Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 232019 AFDPHI AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE MOUNT HOLLY NJ 319 PM EST TUE DEC 23 2014 .SYNOPSIS... LOW PRESSURE, DEVELOPING ALONG A TRAILING COLD FRONT ACROSS THE GULF COAST TONIGHT, WILL TRACK UP THE OHIO VALLEY AND INTO THE GREAT LAKES WEDNESDAY AND WEDNESDAY NIGHT WHILE STRENGTHENING. AS THIS OCCURS, A COLD FRONT WILL SWEEP THROUGH OUR AREA WEDNESDAY NIGHT INTO THURSDAY MORNING. HIGH PRESSURE WILL THEN SETTLE TO OUR SOUTH THURSDAY NIGHT AND FRIDAY, BEFORE SHIFTING OFFSHORE INTO SATURDAY. LOW PRESSURE IS FORECAST TO TRACK ACROSS THE GREAT LAKES INTO CANADA SATURDAY INTO SUNDAY, WITH ITS COLD FRONT CROSSING OUR AREA BY SUNDAY MORNING. AS HIGH PRESSURE BUILDS ACROSS THE GREAT LAKES, LOW PRESSURE MAY DEVELOP ALONG THE FRONT TO OUR SOUTH SUNDAY AND MONDAY THEN HIGH PRESSURE ARRIVES FOR TUESDAY. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... INITIAL LOW CONTINUES TO LIFT NE AND FILL. AS IT DOES SO, SHOULD SEE WINDS, ALTHOUGH STILL LIGHT, TURN MORE SOUTHERLY. WITH PERSISTENT LOW CLOUD COVER, WILL LIKELY NOT SEE MUCH COOLING OVERNIGHT. LOWS WILL ONLY BE A FEW DEGREES LOWER THAN AFTERNOON HIGHS. BY EARLY TOMORROW MORNING, SHOULD SEE THE WARM FRONT BEGIN TO APPROACH FAR SOUTHERN PORTIONS OF THE REGION. AS IT GETS CLOSER, EXPECT ISENTROPIC LIFT TO DEVELOP OVER THE REGION LEADING TO AN INCREASE IN PRECIPITATION ACROSS THE REGION. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/... WARM FRONT MOVES THROUGH THE REST OF THE REGION. THEREFORE, CONTINUED WET WEATHER THROUGH THE DAY. INCREASING PRESSURE GRADIENT WILL LEAD TO BREEZY CONDITIONS ESPECIALLY NEAR THE COAST. HOWEVER, ABUNDANT CLOUD COVER SHOULD INHIBIT MIXING SOMEWHAT AND THUS LIMIT WIND SPEEDS. WILL HAVE AMPLE LOW AND MID LEVEL MOISTURE, COMBINED WITH SYNOPTIC SCALE AND CONTINUED ISENTROPIC LIFT. THEREFORE EXPECT WIDESPREAD RAIN, POSSIBLY HEAVY RAIN AT TIMES. WILL KEEP IN MENTION OF THUNDER AS MODEL SOUNDINGS SHOW MEAGER ELEVATED INSTABILITY (FOR PARCELS ABOVE THE INVERSION), BUT OVERALL THINK THE THUNDERSTORM POTENTIAL IS LOW. && .LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/... THE SYNOPTIC SETUP IS COMPRISED OF A SHARP UPPER-LEVEL TROUGH LIFTING ACROSS THE EAST WEDNESDAY NIGHT INTO THURSDAY. THIS WILL SWEEP A STRONG COLD FRONT ACROSS OUR AREA, THEN THIS UPPER-LEVEL FEATURE LOOKS TO BE PROGRESSIVE AS THE FLOW RELOADS ACROSS THE WEST AND CENTRAL PLAINS. THIS ENERGY WILL GRADUALLY CARVE OUT A TROUGH TO OUR WEST, HOWEVER THIS SETUP HAS THE POTENTIAL TO BE AN OVERRUNNING EVENT LATE IN THE WEEKEND AND EARLY NEXT 