Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000
FXUS61 KPHI 040822
AFDPHI

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ
422 AM EDT TUE AUG 4 2015

.SYNOPSIS...
A COLD FRONT FROM THE GREAT LAKES WILL SLOWLY APPROACH FROM
WESTERN PENNSYLVANIA THIS MORNING. THE FRONT WILL MOVE THROUGH
OUR REGION THIS AFTERNOON AND TONIGHT BEFORE STALLING JUST SOUTH
OF OUR AREA THROUGH THE END OF THE WEEK. WAVES OF LOW PRESSURE
WILL RIDE ALONG THE BOUNDARY WITH THE FINAL WAVE OF LOW PRESSURE
LOOKING TO PASS SOUTHEAST OF THE REGION ON SATURDAY. HIGH
PRESSURE BUILDS IN FROM FROM EASTERN CANADA SUNDAY AND MONDAY.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
OVERNIGHT SURFACE ANALYSIS SHOWS A COLD FRONT POSITIONED OVER
WESTERN NY AND THE OH VALLEY. MEANWHILE, A SURFACE TROUGH WAS
LOCATED JUST EAST OF I-95 IN THE MID-ATLANTIC REGION. WINDS ARE OUT
OF THE S-SW (NW) AND DEWPOINTS IN THE LOW 70S (LOW 60S)TO THE EAST
(WEST) OF THE TROUGH.

UPPER-LEVEL DIVERGENCE IN THE RIGHT ENTRANCE REGION OF AN UPPER-
LEVEL JET STREAK OVER THE NORTHEAST STATES AND DPVA FROM A SUBTLE
MID-LEVEL SHORTWAVE TROUGH HAS BEEN THE SOURCE OF LIFT FOR NOCTURNAL
CONVECTION THAT WAS MOVING ACROSS EASTERN PA AND THE NORTHERN HALF
OF NJ. HI-RES CAM GUIDANCE HAD A POOR HANDLE ON THE OVERNIGHT
CONVECTION, SO FORECAST THRU EARLY THIS MORNING BASED MORE OFF OF AN
EXTRAPOLATION OF CURRENT SATELLITE/RADAR AND FROM MESOANALYSIS
TRENDS. THERE IS ENOUGH ELEVATED INSTABILITY TO KEEP THE SHOWERS AND
STORMS GOING EARLY MORNING AS THEY MOVE EASTWARD THIS MORNING. WE
ARE PARTICULARLY KEEPING AN EYE ON A CLUSTER OF STORMS NEAR THE MASON-
DIXON LINE JUST TO OUR WEST AS THEY CURRENTLY HAVE SEVERE
THUNDERSTORM WARNINGS WITH IT. AT 07Z, THERE WAS 2000+ J/KG OF
ELEVATED CAPE DOWNSTREAM OF THESE STORMS.

