-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KPHI 210121
AFDPHI

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ
921 PM EDT MON MAY 20 2013

.SYNOPSIS...
A FRONTAL BOUNDARY WILL BE LOCATED NORTH OF THE REGION THROUGH
WEDNESDAY WITH HIGH PRESSURE OFFSHORE. A COLD FRONT WILL MOVE THROUGH
THE REGION THURSDAY INTO FRIDAY. THIS FRONT WILL BE FOLLOWED BY A
HIGH PRESSURE SYSTEM WHICH WILL BE IN PLACE THROUGH HOLIDAY THE
WEEKEND.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
-- Changed Discussion --
THE 9-930 PM UPDATES: 630 PM FCST UPDATE ON TRACK. A QUIET NOW AND
WAITING TO SEE HOW EXTENSIVE THE FOG WILL BE.

HAVING REVIEWED THE 12Z/20 ECMWF...THE 12 AND 18Z GFS AND 18Z NAM...THE
FCST SCENARIO TONIGHT AND TUESDAY IS BASED ON A BLEND OF THE ECMWF/NAM
SINCE THE GFS HAS NOT DEFINED THE LOW LVL MOISTURE AS WELL AS THE
NAM/ECMWF.

ITS DOUBTFUL WHETHER ANY MORE SHOWERS WILL OCCUR IN OUR AREA THIS
EVENING. THE SECOND SMALL GROUP OF ISOLATED SHOWERS IN CUMBERLAND
COUNTY AROUND 8 PM HAS SINCE DISSIPATED.

THE LACK OF MID AND UPPER LVL CLOUDINESS OVERNIGHT OPENS THE DOOR
FOR AT LEAST PATCHY DENSE FOG AFTER MIDNIGHT. SINCE TEMP/DEWPOINTS
SPREADS AT 22Z WERE STILL LARGE...DELAYED THE FOG FORMATION ABOUT
6 HOURS. ITS SUMMERLIKE SITN SO NOT SURE HOW EXTENSIVE THE FOG BUT
WITH LIGHT WIND AND DECENT RADIATION CONDITIONS WE SHOULD SEE
AREAS OF DENSE FOG DEVELOP LATE.
-- End Changed Discussion --

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH 6 PM TUESDAY/...
HIGH PRESSURE WILL REMAIN WELL OFF THE COAST...PUMPING IN WARM AND
HUMID AIR ACROSS THE REGION. A WEAK UPPER LOW WILL BE MEANDERING
ACROSS VA/MD DURING THE DAY. UPPER (H5/H7) HEIGHTS WILL BE SLOWLY
BUILDING EARLY...THEN BEGINNING TO LOWER LATE.

SLOW IMPROVEMENT IN THE CIGS/VSBYS EXPECTED TUE. AREAS OF FOG WILL
DISSIPATE BY MID MORNING AND SKIES SHOULD BECOME PARTLY SUNNY BY
AFTERNOON. THERE WILL BE MORE INSTABILITY THAN TODAY...SO A CHC FOR
SHOWERS/TSTMS WILL BE KEPT IN THE FCST FOR TUE. HIGHS WILL CLIMB
INTO THE LOW 80S NORTH AND MID 80S ACROSS THE METRO PHILADELPHIA
AREA. WINDS WILL BE SOUTHWEST AT AROUND 10 MPH.

&&

.LONG TERM /TUESDAY NIGHT THROUGH MONDAY/...
A FRONTAL BOUNDARY WILL BE LOCATED NORTH OF THE REGION THROUGH
WEDNESDAY, WITH HIGH PRESSURE OFFSHORE. MOISTURE ADVECTION WILL
ALSO BE ABUNDANT INTO THE REGION. THIS ADVECTION, ALONG A WITH A
WARM AND MUGGY AIRMASS WILL CONTRIBUTE TO SOME INSTABILITY DURING
THE DAY WEDNESDAY. THE INSTABILITY MAY BE ON THE MAGNITUDE OF 1000
J/KG COUPLED WITH STEEP LAPSE RATES. SHEAR WILL BE VERY WEAK ON
WEDNESDAY WITH LIMITED STORM MOTIONS. PULSE TYPE CONVECTION MAY BE
POSSIBLE PARTICULARLY CLOSER TO THE FRONT DURING THE DAY. FEEL
THIS ACTIVITY WILL BE SCATTERED BUT THE DEGREE OF INSTABILITY MAY
ALLOW FOR A SCATTERED STRONG STORM. THE OVERALL LACK OF SHEAR WILL
LIMIT ANY SEVERE THREAT WEDNESDAY. WITH THE SCATTERED NATURE OF
CONVECTION HIGH TEMPERATURES MAY DIFFER IN SPOTS. HAVE GONE ON THE
WARM SIDE GIVEN THE STRONG SOUTHWEST FLOW INTO THE REGION.

