Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 311531 AFDPHI AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE MOUNT HOLLY NJ 1031 AM EST SAT JAN 31 2015 .SYNOPSIS... STRONG LOW PRESSURE WILL CONTINUE TO TRACK INTO THE CANADIAN MARITIMES, MEANWHILE HIGH PRESSURE WILL GRADUALLY BUILD INTO OUR AREA FROM THE OHIO VALLEY INTO TONIGHT. THIS HIGH WILL THEN MOVE OFFSHORE LATE TONIGHT AND SUNDAY. BY SUNDAY NIGHT, LOW PRESSURE WILL MOVE INTO THE OHIO VALLEY THEN WILL TRACK INTO OUR REGION ON MONDAY. ANOTHER AREA OF HIGH PRESSURE WILL MOVE INTO THE REGION TUESDAY AND WEDNESDAY WITH LOW PRESSURE TRACKING JUST OFFSHORE FOR THE LATER HALF OF THE WEEK. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... STRONG LOW PRESSURE WILL CONTINUE TO MOVE INTO THE CANADIAN MARITIMES TODAY. MEANWHILE, HIGH PRESSURE CENTERED ACROSS THE SOUTHEASTERN STATES EXTENDS INTO THE EASTERN GREAT LAKES. THIS FEATURE WILL GRADUALLY BUILD EASTWARD THROUGH THIS AFTERNOON, AND AS A RESULT THE PRESSURE GRADIENT WILL RELAX. THIS WILL ALLOW THE GUSTY WINDS TO GRADUALLY DIMINISH THROUGH THE AFTERNOON FROM WEST TO EAST. AN ARCTIC AIRMASS REMAINS IN PLACE TODAY, AND WITH THE GUSTY WINDS THE ADDED WIND CHILL EFFECT WILL BE NOTICEABLE. THE GUSTY WINDS HAVE BEEN PRODUCING SOME BLOWING AND DRIFTING SNOW MAINLY ACROSS OUR FAR NORTHWEST ZONES, AND THIS MENTION WAS CONTINUE A BIT LONGER. IT APPEARS THAT THE LOWEST WIND CHILL VALUES ARE TIED TO THE HIGHEST TERRAIN, WITH OTHER AREAS BELOW CRITERIA. GIVEN THIS AND BASED ON THE WINDS DIMINISHING THROUGH THE AFTERNOON, THE WIND CHILL ADVISORY FOR OUR FAR NORTHWEST ZONES WAS CANCELLED. NO MAJOR CHANGES WERE MADE TO THE GOING FORECAST, OTHER THAN TO ADJUST THE CURRENT CONDITIONS BASED ON THE OBSERVATIONS AND THEN THE LAMP/LAV GUIDANCE WAS USED TO HELP ASSIST THROUGH THE AFTERNOON. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/... AS THE SURFACE LOW SHIFTS EVEN FURTHER EAST, PULLING AWAY FROM THE COAST, WINDS IN OUR REGION WILL SHIFT TO WESTERLY, BUT WILL GENERALLY BE NEAR OR BELOW 10KT. DESPITE THE LIGHT WINDS, THE SLIGHT DOWNSLOPE, ALONG WITH INCREASING MID AND HIGH LEVEL CLOUDS SHOULD HELP TO AT LEAST SOMEWHAT LIMIT RADIATIONAL COOLING OVERNIGHT. THEREFORE, LOWS TONIGHT, WHILE STILL SLIGHTLY BELOW NORMAL, SHOULD BE 5 TO 10 DEGREES HIGHER THAN LOWS LAST NIGHT. TOWARDS THE EARLY MORNING HOURS ON SUNDAY, THE COLD FRONT SHOULD BEGIN TO APPROACH THE REGION FROM THE NORTHWEST. HOWEVER, AT THIS POINT, EXPECT IT TO REMAIN NORTHWEST OF THE REGION THROUGH THE EARLY MORNING HOURS. && .LONG TERM /SUNDAY THROUGH FRIDAY/... SUNDAY: LOOKS TO BE FAIRLY QUIET DAY WITH HIGH PRESSURE MOVING OFFSHORE. THIS WILL BRING SOUTHERLY FLOW AND