038 FXUS61 KPHI 271051 AFDPHI AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE MOUNT HOLLY NJ 551 AM EST SUN JAN 27 2013 .SYNOPSIS... A HIGH PRESSURE SYSTEM CRESTING THE APPALACHIANS WILL REACH THE FORECAST AREA THIS AFTERNOON AND MOVE OFFSHORE TONIGHT. A WARM FRONT IS FORECAST TO MOVE SLOWLY NORTHEAST THROUGH THE REGION LATER MONDAY INTO TUESDAY. A STRONG COLD FRONT WILL SWEEP THROUGH THE MIDDLE ATLANTIC STATES ON WEDNESDAY NIGHT. REINFORCING COLD FRONTS ARE THEN FORECAST TO MOVE THROUGH THE NORTHEAST FRIDAY AND SATURDAY NIGHT. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... PERHAPS THE FORECAST PROBLEM OF THE DAY WILL BE THE EXTENT OF THE STRATOCUMULUS LEVEL CLOUDINESS ACROSS PORTIONS OF SOUTHEAST PENNSYLVANIA...NORTHEAST MARYLAND AND NORTHERN DELAWARE EARLY. THE OVERALL TREND HAS BEEN FOR THE CLOUD COVER TO SHRINK...AS THE INVERSION SEEN ON THE 0000 UTC KIAD SOUNDING CONTINUES TO EASE. IT APPEARS AS THOUGH THE CLOUDINESS COMING OFF LAKES ERIE AND ONTARIO REMAINS NORTH AND WEST OF THE SOUTHERN POCONOS AND NORTHWEST NEW JERSEY. THE LOW LEVEL FLOW SHOULD CONTINUE TO BACK...KEEPING THESE CLOUDS OUT OF THE AREA. TRENDS WILL BE MONITORED...BUT THE CLOUDINESS IS NOT EXPECTED TO BE A PROBLEM HERE. OTHERWISE...JUST SOME HIGH CLOUDINESS FROM THE OHIO VALLEY IS EXPECTED TODAY...AS THE COLUMN LOOKS TOO DRY FOR CUMULUS DEVELOPMENT. MODEL SOUNDINGS SUGGEST THAT WE MIX FROM ABOUT 900 MB....AND THAT WOULD SUPPORT HIGHS FROM THE MID 20S IN THE TERRAIN TO THE MID 30S SOUTH. THIS IS MORE OR LESS IN LINE WITH THE MOS BLEND...SUGGESTING THAT PHILADELPHIA COULD CRACK THE FREEZING MARK FOR THE FIRST TIME IN ALMOST A WEEK. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM MONDAY/... MID LEVEL RIDGING CRESTS OVER THE REGION IN THE HOURS AROUND MIDNIGHT. THE RIDGING PROBABLY KEEPS THE MID LEVEL CLOUDS AT BAY...BUT CIRRUS SHOULD STREAM OVER THE TOP OF THE RIDGE. WINDS WILL BE LIGHT FOR MUCH OF THE NIGHT. THE COMBINATION OF LIGHT WINDS... MAINLY CLEAR SKIES THIS EVENING AND LOW DEW POINTS RAISES THE SPECTER OF A QUICK TEMPERATURE DROP IN THE EVENING. IT WOULD APPEAR AS THOUGH THE CIRRUS REMAINS THIN ENOUGH FOR THIS TO OCCUR. PROTECTED AREAS SHOULD PLUMMET THIS EVENING...THEN RECOVER SLOWLY LATE THIS EVENING AND OVERNIGHT. BECAUSE OF THIS...THE SLIGHTLY COLDER GFS MOS NUMBERS WERE FAVORED FOR LOWS. IN REALITY... LOWS PROBABLY OCCUR LATE THIS EVENING OR EARLY MONDAY MORNING...THEN REMAIN STEADY OR