Areal Flood Advisory 44, NWS Philadelphia/Mount Holly

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ
332 PM EDT WED JUL 29 2009

.SYNOPSIS...
A WEAK COLD FRONT WILL MOVE THROUGH MOST OF OUR AREA TONIGHT AND
DISSIPATE THURSDAY MORNING. ANOTHER FRONT WILL APPROACH OUR AREA
FROM THE WEST FRIDAY AND MOVE JUST OFF THE COAST EARLY SATURDAY AS
WEAK HIGH PRESSURE ARRIVES. MORE LOW PRESSURE IS EXPECTED AT TIMES
FROM LATER SUNDAY THROUGH THE MIDDLE OF NEXT WEEK.

&&

.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
THE FIRST ROUND OF SEVERE WEATHER OCCURRED OVER OUR NORTHWESTERN
AND NORTHERN ZONES THIS AFTERNOON SO FAR. ANOTHER SHORT WAVE IS
SEEN APPROACHING FROM THE SOUTHWEST, AS ILLUSTRATED BY THE
DEVELOPING CONVECTION OVER NORTHEASTERN MARYLAND. THE AIRMASS MAY
HAVE BEEN SOMEWHAT STABILIZED WHERE PRIOR CONVECTION OCCURRED, BUT
WITH A LINE OF CONVECTION STILL JUST WEST OF THERE, MORE COULD
COULD OCCUR SO THE SEVERE THUNDERSTORM WATCH WILL REMAIN INN
EFFECT WHERE IT WAS ISSUED PREVIOUSLY THROUGH 00Z. FURTHER
SOUTHWEST FROM PHILADELPHIA AND SOUTHEASTWARD, THE AIRMASS
REMAINS PRIMED FOR CONVECTIVE ACTION. AT LEAST MODERATE
INSTABILITY, QUITE HIGH LOW LEVEL MOISTURE, FAVORABLE UPPER
DYNAMICS, AND THE APPROACH OF A LOW LEVEL FORCING MECHANISM
INDICATE THE POTENTIAL FOR STRONG TO SEVERE STORMS THIS EVENING.
HIGH PRECIPITATION CHANCES SHOULD DROP PRECIPITOUSLY WITH THE
PASSAGE OF THE UPPER DISTURBANCE AFTER MIDNIGHT, AND THE WANING
DIURNAL EFFECTS. HOWEVER, A CHANCE OF A SHOWER OR THUNDERSTORM
WILL BE KEPT. LATER FORECASTERS MAY BE ABLE TO REDUCE IT TO SLIGHT
CHANCE OR REMOVE IT ALTOGETHER DEPENDING ON THE EVOLUTION OF
EVENTS.

WITH HIGH GROUND AND LOW LEVEL MOISTURE OVERNIGHT, PATCHY FOG WAS
ADDED TO THE WEATHER GRIDS LATE TONIGHT.

&&

.SHORT TERM /6 AM THURSDAY MORNING THROUGH FRIDAY/...
ANOTHER CONVECTIVE EVENT IS SHAPING UP FOR FRIDAY. AGAIN, HIGH
HUMIDITY, GOOD UPPER DYNAMICS, AND AN APPROACHING SURFACE FRONT
LOOK TO ACT ON AN UNSTABLE ATMOSPHERE. THE AREA IS ALREADY IN SEE
TEXT FROM SPC.

OTHERWISE, SMALL CONVECTIVE CHANCES WILL REMAIN THROUGHOUT THE
SHORT TERM PERIOD DUE TO DIURNAL AND TERRAIN EFFECTS.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
AGAIN, MOST PERIODS WILL CONTINUE TO HAVE AT LEAST LOW RAIN
CHANCES AS PERIODIC LOW PRESSURE WILL AFFECT OUR AREA THROUGH THE
MIDDLE OF NEXT WEEK. LATER PERIODS WERE PEGGED AS SLIGHT CHANCE,
SO THEY WERE INVISIBLE IN THE WORDED FORECASTS. HPC TEMPERATURES
AND WINDS WERE USED AS THEY LOOKED REASONABLE AND BLENDED WITH
SURROUNDING OFFICES. GENERALLY, NORMAL TO SLIGHTLY ABOVE NORMAL
TEMPERATURES APPEAR REASONABLE.

