294  
FXUS61 KPHI 250159  
AFDPHI  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ  
959 PM EDT FRI MAR 24 2017  
   
SYNOPSIS
 
 
A WARM FRONT OVER THE SOUTHERN TIER OF NEW YORK WILL BECOME  
NEARLY STATIONARY NEAR OUR REGION THRU TUESDAY, AS SEVERAL WAVES  
OF LOW PRESSURE MOVE ALONG IT. A COLD FRONTAL PASSAGE IS  
EXPECTED TUESDAY NIGHT, FOLLOWED BY HIGH PRESSURE FROM CANADA,  
WHICH WILL BUILD INTO OUR AREA WEDNESDAY THROUGH THURSDAY. A  
WARM FRONT MAY APPROACH OUR REGION BY FRIDAY.  
 

 
   
NEAR TERM /UNTIL 6 AM SATURDAY MORNING/
 
 
HIGH PRESSURE WAS ANCHORED OFF THE SOUTHEAST COAST THIS EVENING.  
STRONG WAA OCCURRED EARLIER TODAY, RESULTING A LATE DAY  
TEMPERATURE SPIKE. SWLY RETURN FLOW AROUND THE OFFSHORE HIGH  
WILL RESULT IN A MUCH MILDER NIGHT THAN IN RECENT NIGHTS.  
NONETHELESS, THERE WILL STILL BE A WIDE RANGE IN TEMPERATURES  
ACROSS THE FORECAST AREA AS SHELTERED VALLEYS ACROSS THE FAR  
NORTHWESTERN NJ AND NORTHEASTERN PA HAVE STARTED TO DECOUPLE  
TONIGHT. TEMPERATURES IN THESE COLDER SPOTS HAVE ALREADY DROPPED  
INTO THE UPPER 30S AND LOWER 40S. MEANWHILE, A 10 MPH S-SW WIND  
AT THE SURFACE HAS HELPED KEEP THE BOUNDARY LAYER MIXED ACROSS  
SOUTHEASTERN PA-CENTRAL NJ SOUTHWARD WHERE TEMPERATURES ARE  
STILL IN THE LOW TO MID 50S.  
 
BASED ON CURRENT TEMPERATURE/DEWPOINT OBSERVATIONS THIS  
EVENING, FOG IS FAVORED NORTH OF I-78, ESPECIALLY IN THE RIVER  
VALLEYS. FOG DEVELOPMENT IS FAVORED LATE TONIGHT AND INTO THE  
EARLY MORNING HOURS WHEN A BACKDOOR COLD FRONT PROGRESSES  
SOUTHWARD INTO OUR FAR NORTHERN ZONES.  
 

 
   
SHORT TERM /6 AM SATURDAY MORNING THROUGH 6 PM SATURDAY/
 
 
THE WARM FRONT THAT LIFTED THROUGH OUR REGION EARLIER TODAY, IS  
EXPECTED TO STALL AND THEN POSSIBLY RETREAT SLIGHTLY SOUTH, THOUGH  
STILL STAYING JUST NORTH OF OUR REGION. AS IT DOES SO HOWEVER, A  
WEAK LOW WILL PROPAGATE ALONG THE FRONT, BRINGING A CHANCE FOR  
PRECIPITATION ON BOTH SIDES OF THE FRONT. AS WITH TODAY, THE GFS  
SHOWS THE MOST EXTENSIVE QPF AS COMPARED TO THE OTHER MODELS. GIVEN  
HOW WEAK THE LOW IS THAT IS EXPECTED TOMORROW, THINK THIS IS  
OVERDONE.  
 
TEMPERATURE WISE, WITH OUR REGION FIRMLY IN THE WARM SECTOR WE  
SHOULD BE WELL ABOVE NORMAL WITH HIGHS RANGING FROM THE MID 50S TO  
LOWER 70S DESPITE PERSISTENT CLOUD COVER. ONE CAVEAT THOUGH IS THAT  
IF THE NORTHEASTERLY FLOW DEVELOPS EARLIER THAN FORECAST (AT THIS  
POINT WE DON'T EXPECT TO SEE THAT WIND SHIFT UNTIL LATER IN THE  
DAY), IT COULD TEMPER THE WARMING TREND.  
 

