145  
FXUS61 KPHI 252226  
AFDPHI  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ  
626 PM EDT SUN JUN 25 2017  
   
SYNOPSIS
 
 
A LARGE AREA OF HIGH PRESSURE WILL SLOWLY MOVE THROUGH THE EASTERN  
U.S. EARLY THIS WEEK BEFORE MOVING OFFSHORE WEDNESDAY AND THURSDAY.  
A SERIES OF SURFACE LOWS WILL MOVE FROM THE NORTHERN AND CENTRAL  
PLAINS TO THE GREAT LAKES AND NORTHEAST FROM MID TO LATE WEEK,  
USHERING IN WARMER AND MORE HUMID AIR LATE THIS WEEK AND THE  
UPCOMING WEEKEND.  
 

 
   
NEAR TERM /UNTIL 6 AM MONDAY MORNING/
 
 
THE MID AND UPPER LEVEL SHORTWAVE TROUGH NOW OVER THE EASTERN  
GREAT LAKES REGION WILL LIFT NORTHEAST THROUGH THE EVENING AND  
EARLY OVERNIGHT HOURS. A FEW ADDITIONAL SHOWERS ARE POSSIBLE  
ACROSS PORTIONS OF THE CWA THROUGH THE EARLY EVENING HOURS.  
HOWEVER, AFTER 00Z, THE DIURNALLY DRIVEN ACTIVITY WILL  
DISSIPATE. THE LATEST RUN OF THE HRRR INDICATES THAT EVEN THE  
MORE WIDESPREAD SHOWER ACTIVITY ASSOCIATED WITH THE MAIN TROUGH  
AXIS NOW IN WESTERN PENNSYLVANIA WILL ALSO DISSIPATE BY  
MIDNIGHT.  
 
TEMPERATURE WISE, AS THE DRIER AIR CONTINUES TO FILTER INTO OUR  
AREA, EXPECT EFFICIENT RADIATIONAL COOLING TO RESULT IN LOWS  
SLIGHTLY BELOW NORMAL, RANGING FROM THE LOWER 50S TO MID 60S.  
 

 
   
SHORT TERM /6 AM MONDAY MORNING THROUGH 6 PM MONDAY/
 
 
MAIN STORY THROUGH THIS PERIOD IS A COLD FRONT AT THE SURFACE WHICH  
WILL APPROACH OUR REGION LATE IN THE DAY. THERE MAY BE ENOUGH LIFT  
WITH THIS TO ONCE AGAIN RESULT IN ISOLATED CONVECTIVE INITIATION  
ACROSS THE HIGHER TERRAIN OF EASTERN PA AND NW NJ.  
 
IN THE MID AND UPPER LEVELS, AN APPROACHING TROUGH WILL RESULT IN  
DECREASING 1000 TO 500 MB THICKNESSES WHICH SHOULD ALSO TRANSLATE TO  
SLIGHTLY LOWER TEMPERATURES THAN WHAT WE HAVE SEEN LATELY. HIGHS ARE  
EXPECTED TO RANGE FROM THE UPPER 60S IN THE HIGHER TERRAIN OF THE  
POCONOS TO THE LOWER 80S ACROSS CENTRAL DELMARVA.  
 

 
   
LONG TERM /MONDAY NIGHT THROUGH SUNDAY/
 
 
MAIN FORECAST CHALLENGES FOR THE LONG TERM INCLUDE A DEEP TROUGH  
PASSAGE AROUND MIDWEEK AND INCREASING WARMTH AND PRECIPITATION  
CHANCES LATE WEEK INTO THIS WEEKEND.  
 
