301 FXUS61 KPHI 212110 AFDPHI AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE MOUNT HOLLY NJ 410 PM EST MON JAN 21 2013 .SYNOPSIS... A SECONDARY COLD FRONT WITH WEAK LOW PRESSURE WILL MOVE ACROSS THE AREA TONIGHT. THE LOW WILL BEGIN TO STRENGTHEN AFTER IT PASSES EAST. ANOTHER REINFORCING COLD FRONT WITH WEAK LOW PRESSURE MAY PASS ACROSS THE AREA ON WEDNESDAY NIGHT. A POTENTIALLY MORE SIGNIFICANT AREA OF LOW PRESSURE IS FORECAST TO CONSOLIDATE TO OUR SOUTHWEST THURSDAY NIGHT OR FRIDAY AND MOVE OFF THE MID ATLANTIC COAST LATE FRIDAY OR FRIDAY EVENING. HIGH PRESSURE FROM THE WEST IS EXPECTED TO FOLLOW FOR THE WEEKEND. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/... AN ARCTIC AIRMASS TO SWEEP INTO THE AREA TONIGHT WITH EVEN SOME SNOW SHOWERS. A LOCAL BURST OF SNOW SHOULD OCCUR WITH ITS ARRIVAL. A 19Z SURFACE ANALYSIS PLACED WEAK LOW PRESSURE OVER THE EASTERN GREAT LAKES WITH A SECONDARY LOW IN EASTERN VIRGINIA. AN ARCTIC FRONT WAS DRAPED FROM LAKE ONTARIO SOUTHWARD ACROSS CENTRAL PENNSYLVANIA AND INTO WESTERN VIRGINIA. AN UPPER AIR ANALYSIS PLACED A STRONG TROUGH FROM EASTERN CANADA ACROSS THE GREAT LAKES/MIDWEST AND INTO THE NORTHEAST. SOME WEAK WAA AT 850 MB WAS NOTED ACROSS THE NORTHERN MID ATLANTIC. THE CLOUD COVER HAS HELD TEMPERATURES DOWN SOME TODAY ACROSS MUCH OF THE AREA, ALTHOUGH THIS SHIELD OF CLOUDS HAS BEEN SLOWLY ERODING ACROSS THE WASHINGTON/BALTIMORE AREAS THIS AFTERNOON WITH SOUTHERN DELAWARE HAVING LOTS OF SUNSHINE FOR MUCH OF THE DAY. LOW PRESSURE NEAR THE EASTERN GREAT LAKES WILL SLIDE EAST AS A SECONDARY LOW DEVELOPS NEAR THE DELMARVA. THIS SECONDARY LOW WILL BE THE MAIN SYSTEM AS IT STRENGTHENS WHILE TRACKING NORTHEAST IN RESPONSE TO A POTENT MID LEVEL JET AND AN INCREASING THERMAL GRADIENT OVER THE OCEAN. A STRONG SHORT WAVE ROUNDING THE BASE OF THE UPPER-LEVEL TROUGH WILL TEAM UP WITH THE ARCTIC FRONT AND STRENGTHENING MID LEVEL FLOW. WHILE THE AIRMASS IS NOT ALL THAT MOIST, ENOUGH ISENTROPIC LIFT AIDED BY SOME WAA WILL RESULT IN SNOW SHOWERS DEVELOPING DURING THE REST OF THE AFTERNOON ESPECIALLY ACROSS THE NORTHERN HALF OF THE CWA. THIS INITIAL LIFT MAY JUST BE SOME LIGHT SNOW AT FIRST. THE LOW LEVELS HAVE BEEN RATHER DRY, THEREFORE ENOUGH EVAPORATIVE COOLING SHOULD RESULT IN MOSTLY SNOW OCCURRING EVEN DOWN SOUTH WITH TIME. THE BEST LIFT IS MAINLY FROM NORTHEASTERN PENNSYLVANIA ON ACROSS NORTHERN NEW JERSEY. THIS SETUP IS BECOMING MORE CONVECTIVE LOOKING AS THE TOTAL TOTALS ARE FORECAST TO INCREASE TO ABOUT THE 40-50 RANGE. SOME OF THE HIGH-RESOLUTION GUIDANCE INDICATES THE POTENTIAL FOR A BAND OF HEAVIER SNOW SHOWERS TO ACCOMPANY THE ARCTIC FRONT AS LAPSE RATES IN THE LOWER LEVELS STEEPEN