082  
FXUS61 KPHI 181021  
AFDPHI  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ  
621 AM EDT THU APR 18 2019  
   
SYNOPSIS
 
 
A WARM FRONT WILL SLIDE NORTHWARD ACROSS OUR AREA TODAY INTO  
TONIGHT. LOW PRESSURE TRACKING UP THE OHIO VALLEY FRIDAY WILL SLOWLY  
PUSH A STRONG COLD FRONT TOWARD OUR AREA. AS THE STRONG ENERGY ALOFT  
ARRIVES LATER SATURDAY INTO SUNDAY, THE STRONG COLD FRONT WILL MOVE  
THROUGH. HIGH PRESSURE WILL THEN BUILD INTO THE REGION FROM THE  
SOUTH SUNDAY NIGHT INTO MONDAY, FOLLOWED BY A COLD FRONT MOVING  
THROUGH LATE MONDAY INTO TUESDAY. HIGH PRESSURE THEN BUILDS TO OUR  
NORTH TUESDAY INTO WEDNESDAY.  
 

 
   
NEAR TERM /UNTIL 6 PM THIS EVENING/
 
 
A FEW SPRINKLES OR LIGHT SHOWERS WILL BE POSSIBLE THROUGH THIS  
MORNING, ESPECIALLY ACROSS THE LEHIGH VALLEY, SOUTHERN POCONOS, AND  
NW NJ AS WEAK UPSLOPE FLOW CONTINUES AHEAD OF THE WARM FRONTAL  
PASSAGE. WARM AIR IS ALREADY ADVECTING ACROSS THE AREA AT THE 850 MB  
LEVEL, HOWEVER, IT WILL BE SLOW TO WORK ITS WAY NORTHWARD AT THE  
SURFACE ACROSS THE NORTHERN HALF OF THE CWA. THERE WILL LIKELY BE A  
FAIRLY STARK TEMPERATURE GRADIENT AS THE WARM AIR WORKS ITS WAY  
NORTHWARD BY THIS AFTERNOON WITH SURFACE TEMPERATURES WARMING INTO  
THE LOW 70S FROM ROUGHLY I-195 AND SOUTHWARD WHERE THE FRONT PASSES  
NORTH. ON THE OTHER HAND, KEPT TEMPERATURES ON THE COOL SIDE IN THE  
LOW 60S FOR HIGHS IN THE HIGHER TERRAIN OF THE POCONOS AND FAR NW NJ  
AS COOLER UPSLOPE FLOW WILL BE STUBBORN TO BREAK. WINDS WILL TURN  
SOUTHERLY THROUGH THE AFTERNOON AS THE FRONT WORKS ITS WAY NORTHWARD.  
 

 
   
SHORT TERM /6 PM THIS EVENING THROUGH 6 AM FRIDAY/
 
 
THE WARM FRONT WILL FINALLY WORK ITS WAY THROUGH THE REST OF THE  
REGION TONIGHT. WILL BE A QUIET NIGHT WITH ONLY A FEW SCATTERED  
LIGHT SHOWERS OR SPRINKLES ACROSS THE NORTHERN THIRD OF THE  
FORECAST AREA AS THE FRONT LIFTS ACROSS THE REST OF THE REGION.  
SOUTHERLY FLOW WILL CONTINUE THROUGH THE NIGHT, ADVECTING WARMER  
AND MOISTER AIR NORTHWARD. LOWS WILL BE ON THE MILDER SIDE  
RANGING FROM THE LOW 60S FROM THE PHILADELPHIA METRO SOUTHWARD  
TO THE MID 50S ACROSS THE POCONOS AND NORTHERN NJ.  
 

 
   
LONG TERM /FRIDAY THROUGH WEDNESDAY/
 
 
SUMMARY...WARM AIR AHEAD OF A STRONG SYSTEM FRIDAY AND SATURDAY THEN  
SOME COOLING SUNDAY. HEAVY RAIN AND POTENTIALLY SOME GUSTY  
THUNDERSTORMS MOVE THROUGH LATER FRIDAY INTO SATURDAY. ANOTHER SURGE  
OF WARM AIR MONDAY BEFORE SOME COOLING TAKES PLACE AGAIN TUESDAY AND  
WEDNESDAY.  
 