A POTENT SHORTWAVE TROUGH IS POISED TO MOVE THROUGH THE LARGER-  
SCALE TROUGH IN EASTERN NORTH AMERICA EARLY THIS WEEK, WITH  
PASSAGE THROUGH THE NORTHEASTERN U.S. IN THE TUESDAY-WEDNESDAY  
TIME FRAME. LARGE-SCALE ASCENT IN ADVANCE OF THE ATTENDANT  
VORTICITY MAXIMUM WILL ALLOW FOR INCREASED CLOUD COVER ON  
TUESDAY AND FAIRLY COOL TEMPERATURES FOR THIS TIME OF YEAR.  
FORECAST HIGHS ARE AROUND 5-10 DEGREES BELOW SEASONAL AVERAGES.  
 
FORECAST CONFIDENCE IS A LITTLE BELOW AVERAGE ON TUESDAY,  
HOWEVER, GIVEN SOME LINGERING DISAGREEMENT AMONG THE OPERATIONAL  
MODELS REGARDING STRENGTH/ORIENTATION OF THE TROUGH AXIS,  
EXTENT OF LARGE-SCALE ASCENT IN ADVANCE OF THE TROUGH IN THE  
MID-ATLANTIC, AND ASSOCIATED FRONTAL TIMING. THE 12Z GFS/CMC ARE  
FAIRLY DRY ACROSS THE AREA, BUT THE 12Z ECMWF DOES GENERATE  
SOME PRECIPITATION ALONG THE FRONT AND OFFSHORE. THOUGH THERE IS  
MUCH IMPROVED AGREEMENT ON THE SPECIFICS OF A DEVELOPING LOW  
OFF THE COAST WITH THE 12Z SIMULATIONS, THE QPF DISCREPANCIES DO  
NOT BODE MUCH CONFIDENCE IN THE SENSIBLE WEATHER DETAILS,  
PARTICULARLY IN SOUTHERN/EASTERN PORTIONS OF THE AREA -- WHICH  
WILL BE IN CLOSER PROXIMITY TO THE DEVELOPING LOW AND THE LAST  
TO BE PASSED BY THE REMNANT SURFACE BOUNDARY MOVING THROUGH THE  
REGION. DECIDED NOT TO STRAY MUCH FROM FORECAST CONTINUITY FOR  
THIS PERIOD.  
 
UPSTREAM RIDGING WILL MOVE INTO THE EAST ON WEDNESDAY, WITH A  
BROAD SURFACE HIGH MIGRATING TO THE COAST BY 00Z THURSDAY. THIS  
SHOULD ALLOW FOR A DRY DAY WITH CONTINUED PLEASANT (SLIGHTLY  
BELOW AVERAGE) TEMPERATURES.  
 
AS THE SURFACE RIDGE MOVES OFFSHORE BY THURSDAY, RETURN FLOW  
COMMENCES. TEMPERATURES ARE LIKELY TO RAPIDLY WARM IN THIS  
REGIME, WITH SUBSTANTIAL WARM/MOIST ADVECTION OCCURRING IN  
ADVANCE OF ANOTHER POTENT VORTICITY MAXIMUM MOVING INTO THE  
GREAT LAKES AND ADJACENT SOUTHERN/SOUTHEAST CANADA BY THIS TIME.  
MIDLEVEL RIDGING IN ADVANCE OF THE VORT MAX SHOULD PERMIT THE  
WARM SECTOR TO SURGE WELL NORTH OF THE AREA, WHICH WILL PREVENT  
ASSOCIATED SURFACE FRONTS FROM REACHING THE AREA LIKELY THROUGH  
THE END OF THE LONG TERM PERIOD.  
 
THIS MEANS THE MAIN QUESTIONS REGARDING THE FRIDAY-SUNDAY  
FORECAST REVOLVE AROUND HEAT/HUMIDITY AND CHANCES FOR  
PRECIPITATION, AS THE RAPID WARMTH OF THURSDAY WILL LIKELY BE  
FOLLOWED BY MORE GRADUAL INCREASES IN TEMPS/DEW POINTS  
THEREAFTER. ADDITIONALLY, WITH THE MID-ATLANTIC BECOMING MORE  
AND MORE ON THE FRINGES OF MIDLEVEL RIDGING, LARGE-SCALE ASCENT  
(PRONOUNCED IN ADVANCE OF WEST-TO-EAST MOVING VORTICITY MAXIMA)  
WILL AT LEAST GLANCE THE AREA, ESPECIALLY THE NORTHERN/WESTERN  
CWA. AS SUCH, THE MODELS (TO VARYING DEGREES) SUGGEST INCREASING  
CHANCES FOR PRECIPITATION AS A SURFACE BOUNDARY SAGS SOUTHWARD  
FROM CANADA THROUGH THIS PERIOD.  
 
