FXUS61 KPHI 111344
AFDPHI

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ
844 AM EST THU MAR 11 2010

.SYNOPSIS...
A LOW PRESSURE SYSTEM OVER THE CENTRAL MISSISSIPPI VALLEY WILL MEANDER
OVER THE MIDWEST, AS A NEW LOW PRESSURE SYSTEM IS FORECAST TO FORM
ON ITS COLD FRONTAL BOUNDARY OVER THE GULF COAST STATES ON FRIDAY.
THIS LOW IS FORECAST TO REACH VIRGINIA ON SATURDAY AND THEN MOVE
SLOWLY NORTHEAST THROUGH THE REST OF THE WEEKEND. A LARGE HIGH
PRESSURE SYSTEM WILL THEN SLOWLY BUILD INTO THE NORTHERN PLAINS
MONDAY THROUGH WEDNESDAY.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
THE FORECAST HAS BEEN UPDATED TO REFLECT THE LIGHT SHOWERS OR
SPRINKLES MOVING UP THROUGH PARTS OF OUR DELMARVA ZONES AND INTO
FAR SOUTHERN NEW JERSEY. THE RUC SHOWS A SHORT WAVE SLIDING ACROSS
THIS AREA THROUGH MIDDAY, ALTHOUGH ITS LIGHT QPF IS A FEW HOURS
SLOW BASED ON RADAR TRENDS. WE CARRIED SPRINKLES TO COVER THIS,
ALTHOUGH SOME SPOTS MAY PICK UP A BRIEF LIGHT SHOWER. A SHORT TERM
FORECAST /NOW/ WAS ISSUED EARLIER TO ALSO COVER THIS ACTIVITY.
SOME TWEAKS TO THE TEMPERATURES AND DEW POINTS WERE ALSO MADE.
TRENDS WILL BE MONITORED.

OTHERWISE, WHILE THE GFS INITIALIZED BETTER THAN THE WRF-NMM WITH
THE LARGE CLOSED LOW IN THE MIDWEST, ITS 00Z SOLUTION SHOWED VERY
LITTLE RUN TO RUN CONTINUITY. THE WRF-NMM GIVEN ITS WESTWARD
DISPLACED 500MB LOW IS PROBABLY TOO SLOW. IF THIS PAST WINTER'S
PERFORMANCE IS AN INDICATION OF FUTURE TRENDS, THE GFS'S 500MB LOW
PROG WILL VERIFY TOO SOUTH. FOR THIS MODEL RUN WE ARE GOING TO
FOLLOW A UKMET/ECMWF COMPROMISE WHICH PRESERVES FORECAST
CONTINUITY THE BEST AND HAS NOT SHOWN AS MUCH RUN TO RUN
VOLATILITY. ANYWAY ONE SLICES THOUGH, HEAVY RAIN INVOLVED.

THE FIRST FEW VISIBLE SATELLITE IMAGES THIS MORNING SHOW THE
POCKETS OF FOG MAINLY OVER EASTERN NEW JERSEY. THIS MAY BE SLOW TO
COMPLETELY DISSIPATE THEREFORE THE MENTION OF FOG WAS CARRIED A
LITTLE LONGER. MAX TEMPS WILL BE DIFFICULT TO GET CLOSE TODAY AS
WE WILL HAVE THE ONSHORE FLOW OFF OF THE CHILLY ATLANTIC COMPETING
WITH THE STILL RELATIVELY MILD AIR MASS. GFS MOS OF LATE HAS BEEN
TOO COOL, BUT BECAUSE OF THE ONSHORE FLOW WE STAYED CLOSE TO IT
FOR MOST LOCALES.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH FRIDAY NIGHT/...
A WEAK SHORT WAVE IS FORECAST TO START ERODING THE RIDGE AXIS
TONIGHT. COUPLED WITH A CONTINUING ONSHORE FLOW, THIS SHOULD DEVELOP
SOME UPSLOPE PRECIPITATION ACROSS OUR CWA. GREATEST CONFIDENCE AND
HIGHEST POPS IS THUS NW OF THE INTERSTATE 95 CORRIDOR. UNLIKE WHAT
IS IN STORE FOR US IN THE DAYS AHEAD, ANY PRECIPITATION THAT FALLS
TONIGHT SHOULD BE MAINLY DRIZZLE OR LIGHT RAIN. BECAUSE OF THE
UPSLOPE FLOW AND MODEL SOUNDINGS SHOWING PLENTY OF MOISTURE BELOW
5K, WE MAINTAINED THE MENTION OF PATCHY FOG IN THE NWRN HALF OF
OUR CWA TONIGHT AND INTO FRIDAY MORNING. BECAUSE OF THE CLOUDS WE
BELIEVE NAM MOS IS TOO LOW WITH THE MIN TEMPS AND SIDED CLOSER
TO GFS MOS.

