FXUS61 KPHI 220110
AFDPHI

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ
810 PM EST MON DEC 21 2009

.SYNOPSIS...
HIGH PRESSURE WILL BUILD OVER THE REGION TONIGHT THROUGH THURSDAY,
THEN LIFT NORTHEAST EARLY FRIDAY. MEANWHILE...A LOW PRESSURE
SYSTEM FROM THE SOUTHERN ROCKIES IS EXPECTED TO INTENSIFY AS IT
MOVES NORTHEAST INTO THE GREAT LAKES REGION THURSDAY AND FRIDAY.
PRECIPITATION WITH THIS SYSTEM COULD OVERSPREAD THE AREA CHRISTMAS
MORNING AS A WINTRY MIX, TURNING OVER TO ALL RAIN BY AFTERNOON IN 
MANY AREAS. ANOTHER AREA OF HIGH PRESSURE IS THEN EXPECTED FOR THE 
LATTER HALF OF THE UPCOMING WEEKEND.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
AT 8:00 PM, THE SKY OVER OUR FORECAST AREA WAS MAINLY CLEAR,
EXCEPT FOR A FEW PATCHES OF STRATOCUMULUS OVER PARTS OF EASTERN
PENNSYLVANIA AND NORTHERN NEW JERSEY. TEMPERATURES WERE MOSTLY IN
THE UPPER 20S AND LOWER 30S.

AN UPPER LEVEL JET EXTENDED FROM THE GREAT LAKES TO THE SOUTHERN
PART OF OUR REGION THIS EVENING. THE JET SHOULD LIFT TO THE
NORTHEAST OVERNIGHT AND IT IS EXPECTED TO WEAKEN. EVEN WITH A
CONTINUED WEST TO NORTHWEST FLOW OVERNIGHT, THE TREND SHOULD BE
TOWARD LESSENING CLOUD COVER.

WE WILL CONTINUE TO FORECAST LOW TEMPERATURES IN THE TEENS AND
LOWER 20S.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH THURSDAY/...
HIGH PRESSURE WILL BUILD OVER THE REGION THROUGH THE PERIOD,
RESULTING IN GENERALLY DRY WEATHER THROUGH CHRISTMAS EVE. THE ONLY
EXCEPTION COULD BE A FEW FLURRIES LATE TUESDAY AND TUESDAY NIGHT
AS ANOTHER WEAK UPPER AIR DISTURBANCE CROSSES THE CWA IN A ZONAL
FLOW WHICH WILL HAVE ESTABLISHED ITSELF OVER THE EASTERN U.S. BY
THAT TIME. TEMPERATURES THROUGH THE PERIOD WILL BE BELOW NORMAL,
BUT HIGH TEMPERATURES EACH DAY COULD STILL MAX OUT NEAR OR JUST OVER
THE FREEZING MARK, WHICH IN TURN WILL INDUCE SOME ADDITIONAL
DAYTIME MELTING AND NIGHTTIME FREEZING. THIS IS THE PROBLEM WITH
SNOW...IT HANGS AROUND AND CAUSES PROBLEMS LONG AFTER IT FALLS!

&&

.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
AS RIDGING IS FORECAST INTO THE WESTERN UNITED STATES BY THE TIME
THIS PERIOD BEGINS, THIS TENDS TO SUPPORT AN AMPLIFYING TROUGH IN
THE MIDDLE OF THE NATION. IN ADDITION, SOME RIDGING WILL BE
DOWNSTREAM ACROSS THE EAST FOR A TIME. WHILE THE OVERALL MODEL
GUIDANCE SUPPORTS THIS IDEA, THERE ARE SOME DIFFERENCES IN TIMING
AND ALSO THE AMPLIFICATION OF THE UPPER-LEVEL TROUGH. THIS WILL
HAVE AN IMPACT ON THE STRENGTH OF THE SURFACE LOW AS WELL AS THE
DOWNSTREAM SURFACE HIGH IN SOUTHEASTERN CANADA. HPC WENT MORE WITH
A BLEND OF THE ECMWF ENSEMBLE MEAN AND ECMWF, WITH A HIGHER WEIGHT
TOWARD THE MEAN. WE INITIALIZED THE DATABASE WITH HPC GUIDANCE,
HOWEVER SOME MOSTLY MINOR ADJUSTMENTS WERE MADE BASED ON
ADDITIONAL COLLABORATION WITH OUR SURROUNDING OFFICES.