THE AFOREMENTIONED COLD FRONT WILL ADVANCE EASTWARD THRU THE LEHIGH
VALLEY AND NW NJ EARLY THIS AFTERNOON AND I-95 CORRIDOR LATE IN THE
DAY. ONCE THE DEBRIS CLOUDS FROM THE CURRENT CONVECTION MOVES OFF
THE COAST, STRONG HEATING WILL ALLOW THE ATMOSPHERE TO DESTABILIZE
THIS AFTERNOON. A HOT (HIGHS IN THE LOWER 90S) AND HUMID (DEWPOINTS
IN THE UPPER 60S/LOW 70S) AIRMASS WILL BE PRESENT AHEAD OF THE
FRONT. FORECAST SOUNDINGS FROM THE 00Z NAM AND GFS ARE VASTLY
DIFFERENT BUT AFTER ACCOUNTING FOR MOISTURE BIASES FROM BOTH MODELS,
MLCAPE WILL RANGE FROM AROUND 1500 J/KG NEAR THE I-95 CORRIDOR TO
3000 J/KG IN THE LOWER DELMARVA DURING PEAK HEATING. THE 00Z HI-RES
NCAR ENSEMBLES SUPPORT THESE NUMBERS AS WELL. BULK SHEAR WILL ALSO
BE MODERATELY STRONG AS WESTERLY WINDS IN THE MID-LEVELS INCREASE TO
40-50 KT. THE ONE LIMITING FACTOR WILL BE THE ABSENCE OF ORGANIZED
LIFT ALOFT. HOWEVER, SURFACE CONVERGENCE NEAR THE FRONTAL BOUNDARY
SHOULD PROVIDE JUST ENOUGH LIFT TO INITIATE CONVECTION THIS
AFTERNOON. SPC HAS PLACED AREAS EAST OF I-95 IN A SLIGHT RISK FOR
SEVERE STORMS. THE ENVIRONMENT WILL SUPPORT DAMAGING STRAIGHT LINE-
WINDS FROM WET MICROBURSTS AND LARGE HAIL WITH THE STRONGEST CORES.
THE THREAT APPEARS TO BE GREATEST FROM APPROXIMATELY 2-8 PM.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/...
THE COLD FRONT WILL SLOWLY MOVE EASTWARD THRU THE COASTAL PLAIN THIS
EVENING AND EVENTUALLY OFF THE COAST OVERNIGHT. ACCORDINGLY, THE
HIGHEST POPS RESIDE THIS EVENING OVER SOUTHERN NJ AND THE DELMARVA.
PUT A LITTLE MORE WEIGHT IN THE HI-RES GUIDANCE FOR THIS EVENING`S
ROUND OF CONVECTION THAN THE ONGOING ROUND EARLY THIS MORNING AS THE
LATTER ROUND OF CONVECTION IS MAINLY FOCUSED/FORCED ALONG THE WELL-
DEFINED SYNOPTIC COLD FRONT.

POST-FRONTAL NW WINDS WILL BE LIGHT, BUT THERE APPEARS TO BE ENOUGH
DRY AIR ADVECTION TO LIMIT THE FOG POTENTIAL OVERNIGHT. IF THE FRONT
SLOWS DOWN ENOUGH AND SKIES ARE ABLE TO CLEAR OVERNIGHT, THEN FOG
MAY DEVELOP NEAR THE COAST, ESPECIALLY IN AREAS THAT RECEIVE RAIN.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
THE COLD FRONT WILL MOVE OFF THE COAST WEDNESDAY WHILE HIGH PRESSURE
BUILDS IN THE MIDWEST. A RELATIVELY TIGHT PRESSURE GRADIENT FOR THIS
TIME OF YEAR IS FORECAST OVER OUR AREA, LEADING TO A BREEZY NW WIND.
DRY AIR ADVECTION WILL BE MORE PROMINENT THAT COLD AIR ADVECTION
BEHIND THE FRONT. MAX TEMPS RANGE FROM THE UPPER 70S IN THE POCONOS
TO NEAR 90F IN THE MORE URBAN AREAS ALONG AND EAST OF I-95. HOWEVER,
IT WILL FEEL COMFORTABLE WITH DEWPOINTS ONLY EXPECTED TO BE IN THE
50S.

THE EXTENDED PART OF THE FORECAST (THURSDAY NIGHT AND BEYOND)
REMAINS UNTOUCHED. PREVIOUS DISCUSSION FROM THE DAY SHIFT
FOLLOWS...

IN A MODEL INITIALIZATION FLIP FROM THE WEEKEND, THE WRF/NAM
INITIALIZED THE 500MB PATTERN BETTER THAN THE GFS OR ECMWF IN THE
WESTERN CONUS, WHILE IN ERN NOAM THE GFS LOOKED BETTER. THE GFS
LOOKED CLOSER AT 850 MB AND 925MB. DP/DTING THE INITIALIZATION
OVER THE PAST 24 HOURS, THE RIDGING IN MONTANA AND IDAHO IS
STRONGER AND DOWN THE ROAD THIS IS LEADING TO A MORE BIFURCATED
SOLUTION TO HOW THE ENERGY ROUNDS THIS RIDGE, THUS THERE IS NO
DECREASE IN UNCERTAINTY WITH THIS SOUNDING RUN. THE GFS WAS NOT
USED BECAUSE OF ITS WEAKER WESTERN CONUS INITIALIZATION AND POOR
RUN TO RUN CONTINUITY. GEFS MEAN WITH THE WAVE OF LOW PRESSURE IS
FARTHER SOUTH AND THEN A FASTER EXIT ON SATURDAY. OUR FORECAST IS
CLOSEST TO A ECMWF/CAN GGEM/MF-ARPEGE BLEND. IT SHOULD BE NOTED
THAT THE UKMET HAS TRENDED SOUTHWARD WITH A MUCH WEAKER LOW WHILE
THE NOGAPS AND JMA HAVE GONE BACK TO NOT HAVING ANY PCPN REACH OUR
CWA UNTIL SATURDAY.

