Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 010225 AFDPHI AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE MOUNT HOLLY NJ 925 PM EST SAT FEB 28 2015 .SYNOPSIS... HIGH PRESSURE OVERHEAD TONIGHT WILL SHIFT OFFSHORE ON SUNDAY. A COLD FRONT WILL APPROACH FROM THE WEST SUNDAY BEFORE IT MOVES THROUGH THE REGION LATE SUNDAY NIGHT. HIGH PRESSURE BUILDS BACK INTO THE AREA MONDAY AND MONDAY NIGHT. LOW PRESSURE WILL TRACK WELL TO OUR NORTHWEST MIDWEEK. THE COLD FRONT ASSOCIATED WITH THIS SYSTEM APPROACHES TUESDAY AND TUESDAY NIGHT BEFORE IT PASSES THROUGH OUR AREA ON WEDNESDAY. THE FRONT WILL SLOW DOWN AS IT MOVES SOUTH OF THE AREA WEDNESDAY NIGHT INTO THURSDAY. ARCTIC HIGH PRESSURE BUILDS IN FROM THE WEST LATE IN THE WORK WEEK. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/... THE 00Z SFC ANALYSIS SHOWS HIGH PRESSURE CENTERED OVER NJ AND ERN PA RESULTING IN CALM OR LIGHT AND VARIABLE WINDS ACROSS THE AREA. THE LIGHT WINDS AND CLEAR SKIES ALLOW FOR GOOD RADIATIONAL COOLING AND TEMPS WILL CONTINUE TO FALL INTO/THRU THE TEENS...REACHING SINGLE DIGITS IN SOME COLDER LOCATIONS. HWVR...HI/MID-LVL CLOUDS ARE ADVANCING STEADILY EASTWARD AND HAVE REACHED WRN PA AS OF 0200Z OR 900 PM. CLOUDS SHOULD OVERSPREAD OUR FCST AREA STARTING SHORTLY AFTER MIDNIGHT...PUTTING AN END TO MOST OF THE RADIATIONAL COOLING. SNOW LAGS BEHIND THE CLOUDS AND SHOULD BE ENTERING WRN OH SHORTLY. && .SHORT TERM /6 AM SUNDAY MORNING THROUGH 6 PM SUNDAY/... CLOUDS WILL HAVE OVERSPREAD THE ENTIRE AREA BY SUNDAY MORNING. A FAST MOVING WEAK LOW PRESSURE SYSTEM WILL ZIP ACROSS THE AREA SUNDAY AFTERNOON. A LARGE AREA OF MOISTURE WILL ACT UPON THE DISORGANIZED DYNAMICS TO PRODUCE PCPN ACROSS THE REGION...BEGINNING WEST BY MID/LATE MORNING AND DURING THE EARLY AFTERNOON FOR THE DEL VALLEY/SRN NJ. P-TYPE ISSUES ABOUND WITH COLD MORNING TEMPS AND WAA ALOFT...THE USUAL SUSPECTS...ON SCENE. MUCH OF THE PCPN WILL BE SNOW ACROSS THE DEL VALLEY N/W AT THE ONSET WITH SLEET AND FREEZING RAIN AT THE ONSET FAR S/E. THE FROZEN P-TYPE WILL SLOWLY YIELD TO FREEZING PCPN FROM S TO N DURING THE AFTERNOON. IT APPEARS TO BE ALL SNOW THRU 00Z NORTH OF I-80. P-TYPE ARE ALWAYS TRICKY...SO CHECK BACK OVERNIGHT FOR THE LATEST. THE OVERALL QPF AMOUNTS ARE NOT THAT MUCH...USING THE 12Z GFS AS GUIDANCE...TOTALS OF 1/4 TO 1/2 INCH ARE EXPECTED. THESE TOTALS WOULD LEAD TO AN ADVISORY LEVEL SNOW EVENT (NORTH - IN THE ALL SNOW AREA)...AND AT LEAST SOME ICE (ONLY A TRACE IS NEEDED FOR AN ADVISORY) ELSEWHERE. ITS POSSIBLE THAT 1/10 TO 2/10TH OF AN INCH OF ICE MAY OCCUR IN PERSISTENT FREEZING RAIN AREAS. WE