443  
FXUS61 KPHI 211928  
AFDPHI  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ  
328 PM EDT SAT APR 21 2018  
   
SYNOPSIS
 
 
LARGE AREA OF HIGH PRESSURE OVER THE NORTHEASTERN UNITED STATES WILL  
CONTINUE TO SLOWLY BUILD EAST, BUILDING OFF THE NEW ENGLAND COAST BY  
MONDAY. MEANWHILE, A LOW IN THE ARKLATX REGION WILL LIFT INTO THE  
MID ATLANTIC STATES BY MID TO LATE WEEK. ANOTHER AREA OF LOW  
PRESSURE MAY COME IN RIGHT BEHIND IT BY NEXT WEEKEND.  
 

 
   
NEAR TERM /UNTIL 6 AM SUNDAY MORNING/
 
 
AN UPPER AIR ANALYSIS THIS AFTERNOON SHOWED A LINGERING TROUGH IN  
THE NORTHEAST WITH A CLOSED LOW EJECTING INTO THE CENTRAL PLAINS.  
THERE IS SOME RIDGING FROM HUDSON BAY CANADA TOWARD THE MIDWEST. AT  
THE SURFACE, HIGH PRESSURE IS CENTERED FROM THE EASTERN GREAT LAKES  
TO THE MID-ATLANTIC REGION.  
 
WHILE SURFACE HIGH PRESSURE IS PRODUCING SOME LOW-LEVEL SUBSIDENCE  
CROSS OUR REGION, A 250 MB JET WITH ITS CORE POSITIONED JUST TO OUR  
NORTH WILL CONTINUE TO PROVIDE ENOUGH LIFT WITHIN THE FAST NORTHWEST  
FLOW ALOFT TO PRODUCE AREAS OF HIGHER LEVEL CLOUDINESS. THIS IS  
THICKER TO OUR WEST, AND THIS SHOULD MOVE OVER ESPECIALLY THE  
CENTRAL AND SOUTHERN PARTS OF OUR REGION FOR A TIME TONIGHT. THE  
WINDS WILL GO LIGHT AND VARIABLE TO EVEN CALM TONIGHT, WHICH  
COMBINED WITH A DRY AIRMASS IS CONDUCIVE FOR GOOD RADIATIONAL  
COOLING CONDITIONS. HOWEVER, THE PRESENCE OF THE HIGHER LEVEL  
CLOUDINESS MAY PLAY A ROLE IN KEEPING TEMPERATURES FROM REALLY  
DROPPING OFF. BASED ON THE EXPECTATION OF THIS CLOUDINESS AND  
COLLABORATING WITH OUR NEIGHBORING OFFICES, WE OPTED TO MENTION  
PATCHY FROST FOR PARTS OF SOUTHEAST PA AND INTERIOR CENTRAL/SOUTHERN  
NEW JERSEY WHERE TEMPERATURES ARE CURRENTLY FORECAST TO BE COLD  
ENOUGH AND THE GROWING SEASON HAS STARTED. AS A RESULT, NO FROST  
ADVISORY ISSUED AT THIS TIME. WE LEANED MORE TOWARD SOME POTENTIAL  
FROST AS OPPOSED TO A FREEZE BASED ON THE ABOVE AND DEW POINTS  
CREEPING UP SOME DURING THE COURSE OF TONIGHT.  
 
THE HOURLY TEMPERATURE, DEW POINT AND WIND GRIDS WERE ADJUSTED BASED  
ON THE LATEST OBSERVATIONS/TRENDS, THEN BLENDED INTO THE TONIGHT  
PERIOD. LOW TEMPERATURES ARE A MULTI-MODEL BLEND WITH  
CONTINUITY.  
 

 
   
SHORT TERM /6 AM SUNDAY MORNING THROUGH 6 PM SUNDAY/
 
 
AS AN UPPER-LEVEL TROUGH SLIDES ACROSS THE NORTHEAST AND NORTHERN  
MID-ATLANTIC REGIONS DURING SUNDAY, ITS AXIS ALONG WITH SHORT WAVE  
ENERGY CROSSES OUR AREA DURING THE AFTERNOON. AS THIS OCCURS, A 250  
MB JET STREAK WILL SHIFT TO OUR EAST AND THE HIGHER LEVEL CLOUDINESS  
SHOULD SETTLE SOUTH AND EAST WITH IT. MEANWHILE, A CLOSED LOW SLIDES  
ACROSS THE GULF COAST STATES WHICH IS TRACKING UNDER A RIDGE TO ITS  
NORTH. THIS RIDGE WILL KEEP SURFACE HIGH PRESSURE ANCHORED OVER THE  
MID-ATLANTIC AND NORTHEAST THROUGH THE DAY SUNDAY.  
 
