675  
FXUS61 KPHI 181630  
AFDPHI  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ  
1230 PM EDT SAT APR 18 2015  
   
SYNOPSIS
 
 
LOW PRESSURE WILL QUICKLY MOVE ACROSS NEW ENGLAND THIS AFTERNOON,  
PULLING A DRY COLD FRONT ACROSS OUR AREA THIS EVENING. HIGH  
PRESSURE WILL THEN BUILD IN SATURDAY NIGHT INTO SUNDAY BEFORE  
SHIFTING OFFSHORE. LOW PRESSURE WILL MOVE FROM THE GREAT LAKES TO  
NEAR HUDSON BAY MONDAY AND TUESDAY THEN STALL. AN ASSOCIATED WARM  
FRONT WILL LIFT NORTHWARD INTO PORTIONS OF OUR REGION DURING  
MONDAY, FOLLOWED BY A COLD FRONT MONDAY NIGHT. THE NEXT COLD FRONT  
ARRIVES WEDNESDAY NIGHT WITH POTENTIALLY ANOTHER COLD FRONT MOVING  
THROUGH THURSDAY NIGHT INTO FRIDAY.  
 

 
   
NEAR TERM /UNTIL 6 PM THIS EVENING/
 
 
HIGH PRESSURE ACROSS OUR AREA WILL SLIDE EASTWARD LATER TODAY.  
MEANWHILE, A ROBUST MID LEVEL SHORT WAVE TROUGH IS FORECAST TO  
MOVE FROM EASTERN ONTARIO AND SOUTHWESTERN QUEBEC, ACROSS  
NORTHEASTERN NEW YORK AND NEW ENGLAND DURING THE AFTERNOON AND  
EVENING HOURS. THE SHORT WAVE IS EXPECTED TO PUSH A COLD FRONT  
TOWARD OUR REGION LATE IN THE DAY.  
 
A SUNNY DAY ACROSS THE AREA WITH ONLY SOME HIGH CLOUDS MOVING  
ACROSS ER PA AND NJ. MAXIMUM TEMPERATURES ARE FORECAST TO BE  
AROUND 80 DEGREES. READINGS ARE EXPECTED TO GET NO HIGHER THAN THE  
LOWER AND MIDDLE 70S IN THE ELEVATED TERRAIN AND NO HIGHER THAN  
THE 60S ALONG THE IMMEDIATE COAST.PHILADELPHIA'S LAST 80 PLUS  
DEGREE DAY WAS ON SEPTEMBER 28 WHEN THE TEMPERATURE ROSE TO 86  
DEGREES.  
 
WINDS WILL INCREASE THIS AFTERNOON FROM THE WEST WITH HIGHER  
SPEEDS/GUSTS ACROSS THE NORTHERN AREAS. WINDS WILL BECOME ONSHORE  
ACROSS THE NJ AND DE SHORE AREAS.  
 

 
   
SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/
 
 
A COLD FRONT IS EXPECTED TO BEGIN MOVING INTO THE NORTHERN PART  
OF OUR FORECAST AREA THIS EVENING. IT SHOULD REACH THE LEHIGH  
VALLEY AND THE INTERSTATE 78 CORRIDOR IN NEW JERSEY ABOUT 800 PM.  
THE BOUNDARY WILL CONTINUE TO PUSH SOUTHWARD, REACHING  
PHILADELPHIA AROUND 1000 PM AND CAPE MAY SHORTLY AFTER MIDNIGHT.  
 
THERE WILL BE LITTLE MOISTURE ASSOCIATED WITH THE COLD FRONT AND  
THE SKY SHOULD REMAIN MOSTLY CLEAR.  
 
THE WIND IS FORECAST TO SHIFT FROM THE WEST TO THE NORTH WITH THE  
FRONTAL PASSAGE. THE WIND IS THEN ANTICIPATED TO VEER TOWARD THE  
NORTHEAST LATE TONIGHT. TEMPERATURES ARE EXPECTED TO DROP TO  
AROUND 40 DEGREES IN THE ELEVATED TERRAIN UP NORTH AND INTO THE  
MIDDLE 40S TO LOWER 50S RANGE ELSEWHERE.  
 