WILL THEREFORE ISSUE A WSW (ADVISORY) FOR ALL AREAS FROM 15Z SUN THRU 09Z MONDAY. THE END TIME IS PROBABLY WELL AFTER THE PCPN ENDS...BUT THERE COULD BE SOME RESIDUAL SLICK AREAS...AND THE OPTION TO EXTEND INTO THE MON AM DRIVE TIME IS THERE. && .LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/... A COLD FRONT WILL CROSS THE APPALACHIANS SUNDAY EVENING. PRECIP IS EXPECTED TO BE WIDESPREAD EARLY IN THE EVENING AS THE BEST ISENTROPIC LIFT AND LOW-LEVEL JET DYNAMICS IS EXPECTED PROGRESS THRU THE FORECAST AREA. USED THERMAL PROFILES FROM THE COLDER NAM/ECMWF TO POPULATE OUR WX GRIDS AS COLDER AIR ANCHORED NEAR THE SURFACE WILL BE SLOW TO ERODE. THIS WILL ALLOW FREEZING RAIN TO CONTINUE ALONG THE I-95 CORRIDOR WHERE AN ADDITIONAL ICE ACCUMULATION OF UP TO ONE-TENTH ARE FORECAST DURING THE EVENING. FARTHER NORTH AND WEST ACROSS THE I-78, PREDOMINANT PTYPE WILL BE SNOW AND SLEET BUT IT CAN CHANGEOVER TO FREEZING RAIN FOR A SHORT PERIOD. THE HIGHEST SNOWFALL ACCUMULATIONS ARE EXPECTED ALONG THE I-80 CORRIDOR WHERE PRECIP SHOULD REMAIN ALL SNOW. THE COLD FRONT WILL PASS THRU THE REGION OVERNIGHT SUNDAY NIGHT. THE TREND WAS TO END PRECIP FROM WEST TO EAST EARLIER. EXPECT THE LAST OF THE PRECIP TO HAVE MOVED OFFSHORE BEFORE THE START OF THE MORNING COMMUTE ON MONDAY. ONCE THE FRONT MOVES OFFSHORE, A STRONG PRESSURE GRADIENT THAT DEVELOPS BETWEEN THE LOW NEAR THE GULF OF MAINE AND HIGH PRESSURE OVER THE MIDWEST WILL PRODUCE A BRISK W-NW WIND ON MONDAY. FORECAST SOUNDINGS FOR MONDAY MORNING SHOW THE COLUMN DRYING AND COOLING RAPIDLY IN WAKE OF FROPA IN CONCERT WITH THE POST-FRONTAL SURGE. FOR SOME LOCATIONS, HIGHS MAY OCCUR OVERNIGHT (CLOSE TO FROPA TIME) BEFORE TEMPS DECREASE DURING THE DAY. THERE WILL LIKELY BE AT LEAST BKN CAA STRATOCU THAT DEVELOPS DURING THE DAY BUT AT LOCATIONS THAT ARE ABLE TO RECEIVE MORE APPRECIABLE BREAKS IN THE CLOUDS, SOLAR INSOLATION SHOULD OFFSET CAA, THUS ALLOWING TEMPS TO REMAIN STEADY OR EVEN INCREASE SLIGHTLY DURING THE PEAK HEATING HOURS. HIGH PRESSURE BUILDS IN FROM THE OHIO VALLEY MONDAY EVENING BEFORE MOVING OVERHEAD BY EARLY TUESDAY MORNING. THE SETUP FOR RADIATIONAL COOLING WILL BECOME FAVORABLE, ESPECIALLY OVERNIGHT, WHEN THE HIGH MOVES OVERHEAD AND THE WINDS BECOME CALM. THE FORECAST FOR MIN TEMPS MONDAY NIGHT WEIGHTED TOWARD THE COLDER END OF GUIDANCE. IN THIS SETUP (THE HIGH CENTERED RIGHT ON TOP OF US AND A PRE-EXISTING SNOW PACK), WE TYPICALLY TRY TO UNDERCUT EVEN THE COLDEST GUIDANCE BY A FEW DEGREES BUT DID NOT WANT TO GO QUITE THAT AGGRESSIVE WITH MID AND HI CLOUDS SPREADING INTO THE AREA BY EARLY