE WOULD THINK WITH THE BUILT OF INSTABILITY OUT THERE THIS AFTERNOON, SHOWERS AND THUNDERSTORMS WOULD BE POPPING UP. HOWEVER, THE 12Z RAOB FROM STERLING, VIRGINA INDICATED A CAP AT 850 MB AND THIS IS CONFIRMED BY THE FLATNESS OF THE CUMULUS THAT HAS BEEN ABLE TO POP UP DUE TO SURFACE HEATING. THE SURFACE HEATING WILL LIKELY BE ENOUGH TO WEAKEN THE CAP, HOWEVER WE ARE LOOKING FOR A TRIGGER. THE 300 MB JET IS POSITIONED TO OUR NORTH SUCH THAT OUR CWA IS NOT IN A FAVORABLE REGION OF LIFT. THE MODEL GUIDANCE CONTINUES TO INDICATE LITTLE TO NO CONVECTION FORMING OVER OUR CWA AHEAD OF AND ALONG THE FRONT. IT IS RARE, ESPECIALLY IN MAY, TO HAVE A STRONG COLD FRONT MOVE THROUGH OUR CWA WITH NO REAL CONVECTION BUT THAT MAY OCCUR THIS TIME AROUND. OUR CWA REMAINS BETWEEN TWO AREAS OF POTENTIAL SEVERE WEATHER, WITH THE MAIN ACTIVITY REALLY GETTING GOING ACROSS NEW YORK STATE. THE OTHER AREA TO OUR SOUTH IS ASSOCIATED WITH THE SOUTHERN EXTENSION OF THE TROUGH AXIS AND AN EMBEDDED SHORT WAVE. THIS HAS HAD SOME HELP FROM THE TERRAIN WHERE DEVELOPED CONVECTION PROPAGATES INTO THE HIGHER INSTABILITY EAST OF THE MOUNTAINS. SOME OF THIS ACTIVITY MAY IMPACT OUR FAR SOUTHERN ZONES BUT IT APPEARS MOST WILL STAY SOUTH. GIVEN THAT SOME CONVECTION HAS FORMED IN NORTH-CENTRAL PENNSYLVANIA BACK TO SOUTHWESTERN PENNSYLVANIA WE WILL MAINTAIN LOW CHC TO SLIGHT CHC POPS FOR THE REST OF THE AFTERNOON AND FOR A TIME THIS EVENING BEFORE DROPPING POPS ALTOGETHER. THERE IS PLENTY OF WIND WITHIN THE ATMOSPHERIC COLUMN, THEREFORE ANY STRONGER CELL THAT IS ABLE TO FORM COULD PRODUCE LOCALLY GUSTY WINDS AND MAYBE SOME HAIL GIVEN LOWER FREEZING LEVELS. AS OF NOW, THE SEVERE POTENTIAL ACROSS OUR CWA IS LOOKING RATHER LOW AND WILL CONTINUE TO NOT MENTION ANYTHING IN THE HAZARDOUS WEATHER OUTLOOK /HWO/. AS THE FRONT MOVES THROUGH, COOLER AIR WILL OVERSPREAD THE REGION AND THIS WILL BE ACCOMPANIED BY A DECENT WIND FIELD ABOVE THE SURFACE. THIS WIND FIELD HAS ALLOWED FOR BREEZY CONDITIONS ACROSS OUR CWA THIS AFTERNOON. HOWEVER, THE SURFACE WILL BECOME MORE STABLE WITH NOCTURNAL COOLING AND THIS MAY TEND TO KEEP THE STRONGEST WINDS FROM REACHING THE SURFACE DESPITE A PRESSURE RISE MAXIMUM MOVING IN. THE GREATEST PRESSURE RISES ARE FORECAST TO LIFT TO OUR NORTH NEAR THE PARENT SURFACE LOW AND MAIN MID LEVEL WIND FIELD. AS A RESULT, WE WILL