 
   
LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/
 
 
AN UNSETTLED PERIOD OF WEATHER IS IN STORE SUNDAY THRU TUESDAY.  
AT THE MID-LEVELS ACROSS THE CONUS, A GENERALLY ZONAL NORTHERN  
STREAM FLOW WILL BE IN PLACE, WHILE SEVERAL SHORTWAVE  
DISTURBANCES TRAVERSE THE SOUTHERN STREAM. MEANWHILE AT THE  
SURFACE, CANADIAN AND SUB-TROPICAL HIGH PRESSURE SYSTEMS WILL  
MAINTAIN A FRONTAL BOUNDARY IN THE VICINITY OF OUR REGION UNTIL  
A MORE SUBSTANTIAL FRONTAL PASSAGE IN THE TUESDAY NIGHT TIME  
FRAME. TEMPERATURE-WISE, AT OR ABOVE NORMAL THROUGH THE PERIOD  
 
THE PRIMARY UNCERTAINTY WILL BE THE LOCATION OF THE AFOREMENTIONED  
BOUNDARY, ALONG WITH THE TIMING OF SEVERAL AREAS OF LOW PRESSURE  
THAT RIDE ALONG IT, AND ACCOMPANYING AREAS OF PRECIPITATION.  
MOST NOTABLY, THE NAM AND GFS HAVE TRENDED STRONGER AND FURTHER  
SOUTH WITH THE BACKDOOR FRONT, WHICH BRINGS COLDER TEMPERATURES  
AND A MORE ROBUST EASTERLY FLOW TO OUR REGION FOR THE SECOND  
HALF OF THE WEEKEND. FOR THE MOST PART, THE MODELS REMAIN  
CONSISTENTLY TOO WARM FOR FREEZING RAIN SUNDAY NIGHT INTO MONDAY  
MORNING, BUT THE COLD AIR IS CLOSE ENOUGH THAT IT CANNOT BE  
TOTALLY DISCOUNTED. GIVEN THE TRENDS WITH RESPECT TO THE  
PLACEMENT OF THE FRONT WITHIN AN OVERRUNNING REGIME, THERE IS  
NOW AN INCREASING CHANCE OF FREEZING RAIN OR DRIZZLE SATURDAY  
NIGHT INTO SUNDAY MORNING.  
 
IN TERMS HYDROLOGY, GIVEN THE ABOVE NORMAL TEMPERATURES THROUGH  
THE LONG TERM, THE SNOW PACK NORTH OF I-78 WILL CONTINUE TO  
MELT. THE MODEL CONSENSUS BRINGS ADDITIONAL RAINFALL AMOUNTS OF  
ONE TO TWO INCHES ACROSS THIS AREA THROUGH WEDNESDAY, MOST OF  
THIS SPREAD OUT BETWEEN MORE SUBSTANTIAL PRECIP SUNDAY NIGHT-  
MONDAY AND TUESDAY NIGHT. UNLESS THERE ARE SIGNIFICANT CHANGES,  
WE EXPECT RISES ON AREA STREAMS AND RIVERS, BUT NO FLOODING  
CONCERNS AT THIS TIME.  
 
THERE IS ALSO THE POTENTIAL FOR SPOTTY MINOR COASTAL FLOODING  
ALONG THE OCEAN FRONT FOR THE LATE SUNDAY AND EARLY MONDAY HIGH  
TIDES.  
 