SYNOPTIC OVERVIEW...A RIDGE IN THE WESTERN ATLANTIC OCEAN FRIDAY  
WHILE A HIGHLY AMPLIFIED TROUGH FROM THE GREAT LAKES TO THE LOWER  
MISSISSIPPI VALLEY CLOSES OFF DURING FRIDAY. THIS FEATURE WILL ROLL  
EAST-NORTHEASTWARD AND DRIVE SURFACE LOW PRESSURE UP THE OHIO  
VALLEY. A STRONG SHORT WAVE ROLLS NORTHWARD JUST AHEAD OF THE CLOSED  
LOW FRIDAY NIGHT AND SATURDAY MORNING, AND A SECONDARY SURFACE LOW  
MOVES NORTHWARD INTO THE VICINITY OF WESTERN PENNSYLVANIA. THE  
AMPLIFIED AND SLOW MOTION OF THIS STORM WILL DELAY THE ARRIVAL OF  
THE STRONG COLD FRONT INTO OUR AREA, WITH THE ACTUAL FRONTAL PASSAGE  
BECOMING MORE OF AN OCCLUSION WITH TIME. THIS CLOSED LOW WILL CROSS  
OUR AREA SUNDAY BEFORE OPENING UP AS IT LIFTS TO OUR NORTHEAST  
SUNDAY NIGHT INTO MONDAY. THE FLOW ALOFT THEN BECOMES MORE ZONAL FOR  
A TIME EARLY NEXT WEEK.  
 
FOR FRIDAY AND SATURDAY...A RATHER ACTIVE TIME AS AN UPPER-LEVEL  
TROUGH CLOSES OFF AS IT SLOWLY ARRIVES INTO THE EAST. THE  
AMPLIFICATION OF THIS SYSTEM RESULTS IN DEEP SOUTHERLY FLOW  
INCREASING SUBSTANTIALLY WELL AHEAD OF THE COLD FRONT, AND THIS WILL  
RESULT IN AMPLE WARM AIR ADVECTION AND EVENTUALLY ASCENT. SOUTHERLY  
SURFACE WINDS SHOULD GUST TO 30-40 MPH FRIDAY AFTERNOON AND EVENING  
ESPECIALLY IN THE COASTAL PLAIN WITH AN INCREASE IN THE LOW-LEVEL  
JET. WHERE THE BOUNDARY LAYER WARMTH IS MAXIMIZED, THEN THE VERTICAL  
MIXING WILL BE MORE EFFICIENT. FRIDAY HOWEVER MAY START WITH AREAS  
OF LOW CLOUDS ESPECIALLY ACROSS THE NORTHERN AREAS.  
 
LARGE SCALE ASCENT AHEAD OF THE CLOSED LOW AND STRONG SHORT WAVE  
WILL TEAM UP WITH A STRONG LOW-LEVEL JET AND POTENTIALLY DRIVE AN  
EMBEDDED SQUALL LINE EASTWARD FRIDAY NIGHT. THE SYSTEM OVERALL  
CONTINUES TO LOOK SLOWER GIVEN THAT IS CLOSES OFF. THE INITIAL  
SURFACE LOW IS FORECAST TO TRACK UP THE OHIO VALLEY, HOWEVER A NEW  
SURFACE LOW SHOULD DEVELOP NEAR THE TRIPLE POINT AHEAD OF STRONG  
SHORT WAVE ENERGY ASSOCIATED WITH THE CLOSED LOW. THIS LOW SHOULD  
GENERALLY TRACK WEST OF OUR AREA, AND THIS ALL MAY TEND TO DELAY THE  
BULK OF THE FORCING AND THEREFORE SHOWERS/THUNDER UNTIL LATE FRIDAY  
AFTERNOON AND ESPECIALLY OVERNIGHT FRIDAY. OVERALL THE DAYTIME HOURS  
OF FRIDAY MAY HAVE MORE SCATTERED SHOWERS AND PERHAPS SOME THUNDER  
WITHIN THE WARM SECTOR, THEN THE MORE ORGANIZED CONVECTION ARRIVES  
FRIDAY NIGHT.  
 