ANOTHER DRY DAY IS EXPECTED AND WITH A GOOD AMOUNT OF SUNSHINE  
RESULTING IN AMPLE BOUNDARY LAYER WARMING, TEMPERATURES ARE FORECAST  
TO BE AT LEAST A FEW DEGREES WARMER THAN WHAT WAS OBSERVED TODAY  
(SATURDAY). THE FLOW IS ON THE LIGHTER SIDE ESPECIALLY IN THE  
BOUNDARY LAYER, THEREFORE A LIGHT WIND COMBINED WITH WARMING ON THE  
LAND (INCREASING THERMAL GRADIENT BETWEEN LAND AND WATER) IS  
EXPECTED TO RESULT IN THE DEVELOPMENT OF AN AFTERNOON SEA/BAY  
BREEZE. THIS WILL TURN THE WINDS FOR THE COASTAL AREAS TO SOUTH AND  
SOUTHEAST AND PROVIDE SOME AFTERNOON COOLING. HIGH TEMPERATURES ARE  
MAINLY A BLEND OF MOS AND CONTINUITY, ALTHOUGH THE GRADIENT WAS  
TIGHTENED UP SOME ALONG THE COAST DUE TO THE EXPECTED COOLING SEA  
BREEZE.  
 

 
   
LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/
 
 
THE LONG TERM PERIOD BEGINS WITH TRANQUIL PERIOD AS THE DOMINANT  
FEATURE WILL BE THE LARGE AREA OF HIGH PRESSURE THAT CONTINUES  
TO SLOWLY BUILD EAST ACROSS THE NORTHEASTERN U.S.. THIS HIGH  
SHOULD BUILD OFF SHORE BY MONDAY, ALLOWING THE LOW LEVEL FLOW TO  
BECOME ONSHORE, RESULTING IN INCREASING MOISTURE ADVECTION AHEAD  
OF THE NEXT LOW.  
 
THE NEXT LOW CURRENTLY OVER TX, WILL VERY SLOWLY PROGRESS EAST  
AND THEN NORTHEAST, REACHING THE MID ATLANTIC BY MID WEEK. WHAT  
IS REMARKABLE ABOUT THIS LOW IS THAT WHILE I WOULDN'T CONSIDER  
IT A TRUE CUTOFF LOW, IT WILL BE MAKING VERY SLOW PROGRESS EAST  
THROUGH THE WEEK AS COMPARED TO SYSTEMS ALONG THE NORTHERN  
STREAM. AS MENTIONED BY THE PREVIOUS SHIFT, THE MAIN PERIOD OF  
RAIN WITH THIS SYSTEM LOOKS TO BE TUESDAY NIGHT INTO WEDNESDAY,  
COINCIDENT WITH STRONGER LIFT ASSOCIATED WITH THE WARM FRONT.  
DUE TO THE SLOW PROGRESSION OF THE LOW AND LOW LEVEL ON SHORE  
FLOW, THERE IS SOME CONCERN OF HEAVY RAIN, PRIMARILY LATE  
TUESDAY NIGHT INTO WEDNESDAY MORNING. HOWEVER, THERE ARE A FEW  
FACTORS WORKING AGAINST THE HEAVY RAIN THREAT. FIRST, THE CLOUD  
LAYER LOOKS TO BE RATHER SHALLOW AS SOUTHWESTERLY FLOW IN THE  
MID AN UPPER LEVELS MAY KEEP DRIER AIR AT THESE LEVELS.  
SECONDLY, THE WARM CLOUD LAYER IS RELATIVELY SHALLOW (GENERALLY  
8000 TO 9000 FT), THANKS TO CONTINUED BELOW NORMAL TEMPERATURES.  
FINALLY, DEPENDING ON THE TRACK OF THE SURFACE LOW, SOME MODELS  
ARE DEPICTING A DRY SLOT DEVELOPING OVER OUR AREA BY WEDNESDAY  
MORNING. NOT SURE IF THIS IS THE START OF A TREND, OR A  
TEMPORARY CHANGE, BUT SOMETHING WE WILL BE WATCHING OVER THE  
NEXT FEW DAYS.  
 
ONCE THAT LOW LIFTS NORTHEAST AWAY FROM OUR REGION BY THURSDAY,  
WE WILL LIKELY HAVE BRIEF REPRIEVE OF WEAK HIGH PRESSURE, BEFORE  
A MID AND UPPER LEVEL OPEN WAVE TROUGH (THIS TIME FROM THE  
NORTHERN STREAM) APPROACHES OUR REGION NEXT WEEKEND. THE  
ASSOCIATED COLD FRONT COULD SLIDE THROUGH OUR REGION AS EARLY AS  
SATURDAY, THOUGH THE TROUGH MAY BE IN THE PROCESS OF BECOMING  
NEGATIVELY TILTED WHICH COULD ALSO SLOW THE PROGRESS OF THE  
SURFACE LOW AND ASSOCIATED FRONT.  
 
TEMPERATURES DURING THIS TIME WILL GENERALLY BE NEAR OR BELOW  
NORMAL LEVELS FOR MOST OF THE WEEK.  
 