 
   
LONG TERM /SUNDAY THROUGH FRIDAY/
 
 
THE SYNOPTIC SETUP IS COMPRISED OF A CLOSED LOW THAT EJECTS  
ACROSS THE PLAINS INTO SUNDAY. THIS FEATURE BUILDS RIDGING  
DOWNSTREAM, WHICH RESULTS IN DRY WEATHER FOR MUCH OF SUNDAY. AN  
AIR MASS CHANGE THOUGH WILL TAKE PLACE FOR SUNDAY AS HIGH  
PRESSURE BUILDS DOWN FROM THE NORTH. THE INITIAL CLOSED LOW LOOKS  
TO GET ABSORBED INTO A LARGE SCALE TROUGH FROM THE MIDWEST AS IT  
UNDERGOES AMPLIFICATION EASTWARD THROUGH EARLY NEXT WEEK. THIS  
SHOULD THEN RESULT IN ANOTHER CLOSED LOW ACROSS THE GREAT LAKES  
REGION TUESDAY THROUGH THURSDAY, BEFORE SHIFTING CLOSER TO NEW  
ENGLAND FRIDAY. THE MAIN PRECIPITATION EVENT LOOKS TO OCCUR MONDAY  
ALTHOUGH SOME QUICKER ARRIVAL TIME WITH SOME OF THE GUIDANCE WILL  
RESULT IN RAIN DEVELOPING SOONER. GIVEN THE SURGE OF MOISTURE, THE  
RAIN COULD BE HEAVY AT TIMES. THE PATTERN THEREAFTER RESULTS IN  
CYCLONIC FLOW ACROSS THE EAST ALONG WITH COOLING AND A SERIES OF  
COLD FRONTS. WE USED A MODEL BLEND FOR SUNDAY INTO TUESDAY, THEN  
BLENDED IN THE 00Z WPC GUIDANCE. SOME ADJUSTMENTS WERE THEN MADE  
FOLLOWING ADDITIONAL COLLABORATION WITH OUR NEIGHBORING OFFICES.  
 
FOR SUNDAY...HIGH PRESSURE BUILDING DOWN FROM THE NORTH TO START  
SUNDAY THEN IT WILL SHIFT OFFSHORE DURING THE COURSE OF THE DAY.  
THIS WILL ALLOW FOR THE WIND TO BE ONSHORE ALONG WITH A COOLER  
AIR MASS. THE CLOUDS INCREASE MAINLY LATE SUNDAY AS THE NEXT  
SYSTEM BEGINS TO APPROACH FROM THE SOUTHWEST. A STRENGTHENING LOW-  
LEVEL JET AND WAA WILL HELP PUSH A WARM FRONT TOWARD OUR AREA  
OVERNIGHT SUNDAY. THE STRENGTH OF THE LOW-LEVEL JET MOVING IN MAY  
ALLOW FOR STRONGER ASCENT TO ARRIVE SOONER /DEEPER ISENTROPIC LIFT  
OVER THE DEPARTING SURFACE HIGH/, THEREFORE WE INCREASED THE POPS  
QUICKER BY A FEW HOURS. SOME OF THE RAIN MAY BECOME HEAVY AT TIMES  
TOWARD DAYBREAK MONDAY.  
 
FOR MONDAY AND TUESDAY...AS ENERGY EJECTS EASTWARD FROM A CLOSED  
LOW AND ALSO FROM THE MIDWEST, LOW PRESSURE ORGANIZES OVER THE  
GREAT LAKES BEFORE SHIFTING NORTHWARD TOWARD HUDSON BAY. A ROBUST  
LOW- LEVEL JET AT LEAST DURING THE MORNING COMBINED WITH WAA, DEEP  
MOISTURE ADVECTION AND LARGE SCALE ASCENT WILL RESULT IN AN AREA  
OF RAIN. THERE ARE SOME HINTS OF SOME ELEVATED INSTABILITY WITHIN  
THIS, HOWEVER HELD OFF WITH A THUNDER MENTION AS THE INITIAL DEEP  
SATURATION TENDS TO NOT BE CONDUCIVE FOR THUNDER. AN AREA OF PW  
AIR OF NEAR 1.5 INCHES IS FORECAST TO SURGE ACROSS THE CWA,  
THEREFORE THE RAIN COULD BE HEAVY AT TIMES ESPECIALLY IN THE  
MORNING. OTHER THAN SOME TYPICAL PONDING OF WATER ON THE ROADWAYS  
AND POOR DRAINAGE FLOODING DURING HEAVIER RAIN, NO OTHER FLOODING  
IS ANTICIPATED ATTM.  
 