SOME AND AT LEAST SOME INSTABILITY IS INDICATED. THE BEST CHC FOR HEAVIER SNOW SHOWERS LOOKS TO BE NORTH OF A LINE FROM ABOUT WILMINGTON, DE TO ATLANTIC CITY, NJ, ALTHOUGH WE MAY EVEN HAVE TO WATCH PORTIONS OF THE DELMARVA GIVEN MORE BOUNDARY LAYER WARMTH LINGERING AHEAD OF THE ARCTIC FRONT. WE EXPANDED THE LIKELY POPS FARTHER SOUTH AND EAST FOR A TIME THROUGH EARLY THIS EVENING TO INDICATE A BURST OF SNOW WITH THE ARCTIC FRONT. THIS SHOULD BE ABLE TO PUT DOWN A LOCAL QUICK COATING WITH THE GREATEST AMOUNTS ACROSS THE NORTHERN ZONES WITH BETTER LIFT. GIVEN THE GENERAL COLDNESS AND ESPECIALLY DRYNESS OF THE AIRMASS, RATIOS WILL PROBABLY BE 12 TO 1 OR EVEN POSSIBLY INCREASING TO 15 TO 1. AS A RESULT, WE CONTINUED WITH A COUPLE OF INCHES ACROSS THE FAR NORTH THEN TAPERED THE AMOUNTS DOWN WITH A SOUTHWARD EXTENT. THE BURST OF SNOW, IF IT OCCURS, WILL GREATLY REDUCE THE LOCAL VISIBILITY FOR A BRIEF TIME AND MAKE UNTREATED ROADWAYS SLIPPERY. WE THEN TURN OUR ATTENTION TO THE MUCH COLDER AIR THAT OVERTAKES OUR CWA DURING THE COURSE OF THE NIGHT. LOW PRESSURE STRENGTHENING TO OUR NORTHEAST MAY HAVE A SURFACE TROUGH EXTEND BACK TO THE GREAT LAKES AS THE ARCTIC AIR OVER THE WARMER LAKE WATERS HELPS TO LOWER THE PRESSURES. THIS COULD HELP TO FOCUS SOME SNOW SHOWERS ACROSS NORTHERN NEW JERSEY AND TO THE POCONOS. THE LAKE EFFECT SNOW MACHINE WILL THEN BE GEARING UP WITH SOME PLUMES TRYING TO MAKE A RUN AT OUR FAR NORTHWEST ZONES. A FAIRLY POTENT AREA OF CHANNELIZED VORTICITY IS FORECAST TO BE OVER PARTS OF THE AREA, HOWEVER THE AIRMASS MAY BE TO DRY TO GENERATE FLURRIES OR SNOW SHOWERS OVERNIGHT OUTSIDE OF THE NORTHWEST ZONES. SOMETIMES WITH ARCTIC AIR ALONG WITH CHANNELIZED VORTICITY, NARROW AND SHALLOW /LOW-TOPPED/ BANDS OF SNOW OCCURS ACROSS PARTS OF OUR AREA. AS OF NOW, WE HAVE NOT REALLY FACTORED THIS INTO THE FORECAST AS WE WOULD LIKE TO SEE THE VORTICITY MORE ORIENTATED NORTHWEST TO SOUTHEAST. SOMETHING TO PERHAPS WATCH THOUGH AS THE SNOW GROWTH ZONE LOWERS WITH TIME. THE PRESSURE GRADIENT TIGHTENS THROUGH THE NIGHT AS LOW PRESSURE STRENGTHENS TO OUR NORTHEAST AND HIGH PRESSURE BUILDS TOWARD THE OHIO VALLEY. STRONG CAA IN CONJUNCTION WITH THE TIGHTENED PRESSURE GRADIENT WILL RESULT IN RATHER BLUSTERY CONDITIONS DURING THE NIGHT. THIS WILL DRIVE THE WIND CHILL VALUES DOWN INTO THE SINGLE DIGITS ACROSS THE CWA WITH EVEN BELOW ZERO VALUES ACROSS OUR FAR NORTHWEST ZONES. AS FOR LOW TEMPERATURES, WE GENERALLY USED AN EVEN BLEND OF THE GFS/NAM MOS. && .SHORT TERM /6 AM TUESDAY MORNING THROUGH 6 PM TUESDAY/... LOW PRESSURE IS FORECAST TO STRENGTHEN AS IF MOVES AWAY FROM THE NEW ENGLAND COAST. MEANWHILE, HIGH PRESSURE WILL BUILD INTO