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 IS KEY. WE SHOULD BE TURNING COLDER DURING THE SECOND HALF OF THE WEEKEND AND EARLY NEXT WEEK. WE USED A MODEL BLEND APPROACH FOR WEDNESDAY NIGHT THROUGH FRIDAY, THEN WENT WITH THE 12Z WPC GUIDANCE THEREAFTER. SOME ADJUSTMENTS WERE THEN MADE FOLLOWING ADDITIONAL COLLABORATION WITH OUR SURROUNDING OFFICES. FOR WEDNESDAY NIGHT AND THURSDAY...A DEEPENING SURFACE LOW IS FORECAST TO TRACK UP ACROSS THE GREAT LAKES AND INTO CANADA. THIS WILL SWEEP A STRONG COLD FRONT ACROSS OUR AREA BY DAYBREAK THURSDAY. A ROBUST WIND FIELD WILL MOVE ACROSS THE AREA AHEAD OF THE COLD FRONT, AND THE SOUTHERLY FLOW WILL CONTINUE TO ADVECT DEEP MOISTURE ACROSS THE AREA. THIS IS EVIDENT WITH THE IMPRESSIVE 850 MB THETA-E PLUME ACROSS THE AREA WEDNESDAY NIGHT, WHICH TRANSLATES TO PW VALUES NEAR 1.50 INCHES. THE COMBINATION OF WAA, JET INDUCED LIFT AND FRONTAL FORCING WILL RESULT IN PERIODS OF RAIN SOME OF WHICH COULD BE HEAVY AT TIMES. SOME OF THIS GUIDANCE INDICATES THAT WEAK INSTABILITY ADVECTS NORTHWARD AHEAD OF THE FRONT, HOWEVER IT IS NOT QUITE CLEAR IF WE WILL BE ABLE TO ATTAIN CHARGE SEPARATION FOR LIGHTNING. THE STRONG FORCING SHOULD COMPENSATE FOR THAT TO SOME EXTENT, AND THIS MAY RESULT IN A NARROW LOW-TOPPED CONVECTIVE LINE OR BROKEN LINE WHICH WOULD BE EMBEDDED IN ABOUT 50 KNOTS OF WIND AROUND 925 MB. WHILE SMALL, THERE IS THE POTENTIAL FOR LOCALIZED ENHANCED STRONG WIND GUSTS DUE TO ANY CONVECTION. THEREFORE, A BREEZY SOUTHERLY FLOW WITH LOCALLY STRONGER GUSTS POSSIBLE IN DOWNPOURS AND ANY CONVECTIVE ELEMENTS. WE WILL CONTINUE THE ISOLATED THUNDER MENTION FOR A TIME WEDNESDAY NIGHT. THE POPS ARE CATEGORICAL, HOWEVER WE RAPIDLY LOWER THEM TOWARD DAYBREAK THURSDAY. AS OF NOW, THE FLOODING THREAT LOOKS LOW/LOCALIZED AND PROBABLY TO THE TYPICAL POOR DRAINAGE SPOTS DURING BOUTS OF DOWNPOURS. AS A RESULT, NO FLOOD WATCH WILL BE ISSUED. THE COLD FRONT SHOULD BE OFFSHORE TO START THURSDAY WITH ANY SHOWERS QUICKLY ENDING FROM WEST TO EAST EARLY. THERE SHOULD BE A SHARP WIND SHIFT WITH THE FRONT FROM SOUTHERLY TO WEST-NORTHWEST. AS THE DEEP SURFACE LOW TRACKS INTO CANADA THURSDAY AND HIGH PRESSURE BUILDS ACROSS THE SOUTHEAST STATES, THE PRESSURE GRADIENT IS TIGHT ACROSS OUR AREA. THIS COMBINED WITH CAA AND A WELL MIXED BOUNDARY LAYER, THURSDAY LOOKS TO BE BREEZY TO WINDY. THE WINDS IN THE MIXING LAYER ARE GENERALLY TO 35 KNOTS, THEREFORE GUSTS ARE ANTICIPATED TO BE BELOW ADVISORY CRITERIA. IN ADDITION, THE SHOT OF CAA LOOKS TO BE SHORT-LIVED. THE WINDS THEN DIMINISH AT NIGHT AS HIGH PRESSURE BEGINS TO BUILD. FOR FRIDAY AND