Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000
FXUS61 KPHI 312117
AFDPHI

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ
417 PM 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 AM SUNDAY MORNING/...
THE WATER VAPOR IMAGERY SHOWS A STRONG UPPER-LEVEL LOW OFF THE NEW
ENGLAND COAST AND INTO THE CANADIAN MARITIMES, ASSOCIATED WITH A
NEGATIVELY TILTED TROUGH. ANOTHER UPPER-LEVEL LOW IS SLIDING
SOUTHEASTWARD ACROSS NORTHERN MEXICO, MEANWHILE A SHORT WAVE IS
DIVING SOUTHEASTWARD ACROSS THE NORTHERN ROCKIES. AT THE SURFACE,
STRONG LOW PRESSURE IS MOVING INTO THE CANADIAN MARITIMES AND HIGH
PRESSURE IS CENTERED ACROSS THE SOUTHEASTERN STATES BUT EXTENDS
NORTHWARD ACROSS THE EASTERN GREAT LAKES.

AS THE UPPER-LEVEL TROUGH ACROSS THE EAST RELAXES, HIGH PRESSURE
WILL MOVE EASTWARD AND SETTLE OVER OUR AREA LATER TONIGHT. THIS WILL
ALLOW THE PRESSURE GRADIENT TO LESSEN AND THE WINDS ARE EXPECTED TO
DIMINISH QUICKLY THIS EVENING. WE SHOULD DECOUPLE IN MANY PLACES,
ESPECIALLY THOSE AREAS THAT STILL HAVE SNOW COVER. A COLD FRONT
THOUGH WILL BE DROPPING TOWARD OUR AREA LATE TONIGHT FROM THE
NORTHWEST. THIS WILL SET THE STAGE FOR A DEVELOPING THERMAL GRADIENT
ALOFT THAT BECOMES ORIENTATED FROM WEST TO EAST. THE DEVELOPMENT OF
WAA UP THE OHIO VALLEY WILL ALSO INCREASE THE CLOUDS SOME MAINLY
LATE TONIGHT ESPECIALLY ACROSS OUR WESTERN ZONES.

AS FOR THE LOW TEMPERATURES, WE USED A BLEND OF THE NAM/GFS MOS BUT
THEN ADJUSTED THIS DOWN A BIT ACROSS THE TYPICALLY COLDER LOCALES
AND ALSO WHERE SNOW COVER REMAINS.

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH 6 PM SUNDAY/...
A SHORT WAVE SLIDING EAST-SOUTHEASTWARD FROM THE MIDWEST TO
MID-MISSISSIPPI VALLEY AREA WILL SHARPEN AN UPPER-LEVEL TROUGH. IT
IS THIS FEATURE THAT HAS BEEN MOSTLY DRIVING THE CHANGES SEEN IN THE
MODEL GUIDANCE, BUT THE TREND HAS BEEN FOR A SHARPER TROUGH AND
THEREFORE A NORTHWARD TRACK OF THE SURFACE LOW.

THERE WILL BE DOWNSTREAM WAA OCCURRING ALOFT ALTHOUGH THE SURFACE
FLOW WILL INITIALLY BECOME SOUTHERLY. THE EXISTING SNOW COVER MAY
SLOW THE BOUNDARY LAYER WARMING TO SOME EXTENT SUNDAY, HOWEVER AN
INCREASE IN CLOUDS SHOULD ALSO SLOW THE WARMING. THE MODEL GUIDANCE
IS IN PRETTY GOOD AGREEMENT IN QUICKLY DEVELOPING ISENTROPIC LIFT UP
AND OVER THE DEVELOPING 850 MB THERMAL GRADIENT. THIS SHOULD RESULT
IN SOME LIGHT SNOW EXPANDING EASTWARD WITH TIME, AND THIS SHOULD BE
OVERSPREADING AREAS MAINLY WEST OF INTERSTATE 95 BY LATE AFTERNOON.
THE LOW LEVELS SHOULD INITIALLY BE RATHER DRY, THEREFORE WE DID NOT
INCREASE THE POPS ALL THAT FAST EASTWARD DURING THE DAY. IT APPEARS
THAT ANY SNOW ACCUMULATIONS BY EVENING WILL BE LIGHT. THE WARMING
ALOFT WILL PROBABLY RESULT IN SOME RAIN/SLEET ACROSS OUR SOUTHERN
ZONES, IF PRECIPITATION OCCURS THERE BY LATE AFTERNOON.

AS FOR THE HIGH TEMPERATURES, WE USED A BLEND OF THE NAM/GFS MOS
OVERALL. WE ARE EXPECTING A MILDER DAY COMPARED TO TODAY.