NOT PLAY UP THE WINDS TO MUCH BUT DID BUMP THEM UP A LITTLE BIT ESPECIALLY POST FRONTAL. AS FOR TEMPERATURES, WE MAINLY BLENDED THE GFS/NAM MOS WHICH SPLIT THE DIFFERENCE AT SOME LOCATIONS. && .SHORT TERM /6 AM SUNDAY MORNING THROUGH MONDAY NIGHT/... AN AREA OF LOW PRESSURE IS FORECAST TO BE MOVING THROUGH THE CANADIAN MARITIMES ON SUNDAY. A SURFACE TROUGH MAY SWING ACROSS OUR AREA DURING THE DAY, POTENTIALLY ALLOWING EVEN DRIER AIR TO PUSH DOWN INTO OUR CWA. THE MAIN IMPACT ON SUNDAY WILL BE THE WINDS. A TIGHTENED PRESSURE GRADIENT WILL BE ESTABLISHED BETWEEN THE AFOREMENTIONED SURFACE LOW AND HIGH PRESSURE BUILDING INTO THE NORTHERN OHIO VALLEY. A MAXIMUM PRESSURE RISE OF ABOUT 4 MB IN 6 HOURS IS FORECAST TO SLIDE ACROSS OUR AREA DURING THE MORNING THEN THIS WANES. AS A RESULT, THE FLOW AROUND 5,000 FEET IS AROUND 50 KNOTS EARLY IN THE MORNING BUT THEN THIS WEAKENS TO ABOUT 35 KNOTS THROUGH THE AFTERNOON. THE FORECAST SOUNDINGS DO INDICATE STEEPENING LOW-LEVEL LAPSE RATES THROUGH THE DAY AS SURFACE HEATING TAKES PLACE. THIS WILL ALLOW FOR EFFICIENT VERTICAL MIXING, ALLOWING FOR A BREEZY TO WINDY DAY. WE WOULD HAVE TO MIX UP FAIRLY HIGH TO OBTAIN ADVISORY CRITERIA. WE BELIEVE THAT THE MORE STABLE LOW LEVELS EARLY IN THE DAY WILL KEEP THE STRONGEST WINDS ABOVE THE SURFACE. AS A RESULT, WE WILL NOT ISSUE A WIND ADVISORY BUT DID BUMP UP THE GUSTS TO NEAR 30 KNOTS IN THE GRIDDED DATABASE. THE WINDS WILL SETTLE DOWN THROUGH SUNDAY EVENING AS THE PRESSURE GRADIENT WEAKENS AND THE BOUNDARY LAYER COOLS. WITH HIGH PRESSURE AT THE SURFACE BUILDING TOWARD OUR AREA, COUPLED WITH A DRY AIR MASS, LIGHT WINDS AND LITTLE IN THE WAY OF CLOUDS, WILL LEAD TO CHILLY CONDITIONS SUNDAY NIGHT. THERE COULD BE PATCHY FROST TOWARD DAYBREAK MONDAY IN THE MORE SHELTERED AREAS. IF THIS POTENTIAL LOOKS MORE CERTAIN, SOME FROST MAY HAVE TO BE ADDED INTO THE FORECAST FOR SOME AREAS. FOR MONDAY, AN UPPER-LEVEL TROUGH REMAINS ACROSS THE NORTHEAST WITH SOME SHORT WAVE ENERGY EMBEDDED IN THE FLOW. IT APPEARS THAT THE AIR MASS IS TO DRY FOR THE SHORT WAVE TO PRODUCE ANY SHOWERS, WHICH IS CONFIRMED BY THE FORECAST SOUNDINGS. WE DO ESTABLISH STEEPER LOW-LEVEL LAPSE RATES AGAIN MONDAY DUE TO SURFACE HEATING, HOWEVER THE FLOW WITHIN THE BOUNDARY LAYER IS MUCH LESS COMPARED TO SUNDAY. THEREFORE, LESS WIND IS EXPECTED DURING THE DAY