SATURDAY...HIGH PRESSURE SLIDES TO OUR SOUTH FRIDAY AND THEN OFFSHORE INTO SATURDAY. AS THIS OCCURS, 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 SETTLING ACROSS OUR AREA SATURDAY NIGHT. THIS FRONT MAY NOT HAVE MUCH IN THE WAY OF MOISTURE WITH IT AS THE MAIN SURFACE LOW PEELS AWAY FAR TO OUR NORTH. THIS BOUNDARY THOUGH WILL PROBABLY TEND TO START SLOWING ITS SOUTHWARD MOTION AS IT BECOMES MORE PARALLEL TO THE FLOW. OVERALL, A DRY AND MILD FRIDAY INTO SATURDAY THEN SOME LOW POPS FOR LATER SATURDAY AS THE FRONT ARRIVES. FOR SUNDAY AND MONDAY...A POTENTIAL 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 CHALLENGING FLOW ACROSS CANADA. THE MAIN FORECAST CHALLENGE IS A SURFACE LOW THAT DEVELOPS OUT OF THE MAIN TROUGH THEN IS EJECTED NORTHEASTWARD ALONG A STALLED FRONTAL BOUNDARY. WHILE THERE APPEARS TO BE ENOUGH SUPPORT FOR THIS POTENTIAL INCLUDING SOME OF THE ENSEMBLE MEMBERS, THE TRACK AND ASSOCIATED PRECIPITATION SHIELD IS LESS CERTAIN. OF COURSE THE TRACK WILL ALSO DETERMINE THE THERMAL PROFILES AS WELL. WE FAVORED MORE OF WPC GUIDANCE HERE DUE TO UNCERTAINTY IN THE TRACK/DETAILS. THIS RESULTS IN RAISING POPS BUT GIVEN LOW ENOUGH CONFIDENCE WE DID NOT GO HIGHER THAN CHC ATTM. THERE COULD BE ENOUGH COLD AIR SEEPING SOUTHWARD FROM BUILDING HIGH PRESSURE TO RESULT IN SOME SNOW ALONG THE NORTHERN PORTION OF THE PRECIPITATION SHIELD. THIS FEATURE THEN LOOKS TO GRADUALLY MOVE FARTHER AWAY LATER MONDAY. FOR TUESDAY...IT APPEARS ADDITIONAL ENERGY WILL BE EJECTING NORTHEASTWARD AND WHILE THE EARLIER SURFACE LOW SHOULD BE GONE, ANOTHER FRONTAL BOUNDARY LOOKS TO APPROACH. THE MAIN TROUGH LOOKS TO BE STILL TO OUR WEST AND THE ACTUAL TIMING OF THIS FRONT WILL PROBABLY CHANGE. WE GENERALLY WENT WITH A DRY BUT COLDER FORECAST. && .AVIATION /20Z TUESDAY THROUGH SUNDAY/... THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY AND SURROUNDING AREAS. A TOUGH 24 HOURS FOR AVIATION INTERESTS. SHOULD CONTINUE TO SEE WIDESPREAD IFR, AND EVEN AREAS OF LIFR, THROUGH OUT THE TAF PERIOD. EXPECT THE LOWEST CONDITIONS TO BE BETWEEN 03 AND 14Z. AT THIS TIME HAVE VISIBILITIES AS LOW AS 1SM AND CEILINGS AS LOW AS 200 FT AGL IN THE TAFS. HOWEVER, WOULD NOT BE SURPRISED TO SEE LOCALIZED 1/2SM VV001, BUT CONFIDENCE ON WHERE OR WHEN THIS WILL HAPPEN IS LOW. WINDS WILL TURN MORE SOUTHERLY THROUGH THE NIGHT. BY MID MORNING TOMORROW, SHOULD SEE A LOW LEVEL JET DEVELOP, AND WITH IT, LOW LEVEL WIND SHEAR. TH PRIMARY THREAT FOR THIS LOOKS TO BE FROM THE DELAWARE VALLEY AND POINTS EAST, BUT WILL CONTINUE TO WATCH TO SEE IF IT NEEDS TO BE ADDED TO KRDG AND KABE. OUTLOOK... WEDNESDAY