&&

.LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/...
MOST OF THE PREVIOUS WATCH HAS BEEN CONVERTED TO A WARNING, MOSTLY
FOR SNOW, WHILE WE HAVE EXPANDED THE WATCH FARTHER SOUTH TO COVER
THE UNCERTAINTY INVOLVED WITH THE PROGRESSION OF THE RAIN/SNOW LINE.
THIS IS STILL A HIGHLY UNCERTAIN FORECAST SOUTH OF THE LEHIGH
VALLEY, EVEN WITHIN 36 HOURS OF THE START.

SUNDAY EVENING - MONDAY...VERY COMPLEX SYSTEM AND FORECAST TAKING
SHAPE ACROSS OUR REGION LATE SUNDAY EVENING THROUGH MONDAY
AFTERNOON. NOTICED THAT THE GFS/EC, WHEN COMPARED
TO THE NAM INITIALIZED JUST A TAD STRONGER WITH THE MAIN SHORTWAVE
ACROSS THE PAC-NW. THE STRONGER SCENARIO THESE MODELS SHOW, AS THE
SHORTWAVE CONTINUES TO AMPLIFY HEADING SOUTHWARD, CAUSES THE SURFACE
INFLECTION TO BE DISPLACED FARTHER NORTH AS IT APPROACHES OUR
REGION. THIS HAS BEEN NOTED OVER THE PAST COUPLE OF DAYS WITH A
CONTINUED NORTHWARD TRACK OF THE SURFACE LOW...NOT MUCH BLOCKING TO
SUPPRESS THE LOW FARTHER SOUTH.

FOR OUR SENSIBLE WEATHER SUNDAY NIGHT INTO MONDAY. FRONT END THETA-E
AND WAA AHEAD OF THE APPROACHING LOW SHOULD OVERSPREAD LIGHT SNOW
ACROSS MOST OF THE REGION NORTH OF ILG. THE BEST MESOSCALE
LIFT/BANDING POTENTIAL LOOKS TO OCCUR BETWEEN 6-12Z ACROSS THE
NORTHERN ZONES. AT THIS POINT THE BETTER LIFT IN THE DGZ SHOULD BE
CONFINED TO NORTH OF TTN-RDG LINE...BEFORE THE WARM SOUTHERLY AIR
REALLY GETS CRANKING AND WARMS THE COLUMN ENOUGH. THERE IS A DECENT
CHANCE THAT THE POCONOS STAY ALL SNOW THROUGH THE DURATION OF THE
EVENT WITH SOME MIXING AS FAR NORTH AS THE LEHIGH VALLEY. +3 TO +5
850MB TEMPERATURES INDICATE A LACK OF SLEET POTENTIAL EARLY MONDAY
MORNING SOUTH OF A ABE-SMQ LINE WITH THE PHILADELPHIA METRO AREA
SOUTHWARD ALL RAIN. SURFACE TEMPERATURES TOO ARE VERY TRICKY IN THIS
SETUP AS MOST OF THE GUIDANCE WANTS TO BLAST THE SURFACE
TEMPERATURES ABOVE FREEZING BY EARLY MONDAY MORNING. CURRENT
THINKING IS THAT THEY ARE NOT ACCOUNTING FOR THE AMOUNT OF SNOWPACK
CURRENTLY IN PLACE ACROSS SOUTHEAST PENNSYLVANIA AND CENTRAL NEW
JERSEY. THIS WAS THE MAIN REASON THAT WE EXPANDED THE
WATCH...COMBINATION OF FRONT-END SNOW FOLLOWED BY A PERIOD OF
FREEZING RAIN. WE CAN ALWAYS GO TO AN ADVISORY IF NEEDED BUT DID NOT
WANT TO JUMP THE GUN IN CASE THE ICE SIGNAL BECOMES EVEN STRONGER.

BY LATER MONDAY AFTERNOON THE SYSTEM SHOULD BE QUICKLY PULLING TO
OUR NORTHEAST ALLOWING THE COLUMN TO COOL WEST TO EAST WITH ANOTHER
CHANGEOVER BACK TO SNOW SHOWERS.

TUESDAY - 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. SOME MIXING ON TUESDAY MORNING COULD
ALLOW FOR SLIGHTLY WARMER OVERNIGHT LOWS BEFORE THE HIGH PRESSURE
SETTLES IN ON TUESDAY.

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 /21Z SATURDAY THROUGH THURSDAY/...
THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY AND SURROUNDING AREAS.

REST OF THIS AFTERNOON...VFR. NORTHWEST WINDS 10-20 KNOTS WITH GUSTS
TO 25 KNOTS, DIMINISHING TOWARD EVENING.