SLOWLY RISE WITH INCREASED CLOUD COVER. WARM AIR ADVECTION BEGINS IN THE MID LEVELS THIS EVENING...AND PEAKS AFTER 0600 UTC MONDAY. THE INITIAL SHOT OF WARM AIR ADVECTION GOES INTO MOISTENING THE COLUMN...ALLOWING MID LEVEL CLOUDS TO DEVELOP. THE WARM AIR ADVECTION PEAKS BETWEEN 0900 AND 1200 UTC MONDAY...AND THIS MAY BE ENOUGH TO FINALLY MOISTEN THE COLUMN WEST OF THE DELAWARE RIVER. THERE ARE SOME SHORT TERM MODEL DIFFERENCES WITH REGARD THE STRENGTH AND TIMING OF THE RELATIVE WARM AIR ADVECTION PEAK LATE TONIGHT (THE NAM IS ACTUALLY FASTER THAN THE GFS). THE TIMING DIFFERENCE IS MANIFESTED IN THE VERTICAL VELOCITY FIELDS...WHICH ARE ALMOST EXCLUSIVELY DRIVEN BY ISENTROPIC UPGLIDE. AT THIS POINT...WITH DRY AIR IN PLACE...A BLEND OF TIMING WAS USED. THE LEADING EDGE OF THE PRECIPITATION SHOULD BE APPROACHING THE LEHIGH VALLEY BY 0900 UTC...AND OVERSPREAD MUCH OF THE REST OF THE AREA CLOSET O 1200 UTC MONDAY. AT THIS POINT...IT APPEARS AS THOUGH PRECIPITATION TYPE SHOULD BE ALL SNOW...AS THE COLUMN IS DRY AND EVAPORATIVE COOLING WILL BE MAXIMIZED DURING THIS TIME. THE STRENGTH OF THE WARM AIR ADVECTION SUGGESTS THAT SNOW AMOUNTS SHOULD GENERALLY BE LESS THAN AN INCH THROUGH DAYBREAK...WITH THE HIGHEST AMOUNTS IN THE LEHIGH VALLEY. && .LONG TERM /MONDAY THROUGH SATURDAY/... A VERY ACTIVE LONG TERM IS IN STORE FOR OUR CWA WITH A LIKELY ADVISORY LEVEL IMPACT EVENT ON MONDAY AND MONDAY NIGHT, POSSIBLY A MULTI FACETED WIND AND HEAVY RAIN EVENT ON WEDNESDAY NIGHT AND THEN A SERIES OF COLD FRONTS WITH SNOW SHOWERS POSSIBLE LATER IN THE WEEK. THE WRF-NMMB INITIALIZED SLIGHTLY BETTER THAN THE GFS OFF THE 00Z SOUNDINGS. IT NOW JUMPED IN FRONT OF THE EC AS THE COLDEST MODEL. UPWIND THE 850MB TEMPS WERE AROUND 1C COLDER THAN INITIALIZED WHILE THE 925MB TEMPS WERE ABOUT 1C WARMER. OVERALL A SLIGHTLY LESS SLEETIER LOOKING SOUNDING. FOR THIS PACKAGE WE DID TAKE THE FORECAST COLDER ON MONDAY WHICH SEEMS LOGICAL GIVEN HOW COLD, STUBBORN AND DRY THIS AIR MASS IS. AFTER COLLABORATING WITH OUR NEIGHBORING OFFICES, WE DECIDED TO HOLD OFF ON ADVISORIES WITH THIS PACKAGE. THERE REMAINS TIMING ISSUES WITH THE HEAVIER PRECIPITATION (MORNING VS AFTERNOON) BETWEEN THE MODELS. THE LEANING TOWARD SNOW IN A BACK HANDED SENSE POSTPONES THE MOST HAZARDOUS PART OF THIS EVENT, THE ICE AND LENDS SOME UNCERTAINTY AS TO HOW SOUTHEAST TO GO WITH HEADLINES ONCE IT ARRIVES. WE WILL GIVE THE DAY SHIFT A CHANCE TO SEE ANOTHER SOUNDING