Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 300005
AFDPHI

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ
805 PM EDT TUE JUL 29 2014

.SYNOPSIS...
WEAK HIGH PRESSURE OVER THE AREA MERGES WITH THE STRONGER WESTERN
ATLANTIC HIGH PRESSURE LATE THIS WEEK...FORCING THE OFFSHORE FRONT
TO BACK IN TOWARD THE COAST AS A WARM FRONT THIS WEEKEND.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
QUIET WEATHER TONIGHT AS WEAK HIGH PRESSURE BUILDS IN FROM THE WEST.
PW VALUES ARE LOW ENOUGH, BELOW ONE INCH, THAT ANY VORTICITY
IMPULSES TONIGHT ARE NOT EXPECTED TO CREATE ANY PRECIPITATION.

THE MAIN FOCUS FOR TONIGHT WILL BE THE LOW TEMPERATURE FORECAST.
MODEL GUIDANCE IS INDICATING LOW TEMPERATURES GETTING WITHIN A
COUPLE OF DEGREES OF RECORD LOWS FOR VIRTUALLY ALL SITES. THE BIG
QUESTION WILL BE HOW FAST AND HOW MUCH CLEARING TAKES PLACE LATER
THIS EVENING INTO THE OVERNIGHT HOURS. IF THE CURRENT CLOUD COVER
LASTS LONGER INTO THE NIGHT, WE`LL HAVE LESS TIME FOR GOOD
RADIATIONAL COOLING EVEN WITH LIGHT AND VARIABLE/CALM WINDS. IF WE
BREAK OUT FASTER, MUCH MORE COOLING WILL TAKE PLACE. FOR NOW, WE
STAY CLOSE TO MAV/MET MOS AND MOSGUIDE BLEND.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/...
WEAK HIGH PRESSURE CONTINUES TO MOVE ACROSS THE AREA EARLY IN THE
DAY WEDNESDAY. HOWEVER, LATE IN THE DAY, A WEAK SURFACE TROUGH MAY
DEVELOP ACROSS OUR WESTERN ZONES, AND THERE ARE SOME WEAK VORTICITY
IMPULSES THAT MAY SWING THROUGH THE BROAD TROUGH ALOFT AS WELL.
THIS, COMBINED WITH SOME MOISTURE MAY HELP ENHANCE THE
SHOWER/THUNDERSHOWER POTENTIAL LATE IN THE DAY. THERE IS NOT MUCH
INSTABILITY ACROSS OUR AREA, BUT THERE MAY BE JUST ENOUGH FOR AN
ISOLATED THUNDERSHOWER TO OCCUR FOR OUR NORTHWESTERN AREAS. THIS
BEING SAID, WE EXPECT MOST OF THE FOCUSED ACTIVITY TO REMAIN TO OUR
NORTH.

925/850 MB TEMPERATURES INCREASE A DEGREE OR TWO WEDNESDAY, AND
THICKNESSES INCREASE EVER SO SLIGHTLY AS WELL. THEREFORE HIGHS ON
WEDNESDAY SHOULD BE A FEW DEGREES WARMER THAN TUESDAY. WE USED A
MAV/MET MOS, MOSGUIDE, AND MIXED 925MB TEMP BLEND FOR HIGHS ON
WEDNESDAY.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/...
500 MB: THE LARGE TROUGH IN THE EASTERN UNITED STATES WEAKENS LATE
THIS WEEK, THEN PROBABLY FLATTENS EARLY NEXT WEEK.

TEMPERATURES: CALENDAR DAY AVERAGES SEVERAL DEGREES BELOW NORMAL
THURSDAY THEN NEAR NORMAL THEREAFTER POSSIBLY WARMING ABOVE NORMAL
EARLY NEXT WEEK. THIS WILL ALLOW JULY TO AVERAGE NEAR OR SLIGHTLY
BELOW NORMAL AT MOST LOCATIONS EXCEPT AT LEAST A DEGREE BELOW NORMAL
AT KACY.