A COLD FRONT WILL APPROACH THE REGION ON THURSDAY AND RIGHT NOW
THE BEST TIMING FOR FRONTAL PASSAGE IS LATER THURSDAY. AS USUAL
THIS MAY NEED TO BE ADJUSTED AS WE GET CLOSER. RAISED POPS TO HIGH
CHANCE BASED ON CURRENT TIMING. THOUGH SEVERAL OF THE GFS ENSEMBLE
MEMBERS AND THE 12Z ECMWF HINT THE FRONT MAY BE A TOUCH SLOWER
THAN MODEL FCSTS. WENT COOLER ON TEMPERATURES THURSDAY DUE TO
CLOUD COVER. THIS MAY ALSO LIMIT THE INSTABILITY POTENTIAL
THURSDAY AHEAD OF THE FRONT THOUGH SLOWER TIMING COULD LEAD TO A
MORE UNSTABLE ENVIRONMENT. OVERALL WENT WITH ELEMENTS OF THE SREF
AND WPC FOR QPF FOR THE FRONTAL PASSAGE. OVERALL, THURSDAY LOOKS
TO HAVE THE HIGHEST POPS.

BEHIND THE FRONT, STRONG HIGH PRESSURE WILL BUILD IN WITH MORE
SUNSHINE AND COOLER TEMPERATURES. HIGHS GENLY IN THE LOWER TO MID
70S, LIGHT WINDS AND SUNNY SKIES ARE LIKELY TO DOMINATE THE
HOLIDAY WEEKEND! DID NOT STRAY MUCH FROM GUIDANCE HERE. THINK THE
GFS MAY BE TO QUICK WITH ANY SYSTEM THAT COMES IN NEXT MONDAY.

&&

.AVIATION /01Z TUESDAY THROUGH SATURDAY/...
-- Changed Discussion --
THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY AND SURROUNDING AREAS.

TONIGHT...VFR. AREAS OF LIFR FOG AND STRATUS SHOULD DEVELOP BETWEEN
05Z-09Z OVER THE COUNTRYSIDE TAF LOCATIONS AND MOVE CLOSE TO KPHL
BETWEEN 10Z-12Z. LIGHT S-SW WIND.

TUESDAY...IFR/LIFR CONDS IN FOG/STRATUS TO START AT 12Z IMPROVES TO
VFR SCT-BKN CLOUDS AOA 3500 FT IN THE AFTN WITH SCT AFTERNOON
SHOWERS/ISOLATED TSTMS POSSIBLE BUT THOSE SHOWERS WERE NOT INCLUDED
IN THE 18Z/20 TAFS. A LOW PROBABILITY OCCURRENCE. THAT CONVECTION
MAY NOT ARRIVE TIL AFTER DARK?


OUTLOOK...
WEDNESDAY... MAINLY VFR WITH THE POTENTIAL FOR SCATTERED SHOWERS AND
THUNDERSTORMS AT ALL TAF SITES IN THE AFTERNOON TO REDUCE CONDITIONS
TO MVFR OR IFR FOR BRIEF PERIODS. SOUTHWEST WINDS 5 TO 10 KNOTS.

THURSDAY.... VFR TO MVFR IN SHOWERS AND A FEW THUNDERSTORMS.
SOUTHWEST WINDS 5 TO 15 KNOTS TURNING TO THE WEST AT NIGHT.  BEST
CHC FOR SHWRS AND TSTMS THIS WEEK OCCURS THU.

FRIDAY AND SATURDAY... VFR WITH HIGH PRESSURE BUILDING IN AND WINDS
BECOMING LIGHT NORTH OR VARIABLE.
-- End Changed Discussion --

&&

.MARINE...
-- Changed Discussion --
LOWERED THE SWELL A FOOT THROUGH TUESDAY BECAUSE OF A RECENT 1/2
TO 1 FT HIGH BIAS ON THE SWELL. INCREASED THE WNA4 PERIOD SLIGHTLY
TONIGHT SINCE IT WAS UNDERFCSTG BY 1-2 SECONDS.

OVERALL CAPPED SEAS AT 4 FT THROUGH 00Z WED.

THEREFORE NO HEADLINE ANTICIPATED THROUGH AT LEAST TUE AFTN.

THERE MAY BE AREAS OF FOG ON THE WATERS TONIGHT BUT DELAYED THE FOG
BECAUSE OF SW FLOW. CONFIDENCE FOR 2MI OR LESS FOG OVER THE WATER
IS BELOW AVG ON SW ISOBARIC FLOW. A SHOWERS OR TSTM IS POSSIBLE
TUE AFTERNOON BUT CONFIDENCE ON ANY QPF TUESDAY IS BELOW AVG.

OUTLOOK...
SEAS AND WINDS WILL BE BELOW SCA TILL EARLY WEDNESDAY. SEAS ARE
EXPECTED TO BE AT LEAST 5 FEET FROM WEDNESDAY MORNING TILL EARLY
FRIDAY. SEAS AND WINDS WILL DECREASE FRIDAY. WIND WILL ALSO
INCREASE AS THE CDFNT APPROACHES BY THU.  A SCA WILL LIKELY BE
NEEDED FOR A PORTION OF THIS TIME FRAME.
-- End Changed Discussion --

&&

.CLIMATE...
LOW PROB FIRST 90F DAY OF THE SEASON WED AFTN EXCEPTION KILG WHICH
TOUCHED 90F ON 4/10.

&&

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

&&

$$
SYNOPSIS...GAINES/NIERENBERG
NEAR TERM...DRAG 921
SHORT TERM...O`HARA
LONG TERM...GAINES/NIERENBERG
AVIATION...DRAG/NIERENBERG 921
MARINE...DRAG/NIERENBERG 921
CLIMATE...