MONDAY. MONDAY NIGHT WILL BE ANOTHER CHILLY ONE HOWEVER SOME CLOUDS MAY BE AROUND AS THE UPPER-LEVEL TROUGH AXIS APPROACHES. AS FOR TEMPERATURES, WE GENERALLY UTILIZED AN EVENLY WEIGHTED BLEND OF THE GFS/NAM MOS TO SMOOTH OUT SOME DIFFERENCES. && .LONG TERM /TUESDAY THROUGH SATURDAY/... THIS TIME FRAME LOOKS TO BEGIN WITH A TRANQUIL PERIOD AS HIGH PRESSURE SLIDES ACROSS THE AREA. THE UPPER-LEVEL FLOW REGIME IS MORE OF TROUGHINESS AND THE MODEL GUIDANCE IS IN GOOD AGREEMENT TO START OUT. THERE ARE IMPULSES EMBEDDED IN THE FLOW WHICH WILL TOSS AT LEAST ONE COLD FRONT OUR WAY, AND THE TIMING OF THIS FRONT MAY BE A LITTLE TRICKY DEPENDING ON THE UPPER-LEVEL TROUGH AMPLIFICATION AND ASSOCIATED PRIMARY SHORT WAVE. HPC UTILIZED THE ECMWF FOR TUESDAY AND WEDNESDAY, BUT THEN PHASED IN AN ALMOST EQUAL PORTION OF THE GEFS ENSEMBLE MEAN/ECMWF ENSEMBLE MEAN BY FRIDAY AND SATURDAY. THE REASON FOR THIS WAS TO TONE DOWN SOME OF THE HIGHER AMPLITUDE FEATURES IN THE ECMWF. HPC NOTED THAT THE GEFS MEAN AND ECMWF MEAN WERE REASONABLY COMPATIBLE WITH EACH OTHER FOR THURSDAY THROUGH SATURDAY. THE GEFS MEAN ACTUALLY HAD THE BEST RESEMBLANCE TO THE ECMWF WITH THE MAIN SHORT WAVE FEATURES FOR FRIDAY AND SATURDAY. THERE IS SOME GUIDANCE THOUGH THAT HINTS AN INCREASING SPREAD AS A MID LEVEL TROUGH NEARS THE EAST COAST ON FRIDAY. THE GFS ENSEMBLES SHOW SOME DIFFERENCES WITH THE AMPLIFICATION OF THIS TROUGH AS THE UPPER-LEVEL FLOW CONFIGURATION IN THE WEST BECOMES A BIT UNCERTAIN. BASED ON THE ABOVE, HIGH PRESSURE WILL BE SLIDING ACROSS OUR REGION ON TUESDAY, HOWEVER THE PATTERN LOOKS PROGRESSIVE ENOUGH THEREFORE THIS HIGH WILL BE SHIFTING TO OUR NORTHEAST AND EAST ON WEDNESDAY. AN UPPER-LEVEL TROUGH IS FORECAST TO BE MOVING ACROSS THE AREA TUESDAY. THERE DOES NOT APPEAR TO BE MUCH LIFT WITH THIS AS SURFACE HIGH PRESSURE IS NEARBY, HOWEVER COOLER AIR ALOFT COUPLED WITH DAYTIME COULD POP AN ISOLATED SHOWER OR TWO. FOR NOW, WE OPTED TO CARRY A DRY FORECAST GIVEN THE RATHER ISOLATED POTENTIAL AND LOW CONFIDENCE. A WAA REGIME ON THE BACKSIDE OF THIS HIGH SHOULD ALLOW FOR A WARM FRONT TO DEVELOP AND SLIDE NORTHWARD WEDNESDAY NIGHT. WE INCLUDED A SLIGHT CHC POP IN THE DATABASE WEDNESDAY NIGHT FOR NOW. AN AREA OF LOW PRESSURE SLIDING ACROSS SOUTHERN CANADA THURSDAY AND THURSDAY NIGHT WILL DRIVE A COLD FRONT EASTWARD. THIS FRONT MAY