Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 311723 AFDPHI AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE MOUNT HOLLY NJ 123 PM EDT SUN MAY 31 2015 .SYNOPSIS... A COLD FRONT WILL SLOWLY SETTLE SOUTHWARD ACROSS OUR REGION THROUGH MONDAY. AS IT DOES SO, A WEAK AREA OF LOW PRESSURE WILL SLIDE NORTHEASTWARD ALONG THE FRONT. THE FRONT IS EXPECTED TO BE OFFSHORE BY TUESDAY, THEN HIGH PRESSURE BUILDS SOUTHEASTWARD OVER THE REGION FROM WEDNESDAY THROUGH THE END OF THE WEEK. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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A FLASH FLOOD WATCH HAS BEEN ISSUED FOR PORTIONS OF OUR NORTHERN ZONES. WE ARE SEEING THUNDERSTORMS ON THE INCREASE WITH SIGNS OF BACKBUILDING AND TRAINING. THE CONVECTIVE CORES SHOULD BE RATHER EFFICIENT RAINFALL PRODUCERS AND DESPITE THE REGION BEING DRY, SOME RAIN HAS OCCURRED IN PARTS OF THIS AREA RECENTLY AND WITH THE TERRAIN EXCESSIVE RAINFALL RATES COULD LEAD TO SOME FLASH FLOODING. THE WATCH GOES THROUGH 11 PM. OTHERWISE, A COLD FRONT IS DRAPED ACROSS NORTHERN PENNSYLVANIA AND THIS BOUNDARY IS EXPECTED TO DRIFT SOUTHWARD SOME THROUGH THE AFTERNOON. TO THE SOUTH OF THE FRONT, A RATHER WARM AND MOIST AIRMASS IS IN PLACE. THE WATER VAPOR IMAGERY SHOWS AN MCV MOVING ACROSS WESTERN PENNSYLVANIA ATTM AND THIS FEATURE IS FORECAST TO TRACK NORTHEASTWARD. THE ASSOCIATED LIFT WILL HELP INITIATE ADDITIONAL CONVECTION ALONG THE BOUNDARY THIS AFTERNOON. THE MAIN AREA OF ORGANIZATION IS FROM NORTHEASTERN PENNSYLVANIA TO NORTHERN NEW JERSEY. THE BEST SHEAR IS NORTH AND WEST OF OUR AREA WITH SOME INCREASE NOTED AS THE MCV MOVES NORTHEASTWARD. THEREFORE SOME STORM ORGANIZATION SHOULD BE IN THE FORM OF CLUSTERS OR SHORT LINES. THE STORM MOTION COULD BE SLOW ENOUGH IN COMBINATION WITH SOME CELL TRAINING WITHIN A HIGH PW ENVIRONMENT TO RESULT IN LOCALLY VERY HEAVY RAIN. AS FOR A SEVERE THUNDERSTORM THREAT, THE BEST BULK SHEAR IS TO OUR NORTH AND WEST, HOWEVER WITH PLENTY OF INSTABILITY THERE CAN BE ENOUGH STORM ORGANIZATION AND INITIAL ROBUST UPDRAFTS TO RESULT IN LOCAL DOWNBURST/DAMAGING WINDS IN COMBINATION WITH VERY HEAVY RAIN. THIS IS MAINLY FOR OUR WESTERN TO NORTHWESTERN ZONES. FARTHER SOUTH, LESS LIFT SHOULD KEEP ANY CONVECTION MAINLY ISOLATED. THE HOURLY GRIDS WERE ADJUSTED WITH THE LATEST OBSERVATIONS AND THEN THE LAMP/LAV GUIDANCE WAS USED TO HELP ASSIST. NO MAJOR CHANGES TO THE HIGH TEMPERATURES ATTM.