Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 242342 AFDPHI AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE MOUNT HOLLY NJ 642 PM EST WED DEC 24 2014 .SYNOPSIS... LOW PRESSURE STRENGTHENS AS IT LIFTS UP ACROSS THE GREAT LAKES REGION INTO CANADA THROUGH TONIGHT. AN ASSOCIATED COLD FRONT WILL SWEEP ACROSS OUR AREA EARLY THURSDAY MORNING. HIGH PRESSURE WILL THEN SETTLE TO OUR SOUTH THURSDAY NIGHT AND FRIDAY, BEFORE SHIFTING OFFSHORE INTO SATURDAY. A COLD FRONT IS FORECAST TO SETTLE THROUGH OUR AREA SUNDAY, HOWEVER IT MAY THEN STALL NEARBY. LOW PRESSURE SHOULD DEVELOP AND MOVE ALONG THIS FRONT SUNDAY NIGHT AND MONDAY, TRACKING TO OUR SOUTH. HIGH PRESSURE NEAR THE CENTRAL PLAINS THEN STARTS TO GRADUALLY BUILD TOWARD THE MID ATLANTIC TUESDAY AND WEDNESDAY. && .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/... WARM FRONT CONTINUES TO MAKE SLOW PROGRESS NORTH THROUGH THE REGION. THE BIG STORY WILL BE THE COLD FRONT WHICH IS EXPECTED TO SWEEP THROUGH THE REGION DURING THE VERY EARLY MORNING HOURS. SHOULD CONTINUE TO SEE LIGHT TO MODERATE RAIN THROUGH THIS EVENING, THEN AS THE FRONT APPROACHES, THE COMBINATION OF MODEST ELEVATED INSTABILITY (ELEVATED CAPE VALUES AROUND 100 J/KG) AND VERY LARGE SHEAR VALUES LEADS TO AN INCREASED CHANCE FOR ISOLATED THUNDERSTORMS. GIVEN THE LARGE SHEAR VALUES, COULD SEE GUSTY WINDS WITH ANY THUNDERSTORMS. APPEARS TO BE A LULL IN THE PRECIP FOR A T LEAST THE NEXT LITTLE WHILE AND HAVE ADJUSTED POPS DOWN ACCORDINGLY. HRRR SAYS PRECIP SHOULD RAMP BACK UP IN A FEW HOURS. WE`LL SEE. ALSO HAVE ADJUSTED TEMPS, BUT WITH THE WARM FRONT BISECTING THE AREA AND LIGHT RAIN VERSUS NO RAIN IN SOME SPOTS, TEMPS ARE DIFFICULT EARLY THIS EVE. ALSO WATCHING FOG AS AREAS WHERE RAIN HAS STOPPED HAS SEEN VSBYS GO DOWN, BUT FOR NOW THEY DON`T APPEAR TO BE LASTING LONG. BEHIND THE COLD FRONT, THE RAIN SHOULD CLEAR OUT QUICKLY. && .SHORT TERM /6 AM THURSDAY MORNING THROUGH 6 PM THURSDAY/... FOR CHRISTMAS DAY, WE KEEP A RATHER TIGHT PRESSURE GRADIENT OVER THE REGION LEADING TO BREEZY CONDITIONS THROUGH THE DAY TIME HOURS. HOWEVER, WITH THE CONTINUED DRY AIR ADVECTION, SHOULD SEE MOSTLY DRY CONDITIONS EXCEPT NEAR THE COAST WHERE RAIN MAY LINGER THROUGH MID MORNING. DEPENDING ON HOW QUICKLY THE INVERSION BREAKS, COULD EVEN SEE CLOUDS RAPIDLY DISSIPATING THROUGH THE MORNING. THE TEMPERATURE GRADIENT ACROSS THE COLD FRONT IS SOMEWHAT MODEST. THEREFORE, EXPECT HIGHS TOMORROW TO BE 5 TO 10 DEGREES ABOVE NORMAL (RANGING FROM THE LOWER 40S TO LOWER 50S), A FAR CRY FROM THE UPPER 60S THAT DELMARVA HAS THIS AFTERNOON, BUT STILL QUITE A WARM CHRISTMAS DAY. && .LONG TERM /THURSDAY NIGHT THROUGH WEDNESDAY/... THE SYNOPTIC SETUP IS COMPRISED OF AN UPPER-LEVEL TROUGH LIFTING