THE WARM SECTOR MAY GET INTO OUR SOUTHERN AREAS AND PERHAPS TO  
ABOUT THE PHILADELPHIA METRO, AND THEREFORE KEPT A MENTION OF SOME  
THUNDER MONDAY AFTERNOON AND EARLY EVENING BUT DID EXPAND THIS  
NORTHWARD A BIT. THE INSTABILITY LOOKS WEAK ENOUGH TO KEEP THE  
INTENSITY OF ANY THUNDERSTORM ON THE LOWER SIDE, HOWEVER ISOLATED  
STRONG WIND GUSTS AND SMALL HAIL CANNOT BE RULED OUT IF THE WARM  
SECTOR CAN GET FARTHER NORTH /ENOUGH HEATING OCCURS/. THE  
STEADIEST RAIN SHOULD TAPER OFF TO SHOWERS DURING THE LATE MORNING  
AND EARLY AFTERNOON FROM SOUTHWEST TO NORTHEAST AS A WEAK SURFACE  
LOW SHIFTS NORTHEAST NEAR A TRIPLE POINT. A ROBUST LOW-LEVEL JET  
SLIDING ACROSS THE AREA MONDAY MORNING SHOULD ALLOW FOR AN  
INCREASE IN THE SURFACE WINDS SUNDAY NIGHT AND MONDAY MORNING,  
WITH THE STRONGEST GUSTS CLOSER TO THE COAST DUE TO AN INITIAL  
ONSHORE FLOW.  
 
LOW PRESSURE AND A COLD/OCCLUDING FRONT SHOULD BE EXITING TO OUR  
EAST TUESDAY MORNING, WITH ANY SHOWERS ENDING. A DEVELOPING CLOSED  
LOW THOUGH IS FORECAST TO BE AROUND THE GREAT LAKES WITH CYCLONIC  
FLOW RESULTING. A FEW SHOWERS MAY PERSIST TUESDAY MAINLY ACROSS  
THE NORTHERN ZONES.  
 
FOR WEDNESDAY THROUGH FRIDAY...A CLOSED LOW IS FORECAST TO BE IN  
PLACE FROM THE GREAT LAKES TO EVENTUALLY NEW ENGLAND. THIS WILL  
RESULT IN CYCLONIC FLOW ACROSS OUR AREA, ALTHOUGH IT IS NOT REAL  
CERTAIN HOW SHORT WAVE ENERGY WITHIN THE FLOW RESULTS IN SOME  
STRONGER FORCING AS THE MAIN SYSTEM STARTS TO SHIFT TO NEW ENGLAND  
WITH TIME. THESE SHORT WAVES SHOULD SEND A COUPLE OF COLD FRONTS  
OUR WAY, WITH ONE GENERALLY ON WEDNESDAY AND ANOTHER PROBABLY  
THURSDAY NIGHT INTO FRIDAY. THERE MAY BE ENOUGH LIFT WITHIN THE  
CYCLONIC FLOW COMBINED WITH DAYTIME HEATING TO PROMOTE A FEW  
SHOWERS MAINLY ACROSS THE NORTH. OVERALL, WE FAVORED THE WPC  
GUIDANCE/CONTINUITY DURING THIS TIME FRAME WHICH ALSO ADVERTISES  
COOLER TEMPERATURES.  
 

 
   
AVIATION /18Z SATURDAY THROUGH THURSDAY/
 
 
THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG,  
KILG, KMIV, KACY, AND SURROUNDING AREAS.  
 