THERE IS PRETTY DECENT AGREEMENT ON TIMING/LOCATION OF THE FIRST  
VORTICITY MAXIMUM ON THURSDAY (GENERALLY IN NEW YORK/NEW  
ENGLAND), SO KEPT POPS PRETTY LOW IN THE CWA DURING THIS TIME  
FRAME (ONLY MENTIONABLE IN THE SOUTHERN POCONOS AND VICINITY).  
HOWEVER, FRIDAY THROUGH SUNDAY FEATURE INCREASING CHANCES FOR  
CONVECTION AS FASTER SOUTHWEST FLOW SLOWLY EDGES TOWARD THE AREA,  
THOUGH PINPOINTING EXACT LOCATIONS AND TIMING (THE LATTER OF  
WHICH WILL BE CLOSELY TIED TO UPSTREAM VORTICITY MAXIMA THIS  
WEEKEND) IS A DUBIOUS EXERCISE AT THIS TIME RANGE GIVEN THE  
SOMEWHAT UNPREDICTABLE PATTERN (AND ASSOCIATED POOR PHASING  
AGREEMENT AMONG THE ECMWF/GFS/CMC). GENERALLY BROAD-BRUSHED  
SLIGHT CHANCE TO CHANCE POPS THROUGH THE CWA THIS WEEKEND AS A  
RESULT, WITH A GENERAL BLEND OF THE ABOVE-MENTIONED MODELS AND  
HEAVY WEIGHTING TO CONTINUITY/WPC GUIDANCE.  
 
REGARDING HEAT/HUMIDITY, KEPT THE FORECAST FAIRLY TAME DURING  
THIS PERIOD (AND CLOSE TO STATISTICAL GUIDANCE) GIVEN POTENTIAL  
COMPLICATIONS FROM PROXIMITY CONVECTION. HOWEVER, IF THE RIDGE  
REMAINS MORE PREVALENT ACROSS THE AREA, FORECAST COULD BE TOO  
COOL THIS PERIOD. REGARDLESS, A SULTRY WEEKEND IS ANTICIPATED.  
 

 
   
AVIATION /22Z SUNDAY THROUGH FRIDAY/
 
 
THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG,  
KILG, KMIV, KACY AND SURROUNDING AREAS.  
 
MOSTLY VFR CONDITIONS ARE EXPECTED THROUGH THE TAF PERIOD. THERE IS  
A SLIGHT CHANCE SHOWERS AND THUNDERSTORMS AFTER 18Z MONDAY ACROSS  
THE HIGHER TERRAIN (INCLUDING KRDG AND KABE). IF ANY SHOWERS MOVE  
OVER THE TAF SITES, MVFR OR LOWER CONDITIONS ARE POSSIBLE, BUT IT IS  
TOO UNCERTAIN TO INCLUDE IN ANY OF THE TAFS AT THIS TIME.  
 
WINDS WILL STAY MOSTLY LIGHT OUT OF THE WEST OR SOUTHWEST THROUGH  
THE TAF PERIOD. THE ONE EXCEPTION IS KACY, WHERE SOUTH SOUTHEASTERLY  
WINDS ARE EXPECTED TO PERSIST THROUGH AT LEAST 00Z IN THE WAKE OF A  
SEA BREEZE. THERE IS A CHANCE A SEA BREEZE COULD REDEVELOP AFTER  
18Z MONDAY AND ONCE AGAIN SHIFT THE WINDS AT KACY.  
 
OUTLOOK...  
 
