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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