NWS Philadelphia/Mount Holly wrote its last area forecast discussion before the first event at Jul 24, 2:15 PM EST, 3 hours 18 minutes earlier.
Area forecast discussion, Jul 24, 2:15 PM EST, 3 hours 18 minutes before
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ
315 PM EDT FRI JUL 24 2009
.SYNOPSIS...
A WEAKENING COLD FRONT WILL MOVE INTO OUR AREA TONIGHT BEFORE
DISSIPATING ON SATURDAY. AN UPPER AIR DISTURBANCE IS THEN FORECAST
TO DIVE INTO THE GREAT LAKES REGION LATER SATURDAY. THIS FEATURE
WILL HELP DRIVE ANOTHER WEAKENING COLD FRONT INTO OUR REGION LATE
SUNDAY INTO MONDAY. THIS FRONT MAY TEND TO STALL IN OUR VICINITY
MONDAY AND TUESDAY BEFORE DISSIPATING AS ANOTHER COLD FRONT SLOWLY
APPROACHES OUR AREA THURSDAY, BUT THEN MAY MOVE NORTHWARD AS A WARM
FRONT FRIDAY.
&&
.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
AN 18Z SURFACE ANALYSIS PLACED LOW PRESSURE JUST EAST OF CAPE COD
WITH A TRAILING FRONTAL BOUNDARY OFF THE COAST OF NEW JERSEY TO THE
CAROLINAS. ANOTHER COLD FRONT WAS DRAPED FROM THE MIDWEST TO THE
NORTHERN ROCKIES. A WEAK COLD FRONT OR SURFACE TROUGH WAS LOCATED
FROM SOUTHEASTERN CANADA TO NORTHERN VIRGINIA. AN UPPER AIR ANALYSIS
PLACED A MID LEVEL TROUGH FROM THE EASTERN GREAT LAKES TO THE OHIO
VALLEY WITH A PRONOUNCED EMBEDDED SHORT WAVE. THIS SHORT WAVE IS
CLEARLY VISIBLE ON THE WATER VAPOR IMAGERY ATTM FROM WESTERN NEW
YORK TO WESTERN PENNSYLVANIA. A 300 MB JET WAS POSITIONED FROM THE
MIDWEST TO THE MID ATLANTIC, ALTHOUGH THE LATTER IS CYCLONICALLY
CURVED.
THE SHORT WAVE TROUGH /WITH -14C TO -15C AIR AT 500 MB/ AND
ASSOCIATED VORTICITY MAXIMUM WILL BE SLIDING TO OUR EAST THROUGH THE
EVENING. THIS WILL MOSTLY TAKE THE CONVECTION WITH IT AS THE OVERALL
LIFT/FORCING SHIFTS EASTWARD WITH TIME. THE COLD POCKET ALOFT IS
FORECAST TO LIFT MORE NORTHEASTWARD THIS EVENING WITH OUR DELMARVA
ZONES HAVING SLIGHTLY MILDER MID LEVEL AIR, ALTHOUGH LOW-LEVEL
CONVERGENCE IS BECOMING BETTER ACROSS PARTS OF EASTERN MARYLAND
AND DELAWARE. THERE IS ENOUGH INSTABILITY COMBINED WITH THE COLD MID
LEVEL AIR AND DEEP LAYER SHEAR OF ABOUT 20-30 KNOTS TO ALLOW FOR
SOME CONVECTION. STRONG TO SEVERE THUNDERSTORMS /LOCALLY GUSTY
WINDS AND MARGINALLY SEVERE HAIL/ WILL BE POSSIBLE GIVEN THE
KINEMATICS IN PLACE THROUGH ABOUT EARLY THIS EVENING. THESE LOOK
TO MOSTLY PROPAGATE INTO OUR CWA AS THE LAPS DATA INDICATES A
DISTINCT INSTABILITY GRADIENT FROM WEST TO EAST DUE TO LOWER
CLOUDS HANGING TOUGH TODAY. THIS IS GRADUALLY ERODING THOUGH AS
THE LOWER CLOUDS THIN SOME. MLCAPE HOWEVER IS HIGHER AND THIS
SHOULD TEND TO ALLOW FOR STORMS TO THE WEST TO SURVIVE FOR AWHILE.