MORNING, WHICH COULD CURB TEMP DROP OFF). THE HIGH IS FORECAST TO SHIFT OFFSHORE TUESDAY. STRENGTHENING S-SWLY RETURN FLOW AROUND THE HIGH AND AHEAD OF A SYNOPTIC TROUGH THAT WILL BE AMPLIFYING OVER THE CENTRAL CONUS WILL DRAW MILDER AIR UP THE EASTERN SEABOARD TUESDAY AFTERNOON THROUGH WEDNESDAY. PRECIP LOOKS TO OVERSPREAD THE FORECAST AREA IN THE LATE IN THE AFTERNOON (PRIMARILY IN OUR FAR WESTERN ZONES) AND IN THE EVENING WHEN A WARM FRONT LIFTS THRU THE MID ATLANTIC. THE AIRMASS PRECEDING THE ARRIVAL OF THE WARM AIR AND PRECIP WILL BE COLD, SO THERE IS A POTENTIAL FOR IT TO START AS A WINTRY MIX OF SNOW AND SLEET. A QUICK TRANSITION TO FREEZING RAIN AND THEN RAIN IS EXPECTED DURING THE EVENING IN THE DELMARVA AND NEAR THE COAST. THE COLD AIR TRAPPED NEAR THE GROUND IS OFTEN MORE STUBBORN TO RETREAT WEST OF THE FALL LINE THAN MODELS SHOW DUE TO THE COMPLEX OROGRAPHY. AFTER TAKING INTO ACCOUNT THIS MODEL BIAS, THERE IS A CONCERN FOR FREEZING RAIN TUESDAY NIGHT WITH NE PA AND NW NJ FACING A LONGER DURATION OF FREEZING RAIN COMPARED TO THE I-95 CORRIDOR. WE WILL ATTEMPT TO HEIGHTEN THE WORDING FOR AN ICING THREAT IN THE HWO. THE PRE-FRONTAL WARM SECTOR IS EXPECTED TO BE ESTABLISHED OVER THE MID-ATLANTIC REGION ON WEDNESDAY. IF THE AFOREMENTIONED SHALLOW LAYER OF COLD AIR HANGS AROUND A BIT LONGER THAN CURRENTLY FORECAST, THEN FREEZING RAIN COULD PERSIST INTO THE MORNING IN THE SHELTERED VALLEYS OF THE I-80 CORRIDOR. OTHERWISE MOST, IF NOT ALL, OF THE AREA SHOULD SEE PERIODS OF RAIN DURING THE DAY. THE THREAT FOR FLOODING WITH THIS EVENT IS DISCUSSED IN THE HYDRO SECTION OF THE AFD. UNDERCUT TEMP GUIDANCE BY A FEW DEGREES FOR WEDNESDAY ESPECIALLY INLAND SINCE THE COLD AIR WILL NOT GO WITHOUT A FIGHT AND CLOUDS WILL INHIBIT STRONGER HEATING. FORECAST MAX TEMPS RANGE FROM NEAR 40F IN THE POCONOS TO MID 50S IN THE DELMARVA. A COLD FRONT IS FORECAST TO SAG SEWD THRU THE AREA LATE WEDNESDAY. THE LEADING EDGE OF ARCTIC AIR WILL SURGE INTO THE AREA SOMETIME WEDNESDAY NIGHT INTO THURSDAY UNDER NW WINDS BEHIND THE BOUNDARY. HOWEVER, THE FLOW ALOFT IS STILL OUT OF THE SW SINCE THE UPPER TROUGH HANGS BACK TOWARD THE MIDWEST/GREAT LAKES. WHILE THERE IS STILL CONSIDERABLE MODEL DIFFERENCES BETWEEN THE PROGRESSION OF THE COLD AIR, MAJORITY OF GUIDANCE DOES DEVELOP ANAFRONTAL PRECIP (RAIN CHANGING TO SNOW) ON THE BACK END OF THIS EVENT. THE POTENTIAL FOR ACCUMULATING SNOW IS HIGHER UNDER THIS SETUP THAN IN MOST CASES WITH RAIN ENDING AS SNOW. ARCTIC HIGH PRESSURE BUILDS INTO THE AREA THURSDAY NIGHT AND FRIDAY. TEMPS WILL BE 10-20F BELOW NORMAL TO END THE WORK