THE OHIO VALLEY. AN UPPER-LEVEL TROUGH WILL BE ACROSS THE GREAT LAKES AND NORTHEAST, WITH VERY COLD AIR ASSOCIATED WITH IT AS THE 850 MB TEMPERATURES DROP TO AROUND -20C. THE CYCLONIC FLOW AND PLACEMENT OF THE UPPER-LEVEL TROUGH WILL ALLOW THE ARCTIC AIR TO BECOME FIRMLY ENTRENCHED ACROSS OUR CWA. THIS WILL RESULT IN A VERY COLD DAY ACROSS THE ENTIRE CWA. A NORTHWESTERLY FLOW WILL BE MAINTAINED AS A TIGHTENED PRESSURE GRADIENT REMAINS IN PLACE BETWEEN DEPARTING LOW PRESSURE AND INCOMING HIGH PRESSURE. THIS COMBINED WITH THE ARCTIC AIR AND EFFICIENT VERTICAL MIXING WILL RESULT IN A BLUSTERY TO BITTERLY COLD DAY WITH WIND CHILL VALUES IN THE SINGLE DIGITS TO LOW TEENS /REMAINING BELOW ZERO ACROSS THE HIGHER TERRAIN OF THE NORTHWEST ZONES/. THE FORECAST SOUNDINGS SUGGEST THAT WIND GUSTS PEAK MAINLY IN THE 30-35 MPH RANGE. IT APPEARS THAT DESPITE THE ARCTIC AIRMASS IN PLACE /VERY LOW SURFACE DEW POINTS/ THE MAJORITY OF THE CWA REMAINS PRECIPITATION FREE. HOWEVER, THE NORTHWESTERLY FLOW WILL KEEP THE LAKE EFFECT SNOW MACHINE GOING. THERE MAY BE A SECONDARY ARCTIC BOUNDARY DROPPING SOUTHWARD FROM THE GREAT LAKES OR SIMPLY A SURFACE TROUGH, HOWEVER THIS FEATURE MAY ALIGN THE LAKE EFFECT SNOW WEST TO EAST AT FIRST BUT THEN SHIFT IT TOWARD A NORTHWEST TO SOUTHEAST ORIENTATION. THIS SHOULD ALLOW AT LEAST SOME SNOW SHOWERS TO REACH OUR NORTHWEST ZONES. IT TENDS TO NEARLY ALWAYS SNOW IT SEEMS AT LEAST SOME IN THE POCONOS UNDER THIS TYPE OF REGIME, THEREFORE WE CARRIED CHC POPS FOR SCATTERED SNOW SHOWERS. THE SNOW WILL BE RATHER FLUFFY GIVEN THE ARCTIC NATURE OF THE AIRMASS, THEREFORE AN INCH OR SO IS ANTICIPATED WHERE MORE PERSISTENT SNOW SHOWERS OCCUR. ELSEWHERE, WE WILL MAINTAIN A DRY FORECAST ATTM. HIGH TEMPERATURES WERE DERIVED FROM A BLEND OF THE GFS/NAM MOS WITH SOME TWEAKS THEREAFTER. && .LONG TERM /TUESDAY NIGHT THROUGH MONDAY/... VERY COLD TEMPERATURES GENERALLY ARE THE RULE OVER THE LONG TERM. THERE MAY BE A BIT OF A WARM UP WITH A POTENTIAL SYSTEM AROUND FRIDAY, BUT LATEST MODEL RUNS HAVE THAT SYSTEM MOVING OFF THE MID ATLANTIC COAST EITHER AT CENTRAL NEW JERSEY (THE 12Z CANADIAN), SOUTHERN NEW JERSEY (THE 12Z GFS), OR THE OUTER BANKS (THE 12Z ECMWF). AS NOTED BY THE PREVIOUS FORECASTER, THE RECENT TREND HAS BEEN TOWARD A COLDER AND MORE SOUTHERLY TRACK. VERY COLD AIR RETURNS IN THE WAKE OF THE SYSTEM, AND PERHAPS THERE WILL BE A MODERATION IN TEMPERATURES BY MONDAY. DURING THE FIRST PART OF THE LONG TERM, CHANNELED MID LEVEL VORTICITY, VERY LOW CONVECTIVE TEMPERATURES IN AN AIR MASS THAT SHOULD MIX UP TO ABOUT H800 AND A VERY COLD FETCH OFF THE GREAT LAKES MAY RESULT IN FLURRY OR SNOW SHOWER ACTIVITY. THE MODELS SUGGEST THIS