BE RATHER ACTIVE WELL TO OUR WEST, BUT THEN WEAKEN WITH AN EASTWARD EXTENT AS THE BULK OF THE DYNAMICS LIFT WELL TO OUR NORTH. ATTM, WE WILL BRING THE COLD FRONT ACROSS OUR CWA THURSDAY NIGHT AND FRIDAY MORNING. CHC POPS WILL TRAVERSE THE AREA FROM WEST TO EAST THURSDAY INTO THURSDAY NIGHT. WITH A LOW-LEVEL JET POSSIBLY CRUISING ACROSS THE AREA THURSDAY, JUST ENOUGH INSTABILITY MAY OCCUR THEREFORE WE WILL CARRY A THUNDER MENTION AS WELL. HIGH PRESSURE THEN BUILDS SOUTHEASTWARD FROM EASTERN CANADA FRIDAY NIGHT AND SATURDAY. THIS HIGH LOOKS STRONG ENOUGH TO PUSH THE COLD FRONT WELL TO OUR SOUTH SATURDAY. THAT IS ALL FOR NOW. THANKS TO THE SURROUNDING OFFICES/HPC FOR THE COLLABORATION TODAY. HAVE A GOOD ONE! && .AVIATION /19Z SATURDAY THROUGH THURSDAY/... THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY AND SURROUNDING AREAS. VFR CONDITIONS ARE BEING ENJOYED IN THE PER-FRONTAL AIR MASS AHEAD OF THE APPROACHING FRONT. A THICK VEIL OF CI/CS CLOUDS OVER THE TAF SITES ARE KEEPING CONVECTION AT A MINIMUM SO FAR WITH ONLY A FEW-SCT CU ACROSS THE REGION. IT APPEARS THAT THE FRONT WILL CROSS OUR AREA DRY THIS EVENING. THE CHANCES FOR ANY SHOWER LOOK TOO LOW TO INCLUDE IN THE TAFS. A BRISK WEST WINDS AND CLEARING SKIES WILL DEVELOP BEHIND THE FRONT. WE WILL KEEP THE WINDS GUSTY INTO THE OVERNIGHT WITH THE COLDER AIR DRIER AIR MOVING IN. OUTLOOK... A PROLONGED PERIOD OF GOOD FLYING WEATHER IS EXPECTED MUCH OF THE EARLY, AND POSSIBLY INTO THE MIDDLE, OF THE UPCOMING WEEK. && .MARINE... A SMALL CRAFT ADVISORY IS IN EFFECT FOR ALL OF OUR COASTAL WATERS TONIGHT THROUGH SUNDAY NIGHT. A SW WIND AHEAD OF THE APPROACHING FRONT WILL REMAIN GUSTY TONIGHT AND IT WILL SHIFT TO A WRLY DIRECTION AFTER THE FRONT PASSES. THE FRONT SHOULD CROSS AROUND MIDNIGHT. THERE IS A CHANCE FOR A SHOWER OR A THUNDERSTORM UNTIL THE FRONT PASSES, BUT COVERAGE SHOULD BE ISOLATED AT BEST. HIGH PRESSURE WILL BUILD IN SUNDAY AND REMAIN IN CONTROL OF THE WEATHER FOR THE EARLY PART OF THE WEEK. WINDS WILL REMAIN GUSTY SUNDAY AND THEN DECREASE SUNDAY NIGHT. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...NONE. NJ...NONE. DE...NONE. MD...NONE. MARINE...SMALL CRAFT ADVISORY UNTIL 6 AM EDT MONDAY FOR ANZ430-431- 450>455. && $$ SYNOPSIS...GORSE NEAR TERM...GORSE SHORT TERM...GORSE LONG TERM...GORSE/HPC AVIATION...O'HARA MARINE...O'HARA