TONIGHT...VFR, WITH SOME INCREASE IN MID TO HIGH LEVEL CLOUDS LATE.
WEST-NORTHWEST WINDS QUICKLY DIMINISHING TO AROUND 5 KNOTS IN THE
EVENING, THEN LOCALLY BECOMING SOUTHWESTERLY.

SUNDAY...VFR TO START WITH INCREASING CLOUDS. THE CEILINGS MAY LOWER
TO MVFR AT KABE AND KRDG TOWARD 18Z. SOME LIGHT SNOW IS POSSIBLE IN
THE AFTERNOON WEST OF THE KPHL METRO. THE TIMING OF THIS IS OF LOWER
CONFIDENCE AT THIS TIME, THEREFORE IT WAS NOT INCLUDED. LIGHT
SOUTHWEST WINDS MAY BECOMING LIGHT AND VARIABLE AT MOST TERMINALS,
THEN TURN SOUTHEAST BY EVENING.

OUTLOOK...
SUNDAY NIGHT - MONDAY...WIDESPREAD MVFR/IFR IN AREAS OF
RAIN/SNOW/SLEET. EXPECTING MOSTLY SNOW AT RDG/ABE WITH SNOW TURNING
TO RAIN/SLEET AT PHL/ILG/TTN/PNE...RAIN AT MIV/ACY BY MONDAY
MORNING. COULD BE A PERIOD OF FREEZING RAIN IN SOUTHEAST
PENNSYLVANIA INTO CENTRAL NEW JERSEY MONDAY MORNING. LINGERING
MOISTURE MONDAY AFTERNOON SHOULD TURN INTO SNOW SHOWERS.

TUESDAY - WEDNESDAY...VFR. GUSTY NORTHWEST WINDS TUESDAY DECREASE ON
WEDNESDAY. WINDS TURN MORE TOWARDS THE SOUTHWEST BY WEDNESDAY AHEAD
OF THE NEXT POTENTIAL SYSTEM.

&&

.MARINE...
HIGH PRESSURE WILL BUILD INTO THE AREA THROUGH TONIGHT, THEN TO OUR
EAST SUNDAY. THIS WILL ALLOW THE PRESSURE GRADIENT TO RELAX AND THE
WINDS TO DIMINISH. THE DOWNWARD TREND IN THE WINDS CONTINUES TO
OCCUR, AND THEREFORE WE WILL REPLACE THE GALE WARNING WITH A SMALL
CRAFT ADVISORY. THIS WILL RUN THROUGH 03Z ON DELAWARE BAY AND THEN
05Z ON THE COASTAL WATERS AS SEAS WILL TAKE A LITTLE LONGER TO
SUBSIDE. THE AVAILABLE BUOY DATA INDICATES THAT WHILE FREEZING SPRAY
IS OCCURRING THIS AFTERNOON, THE RATE OF ACCRETION HAS REALLY
DROPPED OFF. THEREFORE, WE WILL CANCEL THE FREEZING SPRAY ADVISORY.
THE WINDS WILL BE LIGHTER SUNDAY AND TURN MORE SOUTHERLY.

OUTLOOK...
SUNDAY NIGHT: SUB SCA SEAS AND WINDS WITH SEAS BUILDING TOWARD
SUNRISE MONDAY.

MONDAY THROUGH TUESDAY: SCA SEAS THROUGH TUESDAY MORNING AND
POSSIBLE NORTHWEST GALE GUSTS LATE MONDAY AFTERNOON AND NIGHT.

TUESDAY NIGHT AND WEDNESDAY: SUB SCA SEAS AND WINDS.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...WINTER STORM WARNING FROM 7 PM SUNDAY TO 1 PM EST MONDAY FOR
     PAZ054-055-060>062.
     WINTER STORM WATCH FROM SUNDAY EVENING THROUGH MONDAY
     AFTERNOON FOR PAZ101-103-105.
NJ...WINTER STORM WARNING FROM 7 PM SUNDAY TO 1 PM EST MONDAY FOR
     NJZ001-007-008.
     WINTER STORM WATCH FROM SUNDAY EVENING THROUGH MONDAY
     AFTERNOON FOR NJZ009-010-012.
DE...NONE.
MD...NONE.
MARINE...SMALL CRAFT ADVISORY UNTIL MIDNIGHT EST TONIGHT FOR ANZ450>455.
     SMALL CRAFT ADVISORY UNTIL 10 PM EST THIS EVENING FOR ANZ430-
     431.

&&

$$
SYNOPSIS...GAINES/GORSE
NEAR TERM...GORSE
SHORT TERM...GORSE
LONG TERM...GAINES/HEAVENER
AVIATION...GAINES/GORSE/HEAVENER
MARINE...GAINES/GORSE/HEAVENER





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