164  
FXUS61 KPHI 141256  
AFDPHI  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ  
756 AM EST THU DEC 14 2017  
   
SYNOPSIS
 
 
LOW PRESSURE WILL MOVE QUICKLY TO THE NORTH AND EAST OF THE  
AREA THIS MORNING. HIGH PRESSURE WILL BUILD IN FROM THE WEST FOR  
THE REST OF TODAY THROUGH FRIDAY. A LOW PRESSURE SYSTEM WILL  
MOVE UP THE MID ATLANTIC COAST ON FRIDAY, REMAINING WELL  
OFFSHORE. HIGH PRESSURE THEN RETURNS FOR THE WEEKEND. A WEAK LOW  
PRESSURE SYSTEM WILL MOVE THROUGH SUNDAY NIGHT INTO MONDAY WITH  
ANOTHER SYSTEM EXPECTED AROUND MIDWEEK.  
 

 
   
NEAR TERM /UNTIL 6 PM THIS EVENING/
 
 
745 AM UPDATE: THE REMAINDER OF THE WINTER WEATHER ADVISORY HAS  
BEEN CANCELLED WITH SNOW MOVING OUT OF NORTHEASTERN PA AND  
NORTHERN NJ. THERE ARE ADDITIONAL SNOW SHOWERS FARTHER SOUTH  
ACROSS DE AND SOUTHERN NJ BUT WITH PRECIP MOVING QUICKLY TO THE  
EAST AND WITH TEMPERATURES ABOVE FREEZING, DO NOT EXPECT MUCH  
IN THE WAY OF SNOWFALL ACCUMULATIONS AND IMPACTS ON TRAVEL. THE  
FORECAST (POPS AND WX) HAS BEEN ADJUSTED ACCORDINGLY.  
 
PREVIOUS DISCUSSION...  
 
A DECENTLY STRONG CLIPPER LOW IS MOVING THROUGH THE NORTHERN MID-  
ATLANTIC THIS MORNING, WITH MOST OF THE AREA RECEIVING A DUSTING TO  
AROUND AN INCH AND A HALF OF SNOW WITH WARM-AIR ADVECTION GENERATED  
PRECIPITATION THAT MOVED THROUGH THE AREA EARLIER. LINGERING LIGHT  
SNOW PERSISTS NORTH OF I-78 AS THE SURFACE LOW REMAINS TO THE WEST.  
HOWEVER, IT WILL QUICKLY REDEVELOP OFFSHORE AS SYNOPTIC-SCALE LIFT  
QUICKLY INCREASES IN FAVORABLE LEFT-FRONT QUADRANT JET DYNAMICS  
COMBINED WITH DIFFERENTIAL CYCLONIC VORTICITY ADVECTION FROM THE  
ATTENDANT MIDLEVEL VORT MAX. THIS MEANS THAT THE ZONALLY-ORIENTED  
BAND OF SNOW ACROSS MOST OF PENNSYLVANIA SHOULD GENERALLY DIMINISH  
WITH TIME WHILE RAIN/SNOW DEVELOP OFFSHORE DURING THE NEXT FEW  
HOURS. HOWEVER, AS THE SURFACE LOW TO THE WEST PROPAGATES CLOSER TO  
THE WESTERN CWA, THERE SHOULD BE A FINAL PUSH OF SNOW ACROSS  
NORTHERN PORTIONS OF THE CWA, WHERE UP TO ANOTHER INCH OR SO IS  
POSSIBLE THROUGH THE MORNING RUSH HOUR.  
 
AS SUCH, THE WINTER WEATHER ADVISORY WAS NOT CHANGED WITH THIS  
UPDATE, THOUGH SNOW TOTALS THEMSELVES WILL LIKELY BE BELOW CRITERIA  
(AT LEAST IN AN AREAL AVERAGE SENSE) WITH THIS EVENT. I MAY CHIP  
AWAY AT THE ADVISORY FROM SOUTH TO NORTH DURING THE NEXT FEW HOURS  
SHOULD THE SNOW BE AS FAR NORTH AS THE LATEST HRRR SIMULATIONS  
SUGGEST (VERSUS THE NAM). FOR CONTEXT, THE PRECIPITATION AS  
SIMULATED YESTERDAY WAS UNDERDONE AND TOO FAR NORTH IN THE WAA  
REGIME (BY ABOUT 100 MILES), SO I DO NOT HAVE MUCH CONFIDENCE IN THE  
PLACEMENT OF THE FINAL PUSH OF SNOW THIS MORNING. WITH SUBFREEZING  
CONDITIONS CONTINUING, EXPECT SLICK ROADS FOR THE MORNING COMMUTE,  
ESPECIALLY THOSE THAT REMAIN UNTREATED.  
 