SO THE ACTIVE MIDDLE STRETCH OF THE LONG TERM CONTINUES WITH
TEMPERATURES CLOSER TO THE NORMAL. THE POTENTIAL REMAINS FOR A
SOAKING RAIN CENTERED AROUND FRIDAY, ALTHOUGH THERE REMAINS
UNCERTAINTIES AS TO THE IMPACT WITHIN OUR CWA. LAST NIGHT`S NAEFS
ENSEMBLE MEAN CONTINUE TO SHOW AN ANOMALOUSLY STRONG SFC LOW
PRESSURE SYSTEM FOR EARLY AUGUST WITH A FIVE YEAR RETURN PERIOD
AND SD OF BETWEEN 2 AND 3 AS IT CROSSES THE CENTRAL APPALACHIANS.

THURSDAY, POPS WERE MOVED BACK APPROXIMATELY 12 HOURS. TOO LATE
IN ITS FORECAST CYCLE TO HAVE CONFIDENCE IN THE WRF/NAM TIMING,
BUT MANY OF THE MODELS DO NOT HAVE ANY MEASURABLE AT ALL THURSDAY
DAY BASED ON THE MORE ROUNDABOUT APPROACH FOR THE SHORT WAVES.
BECAUSE OF THE DRIER TRENDING, WE WENT ABOVE STAT GUIDANCE FOR MAX
TEMPS.

THURSDAY NIGHT THROUGH FRIDAY NIGHT THERE UNFORTUNATELY REMAINS
UNCERTAINTY AS TO THE EVOLUTION OF THE SYSTEM, BUT THE SYNOPTIC
SCALE SET-UP STILL POINTS TO HEAVY RAIN, EVEN IF IT DOES NOT
NECESSARILY OCCUR OR AFFECT OUR CWA THE MOST. THE HEAVY RAIN WILL
BE TIED TO CONVECTION AND THUS HOW UNSTABLE ALOFT IT BECOMES
WITHIN OUR CWA WILL DETERMINE THE FINAL OUTCOME. HIGHEST POPS ARE
THURSDAY NIGHT AND FRIDAY WITH THUNDER EMPHASIZED MORE FOR THE
SOUTHERN PART OF OUR CWA. COME FRIDAY NIGHT THE PREDICTED
INSTABILITY (REALLY LACK THEREOF) ALOFT DOES NOT SUBSTANTIATE THE
ECMWF`S PCPN/QPF SOLUTION AND WAS NOT USED. THIS DOES NOT MEAN
HEAVIER RAIN COULD NOT HAPPEN EARLIER. MAX TEMPS MAY VERY WELL BE
OVERSTATED ON FRIDAY IF THIS TIMING HOLDS.

BECAUSE OF TIMING DISCREPANCIES WE KEPT IN A LOW CHANCE FOR
PRECIPITATION ON SATURDAY. AFTER ABOVE, THE END OF THE LONG TERM
APPEARS QUIET AS HIGH PRESSURE NOSES ITS WAY SOUTHWARD FROM CANADA
AND THE FRONTAL BOUNDARY AND LOW PRESSURE SYSTEMS MOVE OFFSHORE.

&&

.AVIATION /08Z TUESDAY THROUGH SATURDAY/...
THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY AND SURROUNDING AREAS.

FIRST ROUND OF SHOWERS AND STORMS HAS ALREADY EXITED THE AREA EARLY
THIS MORNING. ANOTHER ROUND IS MOVING INTO RDG AT 08Z AND COULD
IMPACT THE PHILLY TERMINALS BETWEEN 09Z-11Z. A BRIEF PERIOD OF MVFR
TO LOCALLY IFR VSBYS IS POSSIBLE WITH THIS ACTIVITY. CLOUDS SHOULD
LIMIT FOG DEVELOPMENT EARLY THIS MORNING.