&&

.AVIATION /19Z WEDNESDAY THROUGH MONDAY/...
THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY AND SURROUNDING AREAS.

THE 18Z TAFS WILL BE KEYING ON THE NEXT UPPER AIR DISTURBANCE TO
PRODUCE AN ORGANIZED AREA OF CONVECTION MAINLY DURING THE EVENING
HOURS. THIS IS THE CONVECTION THAT IS CURRENTLY IN VIRGINIA. IFR
CONDITIONS ARE POSSIBLE WHEN THIS BATCH OF TSRAS MOVE THROUGH. SOME
STRONG GUSTY WINDS ARE ALSO POSSIBLE, THE SOONER THEY ARRIVE THE
HIGHER THE CHANCES OF THE LATTER OCCURRING. PRIOR TO THAT DURING
THE REST OF THE AFTERNOON WITH THE FIRST UPPER AIR DISTURBANCE
EXITING NE OF THE KABE AND KTTN TERMINALS AND THERE SHOULD BE A
RELATIVE LULL. CONVECTIVE TEMPS ARE BEING REACHED AND WIDELY
SCATTERED CONVECTION SHOULD OCCUR PRIOR TO THAT FEATURE.

CONVECTION SHOULD WANE TONIGHT, BUT THE AIR SHOULD REMAIN HUMID AND
MOST LIKELY IFR CONDITIONS WILL DEVELOP DUE TO LOW CLOUDS AND OR
FOG. CONDITIONS SHOULD BEGIN TO IMPROVE AGAIN AFTER DAYBREAK
THURSDAY WITH GENERALLY VFR CONDITIONS EXPECTED DURING THE REST OF
THE DAY.

OUTLOOK...A SERIES OF WEAKENING COLD FRONTS SHOULD BRING ORGANIZED
CONVECTION AND MORE WIDESPREAD IFR OR MVFR CONDITIONS LATER FRIDAY
AND LATER SUNDAY INTO MONDAY. AROUND THOSE PERIODS MORE ISOLATED
OR WIDELY SCATTERED CONVECTION IS POSSIBLE, BUT THE IMPACT SHOULD
NOT BE WIDESPREAD IN EITHER TIME OR AREAL EXTENT.

&&

.MARINE...
WINDS AND SEAS ARE STILL EXPECTED TO INCREASE IN ADVANCE OF THE
APPROACHING FRONT. GUIDANCE FORECASTS FOR THE OFFSHORE BUOYS ARE
MARGINAL IN TERMS OF WIND SPEEDS, WHILE WAVEWATCH GUIDANCE MOVES THE
SEAS FIRMLY INTO ADVISORY TERRITORY TONIGHT. GUIDANCE SEEMS TO HAVE
CONDITIONS BACKING DOWN BELOW ADVISORY CRITERIA DURING THE DAY ON
THURSDAY, AND WE WILL CONTINUE TO END THE ADVISORY ON THE OCEAN AT
NOON. OVER THE LOWER BAY, THE BEST CHANCE OF ADVISORY CONDITIONS
WILL BE NEAR THE MOUTH OF THE BAY /INDICATIONS FROM BRANDYWINE
NOTWITHSTANDING/, AND WE WILL CONTINUE TO WORD IT THAT WAY IN THE
TEXT PRODUCT. IT REMAINS UP THERE FOR TONIGHT ONLY.

A CAVEAT IS THAT STRONG THUNDERSTORMS MAY MOVE OVER THE WATERS
THROUGH TONIGHT, AND THEY WOULD CREATE LOCALLY HAZARDOUS
CONDITIONS.