BASED ON THE ABOVE, HIGH PRESSURE IS FORECAST TO BE WEDGED DOWN INTO 
THE MID ATLANTIC THURSDAY NIGHT AS UPPER-LEVEL RIDGING AMPLIFIES 
ACROSS THE EAST. A DEEPENING SURFACE LOW IS FORECAST TO TRACK FROM 
IOWA FRIDAY MORNING TO NEAR LAKE SUPERIOR SATURDAY MORNING. A 
STRENGTHENING SOUTHWESTERLY FLOW ALOFT AHEAD OF THIS SYSTEM WILL 
TRANSPORT GULF OF MEXICO MOISTURE NORTHWARD INTO OUR AREA ON 
CHRISTMAS DAY. SOME WAA-INDUCED PRECIPITATION MAY BEGIN TO EDGE 
CLOSER TO OUR WESTERN CWA CHRISTMAS EVE NIGHT HOWEVER. THERE APPEARS 
TO BE AT LEAST SOME COLD AIR DAMMING TAKING PLACE, GIVEN THE TRACK 
OF THE SURFACE LOW SO FAR TO OUR WEST. THE COLD AIR RIGHT AT THE 
SURFACE MAY ALSO BE HELD IN PLACE LONGER DUE TO THE SNOW COVER, 
WHICH SHOULD STILL BE DEEP ENOUGH BY THIS TIME /ALTHOUGH SOME 
MELTING WILL OCCUR PRIOR/. GIVEN THE WAA ALOFT BUT SOME LINGERING 
COLDER AIR AT THE SURFACE, THIS SETUP LOOKS TO BE MORE OF A MIX/ICE 
SCENARIO BEFORE CHANGING OVER TO PLAIN RAIN. THE DETAILS WILL HAVE 
TO BE PINNED DOWN MORE AS WE GET CLOSER, HOWEVER AREAS ESPECIALLY 
FROM INTERSTATE 95 NORTH AND WEST COULD GET A PERIOD OF ICE/MIX 
BEFORE WARMER AIR GETS DRAWN IN TO WARM THE SURFACE TEMPERATURES 
ABOVE FREEZING. JUST TO NOTE, THE 12Z ECMWF IS SLOWER, WHICH IF 
CORRECT, MAY THEN LOWER THE ICE/MIX POTENTIAL. AT THIS POINT, WE 
UTILIZED MOSTLY THE TOP-DOWN METHOD TO DRAW IN THE MIX/ICE 
POTENTIAL. SOMETHING TO WATCH IS THE POTENTIAL DEVELOPMENT OF A 
SECONDARY SURFACE LOW ALONG THE OCCLUDED FRONT IN OUR VICINITY. IF 
THIS SYSTEM DOES DEVELOP AND IF IT IS FARTHER TO THE SOUTH, THE 
LOW-LEVEL COLD AIR MAY BE TRAPPED LONGER IN OUR NORTHERN AND WESTERN 
ZONES AS THE LOW-LEVEL FLOW MAY REMAIN AGEOSTROPHIC LONGER. WE DID 
RAMP UP POPS FRIDAY INTO SATURDAY, BUT GIVEN SOME TIMING 
UNCERTAINTY, WE WENT NO HIGHER THAN LIKELY ATTM.

AS THE SURFACE LOW TRACKS WELL TO OUR NORTH LATE SATURDAY INTO 
SUNDAY, THE ASSOCIATED SURFACE FRONT WILL CLEAR OUR AREA. AN 
UPPER-LEVEL TROUGH THEN SETTLES INTO THE NORTHEAST WITH A COLDER 
CYCLONIC FLOW REGIME SETTING UP INTO MONDAY. THERE COULD BE SOME 
RESPONSE FROM THE EASTERN GREAT LAKES, THEREFORE WE CARRIED A CHC
OF FLURRIES IN THE POCONOS AND FAR NORTHWESTERN SUSSEX COUNTY, NEW 
JERSEY SATURDAY NIGHT THROUGH SUNDAY NIGHT.

&&

.AVIATION /01Z TUESDAY THROUGH SATURDAY/...
THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG, 
KMIV, KACY AND SURROUNDING AREAS.

AS AN UPPER-LEVEL WEATHER DISTURBANCE GRADUALLY LIFTS FARTHER TO THE 
NORTH AND OUT OF OUR AREA, THE STRATOCUMULUS HAS BEEN THINNING OUT. 
THIS TREND IS EXPECTED TO CONTINUE AS THE STRATOCUMULUS BECOMES 
MOSTLY CONFINED TO OUR WEST, ALTHOUGH A FEW NARROW STREAMERS CANNOT 
BE RULED OUT AT TIMES. HIGH PRESSURE WILL GRADUALLY ARRIVE FROM THE
WEST OVERNIGHT AND TUESDAY, HOWEVER A NORTHWESTERLY FLOW WILL BE 
MAINTAINED. THE LOSS OF DAYTIME HEATING HAS ALLOWED THE WINDS TO 
DIMINISH AT MOST OF THE TERMINALS. WHILE SOME TERMINALS COULD HAVE 
THE WIND GO LIGHT AND VARIABLE, ENOUGH MIXING SHOULD MAINTAIN AT 
LEAST A 5 TO 10 KNOT WIND FROM THE NORTHWEST THROUGH THE NIGHT. AS A 
RESULT, NO GUSTS WERE CARRIED THROUGH THE NIGHT AND EARLY TUESDAY 
MORNING.