FORECAST BASIS:  UNLESS OTHERWISE NOTED THE LONG TERM PERIOD OF THIS
FORECAST IS GENERALLY BASED ON A 50 50 BLEND OF THE 12Z/29 NAM/GFS
MOS FOR THE PERIOD WEDNESDAY NIGHT - THURSDAY NIGHT... THE 12Z/29 GFS
MOS FRIDAY AND FRIDAY NIGHT AND THEN THEREAFTER (SATURDAY ONWARD)
THE 1522Z/29 WPC GRIDDED GUIDANCE WHICH AT TIMES IS BLENDED 50 50
WITH THE 12Z/29 MEXMOS. ALL THIS IS EVENTUALLY CHECKED AGAINST THE
12Z/29 ECMWF TO SEE IF WE`RE IN THE BALLPARK WITH ECMWF CYCLICALLY
PERSISTENT 18Z 2METER TEMPERATURES. THE PRECIPITATION PROBABILITY
GUIDANCE IS CHECKED AGAINST THE 09Z/29 SREF POPS FOR .01 IN 3 HRS
THROUGH 00Z FRIDAY AND THEREAFTER THE 12Z/28 GEFS 6HR POPS FOR
0.05

THE DAILIES...

WEDNESDAY NIGHT - THURSDAY EVENING...CONTINUING THE PRIOR MID SHIFT
FORECAST. MOSTLY DRY ACROSS THE REGION OUTSIDE THE POSSIBILITY OF
ISOLATED INSTABILITY SHOWERS ACROSS THE NORTHERN PORTIONS OF THE
CWA THURSDAY AFTERNOON. COLD POOL INTERACTION WITH A PASSING
SHORTWAVE MAY BE ENOUGH TO GET AIRMASS SHOWERS TO OCCUR.

FRIDAY...TRANSITIONAL DAY WITH THE LOOKS OF AN INVERTED TROUGH
SITTING OFF THE ATLANTIC COAST...THE OLD BAROCLINIC ZONE THAT PUSHED
THROUGH ON MONDAY. INTERIOR INSTABILITY AND WARM AIR ADVECTION ASSOCIATED
WITH SHORT WAVES MOVING NEWD THROUGH THE MID ATLC STATES APPEAR
TO MAKE SHOWERS PROBABLE FOR EASTERN PA. IT SHOULD BE UNSTABLE
ENOUGH FOR THUNDER THERE DURING THE AFTN.


FRIDAY NIGHT - WEEKEND...STILL UNCERTAINTY IN SHOWER TIMING DETAILS
WHICH WILL AFFECT MAX TEMPS THIS WEEKEND BUT IT APPEARS THAT THERE HAS
TO BE A PERIOD OF SHOWERS...SOME BRIEFLY HEAVY...WITH ISOLATED EMBEDDED
TSTMS. THIS IN ASSN WITH A PAIR OF SHORT WAVES MOVING NEWD THROUGH
THE TROUGH. AGAIN...TIMING THE PRIMARY PERIOD OF SHOWERS, IN MY
OPINION, IS STILL AS OF THIS WRITING...QUITE UNCERTAIN. MAX TEMPS
SATURDAY COULD DIFFER 5 DEGREES EITHER WAY OF OUR CURRENT FCST...DEPENDING
ON WHETHER ITS RAINING BETWEEN 11AM AND 3 PM. I MAINTAINED OUR
PRIOR MID SHIFT FCST TEMPS FOR THIS DAY...NOT USING THE COOLER WPC
GUIDANCE, NOR THE MUCH WARMER GFS GUIDANCE. THE ECMWF IS EVEN COOLER
THAN THE WPC GUIDANCE.

MONDAY - TUESDAY...A SLIGHT DRYING WARMING TREND THOUGH NOT ENTIRELY
VOID OF ANY SHOWERS WITH A FEW MORE WEAK SHORTWAVES SHEARING NEWD
NEARBY OR OVERHEAD.