SPCS DAY ONE OUTLOOK HAS A SLIGHT RISK OF SEVERE STORMS IN A GOOD
PART OF OUR CWA, THEREFORE WE WILL CONTINUE TO CARRY SOME ENHANCED
WORDING FOR A TIME THIS EVENING. THE WEAK SURFACE COLD
FRONT/TROUGH LOOKS TO MOVE INTO OUR AREA TONIGHT, HOWEVER IT WILL
TEND TO SLOW ITS FORWARD PROPAGATION TOWARD DAYBREAK AS IT WEAKENS
FURTHER. ANOTHER SHORT WAVE IS FORECAST TO ARRIVE TOWARD DAYBREAK,
ALTHOUGH THIS ONE IS MUCH WEAKER THAN THE PREVIOUS. THE GFS IS A
BIT MORE ROBUST WITH THIS FEATURE COMPARED TO THE NAM/WRF, AND THE
GFS DEPICTION LOOKS A BIT OVERDONE AS IT MAY HAVE SOME GRIDSCALE
FEEDBACK INVOLVED DUE TO UPWIND CONVECTION. THE FLOW ALOFT WILL BE
MORE AND MORE FROM THE SOUTHWEST, THEREFORE THE SHORT WAVE WOULD
TEND TO SLIDE NORTHEASTWARD AS OPPOSED TO THE SOUTHEAST. OVERALL
THOUGH, IT APPEARS LITTLE IN THE WAY OF LIFT WILL BE IN PLACE
ALONG WITH LITTLE INSTABILITY TO PROMOTE SHOWER AND THUNDERSTORM
ACTIVITY TO BE CARRIED WELL INTO THE OVERNIGHT HOURS. AS A RESULT,
WE WILL CARRY SOME POPS THIS EVENING FOR A TIME, THEN LOWER BELOW
SLIGHT CHC.
TEMPERATURES WERE MOSTLY A BLEND OF THE GFS/NAM MOS. ENOUGH
LOW-LEVEL MOISTURE COMBINED WITH LIGHT WINDS OVERNIGHT /EARLY
SATURDAY MORNING/ MAY LEAD TO PATCHY FOG.
&&
.SHORT TERM /6 AM SATURDAY MORNING THROUGH SUNDAY NIGHT/...
AS YET ANOTHER UPPER-LEVEL LOW SETTLES ACROSS THE GREAT LAKES REGION
DURING SATURDAY, A SOUTHWESTERLY FLOW REGIME WILL BE MAINTAINED
ACROSS THE NORTHEAST AND MID ATLANTIC STATES. OUR REGION LOOKS TO BE
BETWEEN SYSTEMS ON SATURDAY AS LITTLE PVA IS FORECAST TO EJECT
NORTHEASTWARD ON THE EAST SIDE OF THE UPPER-LEVEL TROUGH. THIS IS IN
RESPONSE TO THE UPPER-LEVEL LOW STILL WELL TO OUR NORTHWEST. WHILE
THE PRECIPITATION CHCS ARE RATHER LOW ON SATURDAY, A FEW ISOLATED
STORMS CANNOT BE RULED OUT. THE ORGANIZATION POTENTIAL THOUGH IS
EXPECTED TO BE RATHER LOW GIVEN NO REAL FOCUSED SYNOPTIC SCALE LIFT.
WE MAY ACTUALLY HAVE MORE INSTABILITY IN THE MORNING AS THE SURFACE
DEW POINTS MAY TEND TO POOL SOME, HOWEVER WITH VERTICAL MIXING
OCCURRING DESPITE A SOUTHWEST FLOW, THE DEW POINTS MAY ACTUALLY
LOWER THROUGH THE DAY. THE NAM/WRF FORECAST SOUNDINGS SHOW THIS
POTENTIAL WITH THE MOS ALSO LEANING THIS WAY. FOR DEW POINTS, WE
LEANED MORE TOWARD THE NAM GUIDANCE AS IT TYPICALLY DOES BETTER WITH
THESE SETUPS. THE GFS FORECAST SOUNDINGS ARE NOT AS ROBUST WITH THIS
IDEA, ALTHOUGH THEY DO SHOW SOME POTENTIAL FOR SOMEWHAT LOWER DEW
POINTS DURING THE AFTERNOON FOR A TIME. THE NAM/WRF ALSO CONTINUES
TO SHOW A BIT OF A CAP WHERE THE GFS DOES NOT. REGARDING POPS, AS OF
NOW OUR HIGHER TERRAIN AREAS MAY BE WERE ISOLATED CONVECTION FORMS
WHICH MAY THEN PROPAGATE EASTWARD INTO THE EVENING HOURS. WE WILL
CARRY SOME SLIGHT CHC POPS AS A RESULT FOR NOW. SOME ACTIVITY MAY
ALSO DEVELOP ACROSS NORTHERN VIRGINIA IN THE AFTERNOON IN RESPONSE
TO MORE INSTABILITY AND SOME LOW-LEVEL CONVERGENCE.
