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Area Forecast Discussion
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000
FXUS61 KPHI 312012
AFDPHI

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ
312 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/...
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 /20Z 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 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...
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 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...GORSE
LONG TERM...GAINES
AVIATION...GAINES/GORSE
MARINE...GAINES/GORSE




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