TO THE SOUTH, THE DRY SLOT HAS EFFECTIVELY ENDED THE PRECIPITATION  
(MAINLY ALONG/SOUTH OF I-76). MODELS ARE MUCH TOO COLD IN THIS AREA,  
AND HOURLY TEMPERATURES HAVE BEEN A REAL STRUGGLE AS A RESULT.  
STATISTICAL GUIDANCE HAS TRENDED WARMER TODAY, BUT I SUSPECT THIS IS  
TOO AGGRESSIVE WHERE ACCUMULATING SNOW HAS FALLEN. NEVERTHELESS,  
GIVEN THE COLD BIAS IN THE SOUTHERN AREAS THIS MORNING, I INCHED UP  
TEMPERATURES TODAY IN GENERAL.  
 
ONCE THE LOW PASSES THIS MORNING, WEST WINDS SHOULD PICK UP AND  
BECOME GUSTY AGAIN, ESPECIALLY THIS AFTERNOON. GUSTS SHOULD BE MUCH  
LOWER TODAY VERSUS YESTERDAY, AND WITH WARMER TEMPERATURES, IT WILL  
SIMPLY BE UNCOMFORTABLE VERSUS BONE-CHILLING.  
 

 
   
SHORT TERM /6 PM THIS EVENING THROUGH 6 AM FRIDAY/
 
 
HIGH PRESSURE MOVES INTO THE MID-ATLANTIC TONIGHT, AND THIS WILL  
CAUSE THE WINDS TO SUBSIDE AFTER DARK. MOSTLY CLEAR SKIES TO START  
THE NIGHT WILL SET THE STAGE FOR A SHARP DROP IN TEMPERATURES DURING  
THE EVENING, LIKELY EXACERBATED IN AREAS WHERE REMAINING SNOW EXISTS  
(I.E., ESPECIALLY IN THE NORTH). GENERALLY WENT BELOW GUIDANCE FOR  
LOWS, THOUGH WITH INCREASING CLOUDINESS LIKELY TO OCCUR LATE, THERE  
IS SOME UNCERTAINTY HERE. THE INCREASING CLOUD COVER WILL BE AIDED  
BY A 250-MB JET STREAK ENCOMPASSING MUCH OF THE EASTERN U.S.  
DOWNSTREAM OF ANOTHER MIDLEVEL VORT MAX ENTERING THE GREAT LAKES  
REGION. THE MAIN QUESTION IS TIMING, WITH FASTER APPROACH OF THE  
CLOUDS LIKELY RESULTING IN WARMER LOWS. CURRENT FORECASTS ARE FOR AN  
INCREASE RAPIDLY AFTER MIDNIGHT, WHICH SHOULD BE AMPLE TIME FOR  
TEMPERATURES TO PLUMMET IN THE EVENING.  
 
SPEAKING OF LOWS, THEY ARE FORECAST TO RANGE FROM SINGLE DIGITS IN  
THE POCONOS TO AROUND 20 IN THE PHILLY METRO AREA. BUNDLE UP.  
 

 
   
LONG TERM /FRIDAY THROUGH WEDNESDAY/
 
 
FRIDAY THROUGH FRIDAY NIGHT...HIGH PRESSURE WILL MAKE ITS WAY  
INTO THE AREA OVERNIGHT AND CROSS THE REGION ON FRIDAY. A  
COASTAL LOW DEVELOPS ALONG THE CAROLINA COAST AND MAKES ITS WAY  
UP THE MID ATLANTIC COAST. THE MODELS CONTINUE TO SHOW THIS  
SYSTEM REMAINING OFFSHORE AND OUT OF OUR AREA. HOWEVER, SOME  
MOISTURE MAY REACH INTO OUR AREA AND WE COULD SEE SOME SNOW/RAIN  
FALL TOWARDS THE AFTERNOON AND INTO THE EVENING. ADDITIONALLY,  
A COUPLE OF SHORTWAVES WILL ROTATE THROUGH THE AREA ON FRIDAY  
WITH A ANOTHER CLIPPER SYSTEM STAYING TO THE NORTH OF THE AREA.  
GUIDANCE SHOWS THE MOISTURE FROM THE CLIPPER DRYING OUT BEFORE  
IT REACHES THE REGION BUT WITH A STRONG NORTHWEST FLOW, WE COULD  
SEE SOME STREAMERS MAKE IT FROM THE LAKES TO OUR AREA.  
 