VFR CONDITIONS ARE EXPECTED THROUGH THE TAF PERIOD WITH A MOSTLY  
CLEAR SKY.  
 
THE WIND IS FORECAST TO BECOME WESTERLY FOR THIS AFTERNOON AT  
SPEEDS AROUND 6 TO 12 KNOTS. GUSTS NEAR 20 KNOTS ARE POSSIBLE AT  
OUR NORTHERN TAF SITES, KRDG, KABE AND KTTN. A COLD FRONT IS  
EXPECTED TO DROP THROUGH OUR REGION FROM NORTH TO SOUTH EARLY  
TONIGHT AND THE WIND DIRECTION IS FORECAST TO SHIFT TO THE NORTH.  
 
OUTLOOK...  
SUNDAY...VFR INTO THE EVENING WITH INCREASING CLOUDINESS. THE  
CONDITIONS THEN LOWER TO MVFR/IFR AT NIGHT AS RAIN DEVELOPS FROM  
SOUTHWEST TO NORTHEAST. EAST-SOUTHEAST WINDS 10-15 KNOTS, WITH  
SOME GUSTS TO ABOUT 20 KNOTS ESPECIALLY CLOSER TO THE COAST AND AT  
NIGHT. LOW-LEVEL WIND SHEAR MAY START TO DEVELOP AT NIGHT IN  
ADVANCE OF A WARM FRONT AND AS A ROBUST LOW-LEVEL JET MOVES IN.  
 
MONDAY...MVFR/IFR WITH PERIODS OF RAIN IN THE MORNING TAPERING TO  
SOME SHOWERS, THEN SOME IMPROVEMENT MAY OCCUR IN THE AFTERNOON.  
THERE IS A CHANCE FOR SOME THUNDERSTORMS IN THE AFTERNOON AND  
EARLY EVENING, ESPECIALLY FROM ABOUT THE KPHL METRO TO SOUTH AND  
EAST. LOW- LEVEL WIND SHEAR MAINLY IN THE MORNING. EAST OR  
SOUTHEAST WINDS AROUND 10 KNOTS, BECOMING WEST-SOUTHWEST.  
 
TUESDAY...MVFR POSSIBLE EARLY, OTHERWISE VFR. A FEW SHOWERS  
REMAIN POSSIBLE MAINLY FROM KTTN TO KABE NORTHWARD. WESTERLY WINDS  
MAINLY 10 KNOTS OR LESS.  
 
WEDNESDAY...MAINLY VFR. SOME AFTERNOON SHOWERS ARE POSSIBLE,  
MAINLY FROM KPHL AND TO THE NORTH AND WEST. WESTERLY WINDS  
GENERALLY 10 KNOTS OR LESS.  
 

 
   
MARINE
 
 
HIGH PRESSURE IS FORECAST TO INFLUENCE THE COASTAL WATERS OF NEW  
JERSEY AND DELAWARE FOR TODAY. A COLD FRONT FROM THE NORTH IS  
EXPECTED TO PASS THROUGH THE REGION TONIGHT.  
 
THE WIND SHOULD BE LIGHT AND VARIABLE FOR TODAY AS WARM AIR FROM  
THE WEST RIDES OVER THE COLD WATER. THE WIND DIRECTION SHOULD  
SETTLE INTO THE NORTH AND NORTHEAST TONIGHT AFTER THE COLD FRONT  
PASSES. SPEEDS ARE ANTICIPATED TO PICK UP TO AROUND 15 KNOTS AT  
THAT TIME WITH WAVES ON OUR OCEAN WATERS BUILDING TO 3 TO 4 FEET.  
 
OUTLOOK...  
SUNDAY...A STRENGTHENING LOW-LEVEL JET ARRIVES AT NIGHT,  
THEREFORE A NORTHEAST WIND WILL SHIFT TO EASTERLY AT NIGHT AND  
INCREASE. THIS MAY BE STRONG ENOUGH FOR GUSTS TO AROUND 25 KNOTS  
LATER AT NIGHT. THE ONSHORE FLOW WILL ALSO START TO BUILD THE  
SEAS.  
 