BOTH THE GFS AND WRF-NMM ARE FORECASTING DECENT OMEGA FOR FRIDAY
AFTERNOON AND THIS SHOULD BE THE START OF THE STEADIER AND
EVENTUALLY HEAVIER RAIN FOR OUR CWA. THIS AREA OF RAIN IS FORECAST
TO FORM OVER THE OLD FRONTAL BOUNDARY EAST OF OUR CWA AND ADVECT
WESTWARD. NAM MOS REMAINS CONSIDERABLY LOWER THAN GFS WITH MAX TEMPS
ON FRIDAY. WE TRENDED MAX TEMPS DOWNWARD TOWARD IT, MORE SO AT THE
COAST AND A LITTLE BIT LESS INLAND. EVEN THOUGH WATER TEMPS ARE
QUITE CHILLY, THE AIR OVER IT SOUTHEAST OF OUR CWA IS STILL
RELATIVELY WARMER AND THIS ADDS A LAYER OF UNCERTAINTY TO THE MAX
TEMP FCSTS.

THE FIRST POTENTIAL SHOT OF HEAVIER RAIN IS FORECAST TO ARRIVE
DURING THE SECOND HALF OF THE NIGHT FRIDAY NIGHT. THE NAM'S
SCENARIO OF SEVERAL SHOTS OF HEAVIER RAIN GIVEN THE SLOW MOVEMENT
AND DURATION OF THIS EVENT SEEMS LOGICAL. THIS FIRST SHOT TOWARD
SATURDAY MORNING AS OF TODAY HAS STRONG FORECAST OMEGA AND MODEL
QPF MIGHT BE A BIT ON THE LIGHT SIDE. FORECAST SOUNDINGS THROUGH
12Z SATURDAY DO NOT SUPPORT THUNDER YET. WE BELIEVE NAM MOS IS TOO
LOW WITH MINS FRIDAY NIGHT. IF WE ARE WRONG THERE MIGHT BE SOME
ICE IN THE POCONOS, BUT RAW WRF-NMM MODEL DATA DOES NOT SUPPORT
THIS LATTER THOUGHT.

&&

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
THERE HAS BEEN NOT MUCH RUN TO RUN CONTINUITY WITH THE GFS OVER THE
LAST FOUR FORECASTS. PLUS ITS 00Z ENSEMBLE MEAN IS FARTHER NORTH
AND FASTER WITH THE SURFACE LOW. THIS POINTS TOWARD CONTINUING A
SOLUTION CLOSER TO THE LATEST ECMWF. WHILE THE GFS'S TIMING MAY BE
OFF, ITS DETAILS BELOW STILL HOLD.