OUT OF NEW ENGLAND THURSDAY NIGHT AS ENERGY RELOADS ACROSS THE WEST AND CENTRAL PLAINS. THIS ENERGY WILL AMPLIFY A TROUGH WELL TO OUR WEST FRIDAY THROUGH SUNDAY. THE INITIAL ENERGY EJECTS NORTHEASTWARD BETWEEN A RIDGE OFF THE SOUTHEASTERN U.S. COAST AND BUSIER FLOW ACROSS CANADA. THIS SETUP HAS THE POTENTIAL TO BE AN OVERRUNNING EVENT LATE SUNDAY AND MONDAY FOLLOWING A SECONDARY COLD FRONTAL PASSAGE. THESE TYPES OF SETUPS ESPECIALLY FARTHER OUT IN TIME CARRY MORE UNCERTAINTY AS THE TIMING AND TRACK OF THE ENERGY AND ASSOCIATED SURFACE LOW EJECTING NORTHEASTWARD ARE KEY. AS ENERGY CONTINUES TO DIVE INTO THE WEST AND PLAINS, THE TROUGH CONTINUES TO EXPAND EASTWARD AND THIS APPEARS TO BE THE START OF A TURN TO COLDER WEATHER IN THE EAST. WE USED A MODEL BLEND APPROACH FOR THURSDAY NIGHT THROUGH SATURDAY, THEN WENT WITH THE 12Z WPC GUIDANCE THEREAFTER. SOME ADJUSTMENTS WERE THEN MADE FOLLOWING ADDITIONAL COLLABORATION WITH OUR NEIGHBORING OFFICES. FOR THURSDAY NIGHT...HIGH PRESSURE IS FORECAST TO BUILD INTO OUR AREA FROM THE SOUTHWEST. THIS WILL ALLOW THE PRESSURE GRADIENT TO RELAX AND AS THE CAA WANES WITH EVEN SOME MODERATION AT 850 MB, THE MIXING WILL WEAKEN AND THUS THE WINDS ARE EXPECTED TO DIMINISH FAIRLY QUICKLY DURING THE EVENING. THIS WILL SET THE STAGE FOR A COLDER NIGHT. FOR FRIDAY AND SATURDAY...THE CENTER OF HIGH PRESSURE SLIDES TO OUR SOUTH FRIDAY AND THEN OFFSHORE INTO SATURDAY. AS THIS OCCURS, SOME RIDGING ALOFT IS FORECAST TO BUILD ACROSS THE EAST DOWNSTREAM OF AN AMPLIFYING TROUGH ACROSS THE WEST AND INTO THE PLAINS. THIS IS DIVIDED BY A COLD FRONT THAT IS FORECAST TO BE APPROACHING OUR AREA SATURDAY NIGHT. THIS FRONT MAY NOT HAVE MUCH IN THE WAY OF MOISTURE WITH IT AS THE MAIN SURFACE LOW TRACKS FAR TO OUR NORTH. OVERALL, A DRY AND MILD TIME FRAME WITH PERHAPS SOME INCREASE IN CLOUDS LATE SATURDAY. FOR SUNDAY AND MONDAY...THE POTENTIAL REMAINS FOR AN OVERRUNNING SETUP. THIS IS DUE TO THE MAIN UPPER-LEVEL TROUGH SITTING BACK WELL TO OUR WEST WITH THE EAST UNDER MORE OF A SOUTHWESTERLY FLOW ALOFT BETWEEN THE SOUTHEASTERN RIDGE AND THE MORE BUSY FLOW ACROSS CANADA. THE FORECAST CHALLENGE IS A SURFACE LOW THAT DEVELOPS OUT OF THE MAIN TROUGH THEN IS EJECTED NORTHEASTWARD ALONG A STALLED FRONTAL BOUNDARY. WHILE THE SIGNAL IS THERE FOR AN OVERRUNNING EVENT, THE TRACK AND ASSOCIATED PRECIPITATION SHIELD IS LESS CERTAIN. OF COURSE THE TRACK WILL ALSO DETERMINE THE THERMAL PROFILES AS WELL, PLUS A BUILDING SURFACE HIGH TO THE NORTH COULD COMPLICATE THINGS SOME. WE FAVORED MORE OF THE WPC GUIDANCE DUE TO UNCERTAINTY IN THE TRACK/DETAILS. THIS RESULTS IN AN AREA OF LOW LIKELY POPS ACROSS THE SOUTHERN AREAS LATER SUNDAY WHERE THERE IS ENOUGH CONSENSUS, AND ALSO TO PRESERVE