THE UPPER-LEVEL FEATURE WILL DRIVE ANOTHER SURFACE COLD FRONT OUR WAY
SATURDAY NIGHT AND SUNDAY. AS THIS SYSTEM AMPLIFIES TO OUR WEST
DURING SUNDAY, THE MID LEVEL FLOW IS FORECAST TO INCREASE TO ABOUT
40 KNOTS OWNING TO DEEP LAYER SHEAR FOR STORM ORGANIZATION. THERE
APPEARS TO BE NO SHORTAGE OF INSTABILITY WITH SBCAPE ABOUT 1,000
J/KG ALTHOUGH THE GFS HAS SBCAPE AROUND 2,000 J/KG. THE FREEZING
LEVEL IS AROUND 10,000 FEET THEREFORE IF THE INSTABILITY CAN BE
REALIZED, UPDRAFT STRENGTH WOULD BE GOOD AND ALLOW FOR HAIL
PRODUCTION AND EFFICIENT CHARGE SEPARATION. PW VALUES ARE AROUND
1.5 INCHES, WHICH WOULD ALLOW FOR LOCALLY HEAVY RAIN. OUR CWA IS
RATHER CLOSE TO THE RIGHT ENTRANCE REGION OF A 250 MB JET. A
POSSIBLE FLY IN THE OINTMENT WILL BE UPSTREAM ACTIVITY AS SOME MODEL
GUIDANCE SUGGEST THAT A CONVECTIVE CLUSTER MAY BE ONGOING. THIS MAY
TEND TO WEAKEN AS IT MOVES ACROSS NEW YORK AND PENNSYLVANIA IN THE
MORNING, BUT SHOULD INTENSIFY/REORGANIZE WHEN ENCOUNTERING MORE
INSTABILITY WITH DAYTIME HEATING. WE ARE NOT REAL CERTAIN ATTM HOW
ACTIVE THE DAY WILL BECOME, THEREFORE WE WILL GO WITH CHC POPS FOR
SCATTERED CONVECTION ESPECIALLY IN THE AFTERNOON AND TRY TO DEPICT A
WEST TO EAST MOVEMENT DURING THE DAY.
AS WE MAINTAIN A SOUTHWESTERLY FLOW ALOFT REGIME, THE SURFACE FRONT
SHOULD TEND TO SLOW AS IT IS IN OUR CWA AND ANY LOWERING DEW POINTS
ARE ANTICIPATED TO RECOVER SATURDAY NIGHT AND SUNDAY. THE BOUNDARY MAY
ACTUALLY TEND TO STALL AND LINGER TO START THE LONG TERM. THE MODEL
GUIDANCE IS SUGGESTING THAT INSTABILITY IS MAINTAINED THROUGH SUNDAY
NIGHT. AS THE BOUNDARY IS IN THE VICINITY ALONG WITH AN UPPER-LEVEL
TROUGH AXIS STILL TO OUR WEST, ANY EMBEDDED SHORT WAVES MAY BE ABLE
TO POP A SHOWER OR THUNDERSTORM. FOR THIS REASON, WE WILL TAKE THE
CHC POPS IN THE EVENING FOR SCATTERED ACTIVITY AND LOWER TO SLIGHT
CHC FOR ANY ISOLATED ACTIVITY THAT MAY LINGER.
AS FOR TEMPERATURES, WE GENERALLY UTILIZED A GFS/NAM MOS BLEND. THE
SOUTHWEST LOOKS STRONG ENOUGH WHERE 90F BOTH SATURDAY AND SUNDAY
AFTERNOONS ESPECIALLY IN THE MORE URBAN AREAS AND THE COASTAL PLAIN.
THE SOUTHWEST FLOW LOOKS TO BE STRONG ENOUGH AS WELL TO ALLOW
COASTAL DELAWARE TO GET WELL INTO THE 80S, ALTHOUGH FOR COASTAL NEW
JERSEY THE LOW-LEVEL FLOW MAY TEND TO BACK TO THE SOUTH.
&&
.LONG TERM /MONDAY THROUGH FRIDAY/...
WHILE AN UPPER LEVEL TROUGH WILL PERSIST OVER EASTERN CANADA THROUGH
THE PERIOD, A BERMUDA HIGH IS EXPECTED TO BUILD WESTWARD TOWARD THE
SOUTHEAST U.S. COAST EARLY NEXT WEEK. WITH THIS TYPE OF PATTERN,
SHOWERS AND THUNDERSTORMS ARE POSSIBLE EACH DAY, ESPECIALLY DURING
THE AFTERNOON AND EVENING HOURS. HOWEVER, MUCH OF THE TIME WILL BE
RAIN FREE. TEMPERATURES WILL WARM UP INTO THE UPPER 80S EARLY NEXT
WEEK, AND THE 90 DEGREE MARK MAY EVEN BE REACHED BY MIDWEEK IN THE
PHILADELPHIA METRO AREA.