AS THE FRONTAL BOUNDARY MOVES SOUTHWARD IN RESPONSE TO CANADIAN HIGH  
PRESSURE BUILDING TO THE NORTH, SCATTERED SHOWERS ARE POSSIBLE  
ACROSS OUR ENTIRE AREA SATURDAY NIGHT INTO SUNDAY. MODEL  
SOUNDINGS DEPICT RICH LOW-LEVEL MOISTURE WITHIN THE POST-FRONTAL  
MARITIME AIR MASS, WHICH WILL BE CONDUCIVE TO LOW CLOUDS,  
DRIZZLE, AND PERHAPS PATCHY FOG. TWO METER TEMPERATURES MAY BE  
CLOSE TO FREEZING SATURDAY NIGHT INTO SUNDAY MORNING, MAINLY  
NEAR AND TO THE NORTH OF I-80, PER THE HIGHER RESOLUTION NAM,  
WRF-ARW, AND NMM. WHILE ANTECEDENT CONDITIONS ARE NOT THAT COLD,  
THERE IS SOME POTENTIAL FOR SHOWERS AND DRIZZLE TO RESULT IN A  
LIGHT ICE GLAZE ON ELEVATED SURFACES.  
 
THERE IS UNCERTAINTY WITH REGARD TO ICE GLAZE SATURDAY NIGHT  
INTO SUNDAY MORNING, BUT THE PROBABILITY HAS INCREASED, AND WE  
HAVE INCLUDED A MENTION IN THE HWO (PER OUR DIRECTIVES). THIS  
WILL NEED TO REASSESSED WITH FUTURE FORECAST PACKAGES, AS WE'RE  
LOOKING FOR CONTINUED RUN TO RUN MODEL CONSISTENCY. IF CONFIDENCE  
INCREASES, AN ICE MAP WILL BE POSTED TO OUR WEBSITE, AND  
FURTHER ADVISORY HEADLINES WILL BE CONSIDERED.  
 
AS LOW PRESSURE MOVES THROUGH THE OHIO VALLEY INTO THE GREAT LAKES  
FROM SUNDAY NIGHT INTO MONDAY, MORE SUBSTANTIAL OVERRUNNING  
PRECIP IS LIKELY DURING THIS TIME FRAME.  
 
AS MENTIONED PREVIOUSLY, THE MODELS HAVE EXHIBITED RUN-TO-RUN  
CONSISTENCY WITH REGARD TO THE THERMAL PROFILE SUNDAY NIGHT INTO  
MONDAY MORNING, WHICH DOES NOT FAVOR FREEZING RAIN. IN  
ADDITION, BOUNDARY LAYER WINDS ARE SUFFICIENTLY STRONG ENOUGH TO  
ADVECT WARM AIR NORTHWARD THROUGH THE LOW-LEVEL COLUMN, DESPITE  
SOMEWHAT OF AN IN-SITU COLD AIR DAMMING SIGNATURE. IN ADDITION,  
THE LACK OF LOW-LEVEL DRY AIR WILL LIMIT ANY EVAPORATIONAL  
COOLING. AGAIN, GIVEN THE PROXIMITY OF THE COLD AIR, WE CANNOT  
RULE OUT FREEZING RAIN, BUT THE PROBABILITY OF IT OCCURRING IS  
LOW.  
 
EXPECT SHOWERS TO CONTINUE MONDAY AFTERNOON INTO WEDNESDAY,  
WITH ANOTHER ROUND OF MORE SUBSTANTIAL PRECIP LIKELY CENTERED ON  
TUESDAY NIGHT, ASSOCIATED WITH A COLD FRONTAL PASSAGE. WE DO  
NOT ANTICIPATE ANY P-TYPE ISSUES DURING THIS PERIOD. THERE IS  
UNCERTAINTY WITH THE TIMING OF THIS FRONT, PARTICULARLY GIVEN  
THE TIME RANGE, WITH THE GFS BEING THE MOST PROGRESSIVE. AT THIS  
TIME, WE EXPECT A GRADUAL RETURN TO FAIR WEATHER ON WEDNESDAY,  
CONTINUING INTO THURSDAY. BY FRIDAY, A WARM FRONT MAY APPROACH  
THE REGION, ALONG WITH AN INCREASE IN CLOUDINESS AND A CHANCE OF  
SHOWERS.  
 

