556  
FXUS61 KPHI 230800  
AFDPHI  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ  
400 AM EDT FRI APR 23 2021  
   
SYNOPSIS
 
 
HIGH PRESSURE OVER THE SOUTHEASTERN U.S. WILL SHIFT OFF SHORE  
TONIGHT. A SURFACE LOW WILL LIFT FROM THE LOWER MISSISSIPPI VALLEY  
INTO OUR REGION SATURDAY INTO SUNDAY. A COLD FRONT IS EXPECTED IN  
OUR REGION SUNDAY. HIGH PRESSURE BEGINS TO BUILD IN ON MONDAY AND  
WILL LIKELY DOMINATE THE WEATHER FOR MOST OF NEXT WEEK.  
 

 
   
NEAR TERM /THROUGH TONIGHT/
 
 
ANOTHER DRY AND BREEZY DAY IS EXPECTED ON TODAY, ALTHOUGH IT IS NOT  
EXPECTED TO BE AS WINDY AS THURSDAY. LOW PRESSURE CONTINUES TO  
SLOWLY DRIFT ACROSS THE CANADIAN MARITIMES, WHILE HIGH PRESSURE  
BUILDS OFFSHORE OF THE SOUTHEAST COAST. THIS KEEPS A STEADY FLOW  
ACROSS THE AREA, AND WITH GOOD MIXING, GUSTY WINDS ARE EXPECTED  
DURING THE DAY BETWEEN 20-30 MPH. TEMPERATURES TODAY WILL BE  
SUBSTANTIALLY WARMER THAN THURSDAY. AFTER THE FROST ADVISORIES AND  
FREEZE WARNINGS END THIS MORNING, TEMPERATURES WILL WARM CLOSE TO,  
BUT END UP A COUPLE OF DEGREES BELOW NORMAL.  
 
A WEAK SURFACE TROUGH IS EXPECTED TO MOVE ACROSS THE AREA DURING THE  
EVENING AND OVERNIGHT, WHILE A SHORT WAVE/VORTICITY IMPULSES MOVES  
ACROSS THE AREA. HOWEVER, DRY CONDITIONS ARE EXPECTED WITH ONLY  
ADDITIONAL CLOUD COVER EXPECTED. TEMPERATURES ARE EXPECTED TO BE  
CLOSE TO NORMAL TONIGHT. WIDESPREAD TEMPERATURES IN THE LOW 30S ARE  
NOT EXPECTED TONIGHT, SO NO FROST ADVISORIES ARE EXPECTED AT THIS  
TIME, ALTHOUGH SOME AREAS ACROSS NORTHEAST PENNSYLVANIA AND  
NORTHWEST NEW JERSEY COULD DIP INTO THE MID 30S.  
 

 
   
SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/
 
 
THE MAIN PERIOD OF INTEREST FOR THE NEXT WEEK IS SATURDAY INTO  
SUNDAY AS A SOUTHERN STREAM LOW PROGRESSING FROM THE LOWER  
MISSISSIPPI VALLEY SHOULD BE JUST OFF THE NJ COAST BY SUNDAY  
MORNING. THE MAIN CHANGES WITH THIS FORECAST WERE TO INCREASE THE  
RAIN TOTALS, AND INCREASE RAIN CHANCES ON SUNDAY.  
 
THE MAIN SOURCE OF UNCERTAINTY WITH THIS PERIOD IS HOW THE NORTHERN  
STREAM AND SOUTHERN STREAM LOWS WILL INTERACT WITH ONE ANOTHER.  
WHILE MOST MODELS DEPICT THE SOUTHERN STREAM LOW DEEPENING AND  
INTENSIFYING FASTER THAN THE NORTHERN STREAM TROUGH. HOWEVER, EVEN  
MINOR DIFFERENCES IN HOW QUICKLY THE NORTHERN STREAM TROUGH DEEPENS,  
COULD MEAN THE SOUTHERN STREAM LOW IS SLOWER TO LIFT OUT OF THE  
REGION. BASED ON THE LATEST TIMING WITH MOST OPERATIONAL MODELS, I  
INCREASED POPS ON SUNDAY, AS THE LOW COULD LINGER NEAR OUR REGION  
UNTIL SUNDAY AFTERNOON.  
 