&&
.AVIATION /19Z FRIDAY THROUGH WEDNESDAY/...
THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY AND SURROUNDING AREAS.
NEW TAFS REFLECT THE CURRENT CONDITIONS THAT SHOWED ANY OF THE
LEFTOVER CLOUDS BELOW 3 THSD FT DISSIPATING AND GETTING REPLACE BY
CLOUDS AROUND 5 THSD FT DUE TO HEATING. THE ONLY PLACES THAT VCTS
AND CB WERE IN THE INITIAL FCST WERE ABE AND RDG SINCE RADAR
SHOWED SOME POP-UP ECHOES IN THAT REGION. OTHERWISE, VFR CONDITIONS
ARE FCST AT ALL SITES AS OF 18Z.
BY 20-22Z, CONVECTION IS EXPECTED TO DEVELOP WITH INCREASING
COVERAGE. THIS IS DUE TO ADDITIONAL HEATING THAT WILL HELP
DESTABILIZE THE AIRMASS AS WELL AS A DISTURBANCE ALOFT APPROACHING
FROM THE SW AND W. THIS DISTURBANCE IS FCST TO MOVE THROUGH AROUND
00Z. DUE TO THE DISTURBANCE TSTMS SHOULD BECOME MORE WIDESPREAD
AND MAY DEVELOP INTO LINES. ALSO, THERE IS A POSSIBILITY OF SOME
TSTMS BECOMING SVR. SPC HAS PLACED MOST OF THE AREA UNDER A SLIGHT
RISK FOR THIS EVENING.
BETWEEN 00Z AND 03Z, ANY SHOWERS AND TSTMS SHOULD HAVE MOVED OFF
TO THE EAST AND SKIES SHOULD CLEAR. HOWEVER, DURING THE EARLY
MORNING HOURS SOME FOG MAY FORM DUE TO THE CLEARING SKIES AND
LIGHT WINDS AS WELL AS ANY RAIN THAT FALL THIS EVENING. FOR NOW,
MVFR CONDITIONS FOR FOG WILL BE FCST, BUT AREAS THAT GET RAIN MAY
DROP TO IFR FOR A COUPLE HOURS AROUND OR JUST AFTER SUNRISE SAT.
ON SATURDAY, VFR CONDITIONS SHOULD RETURN QUICKLY DURING THE
MORNING AND REMAIN IN PLACE ALL DAY.
OUTLOOK...
THE NEXT THREAT OF TSTMS IS SUNDAY AND THERE IS THE POSSIBILITY OF
TSTMS EACH DAY THROUGH EARLY NEXT WEEK.
&&
.MARINE...
SEAS HAVE COME DOWN BELOW 5 FT IN ALL AREAS SO SCA HAVE BEEN
DISCONTINUED.
A S TO SW FLOW IS FORECAST THROUGH EARLY NEXT WEEK AS ANY FRONTAL
BOUNDARIES SHOULD WEAKEN AND DISSIPATE BEFORE GETTING TO THE REGION.
ON SUNDAY, ONE OF THESE APPROACHING FRONTS SHOULD CAUSE THE
SOUTHERLY WINDS TO INCREASE TO 15 KT AND OCCASIONAL GUSTS TO 25 KT
COULD OCCUR ON THE OCEAN. SINCE IT IS TOO EARLY TO CONSIDER SCA FOR
THIS SETUP, NO HEADLINES WILL BE ISSUED.
SATURDAY SHOULD SEE A RESPITE FROM THE THREAT OF SHOWERS AND TSTMS,
BUT FROM SUNDAY THROUGH EARLY NEXT WEEK, SHOWERS AND TSTMS ARE AGAIN
POSSIBLE, MAINLY EACH AFTN AND EVENING.
&&
.TIDES/COASTAL FLOODING...
TIDAL DEPARTURES HAVE DROPPED AND NOT ENOUGH TO CAUSE FLOODING
CONSIDERING THE ASTRONOMICAL TIDE HEIGHTS. THE TREND THE NEXT COUPLE
DAYS IS FOR NEGATIVE DEPARTURES AND WITH THE ASTRONOMICAL TIDES
COMING DOWN, WE DO NOT EXPECT FLOODING.
&&
.PHI WATCHES/WARNINGS/ADVISORIES...
PA...NONE.
NJ...NONE.
DE...NONE.
MD...NONE.
MARINE...NONE.
&&
SYNOPSIS...GORSE
NEAR TERM...GORSE
SHORT TERM...GORSE
LONG TERM...MIKETTA
AVIATION...STAUBER
MARINE...STAUBER
TIDES/COASTAL FLOODING...STAUBER