THE CONSENSUS OF THE MODELS, INCLUDING THE ECMWF IS THAT THERE IS
GOING TO BE A HEAVY RAINFALL SOMEWHERE IN THE SATURDAY TO SATURDAY
NIGHT TIME PERIOD OVER OUR AREA. H8 DEWPOINTS OF 10 DEG C CLIP OUR
SOUTHERN ZONES AND THE H8 THETA E ADVECTION ON A STIFFENING
SOUTHEAST FLOW LOOKS QUITE STRONG. A VERY PRONOUNCED TROWAL HAS
BEEN PROGGED, AS HAS STRONG LOWER LEVEL FRONTOGENESIS, STRONG
Q-VECTOR CONVERGENCE ALOFT, NEGATIVE OR NEAR-NEGATIVE EQUIVALENT
POTENTIAL VORTICITY ALOFT, AND EVEN SOME ELEVATED UPRIGHT
INSTABILITY OVER THE SOUTH. WE ARE NOT ADDING THUNDER AT THIS
TIME, BUT WE ARE ADDING THE MENTION OF HEAVY RAIN CENTERED ON
SATURDAY AND SATURDAY EVENING. THIS SYSTEM IS SLOW TO MOVE OUT AS
IT ALL WORKS AROUND A VERY SLOW MOVING MID LEVEL LOW PRESSURE AREA
/WE DON'T DROP ALL MENTION OF PRECIPITATION UNTIL MONDAY NIGHT/,
BUT IT STILL APPEARS THAT THE WORST OF THE PRECIPITATION ENDS BY
DAYBREAK SUNDAY. FLOODING REMAINS A REAL CONCERN, NOT JUST BECAUSE
SOME SNOW PACK WITH HIGH WATER CONTENT REMAINS OVER THE FAR
NORTHWEST. WINDS AROUND SATURDAY, ESPECIALLY NEAR THE COAST, MAY
ALSO BE OF SOME CONCERN. THERE MAY BE A CHANGEOVER TO SNOW OR
MIXED PRECIPITATION OVER THE FAR NORTHWEST BEFORE WE ARE DONE WITH
THIS SYSTEM, BUT IT DOES NOT LOOK TO BE SIGNIFICANT AT THIS TIME.

TEMPERATURES IN THE PERIOD START OFF WELL ABOVE NORMAL /THERE MAY BE
SOME RISING TEMPERATURES ON FRIDAY NIGHT, BUT WE WON'T FORECAST THAT
FOR THE TIME BEING AS THE LOW LEVEL FLOW IS SOUTHEAST RATHER THAN
SOUTH/, THEN MOVE TOWARD NORMAL BUT REMAIN ON THE WARM SIDE OF
CLIMATOLOGY.

&&

.AVIATION /14Z THURSDAY THROUGH MONDAY/...
THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY AND SURROUNDING AREAS.

A LARGE SHIELD OF MID LEVEL CLOUDINESS IS ACROSS THE TAF SITES EARLY
TODAY. SOME FOG AND SOME MARINE STRATUS HAS DEVELOPED ACROSS SRN
NJ WHICH IS AFFECTING THE DELAWARE VALLEY TAF SITES AND KACY TOO.
THE DEGREE/INTENSITY OF MARINE FOG/STRATUS TODAY IS PROBLEMATIC
WITH THE LIGHT ONSHORE FLOW AND HIGHER DEW POINTS ARRIVING OVER
THE COLDER WATERS. FOR NOW...I HAVE BROUGHT VFR CONDITIONS BACK TO
THE AREA AFTER 14-15Z. OUTSIDE OF THE STRATUS...THE TRENDS FOR
CIG HEIGHTS WILL BE DECREASING AS THE ONSHORE FLOW INCREASES AND
A WEALTH OF LOW CLOUDS ARE EXPECTED LATER TODAY AND TONIGHT.
CONDITIONS WILL DETERIORATE TO MVFR THEN QUICKLY TO IFR EARLY
TONIGHT. THE LOW CONDITIONS SHOULD PERSIST INTO TONIGHT.

OUTLOOK...
WIDESPREAD IFR CONDITIONS WITH FREQUENT RAINS AND FOG WILL BE ACROSS
THE AREA FRIDAY AND INTO THE WEEKEND. LOW LEVEL WIND SHEAR MAY OCCUR
LATE FRI NGT INTO SAT WITH A DECENT LOW LEVEL JET EXPECTED. BETTER
CONDITIONS WILL HOPEFULLY ARRIVE IN TIME FOR THE NEW WORK WEEK.

&&

.MARINE...
CONDITIONS ON THE WATERS WILL BE TRANQUIL TODAY AND TONIGHT. WINDS
AND SEAS WILL INCREASE FRIDAY INTO THE WEEKEND AS LOW PRESSURE MOVES
IN FROM THE SW. A SMALL CRAFT FLAG HAS BEEN ISSUED FOR FRI WITH
GUSTS OVER 25 KTS AND 5 FT PLUS EXPECTED TO DEVELOP. THE SCA FLAG
WILL LIKELY BE ONGOING THROUGH THE WEEKEND. GALE CONDITIONS MAY
OCCUR SAT. NOT ENOUGH CONFIDENCE AT THIS TIME FOR A GALE WATCH.