THE SEVERE WEATHER POTENTIAL DEPENDS ON THE AMOUNT OF INSTABILITY AS  
LAPSE RATES ALOFT DO NOT LOOK AS STEEP ALONG WITH PERHAPS SOME LOWER  
CLOUDS BENEATH AN INVERSION. THE TIMING LOOKS LATER AND THEREFORE  
NOT AS FAVORABLE REGARDING PEAK INSTABILITY, HOWEVER DEEP SOUTHERLY  
FLOW MAY COMPENSATE SOMEWHAT EVEN AT NIGHT AND THE SHEAR LOOKS  
IMPRESSIVE. THE STRENGTH OF THE SHEAR ALONE CAN CERTAINLY COMPENSATE  
FOR THE LOWER INSTABILITY ESPECIALLY AS CONVECTION ENCOUNTERS THE  
LOW-LEVEL JET, THEREFORE A SQUALL LINE OR A BROKEN BAND OF LOWER  
TOPPED CONVECTION MAY ROLL THROUGH FRIDAY NIGHT WITH LOCALLY STRONG  
TO DAMAGING WIND GUSTS. THE GREATEST SEVERE THREAT CONTINUES TO BE  
DELMARVA SOUTHWARD, HOWEVER FURTHER NORTH THE STRONG SHEAR MAY STILL  
RESULT IN SOME ROBUST CONVECTIVE WINDS REACHING THE SURFACE. AS  
SUCH, SPC HAS EXPANDED THE MARGINAL RISK NORTHWARD.  
 
THE GUIDANCE INDICATES A NARROW BUT INTENSE PLUME OF MOISTURE AHEAD  
OF THIS CLOSED LOW RESULTING IN PW VALUES INCREASING TO 1.5-2.0  
INCHES. THE FREEZING LEVEL IS FORECAST TO BE RATHER HIGH, THEREFORE  
WARM RAIN PROCESSES WILL RESULT IN MORE EFFICIENT RAINFALL RATES.  
THE FLOW INCREASINGLY MORE MERIDIONAL WILL RESULT IN HEAVY RAIN AND  
POTENTIALLY SOME FLASH FLOODING ESPECIALLY IF CELLS TEND TO TRAIN IN  
A SOUTH TO NORTH FASHION. GIVEN UNCERTAINTY ON THE MORE FOCUSED AREA  
OF HEAVIEST RAIN, HELD OFF ON ANY WATCHES FOR NOW. A MENTION  
CONTINUES IN HAZARDOUS WEATHER OUTLOOK. THE SURFACE SYSTEM IS  
FORECAST TO LIFT EAST AND NORTH OF OUR AREA SATURDAY, HOWEVER THE  
CLOSED LOW ALOFT WILL BE ARRIVING THEREFORE AFTER A WET START TO  
SATURDAY THERE MAY STILL BE SOME INSTABILITY SHOWERS DURING THE  
AFTERNOON AND AT NIGHT.  
 
FOR SUNDAY AND MONDAY...THE UPPER LOW MOVES ACROSS OUR AREA SUNDAY,  
THEN IT SHOULD GRADUALLY BE WEAKENING AND LIFTING TO OUR NORTHEAST  
SUNDAY NIGHT. THERE SHOULD STILL BE SOME INSTABILITY SHOWERS AROUND  
ESPECIALLY ON SUNDAY. HIGH PRESSURE THEN STARTS TO BUILD IN FROM THE  
SOUTH, HOWEVER A WEAK COLD FRONT MOVES THROUGH LATER MONDAY INTO  
TUESDAY. THE TIMING OF THIS FRONT WILL DEPEND ON HOW FAST THE CLOSED  
LOW OPENS UP AND LIFTS OUT. IT WILL TURN MUCH WARMER AGAIN MONDAY  
AHEAD OF THE FRONT ASSUMING THE CLOSED LOW HAS ALREADY EXITED THE  
REGION.  
 
FOR TUESDAY AND WEDNESDAY...HIGH PRESSURE IS FORECAST TO BUILD TO  
OUR NORTH DURING THIS TIME FRAME RESULTING IN MORE OF AN ONSHORE  
FLOW. THERE IS SOME TIMING DETAILS THAT NEED TO BE WORKED OUT  
REGARDING THE COLD FRONT MENTIONED ABOVE, AS SOME GUIDANCE IS SLOWER  
WITH IT AND BRINGS IT THROUGH ON TUESDAY. WE LEANED CLOSER TO THE  
WPC TIMING. A SHORT WAVE TROUGH MAY START TO ARRIVE BY LATER  
WEDNESDAY, THEREFORE KEPT SOME LOW POPS FOR POSSIBLE SHOWERS IN FOR  
BOTH DAYS.  
 