MONDAY AND TUESDAY...WINDS WILL INCREASE AS A RESULT OF A  
STRENGTHENING LOW-LEVEL JET MOVING THROUGH MOSTLY MONDAY MORNING.  
WHILE THE STABILITY NEAR THE SURFACE SHOULD KEEP MUCH OF THE WIND  
ABOVE THE GROUND, IT DOES APPEAR THERE WILL BE A PERIOD OF TIME  
WHERE THE ONSHORE FLOW GUSTS TO AROUND 30 KNOTS MONDAY BEFORE  
SHIFTING TO THE SOUTH AND DIMINISHING SOME. AS A FRONTAL SYSTEM  
MOVES IN THE FLOW IS EXPECTED TO TURN MORE SOUTHERLY LATER MONDAY  
THEN WESTERLY INTO TUESDAY. SEAS WILL BUILD TO AND ABOVE 5 FEET ON  
THE OCEAN AND MAY BE SLOW TO SUBSIDE ON TUESDAY. A SMALL CRAFT  
ADVISORY LOOKS TO BE NEEDED.  
 
WEDNESDAY...THE CONDITIONS ARE EXPECTED TO BE BELOW SMALL CRAFT  
ADVISORY CRITERIA.  
 

 
   
FIRE WEATHER
 
 
WE HAVE UPGRADED THE FIRE WEATHER WATCH TO A RED FLAG WARNING FOR  
THE COUNTIES OF BERKS, UPPER MONTGOMERY, BUCKS, MERCER, INLAND  
MONMOUTH AND POINTS NORTHWARD. IT IS IN EFFECT FROM 300 PM UNTIL  
800 PM. RELATIVE HUMIDITY VALUES ARE FORECAST TO DROP INTO THE 20  
TO 25 PERCENT RANGE THERE THIS AFTERNOON AND FINE FUELS WILL  
CONTINUE TO DRY. AS A COLD FRONT APPROACHES FROM THE NORTH, A  
WESTERLY WIND IS ANTICIPATED TO GUST INTO THE 20 TO 25 MPH RANGE  
INCREASING THE THREAT FOR WILDFIRE SPREAD.  
 
THE RELATIVE HUMIDITY LEVELS ARE FORECAST TO DROP INTO THE 20 TO  
30 PERCENT RANGE IN SOUTHEASTERN PENNSYLVANIA, EXTREME  
NORTHEASTERN MARYLAND, NORTHERN DELAWARE AND SOUTHERN NEW JERSEY  
AND FINE FUELS WILL CONTINUE TO DRY THERE, AS WELL. HOWEVER, WIND  
SPEEDS ARE EXPECTED TO BE LOWER THAN THOSE IN AREAS TO THE NORTH,  
LIMITING THE POTENTIAL FOR WILDFIRE SPREAD AND THE NEED FOR A RED  
FLAG WARNING.  
 
RELATIVE HUMIDITY LEVELS SHOULD BE HIGHER AND WIND SPEEDS LOWER  
IN CENTRAL AND SOUTHERN DELAWARE AND IN THE ADJACENT COUNTIES OF  
NORTHEASTERN MARYLAND THAN AT LOCATIONS TO THE NORTH. AS A RESULT,  
THERE ARE NO SIGNIFICANT FIRE WEATHER CONCERNS FOR TODAY IN THOSE  
AREAS.  
 

 
   
PHI WATCHES/WARNINGS/ADVISORIES
 
 
PA...RED FLAG WARNING UNTIL 8 PM EDT THIS EVENING FOR PAZ054-055-  
060>062-103-105-106.  
NJ...RED FLAG WARNING UNTIL 8 PM EDT THIS EVENING FOR NJZ001-  
007>010-012-013-015.  
DE...NONE.  
MD...NONE.  
MARINE...NONE.  
 

 
 

 
SYNOPSIS...GORSE  
NEAR TERM...IOVINO  
SHORT TERM...IOVINO  
LONG TERM...GORSE  
AVIATION...GORSE/IOVINO  
MARINE...GORSE/IOVINO  
FIRE WEATHER...  
 
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