MONDAY NIGHT AND TUESDAY: PREDOMINANTLY VFR, THOUGH  
ISOLATED/SCATTERED SHOWERS/STORMS ARE POSSIBLE. WINDS GENERALLY  
WEST OR SOUTHWEST 5 TO 15 KTS WITH GUSTS TO 20 KTS DURING THE  
DAY. CONFIDENCE AVERAGE.  
 
TUESDAY NIGHT THROUGH WEDNESDAY: VFR. WINDS GENERALLY NORTHWEST  
5 TO 15 KTS. CONFIDENCE ABOVE AVERAGE.  
 
WEDNESDAY NIGHT AND THURSDAY NIGHT: VFR. WINDS GENERALLY  
SOUTHWEST UNDER 10 KTS AT NIGHT AND 10 TO 20 KTS DURING THE DAY.  
CONFIDENCE ABOVE AVERAGE.  
 
THURSDAY NIGHT AND FRIDAY: GENERALLY VFR, THOUGH THERE IS AN  
INCREASING CHANCE FOR SHOWERS/STORMS, ESPECIALLY NORTH/WEST OF  
KPHL DURING THIS PERIOD. WINDS GENERALLY SOUTHWEST 5 TO 15 KTS  
WITH GUSTS TO 20 KTS DURING THE DAY. CONFIDENCE BELOW AVERAGE.  
 

 
   
MARINE
 
 
WINDS AND SEAS ARE EXPECTED TO STAY BELOW SMALL CRAFT ADVISORY  
CRITERIA TONIGHT AND TOMORROW. WESTERLY WIND GUSTS UP TO 20 KT ARE  
POSSIBLE ON THE ATLANTIC COASTAL WATERS OVERNIGHT, BUT SHOULD STAY  
BELOW 25 KT.  
 
OUTLOOK...  
 
MONDAY NIGHT THROUGH WEDNESDAY NIGHT: SUB-ADVISORY CONDITIONS  
EXPECTED. A SLIGHT CHANCE OF SHOWERS/STORMS ON TUESDAY AND  
TUESDAY NIGHT.  
 
THURSDAY THROUGH FRIDAY: ADVISORY WINDS/SEAS POSSIBLE AS  
STRONGER SOUTHWESTERLY WINDS BECOME ESTABLISHED. A SLIGHT CHANCE  
OF STORMS ON FRIDAY.  
   
RIP CURRENTS
 
 
THE OUTLOOK FOR TOMORROW IS THAT THE LOW RISK FOR THE FORMATION OF  
DANGEROUS RIP CURRENTS WILL CONTINUE.  
 

 
   
TIDES/COASTAL FLOODING
 
 
VERY HIGH ASTRONOMICAL TIDES ARE EXPECTED TO CONTINUE TODAY AND  
TOMORROW. THERE IS A CHANCE THAT MINOR TIDAL FLOODING COULD OCCUR  
ESPECIALLY ALONG THE NORTHERN NJ SHORE WITH THE HIGH TIDE CYCLE THIS  
EVENING. HOWEVER, MODELS OVER THE LAST FEW DAYS HAVE HAD A HIGH  
BIAS, AND THE WINDS HAVE BEEN LIGHT THOUGH THE DAY TODAY. THEREFORE,  
DO NOT EXPECT WIDESPREAD TIDAL FLOODING TODAY. TOMORROW WATER LEVELS  
SHOULD BE SLIGHTLY LOWER AS WELL.  
 

 
   
PHI WATCHES/WARNINGS/ADVISORIES
 
 
PA...NONE.  
NJ...NONE.  
DE...NONE.  
MD...NONE.  
MARINE...NONE.  
 

 
 

 
SYNOPSIS...CMS  
NEAR TERM...JOHNSON/MIKETTA  
SHORT TERM...JOHNSON  
LONG TERM...CMS  
AVIATION...CMS/JOHNSON  
MARINE...CMS/JOHNSON/MIKETTA  
TIDES/COASTAL FLOODING...JOHNSON  
 
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