 
   
AVIATION /10Z THURSDAY THROUGH MONDAY/
 
 
THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG,  
KMIV, KACY AND SURROUNDING AREAS.  
 
TODAY...MVFR CIGS THIS MORNING. A BRIEF LIGHT SHOWER OR SPRINKLE  
IS POSSIBLE AT RDG, ABE, OR TTN. IMPROVEMENT TO VFR CONDITIONS  
BY EARLY AFTERNOON AS THE WARM FRONT LIFTS ACROSS THE REGION  
FROM SOUTH TO NORTH. SOUTHEASTERLY WINDS FROM 5 TO 10 KNOTS  
TURNING SOUTHERLY TO EVEN SOUTHWESTERLY AROUND 10 KNOTS THIS  
AFTERNOON. SOME HIGHER GUSTS UP TO 15 KNOTS POSSIBLE, ESPECIALLY  
AT ACY.  
 
TONIGHT...GENERALLY VFR WITH STRENGTHENING SOUTHERLY WINDS EXPECTED.  
MARGINAL MVFR CIGS POSSIBLE TOWARD FRIDAY MORNING. LOW LEVEL  
WIND SHEAR IS EXPECTED TO DEVELOP OVERNIGHT FOR THE DELAWARE  
VALLEY AND COASTAL TAF SITES.  
 
OUTLOOK...  
FRIDAY AND SATURDAY...AREAS OF MVFR CEILINGS FRIDAY BECOMING  
WIDESPREAD MVFR TO IFR FRIDAY NIGHT INTO SATURDAY MORNING, THEN  
IMPROVING TO VFR BY LATER SATURDAY. A SOUTHERLY LOW-LEVEL JET SHOULD  
RESULT IN LOW-LEVEL WIND SHEAR FRIDAY NIGHT ESPECIALLY FOR THE I-95  
TERMINALS ON EASTWARD.  
 
SHOWERS AND SOME THUNDERSTORMS ESPECIALLY LATER FRIDAY AFTERNOON  
INTO SATURDAY MORNING WILL RESULT IN IFR VISIBILITIES AT TIMES.  
SOUTHERLY WINDS 10-20 KNOTS WITH GUSTS OF 25-35 KNOTS.  
 
SUNDAY...VFR CEILINGS MAY TRANSITION TO MVFR CEILINGS AT TIMES WITH  
SOME SHOWERS AROUND. SOUTH TO SOUTHWEST WINDS 10-15 KNOTS, BECOMING  
WEST-SOUTHWEST AT NIGHT.  
 
MONDAY...VFR. WEST-SOUTHWEST WINDS 5-10 KNOTS.  
 

 
   
MARINE
 
 
TODAY...SCA CONDITIONS WITH EASTERLY WINDS 15 TO 20 KNOTS. WINDS  
TURNING TO A SOUTHERLY DIRECTION THROUGH THE AFTERNOON. GUSTS UP TO  
25 KNOTS POSSIBLE OVER THE OCEAN WATERS AND OVER THE BAY. SEAS  
BETWEEN 4 TO 6 FEET.  
 
TONIGHT...SOUTHERLY WINDS INCREASING TO 20 TO 25 KNOTS, WITH GUSTS  
UP TO 30 KNOTS POSSIBLE. SEAS BUILDING TO 5 TO 7 FEET.  
 
OUTLOOK...  
FRIDAY...SMALL CRAFT ADVISORY CONDITIONS EXPECTED AS SEAS BUILD WELL  
ABOVE 5 FEET AND WINDS GUST TO AROUND 30 KNOTS. SOUTHERLY GALE FORCE  
GUSTS ARE POSSIBLE, HOWEVER THIS WILL BE WITHIN A WARM AIR ADVECTION  
REGIME LIMITING MIXING OVER THE COLDER WATERS. FOR NOW, EXTENDED THE  
ADVISORY THROUGH 22Z/6PM FOR NOW AND ALSO ADDED DELAWARE BAY.  
DESPITE THE WINDS, AREAS OF MARINE FOG MAY DEVELOP FOR A TIME AS  
WARM MOIST AIR MOVES OVER THE COOLER WATERS.  
 