RUN AND RECONCILE SOME OF THE ONGOING DIFFERENCES AND HOPEFULLY SEE A STABILIZATION IN THE THERMAL TRENDS. REGARDLESS WE WILL CONTINUE MENTION IN THE HWO AS THE ARRIVAL MAY COME IN TIME TO IMPACT PART OF THE MORNING COMMUTE ON MONDAY, ESPECIALLY WESTERN CWA. BAD TIMING AS THE SNOW ARRIVED JUST IN TIME TO SNARL THE FRIDAY EVENING COMMUTE AND MAY COME BACK JUST IN TIME TO IMPACT THE MONDAY MORNING COMMUTE. AS FOR THE METEOROLOGICAL FORCING, NEITHER THE WRF-NMMB NOR THE GFS WERE INTERNALLY CONSISTENT. THE GFS FCST ISENTROPIC LIFT SUPPORTED THE FASTER WRF-NMMB TIMING WHILE THE WRF-NMMB'S MID LEVEL FGEN FORCING SUPPORTED THE IDEA OF HEAVIER (RELATIVELY SPEAKING) PCPN HOLDING OFF TO THE AFTN LIKE THE GFS HAD. BOTH THOUGH HAD THE ENTRANCE REGION OF THE 250MB JET DEPARTING EARLY IN THE DAY ON MONDAY JUST AS THE PCPN IS SUPPOSE TO ARRIVE. THE VENTING THUS IS NOT THE BEST. (TU BGM) ALSO THE RIDGING IS FORECAST TO INCREASE AND WARMING ALOFT IS NEVER GOOD FOR PCPN. SO THE MODEL PERSPECTIVE OF A QUARTER INCH OR LESS OF WATER EQUIVALENT THROUGH MONDAY EVENING LOOKS REASONABLE. THE THERMAL FIELDS, THE GFS AND ECMWF TRENDED WARMER WHILE THE WRF-NMMB WAS MUCH COLDER, SO MUCH THAT IT NOW HAS THE EVENT NEARLY ALL SNOW IN PHL VS THE PREVIOUS SOUNDING RUN WHICH WAS ALL RAIN. JUST LIKE THAT THE 06Z RUN WENT WARMER AGAIN. THE ANTECEDENT AIR MASS IS DRY, SO WE WILL GIVE THE MORE SNOW POTENTIAL SOME CREDENCE. EVEN THE 00Z WRF-NMMB WARMS THE AIR ALOFT, THE DIFFERENCE BEING IT DOES NOT HAVE MUCH PCPN BEYOND THE MORNING WHILE THE OTHER MODELS TAKE SOME INTO THE AFTERNOON. SO LEANING THE WRF-NMMB WAY TOP DOWN WISE, WE ADDED MORE SNOW EARLY AND THE TRANSITION TO ICE STARTS AROUND NOON. SNOW AMOUNTS THROUGH NOON (PLEASE SEE THE MAPS ON OUR FB PAGE) ARE HALF AN INCH TO AROUND AN INCH. THERE SHOULD BE SOME SLEET MIXED IN, BUT NOT THE MAIN ENTREE. FOR THE AFTERNOON WE TOOK THE SLOWEST MODEL SFC TEMP PROGS AT TRANSITIONING FROM FREEZING RAIN TO RAIN. THE INITIAL TRANSITION INCLUSION WAS CLOSE TO THE I95 CORRIDOR. NO DENYING IT HAS BEEN COLD, SO IT MIGHT TAKE A COUPLE OF DEGREES ABOVE 32F TO STOP THE ICE THREAT. EVEN IF ITS INDIRECT SUN AT LEAST THE CURRENT FORECAST TRANSITION TIMING IS CLOSE TO THE STRONGEST PART OF THE DAY FOR INSOLATION WITH RESIDUAL SALT ON ALL ROADWAYS. BUT THE TRANSITION WILL BE SLOW TO THE NORTH AND SOME FREEZING RAIN SHOULD LAST INTO THE EVENING AT THE LEAST. ICE ACCUMULATIONS SHOULD BE AROUND A