SOME CONTINUITY. THE POPS THEN DECREASE SOME NORTHWARD. THERE IS THE POTENTIAL FOR SOME SNOW ON THE NORTHERN PORTION OF THE PRECIPITATION SHIELD, HOWEVER ANY AMOUNTS AND ASSOCIATED THERMAL PROFILES WILL BE DETERMINED BY THE TRACK OF THE SURFACE FEATURE GIVEN MOSTLY AN OVERRUNNING SCENARIO. THERE IS A CHC THIS FEATURE TRACKS EVEN FARTHER SOUTH IF HIGH PRESSURE TO THE NORTH IS STRONGER WITH ITS SOUTHWARD PUSH. FOR TUESDAY AND WEDNESDAY...IT APPEARS ADDITIONAL ENERGY WILL BE EJECTING NORTHEASTWARD AND WHILE THE EARLIER SURFACE LOW SHOULD BE GONE TO START, A FRONTAL BOUNDARY MAY LINGER NEARBY TUESDAY. AS THE UPPER-LEVEL TROUGH MIGRATES EAST SOME, THERE COULD BE SOME ENERGY STILL SLIDING NORTHEASTWARD ALONG THE FRONT. AS OF NOW, WE FAVORED MORE OF THE WPC GUIDANCE WHICH INDICATES A COLD FRONT SHIFTING EAST TUESDAY AS STRONG HIGH PRESSURE BUILDS TOWARD THE CENTRAL PLAINS AND ALSO EXPANDS TOWARD THE MID ATLANTIC INTO WEDNESDAY. THIS RESULTS IN COLDER AIR SEEPING SOUTH AND EASTWARD. && .AVIATION /00Z THURSDAY THROUGH MONDAY/... THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY AND SURROUNDING AREAS. IFR CONDITIONS WILL PERSIST IN OUR REGION INTO TONIGHT. CONDS HAVE BEEN VARIABLE. THERE HAS BEEN A LULL IN THE PRECIP IN THE NEAR TERM AND THIS SHOULD CONTINUE FOR THE NEXT LITTLE WHILE. UNFORTUNATELY, IN SOME OF THESE AREAS, VSBYS HAVE DROPPED QUITE A BIT, BUT FOR NOW THESE DROPS DON`T SEEM TO LAST ALL THAT LONG. THEREFORE, WILL KEEP PREVAILING IFR CONDITIONS. THIS EVENING AND LATE TONIGHT COULD ALSO SEE ISOLATED TSRA. WITH ANY TSRA COULD SEE VRB AND GUSTY WINDS. HOWEVER, COVERAGE IS TOO LIMITED TO INCLUDE IN THE TAFS AT THIS TIME. A LOW LEVEL SOUTHERLY JET AS DEPICTED BY THE VAD WIND PROFILE FROM KDIX CONTINUES OVER THE COASTAL PLAIN AND SHOULD PERSIST THROUGH AT LEAST 06Z. AS A RESULT, WE HAVE CONTINUED THE MENTION OF LOW LEVEL WIND SHEAR AT KTTN, KPNE, KPHL, KILG, KMIV AND KACY. BOTH KRDG AND KABE SHOULD BE ENOUGH WEST OF THE LOW LEVEL JET. AT THE SURFACE, GENERALLY LIGHT SOUTHERLY WINDS WILL CONTINUE UNTIL THE COLD FRONT ARRIVES. THE COLD FRONT AND ASSOCIATED ABRUPT SHIFT TO WESTERLY WINDS IS EXPECTED TO ARRIVE BETWEEN 06 AND 09Z. BEHIND THE FRONT, EXPECT CONDITIONS TO IMPROVE TO MVFR, THEN VFR. OUTLOOK... THURSDAY NIGHT...VFR. WEST-NORTHWEST WINDS 10-15 KNOTS EARLY, DIMINISHING TO LESS THAN 10 KNOTS. FRIDAY AND SATURDAY...VFR, ALTHOUGH CLOUDS MAY INCREASE LATE SATURDAY AS A COLD FRONT APPROACHES. SOUTHWEST WINDS MAINLY AROUND 10 KNOTS. SUNDAY AND MONDAY...CLOUDS POTENTIALLY LOWER TO SUB-VFR AS A SYSTEM TRACKS TO OUR SOUTH. THIS COULD RESULT IN SOME RAIN OR SNOW, HOWEVER CONFIDENCE IS ON THE LOW SIDE. WINDS SHOULD TURN NORTHERLY. && .MARINE... SOUTHERLY WINDS WILL BE INCREASING