Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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159 FXUS61 KPHI 190758 AFDPHI AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE MOUNT HOLLY NJ 358 AM EDT TUE MAY 19 2015 .SYNOPSIS...
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A COLD FRONT IN OHIO EARLY TODAY WILL PASS THROUGH OUR FORECAST AREA LATE THIS AFTERNOON. WEAK HIGH PRESSURE WILL FOLLOW WEDNESDAY. LOW PRESSURE ALONG THE NORTH CAROLINA COAST LATE THURSDAY SHOULD MOVE OUT TO SEA BY FRIDAY. ANOTHER COLD FRONT IS FORECAST TO ARRIVE FROM THE NORTHWEST FRIDAY AFTERNOON WITH STRONGER HIGH PRESSURE MOVING TO THE MID ATLANTIC COAST THIS WEEKEND. A WARM FRONT MAY APPROACH FROM THE SOUTHWEST FOR SUNDAY NIGHT AND MONDAY.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/... AN UPPER-LEVEL RIDGE WILL CONTINUE TO SHIFT TO OUR EAST EARLY THIS MORNING. THIS WILL TAKE SURFACE HIGH PRESSURE EAST AS WELL, WHICH WILL RESULT IN A LESSENING OF THE MARINE INFLUENCE WITH TIME. BEFORE THIS HAPPENS THOUGH, STRATUS ALONG WITH SOME FOG REMAINS ACROSS THE CWA. THIS IS EXPECTED TO LIFT AND THIN OUT AS THE MORNING AND EARLY AFTERNOON WEARS ON, PLUS AS OUR WINDS VEER FROM ONSHORE TO SOUTHWEST AND WEST BY LATE AFTERNOON. THERE APPEARS TO BE SOME WEAK ISENTROPIC UPGLIDE OVER THE THE MARINE LAYER AND THIS IS PRODUCING SOME SHOWERS. THE CONVECTIVE CLUSTER ACROSS THE DELMARVA CONTINUES TO GRADUALLY WEAKEN AS IT PROGRESSES INTO LESS LINGERING INSTABILITY AND BECOMES MORE ELEVATED WITH A NORTHEASTERN EXTENT. THIS ACTIVITY SHOULD WANE THROUGH THE MORNING, WITH SOME ADDITIONAL SHOWERS EXPECTED FROM THE WEST AHEAD OF A COLD FRONT. THE MAIN SURFACE LOW AND DYNAMICS ARE LIFTING WELL TO OUR NORTH, HOWEVER THE BACKDOOR FRONT SHOULD EITHER LIFT NORTHWARD AS A WEAK WARM FRONT OR DISSIPATE AS THE LOW-LEVEL FLOW VEERS. AS THE FLOW INCREASES SOME ALOFT THROUGH THE DAY WITH AN INCOMING UPPER-LEVEL TROUGH, SOME INCREASE IN SHEAR AND PERHAPS SOME HELP BY EITHER A DIFFERENTIAL HEATING BOUNDARY OR ESPECIALLY A PRE-FRONTAL TROUGH SHOULD BE ENOUGH TO PRODUCE AT LEAST ISOLATED OR SCATTERED CONVECTION THROUGH THE AFTERNOON /AIDED BY SOME GRADUAL COOLING ALOFT/. OUR AREA THOUGH LOOKS TO BE ON THE SOUTHERN EDGE OF THE STRONGER FLOW ALOFT, WHICH PLACES OUR NORTHERN AREAS IN A BIT BETTER SETUP FOR A GUSTY