A MODERATING TREND TO THE REGION. STAYED CLOSE TO MET AND MAV WHICH ARE A FEW DEGREES WARMER THAN MODELED NAM/ECMWF TWO METER TEMPERATURES WHICH SEEM TO COOL. SUNDAY NIGHT THROUGH MONDAY NIGHT: * WINTER STORM POTENTIAL FOR AT LEAST THE NORTHERN HALF OF THE REGION* LOW PRESSURE WILL TRACK INTO THE OHIO VALLEY BY SUNDAY NIGHT AND TOWARD OUR REGION ON MONDAY. THIS LOW PRESSURE SYSTEM WILL TRACK ALONG A COLD FRONT MOVING IN FROM THE NORTHWEST. HOWEVER, SOUTHERLY FLOW WILL CONTINUE TO THE SOUTH OF THIS COLD FRONT ALLOWING FOR WARM ENOUGH TEMPERATURES TO WHERE MIXED PRECIPITATION OR RAIN ARE MORE LIKELY THAN SNOW SOUTH OF THE LOW TRACK AS THE FRONT STALLS OVER THE REGION. THE PAST FEW MODEL CYCLE RUNS A SHORTWAVE WHICH DEVELOPS THE LOW PRESSURE SYSTEM HAS COME INTO THE NATIONAL UPPER AIR NETWORK. THIS INCREASES FORECASTER CONFIDENCE TO SOME DEGREE. THE SHORTWAVE HAS ALSO TRENDED STRONGER AND SHARPER WHICH IN TURN HAS SHIFTED THE TRACK OF THE LOW FURTHER NORTH OVER THE PAST FEW MODEL CYCLES. THE UKMET, NAM AND SREF HAVE BEEN THE MORE AMPLIFIED WHILE THE ECMWF IS MOST SOUTH OF THE MODELS WITH DEEPER LOW LEVEL COLD AIR. PLENTY OF MOISTURE IS DRAWN NORTH AND RUNG OUT ALONG THE FRONTAL BOUNDARY WITH THE POTENTIAL FOR PERIODS OF MODERATE AND HEAVY PRECIPITATION. WENT CLOSE TO MAV GUIDANCE ON TEMPERATURES THROUGHOUT THE STORM. THE NAM SEEMS TO WARM AT THIS JUNCTURE AND THE ECMWF TO COOL. STAYED CLOSE TO WPC ON QPF AND ALSO USED THE SREF FOR THE FRONT END SUNDAY EVENING. PERIODS OF SNOW DEVELOP FOR MOST OF THE REGION WITH RAIN IN THE DELMARVA AROUND 00Z MONDAY. THIS GRADUALLY TRANSITIONS TO RAIN FOR PHILADELPHIA METRO AND A WINTRY MIX JUST NORTHWEST OF PHILADELPHIA. FURTHER NORTH SNOW, MODERATE TO HEAVY AT TIMES CONTINUES FOR THE OVERNIGHT AND MONDAY MORNING HOURS. A VERY SHARP CUTOFF IS LIKELY WITH SNOW ACCUMULATIONS TO OCCUR BASED ON THE VERY SLOW MOVEMENT OF THE CHANGEOVER LINE NORTH. ACCUMULATIONS RANGE FROM AN INCH OR TWO IN PHL TO SIX TO TEN INCHES IN THE LEHIGH VALLEY AND EVEN CLOSE TO A FOOT FOR THE POCONOS. WHERE THIS CUTOFF SETS UP WILL BE DETERMINED ON THE EVENTUAL LOCATION OF THE FRONT AND THE LOW TRACK. ANY SHIFTS IN TRACK COULD ALTER AMOUNTS WITH FUTURE FORECAST UPDATES. A WINTER STORM WATCH HAS BEEN ISSUED FOR THE LEHIGH VALLEY INTO NORTHERN NJ AND THE POCONOS FOR MODERATE TO HIGH CONFIDENCE IN SNOWFALL OVER SIX INCHES FOR THE SUNDAY NIGHT AND MONDAY PERIOD. THE WATCH HAS BEEN EXPANDED TO INCLUDE UPPER BUCKS AND MONTGOMERY COUNTIES BASED ON THE FORECAST SNOW AMOUNTS, ALTHOUGH THERE COULD BE MORE MIXING/ICING HERE. THIS WILL BE LOOKED AT CLOSER WITH THE AFTERNOON FORECAST ISSUANCE. TUESDAY AND WEDNESDAY: BEHIND THE SYSTEM, A PRETTY POTENT BLAST OF COLD COMES INTO THE REGION BY TUESDAY. 