SATURDAY THROUGH SUNDAY MORNING...HIGH PRESSURE SLIDES ACROSS  
THE EASTERN UNITED STATES SATURDAY, BRINGING SOME BEAUTIFUL LATE  
FALL WEATHER. THE WEATHER SHOULD REMAIN QUIET THROUGH AT LEAST  
EARLY SUNDAY BEFORE THE NEXT SYSTEM ARRIVES. SATURDAY MAY BE A  
BIT WINDY, ESPECIALLY ALONG THE COAST. WARMER AIR FILTERS IN  
FROM THE SOUTHWEST BUT WE WON'T REALLY START TO FEEL THE  
DIFFERENCE UNTIL SUNDAY. TEMPERATURES ARE EXPECTED TO BE IN THE  
30S TO LOWER 40S ON SATURDAY AND IN THE UPPER 30S TO UPPER 40S  
ON SUNDAY.  
 
SUNDAY AFTERNOON THROUGH MONDAY...A WEAK LOW PRESSURE SYSTEM  
WILL APPROACH THE REGION SOMETIME AROUND SUNDAY NIGHT INTO  
MONDAY. WHILE THE MODELS ARE NOT TOO ROBUST WITH MOISTURE, IT  
SEEMS MORE LIKELY UNDERDONE AND WE EXPECT A PERIOD OF RAIN OR  
MIXED PRECIPITATION THROUGH THE REGION. HOWEVER, WITH  
DIFFERENCES IN TIMING AND STRENGTH OF THE LOW, CONFIDENCE  
REMAINS LOW.  
 
TUESDAY THROUGH WEDNESDAY...THE MODELS DIVERGE WITH HOW TO  
HANDLE A STRONG LOW PRESSURE SYSTEM TO OUR NORTH AND THE  
ATTENDANT COLD FRONT. INDICATIONS ARE WE WILL SEE SOME  
PRECIPITATION OCCUR SOMETIME IN THE TUESDAY/WEDNESDAY TIMEFRAME  
BUT CONFIDENCE IS LOW WITH RESPECT TO COVERAGE AND TIMING.  
 

 
   
AVIATION /13Z THURSDAY THROUGH MONDAY/
 
 
THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG,  
KMIV, KACY AND SURROUNDING AREAS.  
 
AT 1245Z, CONDITIONS ARE IMPROVING TO VFR WITH THE SN ALREADY  
EXITING THE AREA. HOWEVER, RDG AND ABE ARE STILL REPORTED MVFR  
CIGS IN SN (THOUGH IT IS MORE LIKELY BLSN). W-NW WINDS WILL  
INCREASE LATER THIS MORNING, WHICH COULD POTENTIALLY LEAD TO  
BLSN AND LOCALIZED REDUCED VISIBILITY TO MVFR IN SPOTS WHERE  
FRESH SNOW FELL EARLY THIS MORNING.  
 
GUSTY WEST OR NORTHWEST WINDS WILL OCCUR FROM 15Z TO 22Z FOR  
MOST OF THE AREA (SPEEDS 10 TO 15 KTS WITH GUSTS TO 20 KTS OR  
SO). HIGH CONFIDENCE.  
 
THURSDAY NIGHT...VFR WITH NORTHWEST WINDS AROUND OR BELOW 10 KTS.  
INCREASING MID AND HIGH CLOUDS LATE. HIGH CONFIDENCE.  
 
OUTLOOK...  
 