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM MONDAY/... SHOWER AND THUNDERSTORM ACTIVITY WILL BE ONGOING THIS EVENING ACROSS THE AREA AS THE FRONT WILL BE DRAPED ACROSS THE AREA AND THE LEAD SHORT WAVE/VORTICITY MOVES THROUGH. THERE COULD BE A PERIOD OVERNIGHT WHEN WE WILL BE IN BETWEEN SHORT WAVES AND SHOWER ACTIVITY WANES SLIGHTLY. HOWEVER, AN ADDITIONAL SHORT WAVE/VORTICITY IMPULSE IS FORECAST TO MOVE THROUGH LATER IN THE NIGHT, SO THERE WILL CONTINUE TO BE A CHANCE FOR SHOWERS/THUNDERSTORMS THROUGH THE NIGHT. THE GREATEST CHANCES ARE FOR THE NORTHERN TWO-THIRDS OF THE AREA, WHICH ARE CLOSER TO THE ENHANCED LIFT FROM THE SHORT WAVES. ALTHOUGH INSTABILITY WANES WITH LOSS OF DAYTIME HEATING, THERE WILL STILL BE SOME INSTABILITY, SO THUNDERSTORMS REMAIN IN THE FORECAST. PW VALUES ACTUALLY INCREASE OVERNIGHT TO AROUND 1.75 INCHES, SO THERE WILL REMAIN THE TREAT FOR HEAVY RAINFALL WITH ANY SHOWERS OR THUNDERSTORMS. && .LONG TERM /MONDAY THROUGH SATURDAY/... MONDAY INTO MONDAY NIGHT...THIS LOOKS TO BE THE DAY THAT THE FRONT FINALLY BEGINS TO MAKE SOME PROGRESS INTO OUR REGION. AS WE HAVE BEEN SEEING FOR THE LAST FEW DAYS, THE MAIN HAZARD THROUGH THIS TIME LOOKS TO BE HEAVY RAIN, THOUGH THE EXACT LOCATION OF THE HIGHEST THREAT AND TIMING HAS CHANGED SLIGHTLY FROM YESTERDAYS MODEL RUNS. THE OVERALL PATTERN LOOKS SOMEWHAT SIMILAR TO A MADDOX SYNOPTIC EVENT PATTERN FOR HEAVY RAIN WITH A FEW EXCEPTIONS. THE FACTORS INCREASING THE HEAVY RAIN THREAT INCLUDE A VERY SLOW MOVING FRONT, PWAT VALUES WELL ABOVE NORMAL (EVEN FOR JUNE), SLOW MEAN WIND AND STORM MOTIONS. IN ADDITION, WE COULD SEE STORM INITIATION FOCUSED ALONG THE FRONT. THUS, SOME TRAINING STORMS FOR LOCATIONS CLOSEST TO THE FRONT. ON THE OTHER HAND, THE MID AND UPPER LEVEL FLOW IS MORE PERPENDICULAR TO THE FRONT, NOT PARALLEL AS IS THE CASE WITH THE MADDOX SYNOPTIC EVENTS. IN ADDITION, THE MEAN CLOUD LAYER RH (GENERALLY 80 TO 85 PERCENT) AND THE DEPTH OF THE WARM CLOUD LAYER (AROUND 11K FT) IS LESS THAN WHAT WOULD BE EXPECTED IN MAJOR HEAVY RAIN EVENTS. AS FAR AS WHERE AND WHEN THE HIGHEST HEAVY RAIN THREAT IS, IT LOOKS TO HAVE SHIFTED SW AND LATER FROM YESTERDAYS RUNS WHICH FAVORED THE POCONOS AND NW NJ DURING THE DAY MONDAY. SINCE THE FRONT COULD MOVE THROUGH THOSE LOCATIONS DURING THE DAY MONDAY, NOW EXPECT THE HEAVY RAIN THREAT TO BE ALONG AND SE OF THE DELAWARE VALLEY WHERE THE FRONT MAY NOT ARRIVE UNTIL LATE MONDAY NIGHT. IN ADDITION TO THE HEAVY RAIN THREAT, THERE IS SOME POTENTIAL FOR STRONG STORMS WITH MODEST INSTABILITY, AND CONSIDERABLE LOW LEVEL DIRECTIONAL SHEAR. HOWEVER, GIVEN ABUNDANT CLOUD COVER, AND WEAK ON SHORE FLOW IN THE BOUNDARY LAYER, IT IS UNLIKELY THAT THE MAXIMUM INSTABILITY WILL BE REALIZED UNLESS THERE ARE BREAKS IN THE CLOUDS (WHICH SEEMS QUITE