NIGHT...IFR/LIFR CONDITIONS WITH PERIODS OF RAIN, SOME POSSIBLY HEAVY AT TIMES. THE RAIN IS EXPECTED TO TAPER TO SHOWERS LATE AS A COLD FRONT WORKS ITS AWAY THROUGH, AND THEREFORE SOME IMPROVEMENT SHOULD START TO OCCUR. SOUTHERLY SURFACE WINDS 10-15 KNOTS HOWEVER TURNING SOUTHWEST NEAR 50 KNOTS AT 2000 FEET, THEN BECOMING NORTHWEST LATE. THURSDAY...THE CEILINGS QUICKLY IMPROVE TO VFR EARLY, THEN THIN OUT. WEST-NORTHWEST WINDS INCREASING TO AROUND 15 KNOTS WITH GUSTS TO AROUND 30 KNOTS, THEN DIMINISHING TO NEAR 10 KNOTS AT NIGHT. FRIDAY AND SATURDAY...MAINLY VFR, ALTHOUGH CLOUDS INCREASE LATER SATURDAY. SOME SHOWERS ARE POSSIBLE SATURDAY NIGHT WITH A COLD FRONT. WINDS TURNING SOUTHWEST SATURDAY AND INCREASING TO 10-20 KNOTS, THEN SHIFTING TO THE NORTHWEST AT NIGHT. SUNDAY...CLOUDS POTENTIALLY LOWER TO SUB-VFR AS A SYSTEM TRACKS TO OUR SOUTH. THIS COULD RESULT IN A PERIOD OF RAIN OR SNOW, HOWEVER CONFIDENCE IS ON THE LOWER SIDE. WEST-NORTHWEST WINDS PROBABLY TURNING NORTH. && .MARINE... SCA FOR HAZARDOUS SEAS CONTINUES. SEAS SHOULD HOWEVER BEGIN TO DIMINISH OVER THE NEXT SEVERAL HOURS. ONCE SEAS DROP BELOW 5 FT, SHOULD SEE A LULL OF SEVERAL HOURS BEFORE WINDS BEGIN TO APPROACH SCA CRITERIA WEDNESDAY AFTERNOON. OUTLOOK... WEDNESDAY NIGHT...SMALL CRAFT ADVISORY CONDITIONS ARE EXPECTED WITH A STRONG SOUTHERLY FLOW. THIS WILL BUILD THE SEAS, HOWEVER A WARM AIRMASS MAY LIMIT THE MIXING AND HOLD GUSTS MOSTLY BELOW GALE FORCE. THURSDAY...SMALL CRAFT ADVISORY CONDITIONS CONTINUE BEHIND A COLD FRONTAL PASSAGE RESULTING IN A SHARP WIND SHIFT FROM SOUTH TO WESTERLY. THERE IS THE POTENTIAL FOR SOME LOW-END GALE FORCE GUSTS HOWEVER THE FORECAST SOUNDINGS ARE NOT ALL THAT CONDUCIVE WITH THE LOW-LEVEL MIXING. GIVEN THE UNCERTAINTY, WE WILL HOLD OFF ON A GALE WATCH. THE WINDS AND SEAS ARE EXPECTED TO DECREASE AT NIGHT AS HIGH PRESSURE BEGINS TO BUILD IN. 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 MOVES THROUGH LATER SATURDAY NIGHT. SUNDAY...A NORTHWEST TO NORTH FLOW BEHIND A COLD FRONT MAY INCREASE SOME, HOWEVER THE CONDITIONS APPEAR TO REMAIN BELOW ADVISORY CRITERIA. A LOW PRESSURE SYSTEM MAY TRACK TO OUR SOUTH AT NIGHT. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...NONE. NJ...NONE. DE...NONE. MD...NONE. MARINE...SMALL CRAFT ADVISORY FOR HAZARDOUS SEAS UNTIL 2 AM EST WEDNESDAY FOR ANZ450>453. SMALL CRAFT ADVISORY FOR HAZARDOUS SEAS UNTIL 6 PM EST THIS EVENING FOR ANZ454-455. && $$ SYNOPSIS...GORSE NEAR TERM...JOHNSON SHORT TERM...JOHNSON LONG TERM...GORSE AVIATION...GORSE/JOHNSON MARINE...GORSE/JOHNSON

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