TENTH OF AN INCH OR LESS. WE WERE VERY CONSERVATIVE WITH RAISING TEMPERATURES. MODEL FORECAST SOUNDINGS ARE STILL LOSING ICE POTENTIAL ALOFT ON MONDAY NIGHT SO IT WILL BE EITHER RAIN/DRIZZLE OR FREEZING RAIN OR FREEZING DRIZZLE. MOST OF THE IMPULSES ARE EAST OF THE AREA BY MIDNIGHT, SO WHAT PCPN THERE IS SHOULD BE LIGHT AND BECOME LIGHTER. WE REMAINED SLOWER THAN STAT GUIDANCE IN RISING TEMPERATURES. TUESDAY AND TUESDAY NIGHT HAVE THE MAKINGS OF ANOTHER ONE OF THOSE GRAY DAYS WITH LOW CLOUDS AND MAYBE SOME DRIZZLE OR RESIDUAL LIGHT RAIN. TEMPERATURES WERE KEPT AT OR BELOW STAT GUIDANCE ON TUESDAY AND WERE PRETTY MUCH WAFFLING UNCHANGED OVERNIGHT. THE GFS REMAINS THE MOST ROBUST WITH THE LLJ ON WEDNESDAY WITH FORECAST SPEEDS REACHING AROUND 85 KTS. NOT SURPRISINGLY ITS ALSO THE WETTEST MODEL. FCST PCPN WATER VALUES ARE STILL SUPPOSE TO GET CLOSE TO 1.5 INCHES SO WE WILL HAVE TO CONTINUE TO MONITOR THE POTENTIAL FOR BOTH STRONG OR HIGH WINDS (EAST CWA) AND POSSIBLE FLOODING IF TRAINING ECHOES OCCUR. SPEAKING OF TRAINING, BECAUSE THE GFS AND ECMWF CONTINUE TO SHOW SOME INSTABILITY ALOFT, WE DID ADD ISOLATED THUNDER. UNLESS THE FRONT MOVES IN FASTER, WINDS ALOFT, EVEN 10 TO 20 PERCENT LESS, SHOULD BE ABLE TO SCOUR THE CHILLY AIR NEAR THE GROUND AND WE DID NOT DIFFER FROM STAT GUIDANCE WITH MAX TEMPS. THE COLD FRONT SHOULD MOVE THROUGH ON WEDNESDAY NIGHT AND FOR THE REST OF THE LONG TERM WE FOLLOWED A 50-50 BLEND OF LATEST HPC GUIDANCE AND CONTINUITY. THE TIMING OF SNOW SHOWERS WITH SUBSEQUENT COLD FRONTS WAS CLOSEST TO THE ECMWF. && .AVIATION /12Z SUNDAY THROUGH THURSDAY/... THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY AND SURROUNDING AREAS. VFR CONDITIONS ARE EXPECTED TODAY...WITH JUST SOME HIGH CLOUDINESS. WEST NORTHWEST WINDS WILL AVERAGE BETWEEN 5 AND 8 KNOTS FROM 1400 UTC THROUGH ABOUT 1800 UTC...THEN START TO BACK TOWARD SOUTHWEST TOWARD SUNSET. VFR CONDITIONS ARE EXPECTED TONIGHT...AS WINDS BECOME CALM OR LIGHT AND VARIABLE. HIGH CLOUDINESS IS EXPECTED THROUGH 0600 UTC MONDAY... WITH MID LEVEL CLOUDINESS THEREAFTER. AS SNOW MOVES IN FROM THE WEST...KABE AND KRDG COULD SEE MVFR CONDITIONS AS EARLY AS 1000 UTC MONDAY. AT KPHL...MVFR/IFR CONDITIONS IN LIGHT SNOW AND LOW CEILINGS ARE EXPECTED CLOSE TO 1200 UTC MONDAY. OUTLOOK... MONDAY...BECOMING MVFR TO LOCALLY IFR WITH MIXED PRECIPITATION. THIS WILL BE THE START OF AN UNSETTLED WEATHER PERIOD. MONDAY NIGHT...MVFR POSSIBLY IFR. SOME