THUNDERSTORM. THE FORECAST SOUNDINGS SHOW ONCE THE STRATUS BREAKS, ENOUGH HEATING OCCURS TO BOOST THE INSTABILITY HOWEVER THE AIRMASS ESPECIALLY IN THE MID TO UPPER LEVELS ARE FORECAST TO BE DRYING QUITE A BIT THIS AFTERNOON. THE DRYING MAY LIMIT OVERALL ORGANIZATION, HOWEVER IF SOME STRONG ENOUGH CONVECTIVE CORES CAN DEVELOP THEN LOCALLY GUSTY WINDS /ENHANCED BY A POSSIBLE LOW-LEVEL INVERTED-V SIGNATURE DEVELOPING BY LATE AFTERNOON/ CAN OCCUR. AS FOR THE HIGH TEMPERATURES, WE USED MOSTLY AN EVEN BLEND OF THE GFS/NAM MOS. WE OPTED NOT TO GO ABOVE GUIDANCE OVERALL GIVEN THE EXPECTATION OF STRATUS TO BE AROUND FOR AWHILE THIS MORNING. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/... AN UPPER-LEVEL TROUGH IS FORECAST TO START SHARPENING UP ACROSS SOUTHEASTERN CANADA AND INTO THE NORTHEAST LATER TONIGHT. THIS WILL DRIVE A COLD FRONT OFFSHORE THIS EVENING AND ANY LINGERING CONVECTION WILL EITHER DISSIPATE OR MOVE OUT TO SEA. AS A RESULT, KEPT A MENTION IN FOR A TIME IN THE EVENING BUT QUICKLY TAPER THE POPS DOWN FROM WEST TO EAST. FOLLOWING THE COLD FRONTAL PASSAGE, CAA BEGINS TO OVERTAKE THE AREA THROUGH THE NIGHT. THE PRESSURE GRADIENT SHOULD TIGHTEN SOME THROUGH THE NIGHT AS LOW PRESSURE TRACKS NORTH OF MAINE AND HIGH PRESSURE BUILDS INTO THE OHIO VALLEY. THIS ALONG WITH SOME CAA SHOULD PROMOTE SOME MIXING THROUGH THE NIGHT AND THEREFORE AT LEAST SOME WIND AT THE SURFACE. THE AIRMASS IS DRYING QUITE A BIT THEREFORE OTHER THAN SOME STRATOCUMULUS ACROSS THE NORTHWEST ZONES, MOST AREAS SHOULD HAVE A CLEAR OR MOSTLY CLEAR SKY. AS FOR THE LOW TEMPERATURES, WE USED A NAM/GFS MOS BLEND WITH SOME TWEAKS. && .LONG TERM /WEDNESDAY THROUGH MONDAY/...
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500MB: CONFLUENT WESTERLY FLOW OVERHEAD TO START WEDNESDAY WITH THE NORTHERN BRANCH SLIPPING SOUTHWARD AND BECOMING DOMINANT WITH ITS SHORT WAVE PASSAGE FRIDAY AND COOLER THAN NORMAL HEIGHTS. A VERY WARM RIDGE DEVELOPS ALONG THE EAST COAST SUNDAY THROUGH EARLY NEXT WEEK. TEMPERATURES: THE FIRST 18 DAYS HAVE AVERAGED AROUND 7 DEGREES ABOVE NORMAL. CALENDAR DAY AVERAGES WILL BE BELOW NORMAL WEDNESDAY THROUGH SUNDAY WITH GREATEST NEGATIVE DEPARTURES ALMOST 10 DEGREES BELOW NORMAL ON SATURDAY. (AND POSSIBLY THURSDAY IF IT RAINS). CONSIDERABLE WARMING SHOULD BEGIN AROUND MEMORIAL DAY