AS FOR THE HAZARDS, IT STILL APPEARS THAT OUR REGION WILL BE MOSTLY  
NORTH OF THE CENTER OF THE SOUTHERN STREAM LOW, KEEPING OUR REGION  
WITH A RELATIVELY STABLE ATMOSPHERE. CONSEQUENTLY, EXPECT MORE OF A  
STRATIFORM RAIN EVENT, AND THUNDERSTORM CHANCES WILL PROBABLY BE  
WELL SOUTH OF OUR REGION. THAT BEING SAID, THE RISK FOR HEAVY RAIN  
HAS INCREASED SLIGHTLY THANKS TO MOST MODELS DEPICTING A PERSISTENT  
SOUTHEASTERLY THEN EASTERLY LOW LEVEL JET SATURDAY NIGHT INTO SUNDAY  
MORNING. THIS WILL PROMOTE MOISTURE TRANSPORT THROUGHOUT THE EVENT  
THANKS TO THE MOIST FETCH FROM THE ATLANTIC. FOR NOW, I'VE TRENDED  
PRECIPITATION AMOUNTS UP TO RANGE MOSTLY FROM 0.75 TO 1.25 INCHES  
ALONG THE COASTAL PLAINS, AND 0.5 TO 1 INCH NORTH AND WEST OF THE  
FALL LINE. THAT BEING SAID, SOME GUIDANCE IS MUCH HIGHER. MOST  
NOTABLY THE NAM NEST HAS RAIN AMOUNTS ABOVE 1.25 INCHES IN JUST THE  
6 HOUR PERIOD LATE SATURDAY NIGHT. I THINK THAT IS A BIT OVERDONE AS  
THE NAM NEST ALSO DEPICTS THE LOW LEVEL JET ABOUT 30 KT FASTER THAN  
OTHER GUIDANCE, BUT WILL BE MONITORING THIS OVER THE NEXT FEW  
DAYS. GIVEN THAT THE REGION IS STILL RELATIVELY DRY, THE  
CURRENT FORECAST AMOUNTS SHOULD NOT CAUSE ANY WIDESPREAD  
FLOODING ISSUES. HOWEVER, IF RAIN AMOUNTS KEEP TRENDING UPWARDS,  
ESPECIALLY IF IT OCCURS IN A SHORTER TIME PERIOD AS THE NAM NEST  
IS DEPICTING, THEN SOME FLOODING, PRIMARILY IN AREAS OF POOR  
DRAINAGE IS POSSIBLE.  
 

 
   
LONG TERM /MONDAY THROUGH THURSDAY/
 
 
AS MENTIONED BY THE PREVIOUS SHIFT, THROUGH NEXT WEEK, THE UPPER AIR  
PATTERN ISN'T A TRUE BLOCKING PATTERN, BUT CERTAINLY BECOMES MUCH  
LESS PROGRESSIVE, WITH THE LONGWAVE VERY SLOWLY PROGRESSING EAST.  
WHAT THIS MEANS FOR OUR AREA IS THAT ONCE THE CLOSED LOW  
(RESPONSIBLE FOR THE WEEKEND PRECIPITATION) WEAKENS AND SHIFTS A BIT  
FURTHER EAST, THE MID LEVEL RIDGE WILL BE BUILDING OVER THE EASTERN  
U.S. FOR MOST OF THE WEEK. CONSEQUENTLY, EXPECT DRY WEATHER AND A  
STEADY WARMING TREND FROM MONDAY THROUGH AT LEAST WEDNESDAY. BY  
WEDNESDAY, FORECAST TEMPERATURES ARE 15 DEGREES ABOVE NORMAL, WITH  
HIGHS IN THE 70S AND 80S. SOME GUIDANCE STILL DEPICTS TEMPERATURES  
EVEN HIGHER, WITH WIDESPREAD 80S, AND SOME PLACES GETTING CLOSE TO  
90. FOR NOW, I'VE STAYED CLOSE TO A MEAN OF THE GUIDANCE, AS THIS  
WILL BE DEPENDENT ON HOW QUICKLY THE RIDGE AXIS SHIFTS EAST OVER OUR  
REGION.  
 