SHORTLY, AND SHOULD SEE GUSTS ABOVE 25KT ON ALL THE WATERS THROUGH THE EVENING HOURS. WITH THE INCREASING WINDS, SEAS SHOULD ALSO BUILD BACK ABOVE 5 FT. AFTER MIDNIGHT TONIGHT, LIKELY BETWEEN 2 AND 6 AM LST, A COLD FRONT WILL PUSH THROUGH, BRINGING AN ABRUPT SHIFT TO WESTERLY WINDS. IN ADDITION, COULD SEE ISOLATED THUNDERSTORMS WITH THE FRONT. UNDER ANY THUNDERSTORMS, EXPECT VERY GUSTY WINDS. THE STRONG WESTERLY WINDS WILL CONTINUE ON ALL THE WATERS THROUGH THE DAY TOMORROW. ALONG THE OCEAN WATERS, ESPECIALLY THE NORTHERN NEW JERSEY COASTAL WATERS, COULD SEE A FEW GUSTS ABOVE 30KT. HOWEVER, VERY UNCERTAIN IF WE WILL REACH GALE FORCE GUSTS. THEREFORE, HAVE CANCELED THE GALE WATCH, BUT AN SCA IS IN EFFECT FOR ALL THE WATERS THROUGH EARLY TOMORROW EVENING. HOWEVER, WILL MONITOR WHEN THE NEW GUIDANCE COMES IN THIS EVENING TO SEE IF THE THREAT FOR GALES FOR TOMORROW CHANGES. OUTLOOK... THURSDAY NIGHT...AS THE PRESSURE GRADIENT RELAXES, THE WINDS WILL AS WELL. IT APPEARS THAT THE WINDS AND SEAS DECREASE FAIRLY QUICKLY, THEREFORE A SMALL CRAFT ADVISORY WAS NOT EXTENDED AS OF NOW. FRIDAY AND SATURDAY...THE CONDITIONS SHOULD BE BELOW SMALL CRAFT ADVISORY CRITERIA AS HIGH PRESSURE BUILDS TO OUR SOUTH AND THEN OFFSHORE. A WEAK COLD FRONT APPROACHES SATURDAY NIGHT. SUNDAY AND MONDAY...A COLD FRONT SETTLES SOUTHEASTWARD ACROSS THE AREA SUNDAY BUT THEN MAY STALL NEARBY. THE SOUTHWESTERLY FLOW MAY INCREASE SOME AHEAD OF IT, THEN TURN NORTHWESTERLY IN ITS WAKE. OVERALL, IT APPEARS THAT THE CONDITIONS REMAIN BELOW ADVISORY CRITERIA. A WEAK LOW PRESSURE SYSTEM SHOULD DEVELOP ON THE FRONT AND TRACK TO OUR SOUTH DURING THIS TIME FRAME. && .TIDES/COASTAL FLOODING... STILL A CONCERN FOR COASTAL FLOODING ALONG THE CHESAPEAKE BAY. THOUGH GUIDANCE CONTINUES TO BE A BIT TOO HIGH, EVEN SUBTRACTING THE BIAS, BOTH TIDAL GAUGES ALONG THE UPPER EASTERN SHORE WILL GET CLOSE IF NOT EXCEED MINOR FLOODING THRESHOLDS FOR THE HIGH TIDE THIS EVENING. TO A LESSER EXTENT THERE IS A SMALL THREAT AGAIN TOMORROW MORNING ALONG THE CHESAPEAKE. IT LOOKS LIKE WE SHOULD HAVE THE SHIFT TO WESTERLY WINDS WELL BEFORE THE HIGH TIDE CYCLE, AND THE ASTRONOMICAL TIDE IS LOWER, SO THE THREAT IS TOO LOW TO MENTION IN THE HWO FOR NOW, BUT WILL CONTINUE TO MONITOR. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...NONE. NJ...NONE. DE...NONE. MD...COASTAL FLOOD ADVISORY UNTIL 10 PM EST THIS EVENING FOR MDZ008- 012-015-019-020. MARINE...SMALL CRAFT ADVISORY UNTIL 7 PM EST THURSDAY FOR ANZ430-431- 450>455. && $$ SYNOPSIS...GORSE NEAR TERM...JOHNSON/NIERENBERG SHORT TERM...JOHNSON LONG TERM...GORSE AVIATION...GORSE/JOHNSON/NIERENBERG MARINE...GORSE/JOHNSON TIDES/COASTAL FLOODING...

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