&&

.HYDROLOGY...
THE EURO SOLUTION IS MOST PREFERRED TONIGHT.

PREDICTED RAINFALL, FRIDAY NIGHT THROUGH SUNDAY, A WIDESPREAD 2
TO 4 INCHES LOOKS LIKELY ACROSS THE HSA. HIGHER AMOUNTS ARE
POSSIBLE.

WITH THESE NUMBERS, FLOODING IS LIKELY THIS WEEKEND INTO EARLY NEXT
WEEK. BOTH SMALLER STREAMS AND LARGER RIVERS COULD EXPERIENCE
FLOODING. STREAMFLOW IS ALREADY RUNNING HIGH ACROSS SE PA. SO THEY
DON'T NEED THE SNOWMELT TO FLOOD. ON THE OTHER HAND, RIVERS LIKE THE
DELAWARE WILL NEED THE SNOWMELT TO FLOOD, AND IN THIS CASE, IT
APPEARS THE HEAVY RAIN WILL FALL ACROSS THE HEADWATER SNOWPACK WHICH
IS THREE TIMES NORMAL FOR THIS TIME OF YEAR.

SO IN VERY GENERAL TERMS, 1 TO 2 INCHES OF WIDESPREAD RAINFALL WILL
CAUSE MINOR FLOODING ACROSS THE SMALLER STREAMS, CREEKS AND RIVERS
ACROSS THE PASSAIC, RARITAN, AND DELAWARE BASINS, WITH LITTLE OR NO
FLOODING ALONG THE MAINSTEM RIVERS /LEHIGH, SCHUYLKILL, AND DELAWARE/.
2.0 TO 3.5 INCHES WOULD CAUSE MODERATE FLOODING ALONG THESE SAME
SMALLER CREEKS AND STREAMS, WITH SOME FLOODING POSSIBLE ALONG THE
MAINSTEM RIVERS, ESPECIALLY THE SCHUYLKILL. 3.5 TO 5.0 INCHES OF
RAINFALL, BOTH LOCALLY AND OVER THE EXISTING SNOWPACK IN THE
NORTH, WOULD CAUSE WIDESPREAD MODERATE/MAJOR FLOODING, WITH MINOR
FLOODING ALONG THE MAINSTEM DELAWARE.

THE LEHIGH VALLEY HAS BEEN A BIT DRIER OVER THE LAST 30, 60, AND
90 DAYS. FOR THIS REASON, THIS PARTICULAR AREA CAN TOLERATE A
LITTLE MORE WATER BEFORE FLOODING. THIS FLOOD POTENTIAL WILL
CONTINUE TO BE HIGHLIGHTED IN THE HAZARDOUS WEATHER OUTLOOK AT
THIS TIME.

&&

.TIDES/COASTAL FLOODING...
THE APPROACHING LOW WITH ONSHORE FLOW AND THE (SOON TO BE) NEW MOON
WILL CONTRIBUTE TO HIGHER THAN USUAL TIDES FRIDAY AND THROUGH THE
WEEKEND. THE RECENT SLOWER TREND OF THE SYSTEM IS PROBLEMATIC, SINCE
IT WILL BRING THE GREATER TIDES CLOSER TO THE NEW MOON DATE (15TH).
IT APPEARS NOW THAT (AT LEAST) MINOR TIDAL FLOODING WILL OCCUR ON
SAT...PERHAPS INTO SUN.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...NONE.
NJ...NONE.
DE...NONE.
MD...NONE.
MARINE...SMALL CRAFT ADVISORY FROM 10 AM FRIDAY TO 6 AM EST SATURDAY
FOR ANZ430-431-450>455.

&&

$$
SYNOPSIS...GIGI
NEAR TERM...GIGI/GORSE
SHORT TERM...GIGI
LONG TERM...DELISI/GIGI
AVIATION...O'HARA
MARINE...O'HARA
HYDROLOGY...KRUZDLO
TIDES/COASTAL FLOODING...O'HARA

Back to Main Forecast Discussion Page