THURSDAY...DEPENDING ON HOW QUICKLY THE MID LEVEL RIDGE SHIFTS EAST  
(AND POSSIBLY WEAKENS), A CLOSED LOW LIFTING THROUGH THE GREAT LAKES  
REGION COULD BRING A CHANCE FOR PRECIPITATION AND POSSIBLY A COLD  
FRONT (THOUGH THIS WOULD AT THE EARLIEST PROBABLY NOT ARRIVE UNTIL  
FRIDAY, IF AT ALL). HOWEVER, THERE IS SOME QUESTION ABOUT THIS, AS  
THE ECMWF AND CMC SHOW THE LOW LIFTING WELL INTO SOUTHERN CANADA  
BEFORE THE RIDGE BEGINS TO SHIFT EAST, WHILE THE GFS DEPICTS THE  
RIDGE AXIS SHIFTING OFF SHORE AS EARLY AS THURSDAY. A SOLUTION SUCH  
AS THE ECMWF AND CMC WOULD BE WARMER AND LIKELY DRY THROUGH THE WORK  
WEEK, WHILE WITH THE GFS, WE COULD SEE PRECIPITATION THURSDAY, OR AT  
THE VERY LEAST INCREASED CLOUD COVER LIMITING THE WARMING TREND AS  
EARLY AS THURSDAY. FOR NOW, HAVE STAYED CLOSE TO THE PREVIOUS  
FORECAST AND A BLEND OF GUIDANCE.  
 

 
   
AVIATION /08Z FRIDAY THROUGH TUESDAY/
 
 
THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG,  
KMIV, KACY AND SURROUNDING AREAS.  
 
TODAY...VFR CONDITIONS EXPECTED. WEST WINDS 5-10 KNOTS EARLY,  
INCREASING TO 10-15 KNOTS THIS MORNING INTO THIS AFTERNOON WITH  
GUSTS OF 20-25 KNOTS.  
 
TONIGHT...VFR CONDITIONS EXPECTED. WEST WINDS 5-10 KNOTS, BECOMING  
LIGHT AND VARIABLE THIS EVENING AND OVERNIGHT.  
 
OUTLOOK...  
 
SATURDAY...MOSTLY VFR CONDITIONS EXPECTED THROUGH THE DAY. CLOUDS  
WILL INCREASE AND CEILINGS WILL LIKELY GRADUALLY LOWER IN THE  
AFTERNOON. SOUTHWEST WIND 5 TO 15 KT. HIGH CONFIDENCE.  
 
SATURDAY NIGHT...MVFR CEILINGS ARE LIKELY TO DEVELOP DURING THE  
EVENING. CEILINGS COULD LOWER TO IFR THROUGH THE OVERNIGHT AND SOME  
VISIBILITY RESTRICTIONS ARE POSSIBLE AS RAIN SPREADS OVER THE  
REGION. WINDS GRADUALLY SHIFTING FROM SOUTH TO EAST TO NORTHEAST AT  
AROUND 10 KT. MODERATE CONFIDENCE ON THE OVERALL PATTERN, BUT LOW  
CONFIDENCE ON TIMING DETAILS.  
 
SUNDAY-SUNDAY NIGHT... LINGERING MVFR AND IFR CEILINGS WITH RAIN IS  
POSSIBLE SUNDAY MORNING, BUT RELATIVELY QUICK IMPROVEMENT TO VFR  
CONDITIONS IS LIKELY THROUGH THE AFTERNOON. NORTHEAST WIND OF 5 TO  
10 KT SHIFTING TO NORTHWEST AND INCREASING TO AROUND 15 KT WITH  
GUSTS TO 25 KT IN THE AFTERNOON AND THROUGH THE NIGHT. MODERATE  
CONFIDENCE.  
 
MONDAY-MONDAY NIGHT... VFR CONDITIONS EXPECTED. NORTHWEST WIND 10 TO  
15 KT DECREASING OVERNIGHT AND BECOMING LIGHT AND VARIABLE. HIGH  
CONFIDENCE.  
 
TUESDAY-TUESDAY NIGHT... VFR CONDITIONS EXPECTED. LIGHT WINDS IN THE  
MORNING BECOMING SOUTHWEST AT 5 TO 10 KT, THEN LIGHT AND VARIABLE  
AGAIN OVERNIGHT. HIGH CONFIDENCE.  
 