THE WEATHER PATTERN WILL CONTINUE TO REPEAT ITSELF OVER THE NEXT
COUPLE OF DAYS WITH SIMILAR RESULTS EXPECTED. SMALL ADVISORY
CONDITIONS MORE FOR SEAS THAN WINDS MAY DEVELOP AGAIN ON FRIDAY IN
THE SOUTHERLY FLOW PRECEDING THE NEXT COLD FRONT. SEAS SHOULD THEN
SUBSIDE ON SATURDAY AS THE FRONT EITHER CLEARS OR DISSIPATES OVER
THE REGION AND THE PRESSURE GRADIENT DECREASES. WEAK HIGH PRESSURE
SHOULD THEN BUILD IN FOR LATER SATURDAY INTO SUNDAY BEFORE WE ARE
EXPECTED TO REPEAT THE PROCESS AGAIN (POSSIBLE SMALL CRAFT ADVISORY
MORE FOR SEAS THAN WINDS) LATER SUNDAY INTO MONDAY.

&&

.HYDROLOGY...
THE FLASH FLOOD WATCH WILL CONTINUE THIS EVENING IN VIEW OF THE
HEAVY RAIN THAT FELL OVER THE NORTHWESTERN SECTIONS OF OUR FORECAST
AREA THIS AFTERNOON, AND THE ACTIVITY TAKING SHAPE THAT IS
UPSTREAM WITH THE NEXT SHORT WAVE ALOFT. PRECIPITABLE WATER VALUES
ARE STILL 1.5 TO 2 INCHES. OUR NORTHWESTERN ZONES, EXCLUDING THE
POCONOS FOR NOW, ARE STILL VIEWED AS THE MOST SUSCEPTIBLE AREAS
FOR ADDITIONAL FLASH FLOODING THIS EVENING AND LEADING INTO THE
OVERNIGHT.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...FLASH FLOOD WATCH THROUGH LATE TONIGHT FOR PAZ060>062-067>071.
NJ...FLASH FLOOD WATCH THROUGH LATE TONIGHT FOR NJZ001-007>010-012-
     015.
DE...NONE.
MD...NONE.
MARINE...SMALL CRAFT ADVISORY UNTIL NOON EDT THURSDAY FOR ANZ450>455.
     SMALL CRAFT ADVISORY FROM 8 PM THIS EVENING TO 6 AM EDT
     THURSDAY FOR ANZ431.

&&

SYNOPSIS...
NEAR TERM...
SHORT TERM...
LONG TERM...
AVIATION...GIGI
MARINE...GIGI
HYDROLOGY...

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SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ
443 PM EDT WED JUL 29 2009

NJZ009-015-017>019-PAZ068>071-292145-
BUCKS PA-CAMDEN NJ-DELAWARE PA-GLOUCESTER NJ-HUNTERDON NJ-MERCER NJ-
MONTGOMERY PA-NORTHWESTERN BURLINGTON NJ-PHILADELPHIA PA-
443 PM EDT WED JUL 29 2009

...AN AREA OF SHOWERS PRODUCING HEAVY RAINFALL WILL AFFECT THE
DELAWARE VALLEY NORTHEAST INTO HUNTERDON COUNTY NEW JERSEY...

AT 439 PM EDT...NATIONAL WEATHER SERVICE DOPPLER RADAR INDICATED AN
AREA OF SHOWERS PRODUCING HEAVY RAINFALL 3 MILES WEST OF CAMDEN...
SOUTHWEST TO THE PHILADELPHIA AREA. THEY ARE MOVING RATHER QUICKLY
BUT OVER THE SAME GENERAL AREA WHICH WILL CAUSE SOME STREET AND HIGHWAY
FLOODING THROUGH RUSH HOUR.

THE SHOWERS AND OCCASIONAL THUNDERSTORM WILL LAST ABOUT AN HOUR BUT
MORE ARE EXPECTED TO MOVE INTO THE SAME REGION ADDING TO THE
POTENTIAL FOR FLOODING.

EBERWINE

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