WEEK. THE HIGH IS THEN FORECAST TO SHIFT SOUTHWARD TOWARD THE SOUTHEAST STATES BY SATURDAY. A MODERATING TREND WOULD BE IN STORE FOR START OF THE WEEKEND WITH W-SW FLOW DEVELOPING AROUND THE HIGH. && .AVIATION /02Z SUNDAY THROUGH THURSDAY/... THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY AND SURROUNDING AREAS. THE 00Z TAFS CONTINUE TO FEATURE VFR CONDITIONS THE REST OF THIS EVENING AND TONIGHT. SKY COVER WILL REMAIN MOSTLY SCATTERED CI/CS THROUGH THE EVENING...BEFORE CLOUDS LOWER AND THICKEN TOWARDS DAWN ON SUNDAY MORNING. WE HAVE COMPOSED THE TAFS ALONG THE GFS/LAMP LINES...SO LOWER CONDITIONS ARRIVE MID-MORNING FAR WEST AND THEN DURING THE AFTERNOON DEL VALLEY AND EAST. SNOW WILL DEVELOP OVER THE AREA FIRST...THEN CHANGE TO SLEET/FREEZING RAIN LATER DURING THE AFTERNOON. THE 30-HR PHL TAF HAS A PRELIM TIMELINE FOR THAT CHANGE-OVER...WHICH MAY OCCUR SOMEWHAT SOONER AT MIV/ACY AND LATER OR NOT AT ALL AT ABE/RDG. WINDS LOOK LIGHT THROUGH THE PERIOD... VARIABLE DIRECTION TONIGHT AND SE/E SUNDAY. OUTLOOK... SUNDAY NIGHT...WINTRY MIX CHANGING TO ZR AND THEN TO R FROM SE TO NW IN THE EVENING. RDG AND ABE MAY HAVE ENOUGH COLD AIR FOR S/IP TO CONT INTO MUCH OF THE NGT. FLUCTUATIONS BETWEEN MVFR AND IFR ARE EXPECTED. MONDAY...DRYING TREND WITH A GENERAL IMPROVEMENT TO VFR EXPECTED. HOWEVER, STRATOCU AROUND 3-4 KFT AGL MAY LEAD TO MVFR CIGS FOR SOME LOCATIONS. W-NW WINDS GUSTING TO 25 KT. MONDAY NIGHT AND TUESDAY MORNING...VFR AND LIGHT WINDS. TUESDAY AFTERNOON THROUGH WEDNESDAY...PRECIP OVERSPREADS LATE IN THE AFTN AND EVE. HIGH CONFIDENCE IN CONDITIONS DETERIORATING TO MVFR AND IFR DURING THIS TIME. PRECIP MAY BEGIN AS A WINTRY MIX BEFORE CHANGING TO RA LATE TUESDAY NIGHT AND WED. THE RAIN COULD BE MOD TO HVY AT TIMES. LLWS POSSIBLE, ESPECIALLY TUE NIGHT WITH A 60 KT SWLY JET DEVELOPING AT 2 KFT AGL. WEDNESDAY NIGHT AND THURSDAY...PRECIP MAY TRANSITION BACK TO SN FOR A PERIOD BEFORE ENDING. TIMING OF THIS CHANGEOVER STILL UNCERTAIN. IFR LIKELY IN SN. && .MARINE... WINDS AND SEAS ARE SUB-SCA AND ARE EXPECTED TO REMAIN SO THROUGH TONIGHT AND SUNDAY. HIGH PRESSURE OVER THE WATERS THIS EVENING WILL MOVE EAST OVERNIGHT. WINDS INTO THE EVENING WILL BE MOSTLY NORTH OR NORTHWEST AND THEN THEY WILL SWITCH AROUND TO EAST OR SOUTHEAST SUNDAY. WINDS SPEEDS WILL REMAIN UNDER 10 KNOTS MOST OF THE TIME. SNOW/SLEET AND FREEZING RAIN WILL OVERSPREAD THE AREA SUNDAY AFTERNOON. VSBY RESTRICTIONS POSSIBLE. OUTLOOK... SUNDAY NIGHT...NO MARINE HEADLINES ARE ANTICIPATED. MONDAY AND MONDAY NIGHT...HIGH CONFIDENCE FOR SCA-LEVEL NW WINDS. THE 12Z GFS IS INDICATING THE