VFR THIS MORNING AND EARLY AFTERNOON. SCT SHRA AND TSRA EXPECTED TO
DEVELOP AGAIN DURING THE MID AND LATE AFTERNOON. BASED ON TIMING OF
A COLD FRONT, THE I-95 (TTN/PNE/PHL/ILG) AND SOUTHERN NJ (MIV/ACY)
TERMINALS HAVE THE BEST CHANCE TO SEE STORMS. INCLUDED A PROB30
GROUP IN THE 06Z TAFS FOR THESE TERMINALS. LATER TAF CYCLES WILL
LIKELY ADD A TEMPO AND THEN PREVAILING GROUP FOR TSRA WHEN
CONFIDENCE IN TIMING AND LOCATIONS INCREASES.

OUTLOOK...

WEDNESDAY AND WEDNESDAY NIGHT...MAINLY VFR CONDITIONS.

THURSDAY THROUGH FRIDAY NIGHT...MVFR CONDITIONS DURING SHWRS/
TSTORMS, WITH SOME IFR CONDITIONS POSSIBLE. WHILE CONFIDENCE OF
OCCURRENCE IS INCREASING, TIMING CONFIDENCE REMAINS LOW.

SATURDAY...PREDOMINATELY VFR CONDITIONS EXPECTED TO RETURN AS
SHOWER FREQUENCY DIMINISHES.

&&

.MARINE...
EXTENDED THE SCA THAT WAS ALREADY IN EFFECT FOR TODAY INTO TONIGHT.
S-SW WINDS WILL GUST TO 25 KT TODAY AND EARLY THIS EVENING. THE SEAS
WILL BE ELEVATED TO 4-6 FT DURING THIS TIME. WINDS ARE EXPECTED TO
WEAKEN AFTER SUNSET BUT SEAS WILL TAKE SEVERAL MORE HOURS TO SUBSIDE
TO BELOW SCA CRITERIA.

IN DELAWARE BAY, WE`LL TOP THE WIND GUSTS OUT AT 20 KTS (SUB
SMALL CRAFT).

OUTLOOK...

WEDNESDAY THROUGH THURSDAY...SUB-ADVISORY CONDITIONS ARE
EXPECTED.

THURSDAY NIGHT THROUGH FRIDAY NIGHT...MOSTLY SUB-SCA CONDITIONS;
HOWEVER, A STALLED FRONT AND A WAVE OF LOW PRESSURE MOVING ALONG
IT COULD ENHANCE SEAS AND/OR WINDS FOR THE COASTAL WATERS, MAKING
SCA CONDITIONS POSSIBLE.

SATURDAY...SUB SMALL CRAFT ADVISORY CONDITIONS EXPECTED TO
RETURN. THIS IS BASED ON TIMING OF WAVES OF LOW PRESSURE CLOSE,
CONFIDENCE AT THIS JUNCTURE IS LESS THAN AVERAGE.

&&

.RIP CURRENTS...
THERE IS A MODERATE RISK FOR THE FORMATION OF DANGEROUS RIP
CURRENTS TODAY ALONG THE NEW JERSEY SHORE AND AT THE DELAWARE
BEACHES. SEAS HAVE CONTINUED TO BUILD IN THE STIFF SOUTHERLY FLOW.
WAVE HEIGHTS IN THE SURF ZONE WILL BE UP TO 4 OR 5 FEET.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...NONE.
NJ...NONE.
DE...NONE.
MD...NONE.
MARINE...SMALL CRAFT ADVISORY UNTIL 5 AM EDT WEDNESDAY FOR ANZ450>455.

&&

$$
SYNOPSIS...KLEIN
NEAR TERM...KLEIN
SHORT TERM...KLEIN
LONG TERM...GIGI/KLEIN
AVIATION...GIGI/KLEIN
MARINE...GIGI/KLEIN
RIP CURRENTS...IOVINO/KLEIN


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