&&

.AVIATION /00Z WEDNESDAY THROUGH SUNDAY/...
THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY AND SURROUNDING AREAS.

VFR CONDITIONS WILL CONTINUE ACROSS THE TAF SITES THROUGH WEDNESDAY.
WEST-TO NORTHWEST WINDS THIS AFTERNOON INTO THIS EVENING WILL BECOME
LIGHT AND VARIABLE/CALM OVERNIGHT, BEFORE INCREASING OUT OF THE WEST
TO SOUTHWEST DURING THE DAY WEDNESDAY.

OUTLOOK...

WEDNESDAY NIGHT - THURSDAY...MOSTLY VFR. ISOLATED SHOWER OR THUNDERSTORM
POSSIBLE MAINLY NORTH OF KABE THU AFTN.

FRIDAY - SATURDAY - SUNDAY...MOSTLY VFR CIGS. IFR CONDITIONS IN SHOWERS
AND ISOLATED EMBEDDED TSTMS EXPECTED SOMETIME BETWEEN FRIDAY AFTN AND
MIDDAY SATURDAY.

&&

.MARINE...
SUB-ADVISORY CONDITIONS WILL CONTINUE THROUGH WEDNESDAY AS WINDS
REMAIN 10-15 KNOTS OR LESS AND SEAS REMAIN 2-3 FEET OR LESS.

OUTLOOK...
WEDNESDAY NIGHT - SATURDAY...SUB-SCA CONDITIONS EXPECTED. SHOWERS
AND TSTMS SHOULD OCCUR SOMETIME LATER FRIDAY INTO SATURDAY.

NEXT WEEK: WE ARE MONITORING THE GFS OPERATIONAL RUN DEPICTION
(FOR TWO DAYS OF FORECAST CYCLES, THOUGH TODAYS 12Z/29 RUN BACKED
OFF) OF A TROPICAL SYSTEM IN THE ATLANTIC BASIN FOR EARLY NEXT
WEEK. THE ECMWF OPERATIONAL VERSION HAS BEEN MUCH MORE CONSERVATIVE.
WE COULD BE SEEING A 1 FOOT 15 SECOND SWELL ARRIVE ALONG OUR SHORES
NEXT MONDAY... MAYBE INCREASE TO A 2 FOOT 13 SECOND SE SWELL NEXT
TUESDAY...AGAIN...IF A TROPICAL SYSTEM OCCURS AS OUTLOOKED TODAY.

&&

.CLIMATE...
A COOL TO EVEN LOCALLY CHILLY OVERNIGHT IS ANTICIPATED. THIS MAY
RESULT IN SOME RECORD LOW TEMPERATURES BEING APPROACHED WEDNESDAY
MORNING. AS OF NOW, THE FORECAST LOW TEMPERATURES ARE NOT RECORD
TYING OR SETTING. THE RECORD LOW TEMPERATURES FOR JULY 30TH ARE
LISTED BELOW FOR REFERENCE.

PHILADELPHIA, PA....58 DEGREES SET IN 1981 AND 1914.
WILMINGTON, DE......55 DEGREES SET IN 1956.
ALLENTOWN, PA.......50 DEGREES SET IN 1997 AND 1968.
READING, PA.........52 DEGREES SET IN 1981.
ATLANTIC CITY, NJ...53 DEGREES SET IN 1981 AND 1968.
TRENTON, NJ.........54 DEGREES SET IN 1997.
GEORGETOWN, DE......54 DEGREES SET IN 1981.
MOUNT POCONO, PA....42 DEGREES SET IN 1968.

&&

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

&&

$$
SYNOPSIS...DRAG
NEAR TERM...ROBERTSON
SHORT TERM...ROBERTSON
LONG TERM...DRAG
AVIATION...DRAG/ROBERTSON
MARINE...DRAG/ROBERTSON
CLIMATE...GORSE/BUNKER



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