SATURDAY...SMALL CRAFT ADVISORY CONDITIONS WITH GUSTS OF UP TO 25  
KNOTS, HOWEVER WINDS DIMINISH AT NIGHT. SEAS 6-10 FEET ON THE OCEAN  
WATERS.  
 
SUNDAY...WINDS BELOW ADVISORY CRITERIA, HOWEVER SEAS REMAINING  
ELEVATED ON THE OCEAN ZONES ALTHOUGH DUE SLOWLY SUBSIDE AT NIGHT.  
 
MONDAY...CONDITIONS (SEAS) DROPPING BELOW ADVISORY CRITERIA.  
 

 
   
TIDES/COASTAL FLOODING
 
 
EAST TO SOUTHEAST SURFACE FLOW TURNS MORE SOUTHERLY AND INCREASES BY  
LATER TODAY AND CONTINUES THROUGH FRIDAY NIGHT. THIS WILL INCREASE  
THE SURGE ABOVE ASTRONOMICAL TIDE.  
 
MINOR TIDAL FLOODING IS ANTICIPATED, AND LOCALIZED MODERATE TIDAL  
FLOODING IS POSSIBLE WITH THE HIGH TIDE TONIGHT AND ESPECIALLY  
FRIDAY NIGHT. WITH THE FULL MOON ON FRIDAY, THE ASTRONOMICAL TIDE  
BOTH TONIGHT AND FRIDAY NIGHT IS LESS THAN ONE FOOT BELOW MINOR  
FLOODING THRESHOLDS. THE SURGE IS EXPECTED TO BE AT LEAST ONE FOOT  
ON THE OCEAN FRONT DESPITE MORE OF A SOUTHERLY WIND COMPONENT.  
SPOTTY MINOR TIDAL FLOODING IS POSSIBLE WITH THE DAYTIME HIGH TIDE  
TODAY. HOWEVER, WATER LEVELS WITH THIS TIDE CYCLE ARE EXPECTED TO  
REMAIN BELOW ADVISORY LEVEL.  
 
THERE ARE A FEW FACTORS THAT COULD FURTHER INCREASE TIDAL FLOODING  
CONCERNS. FIRST, HEAVY RAIN IS POSSIBLE FRIDAY NIGHT. WHILE THE  
HEAVIEST RAIN SHOULD BE WEST OF THE COASTAL PLAIN, ANY HIGHER  
RAINFALL RATES NEAR THE TIME OF THE FRIDAY NIGHT HIGH TIDE COULD  
ENHANCE TIDAL FLOODING ABOVE THE CURRENT FORECAST. SECONDLY, THOUGH  
THE FRIDAY MORNING HIGH TIDE IS A LOWER ASTRONOMICAL TIDE AND THERE  
WON'T BE MUCH FLOODING ON THE OCEANFRONT, THERE COULD BE IMPACTS ON  
THE BACK BAYS AS THERE WILL BE LIMITED OPPORTUNITY FOR WATER TO  
EVACUATE AFTER THE THURSDAY NIGHT HIGH TIDE GIVEN STRONG SOUTHERLY  
WINDS.  
 
FOR THE NORTHEASTERN SHORE OF THE CHESAPEAKE BAY, THIS AREA WILL BE  
SLOWER TO SEE ANY TIDAL FLOODING WITH THIS SYSTEM. HOWEVER, IF THE  
CURRENT MODEL TRENDS CONTINUE THEN MINOR TIDAL FLOODING WILL BE  
POSSIBLE WITH THE SATURDAY MORNING HIGH TIDE.  
 

 
   
PHI WATCHES/WARNINGS/ADVISORIES
 
 
PA...NONE.  
NJ...NONE.  
DE...NONE.  
MD...NONE.  
MARINE...SMALL CRAFT ADVISORY UNTIL 6 PM EDT FRIDAY FOR ANZ450>455.  
SMALL CRAFT ADVISORY FROM 2 PM THIS AFTERNOON TO 6 PM EDT  
FRIDAY FOR ANZ430-431.  
 

 
 

 
SYNOPSIS...GORSE  
NEAR TERM...DAVIS  
SHORT TERM...DAVIS  
LONG TERM...GORSE  
AVIATION...DAVIS/GORSE  
MARINE...DAVIS/GORSE  
TIDES/COASTAL FLOODING...  
 
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