 
   
AVIATION /19Z SATURDAY THROUGH THURSDAY/
 
 
THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG,  
KILG, KMIV, KACY AND SURROUNDING AREAS.  
 
REST OF THIS AFTERNOON...VFR WITH HIGH CLOUDS. NORTH-NORTHWEST TO  
WEST-NORTHWEST WINDS MAINLY AROUND 10 KNOTS.  
 
TONIGHT...VFR. CLOUDS ABOVE 10,000 FEET. WINDS BECOMING LIGHT AND  
VARIABLE.  
 
SUNDAY...VFR WITH SOME LINGERING HIGH CLOUDS. LIGHT AND VARIABLE  
WINDS, BECOMING WEST TO SOUTHWEST LESS THAN 10 KNOTS. WINDS ARE  
ANTICIPATED TO BECOME SOUTHEAST AT ACY DURING THE AFTERNOON.  
 
OUTLOOK...  
 
SUNDAY NIGHT THROUGH MONDAY NIGHT...MOSTLY VFR CONDITIONS ARE  
EXPECTED. THERE IS A SMALL CHANCE OF VISIBILITY REDUCED IN FOG AT  
KMIV AND KACY ON MONDAY MORNING. WINDS WILL BE LIGHT AND VARIABLE  
FOR MUCH OF THE PERIOD. HIGH CONFIDENCE ON MOST ASPECTS OF THE  
FORECAST EXCEPT THE DEVELOPMENT OF FOG, WHICH IS LOW CONFIDENCE AT  
THIS TIME.  
 
TUESDAY THROUGH THURSDAY...MVFR AND EVEN LOCALIZED IFR CONDITIONS  
WILL BE POSSIBLE ESPECIALLY ON TUESDAY NIGHT AND WEDNESDAY AS  
PERIODS OF RAIN ARE LIKELY ACROSS THE REGION. EASTERLY WIND GUSTS UP  
TO 20 KT ARE POSSIBLE TUESDAY NIGHT INTO WEDNESDAY. OTHERWISE,  
GENERALLY LIGHT WINDS (LESS THAN 10 KT) ARE EXPECTED. MEDIUM  
CONFIDENCE ON FLIGHT CATEGORIES, BUT LOW CONFIDENCE ON THE TIMING OF  
ANY FLIGHT CATEGORY CHANGES.  
 

 
   
MARINE
 
 
THE CONDITIONS ARE EXPECTED TO REMAIN BELOW SMALL CRAFT ADVISORY  
CRITERIA THROUGH SUNDAY. HIGH PRESSURE OVER THE AREA WILL RESULT IN  
LIGHTER WINDS, WHICH WILL BE SOUTHERLY INTO TONIGHT OVERALL BUT TEND  
TO TURN NORTHERLY DURING THE NIGHT. THE LIGHTER FLOW ON SUNDAY ALONG  
WITH PLENTY OF WARMING ON LAND WILL RESULT IN THE SURFACE WINDS  
TURNING FROM THE SOUTH OR SOUTHEAST MOSTLY IN THE AFTERNOON. SOME  
INCREASE IN THE WINDS IS FORECAST IN THE AFTERNOON AS THE WIND SHIFT  
OCCURS.  
 
OUTLOOK...  
 
SUNDAY NIGHT THROUGH MONDAY NIGHT...WINDS AND SEAS ARE EXPECTED TO  
REMAIN BELOW SCA CRITERIA.  
 
TUESDAY AND WEDNESDAY...EASTERLY WINDS WILL INCREASE RESULTING ALSO  
IN AN INCREASE OF WAVE HEIGHTS LATE TUESDAY THROUGH THE DAY  
WEDNESDAY. CURRENTLY FORECASTING SCA CONDITIONS ONLY ON THE ATLANTIC  
COASTAL WATERS. HOWEVER, WE WILL CONTINUE TO MONITOR MODEL TRENDS ON  
THE DELAWARE BAY AS MODELS TEND TO UNDERESTIMATE ESE WINDS  
ESPECIALLY ON THE LOWER BAY.  
 
WEDNESDAY NIGHT AND THURSDAY...WINDS SHIFT TO NORTHWESTERLY AND  
SUBSEQUENTLY DECREASE. THE MAIN QUESTION WILL BE HOW LONG SEAS  
REMAIN ABOVE 5 FEET ON THE COASTAL WATERS. CURRENT FORECAST IS FOR  
ELEVATED SEAS INTO LATE THURSDAY, BUT THERE IS STILL CONSIDERABLE  
UNCERTAINTY WITH THIS.  
 

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

 
 

 
SYNOPSIS...JOHNSON  
NEAR TERM...GORSE  
SHORT TERM...GORSE  
LONG TERM...JOHNSON  
AVIATION...GORSE/JOHNSON  
MARINE...GORSE/JOHNSON  
 
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