 
   
AVIATION /02Z SATURDAY THROUGH WEDNESDAY/
 
 
THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG,  
KILG, KMIV, KACY AND SURROUNDING AREAS.  
 
VFR CONDITIONS WILL CONTINUE THROUGH AT LEAST 06Z, AND WILL LIKELY  
CONTINUE THROUGH 18Z. A FEW MODELS DEPICT LOW STRATUS OR FOG  
DEVELOPING IN THE SOUTHERN POCONOS AND GETTING CLOSE TO KRDG AND  
KABE AFTER 09Z TONIGHT. CONFIDENCE IN FOG HAS INCREASED  
SLIGHTLY BUT ENOUGH TO ADD A TEMPO IFR VSBY GROUP IN THE 00Z  
TAF FOR RDG AND ABE.  
 
FORECAST SOUNDINGS SHOW MARGINAL LLWS TONIGHT WITH A SHARPENING  
NOCTURNAL INVERSION AND WIND SPEEDS INCREASING TO 35-45 KT AT OR  
ABOVE 1500 FT AGL.  
 
BETWEEN 18Z AND 00Z SATURDAY AFTERNOON/EVENING, CEILINGS AND  
VISIBILITY WILL START TO LOWER ACROSS THE REGION FROM N TO S AS  
RAIN SHOWERS MOVE IN.  
 
SOUTHWESTERLY WINDS 5-10 KT TONIGHT. AFTER 12Z, WINDS WILL  
GRADUALLY SHIFT FROM SOUTHWESTERLY TO WESTERLY THROUGH MIDDAY,  
BEFORE SHIFTING TO NORTHEASTERLY LATE IN THE DAY BEHIND A  
BACKDOOR COLD FRONT.  
 
OUTLOOK...  
 
SATURDAY NIGHT-TUESDAY...MVFR OR IFR CONDITIONS WITH A CHANCE  
OF RAIN POSSIBLE THROUGH THE PERIOD.  
 

 
   
MARINE
 
 
EXTENDED THE SCA FOR THE NJ COASTAL WATERS THRU 8 AM SATURDAY.  
THIS EXTENSION WAS MAINLY FOR SEAS BUT THERE IS STILL A  
DIMINISHING CHANCE FOR 25 KT GUSTS THRU ABOUT MIDNIGHT. GUIDANCE  
HAS BEEN UNDERDONE WITH WAVE HEIGHTS, WHICH ARE CURRENTLY 5-6 FT  
AT OUR OFFSHORE BUOYS. THESE SEAS SHOULD GRADUALLY SUBSIDE  
OVERNIGHT IN RESPONSE TO THE WEAKENING WINDS.  
 
ONCE WINDS AND SEAS DROP BELOW SCA CRITERIA, SUB-SCA CONDITIONS  
ARE EXPECTED THROUGH THE DAY TIME HOURS ON SATURDAY.  
 
OUTLOOK...  
 
SATURDAY-SATURDAY NIGHT...CONDITIONS EXPECTED TO FALL BELOW  
ADVISORY LEVELS.  
 
SUNDAY-SUNDAY NIGHT...SMALL CRAFT ADVISORY LEVELS POSSIBLE.  
 
MONDAY-TUESDAY...WINDS MAY BE BELOW ADVISORY LEVELS, BUT SEAS  
COULD REMAIN ELEVATED LEADING TO SMALL CRAFT ADVISORY LEVELS.  
 

 
   
PHI WATCHES/WARNINGS/ADVISORIES
 
 
PA...NONE.  
NJ...NONE.  
DE...NONE.  
MD...NONE.  
MARINE...SMALL CRAFT ADVISORY UNTIL 8 AM EDT SATURDAY FOR ANZ450>453.  
 

 
 

 
SYNOPSIS...FRANCK  
NEAR TERM...JOHNSON/KLEIN  
SHORT TERM...JOHNSON/KLEIN  
LONG TERM...FRANCK  
AVIATION...JOHNSON/KLEIN/ROBERTSON  
MARINE...JOHNSON/KLEIN/ROBERTSON  
 
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