OR IMMEDIATELY THEREAFTER. LONG TERM HAZARDS: LOW PROBABILITY POSSIBLE COUNTRYSIDE SPOTTY TOUCH OF FROST N OF I-78 THURSDAY MORNING OR MAYBE A SLIGHTLY GREATER CHANCE SATURDAY MORNING? FORECAST BASIS: UNLESS OTHERWISE NOTED, A 50 50 BLEND 00Z/19 GFS/NAM MOS GUIDANCE WEDNESDAY - THURSDAY, 00Z/19 MEX MOS THURSDAY NIGHT AND FRIDAY, THEN THE 0521Z/19 WPC GRIDDED ELEMENTS OF MAX/MIN TEMP, 12 HOUR POP, 6 HOUR DEWPOINT-SKY-WIND FROM FRIDAY NIGHT THROUGH MONDAY. THE DAILIES... WEDNESDAY-WEDNESDAY NIGHT...WEAK HIGH PRESSURE IS EXPECTED TO FOLLOW THE COLD FRONT AND IT SHOULD BRING DRY WEATHER CONDITIONS AND BELOW NORMAL TEMPERATURES WITH PARTLY CLOUDY SKIES OVERALL (DIURNAL CU WED AFTERNOON AND THEN SOME HIGH CLOUDINESS IS POSSIBLE LATE WED NIGHT). WINDS WEDNESDAY NORTHWEST GUSTY 15-20 MPH. NEARLY CALM AT NIGHT. TEMPERATURES MAY FALL INTO THE UPPER 30S IN PARTS OF THE POCONOS REGION AND NORTHWESTERN NEW JERSEY EARLY ON THURSDAY MORNING IF THE HIGH CLOUD COVER DOES NOT ARRIVE PRIOR TO 09Z/21. IT COULD BE CLOSE FOR A TOUCH OF LOCALIZED FROST N OF I-78 IN TYPICALLY COLDEST SPOTS... WALPACK AND PEQUEST NJ AMONG OTHERS? CONFIDENCE: AVERAGE. THURSDAY...PARTLY CLOUDY OR CONSIDERABLE CLOUDINESS WITH THE APPROACHING SHORT WAVE AND MODEL DEPENDENT. GENERALLY WEST OR NORTHWEST WIND GUSTS 15 MPH OR LESS. TEMPERATURES MAY BE FORECAST TOO WARM. THIS DEPENDENT ON SKYCOVER AND THE NORTHERN EXTENT OF THE RAIN SHIELD. SINCE THERE IS UNCERTAINTY, THERE IS ALSO BELOW NORMAL CONFIDENCE IN THE TEMPS AND PROBABILITY OF MEASURABLE RAIN. AT THIS TIME, THINK THIS THURSDAY FORECAST IS ON THE WARMER SIDE OF THE ENVELOPE OF SOLUTIONS. FRIDAY...PARTLY TO MOSTLY SUNNY WITH DIURNAL CU/SC IN THE AFTERNOON. WEST TO NORTHWEST WIND GUSTY 20 MPH BECOME NEARLY CALM LATE AT NIGHT. EC HAS -2C AT 850 MB DOWN TO ABOUT I-80. COULD BE A COUNTRYSIDE RISK OF SPOTTY FROST N OF I-80. SATURDAY...LOOKS SUNNY! LIGHT WIND. SUNDAY...INCREASING CLOUDS WITH WARM AIR ADVECTION. SHOWERS POSSIBLE AT NIGHT? MONDAY...WARMER WITH A CHANCE OF SHOWERS? UNCERTAINTY FOR MEMORIAL DAY AND ITS STILL POSSIBLE IT WILL BE RAINFREE. IT DOES APPEAR THAT WAA SHOWERS SHOULD OCCUR SOMETIME SUNDAY OR MONDAY. PLENTY OF TIME FOR MORE OPTIMISM IN THE MODEL GUIDANCE.
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&& .AVIATION /08Z TUESDAY THROUGH SATURDAY/...