FREEZING PRECIPITATION POSSIBLE NORTH. TUESDAY AND TUESDAY NIGHT...MAINLY MVFR, LOCALLY IFR IN DRIZZLE AND LOWER CIGS. CHANCE OF LLWS TUESDAY NIGHT. WEDNESDAY...MAINLY MVFR IN SHOWERS, VERY STRONG SOUTHWEST WINDS ARE POSSIBLE WITH GUSTS EXCEEDING 30 KTS. CHANCE OF LLWS EARLY. WEDNESDAY NIGHT...MAINLY IFR. SHIFTING STRONG SOUTHWEST TO NORTHWEST WINDS GUSTS COULD EXCEED 30 KTS. THURSDAY...CONDITIONS IMPROVING TO MAINLY VFR NORTHWEST WIND GUSTS OF 20 TO 30 KT POSSIBLE. && .MARINE... THE GRADIENT SUPPORT WEST TO NORTHWEST WINDS BETWEEN 10 AND 15 KNOTS OR MUCH OF THE DAY...DROPPING OF AS THE SURFACE RIDGE AXIS APPROACHES. THE SHORT PERIOD WIND WAVES DIMINISH AS WELL...AS AFTER THE SMALL CRAFT ADVISORY IS DROPPED THIS MORNING...IT WILL NOT BE NEEDED AGAIN TODAY. AS THE SURFACE RIDGE AXIS GETS OFFSHORE...THE GRADIENT FLOW BACKS TO THE SOUTHWEST AND PERHAPS EVEN SOUTH BY DAYBREAK MONDAY. THE FLOW DOES NOT SUPPORT WINDS MUCH ABOVE 10 KNOTS...EVEN AS THE RETURN FLOW BECOMES ESTABLISHED. WINDS AND SEAS ARE EXPECTED TO REMAIN WELL BELOW SMALL CRAFT LIMITS TONIGHT. OUTLOOK... A ROUGHER TIME FOR THE AREA WATERS IS EXPECTED IN THE OUTLOOK PERIOD. MONDAY AND MONDAY NIGHT SHOULD HAVE WIND GUSTS JUST BELOW SMALL CRAFT ADVISORY CONDITIONS WITH THE GREATEST CONCERN ON THE OCEAN. WINDS AND SEAS ON TUESDAY DAY ARE EXPECTED TO BE FURTHER BELOW SMALL CRAFT ADVISORY CRITERIA. FOG MIGHT BE MORE OF AN ISSUE TUESDAY INTO TUESDAY NIGHT. THEN ON TUESDAY NIGHT THE SOUTHERLY FLOW WILL START INCREASING PRECEDING THE COLD FRONT. AS IS TYPICALLY THE CASE, WARM AIR OVER COLDER WATER WILL DAMPEN THE MIXING. SO TUESDAY NIGHT ON THE OCEAN, THE SWELLS MIGHT BE THE PHENOMENON THAT REACHES SMALL CRAFT ADVISORY CRITERIA. ON WEDNESDAY AND WEDNESDAY EVENING, A VERY STRONG LOW LEVEL JET IS FORECAST TO BE PASSING ACROSS THE AREA WATERS AND EVEN WITH THE WARMTH OF THE AIR MASS, THAT MIGHT BE TOO STRONG. WE HAVE WIND GUSTS OUTLOOKED TO REACH GALE FORCE. CONFIDENCE IS HIGHER ABOUT SMALL CRAFT ADVISORY CRITERIA BEING MET. WHILE THERE MAY BE A BIT OF A LULL IN THE WINDS ON WEDNESDAY NIGHT, THE SHOULD INCREASE AGAIN IN ALL LIKELIHOOD TO SMALL CRAFT ADVISORY CRITERIA AGAIN LATER WEDNESDAY NIGHT INTO THURSDAY. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...NONE. NJ...NONE. DE...NONE. MD...NONE. MARINE...NONE. && $$ SYNOPSIS...GIGI NEAR TERM...HAYES SHORT TERM...HAYES LONG TERM...GIGI AVIATION...GAINES/GIGI/HAYES MARINE...GIGI/HAYES