925 AND 850 MB BOTH WILL BE WELL BELOW -10C COUPLED WITH STRONG NORTHWESTERLY WINDS. THIS QUICKLY MODIFIES BY WEDNESDAY AHEAD OF ANOTHER COLD FRONTAL BOUNDARY WITH A RETURN TO SOUTHERLY WINDS. SNOWCOVER LEADS TO GREATER UNCERTAINTY IN TEMPERATURES. OVERALL THINKING IS ECMWF/ MEX MOS AND ENSEMBLE GUIDANCE MAY BE A TOUCH TO WARM TUESDAY THEN TO COOL WEDNESDAY. WEDNESDAY NIGHT THROUGH FRIDAY NIGHT: POTENTIAL EXISTS FOR AN AREA OF LOW PRESSURE TO DEVELOP IN THE GULF OF MEXICO AND TRACK NORTHEAST ALONG THE COLD FRONT WHICH PASSES THROUGH WEDNESDAY NIGHT. LIMITED TIME TO FOCUS ON THIS PERIOD BUT LEFT IN CHANCE OF SNOW THURSDAY INTO FRIDAY. THE ECMWF IS FURTHEST EAST WITH THE GFS, CMC AND UKMET HINTING AT THIS THREAT. && .AVIATION /16Z SATURDAY THROUGH WEDNESDAY/... THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY AND SURROUNDING AREAS. REST OF TODAY...VFR. NORTHWEST WINDS 15-20 KNOTS WITH GUSTS UP TO 30 KNOTS, DIMINISHING BY LATE AFTERNOON. TONIGHT...VFR, WITH SOME INCREASE IN CLOUDS ESPECIALLY FOR KABE AND KRDG. WEST-NORTHWEST WINDS QUICKLY DIMINISHING TO AROUND 5 KNOTS EARLY, THEN LOCALLY BECOMING SOUTHWESTERLY. OUTLOOK... SUNDAY: VFR, INCREASING CLOUDS. SUNDAY NIGHT AND MONDAY: MVFR AND IFR CEILINGS IN RAIN AND SNOW. MAINLY SNOW FOR KRDG, KTTN AND KABE WITH MAINLY RAIN AT OTHER TAF SITES. MONDAY NIGHT THROUGH WEDNESDAY: VFR, NORTHWEST WIND GUSTS AROUND 25 KNOTS MONDAY NIGHT. && .MARINE... ALTHOUGH WINDS WILL SLOWLY SUBSIDE THROUGH MIDDAY, THEY MAY NOT DROP BELOW GALE FORCE UNTIL LATE THIS AFTERNOON. ONCE GUSTS DO DROP BELOW GALES, A SCA MAY BE NEEDED THROUGH AT LEAST THE EVENING HOURS AS WINDS AND SEAS CONTINUE TO SLOWLY FALL. MODERATE FREEZING SPRAY WILL CONTINUE THROUGH THIS AFTERNOON, WITH AN ADVISORY IN PLACE. OUTLOOK... SUNDAY AND SUNDAY NIGHT: SUB SCA SEAS AND WINDS WITH SEAS BUILDING TOWARD SUNRISE MONDAY. MONDAY THROUGH TUESDAY: SCA SEAS THROUGH TUESDAY MORNING AND NORTHWEST SCA WIND GUSTS LATE MONDAY AFTERNOON AND NIGHT. TUESDAY NIGHT AND WEDNESDAY: SUB SCA SEAS AND WINDS. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...WINTER STORM WATCH FROM SUNDAY EVENING THROUGH MONDAY AFTERNOON FOR PAZ054-055-060>062-103-105. NJ...WINTER STORM WATCH FROM SUNDAY EVENING THROUGH MONDAY AFTERNOON FOR NJZ001-007>010. DE...NONE. MD...NONE. MARINE...FREEZING SPRAY ADVISORY UNTIL 6 PM EST THIS EVENING FOR ANZ430- 431-450>455. GALE WARNING UNTIL 6 PM EST THIS EVENING FOR ANZ430-431- 450>455. && $$ SYNOPSIS...GAINES/GORSE NEAR TERM...GORSE SHORT TERM...JOHNSON LONG TERM...GAINES AVIATION...GAINES/GORSE MARINE...GAINES/JOHNSON

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