FRIDAY...MAINLY VFR CONDITIONS EXPECTED. MVFR OR LOWER POSSIBLE  
IN SNOW SHOWERS FRIDAY AFTERNOON AND EVENING. LIGHT WINDS.  
CONFIDENCE: MODERATE  
 
SATURDAY...MAINLY VFR CONDITIONS EXPECTED. WEST WINDS AROUND 10  
TO 15 KNOTS POSSIBLE. GUSTS UP TO 20 KNOTS POSSIBLE, MAINLY AT  
KACY AND KMIV. CONFIDENCE: MODERATE  
 
SUNDAY THROUGH MONDAY...MAINLY VFR CONDITIONS EXPECTED. MVFR  
CONDITIONS IN SNOW/RAIN SHOWERS POSSIBLE SUNDAY AFTERNOON  
THROUGH MONDAY. SOUTHWEST WINDS AROUND 5 TO 10 KNOTS.  
CONFIDENCE: LOW  
 

 
   
MARINE
 
 
630 AM UPDATE: GALE WARNING CONTINUES FOR THE DELAWARE COASTAL  
WATERS, WITH MODELS INDICATING PEAK WINDS BETWEEN NOW AND 10 AM.  
LATEST MODEL SIMULATIONS SHOW CONTINUED ADVISORY-LEVEL GUSTS FOR  
THE NEW JERSEY COASTAL WATERS UNTIL AROUND 10 PM TONIGHT.  
EXTENDED THE ADVISORY HERE, AND WILL REPLACE THE GALE WARNING  
WITH A SMALL CRAFT ADVISORY FOR THE DELAWARE WATERS ONCE THE  
GALES COME DOWN.  
 
PREVIOUS DISCUSSION...  
 
GALES HAVE DEVELOPED AT BUOY 44009 THIS MORNING, SO I HAVE  
ISSUED A SHORT-FUSED GALE WARNING FOR THE DELAWARE COASTAL  
WATERS THROUGH NOON TODAY. ELSEWHERE, STRONG SMALL-CRAFT  
ADVISORY CONDITIONS WILL EXIST THIS MORNING BEFORE A SLOW  
DIMINISHING TREND OCCURS THIS AFTERNOON. ALTHOUGH WINDS MAY  
LINGER NEAR/SLIGHTLY ABOVE ADVISORY CRITERIA EARLY THIS EVENING,  
NOT CONFIDENT ENOUGH TO EXTEND THE ADVISORY AT THIS TIME.  
 
SEAS SHOULD REMAIN ELEVATED THROUGH THE DAY, BUT AS NORTHWEST  
FLOW DEVELOPS THIS AFTERNOON, THERE SHOULD BE A DIMINISHING  
TREND THIS EVENING.  
 
LIGHT SNOW SHOULD CONTINUE ACROSS THE NEW JERSEY COASTAL WATERS  
THIS MORNING, WITH VISIBILITY RESTRICTIONS LIKELY. FAIR WEATHER  
SHOULD SET IN THIS AFTERNOON AND TONIGHT.  
 
OUTLOOK...  
 
FRIDAY...SUB-ADVISORY CONDITIONS ARE EXPECTED.  
 
FRIDAY NIGHT THROUGH SATURDAY NIGHT...SMALL CRAFT ADVISORY  
CONDITIONS POSSIBLE. WEST TO NORTHWEST WINDS WITH GUSTS AROUND  
25 TO 30 KNOTS. GALE FORCE GUSTS POSSIBLE BUT CONFIDENCE IS LOW  
AT THIS TIME. SEAS WILL INCREASE AND NEAR 5 FEET ON SATURDAY.  
 
SUNDAY THROUGH MONDAY...SUB-ADVISORY CONDITIONS EXPECTED.  
 

 
   
PHI WATCHES/WARNINGS/ADVISORIES
 
 
PA...NONE.  
NJ...NONE.  
DE...NONE.  
MD...NONE.  
MARINE...SMALL CRAFT ADVISORY UNTIL 10 PM EST THIS EVENING FOR  
ANZ450>453.  
GALE WARNING UNTIL NOON EST TODAY FOR ANZ454-455.  
SMALL CRAFT ADVISORY UNTIL 6 PM EST THIS EVENING FOR ANZ430-  
431.  
 

 
 

 
SYNOPSIS...MEOLA  
NEAR TERM...CMS/KLEIN  
SHORT TERM...CMS  
LONG TERM...MEOLA  
AVIATION...CMS/MEOLA/KLEIN  
MARINE...CMS/MEOLA  
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab PA Page
The Nexlab NJ Page
The Nexlab DE Page
The Nexlab MD Page Main Text Page