ENDS IN THE WAKE OF A MID LEVEL SYSTEM THAT MOVES BY ON WEDNESDAY NIGHT. THAT SYSTEM IS NOT LOOKING AS ROBUST AS PREVIOUS RUNS OF THE GFS DEPICTED IT, BUT IT HAS A DECENT ASSOCIATED UPPER JET AND IT COULD ENHANCE THE LAST OF ANY FLURRY OR SNOW SHOWER ACTIVITY. THURSDAY LOOKS DRY, AND THURSDAY NIGHT COULD ALSO STAY DRY IF THE SLOWER TREND IN THE ECMWF AND THE ERRATIC BUT STILL SLOWER GFS ARE TO BE BELIEVED. THE CANADIAN IS THE FASTEST OF THE THREE MODELS AND WOULD HAVE PRECIPITATION BREAKING OUT BY DAYBREAK FRIDAY. REGARDING THE POTENTIAL EVENT, WE ARE STILL TOO FAR AWAY TO TALK WITH CERTAINTY ABOUT THE STRENGTH OF Q-VECTOR CONVERGENCE, PHASING OF THE MID LEVEL STREAMS BEFORE THE SYSTEM ARRIVES, A TROWAL, DIFFERENTIAL DIVERGENCE, LOWER LEVEL FRONTOGENESIS, POTENTIAL SLANTWISE CONVECTION OR THE AMOUNT OF AVAILABLE MOISTURE (AND THEREFORE SURFACE CYCLOGENESIS). SO FAR, THE TENDENCY HAS BEEN TO KEEP THE SYSTEM ON THE LESS ROBUST SIDE, BUT THAT MAY CHANGE. THE HIGHER RESOLUTION GFS SOUNDINGS DO CONTINUE TO SUGGEST ENOUGH WARM AIR ABOVE THE SURFACE TO CONTINUE TO MENTION SLEET OR FREEZING RAIN AS A POSSIBILITY IN AT LEAST PART OF THE FORECAST AREA WHERE SURFACE TEMPERATURES WOULD STAY BELOW THE MELTING LEVEL. THE SLOWER MODELS ALSO SUGGEST THERE WOULD BE SOME NASTY RADIATING ON THURSDAY NIGHT BEFORE CLOUDS MOVED IN. THIS FORECAST IS A SLIGHTLY COOLER TAKE ON CONTINUITY, BUT WE DIDN'T APPRECIABLY RAISE THE POPS. AFTER THE LATE WEEK SYSTEM PASSES, IT LOOKS AS THOUGH A MID LEVEL LONG WAVE TROUGH MOVES ACROSS THE EASTERN STATES AND OUT OVER THE WESTERN NORTH ATLANTIC DURING THE WEEKEND. SURFACE HIGH PRESSURE SHOULD BRING COLD AND MAINLY DRY WEATHER FOR SATURDAY AND SUNDAY. AGAIN, PERHAPS TEMPERATURES MODERATE BY MONDAY. && .AVIATION /21Z MONDAY THROUGH SATURDAY/... THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY AND SURROUNDING AREAS. REST OF THIS AFTERNOON...MOSTLY VFR AS THE CLOUDS GRADUALLY LOWER. THERE MAY HOWEVER BE SOME SCATTERING OUT OF THE CEILINGS ACROSS SOUTHEASTERN PENNSYLVANIA ON EASTWARD FOR A TIME INTO THIS EVENING. SOME SNOW SHOWERS DEVELOPING NEAR KABE AND KRDG LATER THIS AFTERNOON WITH LOCAL MVFR VISIBILITIES ESPECIALLY. SOUTH-SOUTHWEST WINDS 5 TO 10 KNOTS. A HEAVIER BURST OF SNOW TO THE WEST SHOULD ARRIVE TOWARD EVENING. TONIGHT...VFR TO A LOCAL PERIOD OF MVFR. SOME SNOW SHOWERS ARE ANTICIPATED TO MOVE ACROSS MOST TERMINALS THROUGH THE EVENING. THIS SHOULD RESULT IN REDUCED VISIBILITIES FOR A TIME. HOWEVER MAINLY FROM KRDG TO KTTN ON NORTHWARD, IFR VISIBILITIES MAY BRIEFLY OCCUR WITH ANY HEAVIER SNOW SHOWERS. NORTHWEST WINDS INCREASE TO 10 TO 20 KNOTS WITH GUSTS TO AROUND 25 KNOTS. TUESDAY...MAINLY