POTENTIAL FOR 35 KT GUSTS ON MONDAY BUT THIS SOLUTION IS AN OUTLIER FOR GALES SO NO WATCH WAS ISSUED ATTM. TUESDAY...NO MARINE HEADLINES ARE ANTICIPATED. TUESDAY NIGHT AND WEDNESDAY...SCA WILL LIKELY BE NEEDED WITH A STRENGTHENING SWLY FLOW. SOUNDINGS OFF THE 12Z GFS SHOWS STORM-FORCE WINDS ONLY 600 FT OFF THE GROUND TUESDAY NIGHT. WARMER AIR MOVING OVER THE COLD WATERS WILL NOT PRODUCE A FAVORABLE MIXING SETUP FOR THESE WINDS ALOFT TO BE TRANSPORTED DOWNWARD BUT CANNOT LOOK OVER MECHANICAL MIXING BRINGING DOWN STRONGER GUSTS GIVEN THE IMPRESSIVE SPEED SHEER. WEDNESDAY NIGHT AND THURSDAY...NW WINDS MAY GUST TO NEAR SCA CRITERIA. && .HYDROLOGY... TWO PRECIPITATION EVENTS ARE EXPECTED OVER THE NEXT FIVE DAYS, ONE ON SUNDAY AFTERNOON/NIGHT AND A SECOND ONE ON TUESDAY AFTERNOON INTO WEDNESDAY NIGHT. THE INITIAL EVENT LOOKS TO BE ONE WITH SNOW OR MIXED WINTRY PRECIPITATION. THE SECOND EVENT ON TUESDAY NIGHT INTO WEDNESDAY NIGHT LOOKS TO BE ON THE LIQUID SIDE WITH TEMPERATURES BY WEDNESDAY ABOVE FREEZING AFTER AN INITIAL WINTRY START. ALTHOUGH THIS SECOND EVENT IS ABOUT 4 DAYS OUT, WE WANTED TO HIGHLIGHT THAT SOME OF THE SOLUTIONS WE`RE SEEING ARE PRODUCING LOCALLY HEAVY RAINFALL. THE COMBINATION OF A BRIEF WARM UP ON WEDNESDAY, LOCALLY HEAVY RAIN, MELTING SNOW, A FROZEN GROUND, ABOVE NORMAL ICE IN CREEKS, STREAMS, AND RIVERS COULD LEAD TO INCREASED RUNOFF AND FLOODING. WHERE THE DEPTH OF THE SNOW PACK IS HIGHEST (IN NW PA AND N NJ), RAPID SNOW MELT IS NOT EXPECTED SINCE TEMPS WILL NOT RISE AS SHARPLY AS AREAS TO THE SOUTH. INCREASED FLOW 0N AREA CREEKS, STREAMS, AND RIVERS COULD ALSO START TO MOVE ICE, AND IN A WORST CASE SCENARIO, CREATE RESTRICTIONS OR JAMS. AGAIN, THIS SECOND EVENT BRINGING WARMTH AND RAIN IS ABOUT 4 DAYS OUT. THINGS CAN CHANGE. CONTINUE TO MONITOR FORECASTS. KEEP IN MIND THAT SNOW MELT AND WARMTH, BY THEMSELVES, TYPICALLY DON`T CAUSE FLOODING. HOW MUCH RAIN WE GET WILL BE THE DETERMINING FACTOR. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...WINTER WEATHER ADVISORY FROM 10 AM SUNDAY TO 4 AM EST MONDAY FOR PAZ054-055-060>062-070-071-101>106. NJ...WINTER WEATHER ADVISORY FROM 10 AM SUNDAY TO 4 AM EST MONDAY FOR NJZ001-007>010-012>027. DE...WINTER WEATHER ADVISORY FROM 10 AM SUNDAY TO 4 AM EST MONDAY FOR DEZ001>004. MD...WINTER WEATHER ADVISORY FROM 10 AM SUNDAY TO 4 AM EST MONDAY FOR MDZ008-012-015-019-020. MARINE...NONE. && $$ SYNOPSIS...KLEIN NEAR TERM...AMC SHORT TERM...O`HARA LONG TERM...KLEIN AVIATION...KLEIN/O`HARA MARINE...KLEIN/O`HARA HYDROLOGY...KRUZDLO/KLEIN

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