 
   
MARINE
 
 
TODAY...SMALL CRAFT ADVISORY REMAINS IN EFFECT, ALTHOUGH WINDS WILL  
DIMINISH THROUGH THE DAY. WINDS MAY BE STRONGEST NEAR THE COAST  
DURING THE DAYTIME WHERE BETTER MIXING WILL LIKELY OCCUR.  
 
TONIGHT...CONDITIONS EXPECTED TO REMAIN BELOW ADVISORY LEVELS.  
 
OUTLOOK...  
 
SATURDAY... WINDS AND SEAS SHOULD STAY BELOW SCA CRITERIA.  
 
SATURDAY NIGHT-SUNDAY NIGHT... SCA CONDITIONS WILL DEVELOP FROM  
SOUTH TO NORTH ON SATURDAY NIGHT AS AN AREA OF LOW PRESSURE PASSES  
OVER OR JUST SOUTH OF THE REGION. SOUTH OR SOUTHEAST WIND GUSTS WILL  
INCREASE TO AROUND 25 KT, WITH SEAS BUILDING TOWARDS 5 FT. SUNDAY  
WILL SEE A WIND SHIFT TO NORTHWEST WITH GUSTS OF 25 TO 30 KT LIKELY  
AND SEAS OF 5 TO 7 FT. LATE SUNDAY NIGHT, DELAWARE BAY MAY FALL  
BELOW SCA LEVELS, BUT THE ATLANTIC COASTAL WATERS WILL SEE CONTINUED  
5 TO 6 FT SEAS AND NORTHWEST WIND GUSTS TO AROUND 25 KT.  
 
MONDAY-TUESDAY... TRENDING TOWARDS SUB-SCA CONDITIONS ON MONDAY AND  
BEYOND, WITH DECREASING WINDS AND SEAS. QUIET CONDITIONS LIKELY ON  
THE WATERS FOR MOST OF NEXT WEEK.  
 

 
   
FIRE WEATHER
 
 
ANOTHER DRY AND BREEZY DAY IS EXPECTED TODAY WITH RH VALUES LOWERING  
INTO THE LOW TO MID 20S, AND WIND GUSTS 20-30 MPH AT TIMES. AFTER  
COORDINATION WITH STATE PARTNERS, IT WAS DETERMINED THAT FUELS ARE  
STILL NOT TO CRITICAL LEVELS TO NEED A RED FLAG WARNING, THEREFORE  
AN SPECIAL WEATHER STATEMENT WILL BE ISSUED TO HIGHLIGHT THE DRY AND  
BREEZY CONDITIONS. FIRE WEATHER CONCERNS SHOULD DIMINISH FURTHER  
OVER THE WEEKEND WITH LIGHTER WINDS ON SATURDAY AND RAIN BY SATURDAY  
NIGHT.  
 

 
   
PHI WATCHES/WARNINGS/ADVISORIES
 
 
PA...FREEZE WARNING UNTIL 9 AM EDT THIS MORNING FOR PAZ060>062-  
101>106.  
FROST ADVISORY UNTIL 9 AM EDT THIS MORNING FOR PAZ070-071.  
NJ...FREEZE WARNING UNTIL 9 AM EDT THIS MORNING FOR NJZ001-007>010-  
012-013-015-020>022-027.  
FROST ADVISORY UNTIL 9 AM EDT THIS MORNING FOR NJZ016>019.  
DE...FROST ADVISORY UNTIL 9 AM EDT THIS MORNING FOR DEZ001>003.  
MD...FROST ADVISORY UNTIL 9 AM EDT THIS MORNING FOR MDZ012-015-019-  
020.  
MARINE...SMALL CRAFT ADVISORY UNTIL 6 PM EDT THIS EVENING FOR  
ANZ450>453.  
SMALL CRAFT ADVISORY UNTIL NOON EDT TODAY FOR ANZ430-431-454-  
455.  
 

 
 

 
SYNOPSIS...JOHNSON  
NEAR TERM...ROBERTSON  
SHORT TERM...JOHNSON  
LONG TERM...JOHNSON  
AVIATION...JOHNSON/ROBERTSON  
MARINE...JOHNSON/ROBERTSON  
FIRE WEATHER...WFO PHI  
 
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