VFR. NORTHWEST WINDS 15 TO 20 KNOTS WITH GUSTS UP TO 30 KNOTS. SOME FLURRIES OR SNOW SHOWERS, MAINLY NORTH AND WEST OF KABE. OUTLOOK... TUESDAY NIGHT THROUGH WEDNESDAY NIGHT...MAINLY VFR. ANY SNOW SHOWER ACTIVITY WOULD CAUSE A TRANSIENT DETERIORATION IN CONDITIONS. CONFIDENCE IS MODERATE. THURSDAY...VFR. CONFIDENCE IS MODERATE. THURSDAY NIGHT...MAINLY VFR. CONDITIONS COULD DETERIORATE LATE. CONFIDENCE IS LOW TO MODERATE. FRIDAY AND FRIDAY NIGHT...WINTRY PRECIPITATION IS POSSIBLE ALONG WITH MVFR/IFR CONDITIONS. CONFIDENCE IS LOW. SATURDAY...MAINLY VFR. CONFIDENCE IS LOW. && .MARINE... AN ARCTIC FRONT WILL SWEEP ACROSS THE AREA TONIGHT. AS THIS OCCURS, STRONG CAA OCCURS MAINLY FROM 00Z THROUGH 06Z AS 850 MB TEMPERATURES DROP TO NEAR -20C BY DAYBREAK TUESDAY. LOW PRESSURE WILL DEVELOP NEAR OUR COAST THEN TRACK NORTHEASTWARD OVERNIGHT AND TUESDAY WHILE STRENGTHENING. THE PRESSURE GRADIENT WILL TIGHTEN AS A RESULT, AND THIS COMBINED WITH STRONG CAA WILL DEEPEN THE VERTICAL MIXING. THE FORECAST SOUNDINGS INDICATE GALE FORCE GUSTS DEVELOPING RIGHT AROUND MIDNIGHT. AS A RESULT, WE WILL MAINTAIN THE GALE WARNING STARTING AT MIDNIGHT /05Z/ TONIGHT FOR ALL OF OUR WATERS. THERE SHOULD BE A PERIOD OF ADVISORY LEVELS WINDS PRIOR TO THE GALES HOWEVER THE RAMP UP SHOULD BE RATHER QUICK. THE GUSTY NORTHWEST WINDS WILL CONTINUE THROUGH TUESDAY. SOME SNOW/RAIN SHOWERS ARE ANTICIPATED WITH THE ARCTIC FRONT MAINLY DURING THE EVENING HOURS. AS THE ARCTIC AIR ARRIVES IN COMBINATION OF THE GALE FORCE GUSTS, CONDITIONS MAY BECOME FAVORABLE FOR SOME FREEZING SPRAY. FOR NOW, WE ADDED IN A CHC FOR THIS STARTING AROUND DAYBREAK TUESDAY. OUTLOOK... STRONG COLD ADVECTION FROM THE NORTHWEST IS ANTICIPATED FOR TUESDAY EVENING. GALE FORCE OR NEAR-GALE FORCE WIND GUSTS ARE LIKELY. A GALE WARNING CONTINUES UNTIL MIDNIGHT TUESDAY NIGHT. WE FORECAST A CHANCE OF FREEZING SPRAY TUESDAY NIGHT AND WEDNESDAY. HIGH PRESSURE FROM THE WEST SHOULD BUILD INTO OUR REGION GRADUALLY THROUGH THURSDAY. WIND GUSTS AROUND 25 KNOTS REMAIN POSSIBLE FROM LATE TUESDAY NIGHT INTO EARLY THURSDAY. WIND SPEEDS AND WAVE HEIGHTS ARE EXPECTED TO DIMINISH LATE THURSDAY AND REMAIN SUB-ADVISORY THURSDAY NIGHT. CONDITIONS MAY INCREASE TO SMALL CRAFT ADVISORY CRITERIA AGAIN ON FRIDAY AND REMAIN INTO SATURDAY ESPECIALLY ON OUR OCEAN WATERS AS LOW PRESSURE ARRIVES FROM THE SOUTHWEST. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...NONE. NJ...NONE. DE...NONE. MD...NONE. MARINE...GALE WARNING FROM MIDNIGHT TONIGHT TO MIDNIGHT EST TUESDAY NIGHT FOR ANZ430-431-450>455. && $$ SYNOPSIS...DELISI NEAR TERM...GORSE SHORT TERM...GORSE LONG TERM...DELISI/IOVINO AVIATION...GORSE/DELISI MARINE...GORSE/DELISI