UNLIKELY). THEREFORE, WILL HOLD OFF ON ANY MENTION OF GUSTY WINDS AT THIS TIME. TUESDAY...BY DAY BREAK TUESDAY THE FRONT SHOULD BE THROUGH MOST OF THE REGION WITH THE EXCEPTION OF DELMARVA AND FAR SOUTHERN NJ. FOR THESE AREAS, THE HEAVY RAIN THREAT COULD CONTINUE FOR MUCH OF THE DAY UNTIL THE FRONT FINALLY MOVES SOUTH OF OUR REGION BY LATE TUESDAY. FOR THE REST OF THE REGION THOUGH, CONTINUED ON SHORE FLOW MEANS THAT LIGHT RAIN COULD CONTINUE INTO TUESDAY NIGHT, BUT SHOULD GENERALLY BE TAPERING OFF THROUGH THIS PERIOD. WEDNESDAY THROUGH SUNDAY...STAYED CLOSE TO THE PREVIOUS FORECAST TO FOCUS ON THE MONDAY AND TUESDAY PERIOD. ALSO THERE ARE CONSIDERABLE DIFFERENCES AMONG THE MAJOR MODELS FOR THIS PERIOD. IN GENERAL THOUGH, EXPECT A HIGH TO BUILD IN OVER THE REGION FOR THE REMAINDER OF THE WEEK. THE COLD FRONT WHICH WAS DUE TO ARRIVE ON SATURDAY ACCORDING TO YESTERDAYS MODEL RUNS, MAY NOT EVEN GET THIS FAR SOUTH BEFORE FRONTOLYSIS TAKES OVER. && .AVIATION /17Z SUNDAY THROUGH THURSDAY/...
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&& .MARINE... SOUTHERLY WINDS WILL CONTINUE AHEAD OF THE APPROACHING COLD FRONT TODAY INTO TONIGHT. ONCE THE FRONT MOVES ACROSS THE NORTHERN WATERS OVERNIGHT, THE WINDS WILL BACK MORE TO THE SOUTHEAST. WINDS WILL GENERALLY BE 10-15 KNOTS, WITH GUSTS IN THE LOW 20S. IT IS POSSIBLE THAT AN OCCASIONAL GUST AROUND 25 KNOTS MAY OCCUR SIMILAR TO YESTERDAY, BUT WE HAVE HELD OFF ON ANY ADVISORIES. SEAS SHOULD REMAIN AROUND 3-4 FEET THROUGH THE DAY AS WELL. OUTLOOK... MONDAY AND TUESDAY...THOUGH A FEW GUSTS ABOVE 20 KT ARE POSSIBLE, ESPECIALLY MONDAY AND MONDAY NIGHT...WINDS AND SEAS SHOULD REMAIN BELOW SCA CRITERIA. HOWEVER, GUSTY WINDS WILL BE POSSIBLE IN THE VICINITY OF ANY THUNDERSTORMS THROUGH THIS PERIOD. WIND SHIFT EXPECTED BEHIND A COLD FRONT ON TUESDAY. WEDNESDAY AND THURSDAY...SEAS NEAR OR AT 5 FT ARE POSSIBLE ON THE ATLANTIC OCEAN WATERS. HOWEVER, THERE IS CONSIDERABLE UNCERTAINTY AT THIS TIME. && .RIP CURRENTS... THE RIP CURRENT RISK FOR THE JERSEY SHORE AND THE DELAWARE BEACHES IS MODERATE THROUGH THIS EVENING, BASED IN PART ON A CONTINUED SOUTHERLY FLOW AND LONGER WAVE PERIOD. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...FLASH FLOOD WATCH UNTIL 11 PM EDT THIS EVENING FOR PAZ054-055- 062. NJ...FLASH FLOOD WATCH UNTIL 11 PM EDT THIS EVENING FOR NJZ001-007- 008. DE...NONE. MD...NONE. MARINE...NONE. && $$ SYNOPSIS...GORSE/JOHNSON NEAR TERM...GORSE SHORT TERM...ROBERTSON LONG TERM...JOHNSON AVIATION...GORSE/JOHNSON/ROBERTSON MARINE...JOHNSON/ROBERTSON RIP CURRENTS...

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