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THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY AND SURROUNDING AREAS. TODAY...IFR/LIFR CEILINGS IMPROVING TO MVFR TOWARD LATE MORNING, THEN VFR DURING THE AFTERNOON AS A COLD FRONT APPROACHES. MOSTLY MVFR FOG AT TIMES IS EXPECTED TO IMPROVE THROUGH THE MORNING. LIGHT EAST TO SOUTHEAST WINDS, TURNING TOWARDS THE SOUTH AND SOUTHWEST AROUND 10 KNOTS IN THE AFTERNOON. SOME SHOWERS AND THUNDERSTORMS /AFTERNOON/ ARE EXPECTED TO OCCUR AHEAD OF THE COLD FRONT, HOWEVER THESE MAY BE MAINLY SCATTERED. LOWER CONFIDENCE ON THE TIMING OF IMPROVEMENT TO MVFR/VFR, AND ALSO SHOWER/THUNDER COVERAGE ESPECIALLY THIS AFTERNOON. TONIGHT...A FEW SHOWERS OR A THUNDERSTORM EARLY, OTHERWISE VFR. WEST TO NORTHWEST WINDS MOSTLY 5-15 KNOTS. OUTLOOK... WEDNESDAY...VFR. SCT-BKN AOA 5000 FT. NORTHWEST GUSTY 15 TO 20 KT DURING THE DAY BECOME NEARLY CALM AT NIGHT. THURSDAY...VFR. SCT-BKN AOA 5000 FT. GENERALLY WEST WIND GUST UNDER 15 KT. SMALL CHC OF SHOWERS PHL SEWD, ESPECIALLY S DE AND COASTAL SE NJ. FRIDAY...VFR SCT-BKN AOA 5000 FT MAY BECOME COMPLETELY CLEAR NIGHT. WEST-NORTHWEST WIND GUSTY 20 KT SHOULD DIMINISH AT NIGHT. SATURDAY...VFR NEARLY CLEAR. LIGHT WIND.
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&& .MARINE...
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A LIGHT ONSHORE FLOW TO START TODAY WILL VEER TO SOUTH THEN TO SOUTHWEST AND WEST BY LATE AFTERNOON. A COLD FRONT WILL MOVE THROUGH THIS EVENING WITH CAA DEVELOPING IN ITS WAKE. THE CONDITIONS ARE EXPECTED TO BE BELOW SMALL CRAFT ADVISORY CRITERIA TODAY INTO TONIGHT, HOWEVER THE CAA LATE TONIGHT AND MAINLY TOWARD DAYBREAK WEDNESDAY MAY ALLOW FOR WIND GUSTS TO NEAR 25 KNOTS. THIS LOOKS SHORT IN DURATION THOUGH IF IT WERE TO OCCUR. SOME SHOWERS AND THUNDERSTORMS ARE EXPECTED TODAY INTO TONIGHT AHEAD OF THE COLD FRONT. OUTLOOK... WEDNESDAY...NO SCA HEADLINE ANTICIPATED AT THIS TIME. NW WIND GUSTS AROUND 20 KT DURING THE DAY DIMINISH AT NIGHT WITH STRONGEST DAYTIME WIND PROBABLY ALONG THE NEARSHORE WATERS. THURSDAY....SCA NOT ANTICIPATED AT THIS TIME, THOUGHT KEEPING AN EYE ON THE NORTH CAROLINA SFC LOW. FRIDAY....SMALL CHANCE OF SCA FOR NEARSHORE WEST TO NORTHWEST WIND GUSTS NEAR 25 KT DURING THE DAY DIMINISHING AT NIGHT. SATURDAY...NO MARINE HEADLINE ANTICIPATED.
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&& .FIRE WEATHER...
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WIND GUSTS SHOULD BE AROUND 20 MPH FROM THE NORTHWEST ON WEDNESDAY AFTERNOON. RELATIVE HUMIDITY VALUES MAY FALL TO NEAR 30 PERCENT ESPECIALLY IN PARTS OF EASTERN PENNSYLVANIA AND NEW JERSEY. FRIDAY AFTERNOON MAY BE SIMILAR TO WEDNESDAY AFTERNOON.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...NONE. NJ...NONE. DE...NONE. MD...NONE. MARINE...NONE. && $$ SYNOPSIS...DRAG 358 NEAR TERM...GORSE SHORT TERM...GORSE LONG TERM...DRAG 358 AVIATION...DRAG/GORSE 358 MARINE...DRAG/GORSE 358 FIRE WEATHER...358

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