THERE SHOULD BE SOME STRATOCUMULUS DEVELOPMENT ONCE AGAIN TUESDAY AS 
THE DAYTIME HEATING GETS UNDERWAY, HOWEVER IT APPEARS IT WILL NOT BE 
AS WIDESPREAD AS TODAY. WE CARRIED SCATTERED VFR COVERAGE FROM THE 
KPHL METRO AREA ON WESTWARD WITH FEW TO THE EAST. A NORTHWEST WIND 
WILL CONTINUE AND IS EXPECTED TO INCREASE DURING THE MORNING. BASED 
ON THE VERTICAL MIXING POTENTIAL FROM ABOUT MID MORNING THROUGH MID 
AFTERNOON, WE INCREASED THE GUSTS A LITTLE BIT FROM CONTINUITY AT 
MOST TERMINALS. THESE WINDS ARE EXPECTED TO DIMINISH FAIRLY QUICKLY 
TOWARD EVENING.

OUTLOOK...
NO SIGNIFICANT WEATHER IS EXPECTED TUESDAY NIGHT THROUGH WEDNESDAY 
AS HIGH PRESSURE SLIDES ACROSS OUR AREA. A POSSIBLE EXCEPTION IS 
WEDNESDAY AFTERNOON AND NIGHT, AS ENOUGH MOISTURE COULD BECOME 
TRAPPED UNDER A STRENGTHENING INVERSION TO POSSIBLY BRING MOSTLY 
STRATOCUMULUS. THE NEXT STORM SYSTEM IS FORECAST TO ARRIVE LATE IN 
THE WEEK, PROBABLY IN TIME FOR CHRISTMAS DAY. THERE COULD BE A 
PERIOD OF MIXED PRECIPITATION/ICE MOSTLY NORTH AND WEST OF KPHL, 
HOWEVER THE TIMING WILL BE KEY REGARDING WHAT THE SURFACE 
TEMPERATURES END UP BEING. OTHERWISE, ANY MIXED OR ICE SHOULD CHANGE
TO PLAIN RAIN DURING CHRISTMAS DAY AS MILDER AIR WORKS IN. THIS 
SYSTEM WILL BRING REDUCED CEILINGS AND VISIBILITIES, ALTHOUGH THE 
TIMING MAY CHANGE. THE MILDER AIR ARRIVING OVER THE REMAINING SNOW 
COVER COULD LEAD TO FOG DEVELOPMENT. THIS SYSTEM MAY LINGER ON 
SATURDAY BEFORE GRADUAL IMPROVEMENTS OCCUR.

&&

.MARINE...
AN INTENSE STORM EAST OF THE CANADIAN MARITIMES CONTINUES MOVING
OFF TO THE NORTHEAST, AND HIGH PRESSURE TRIES TO PUSH IN FROM THE
WEST. A NORTHWESTERLY FLOW WILL CONTINUE ACROSS OUR WATERS, AND
THE WINDS HAVE ACTUALLY INCREASED ACROSS OUR FAR NORTHERN WATERS
DURING THE PAST FEW HOURS WITH SOME GALE FORCE GUSTS. THIS APPEARS
TO BE GRAZING OUR NORTHERN OCEAN ZONE WITH THE AVAILABLE
OBSERVATIONS INDICATING SOME GALE FORCE GUSTS FARTHER OFFSHORE.
FOR NOW, WE WILL CONTINUE WITH THE SMALL CRAFT ADVISORY AND BUMP
THE WINDS UP A LITTLE BIT.

A WEAK UPPER LOW DROPPING DOWN ACROSS THE GREAT LAKES WILL LACK
SIGNIFICANT WEATHER BUT WILL REINFORCE THE COLD AIR AND TEND TO
KEEP THE PRESSURE GRADIENT FAIRLY TIGHT THROUGH TUESDAY NIGHT. IT
IS POSSIBLE THAT A FEW GUSTS COULD REACH TO NEAR 35 KNOTS AT TIMES
THROUGH TUESDAY NIGHT ON OUR OCEAN WATERS BUT NOT FREQUENT ENOUGH
TO ISSUE A GALE WARNING. THE SMALL CRAFT ADVISORY WILL EXTENDED ON
LOWER DELAWARE BAY UNTIL 6 AM TUESDAY AND UNTIL 6 AM WEDNESDAY ON
THE OCEAN WATERS. HIGH PRESSURE WILL MOVE ACROSS OUR REGION
WEDNESDAY THROUGH THURSDAY BEFORE A LOW PRESSURE SYSTEM, MOVING
ACROSS THE GREAT LAKES, DRAGS A COLD FRONT ACROSS OUR AREA LATE
CHRISTMAS NIGHT OR EARLY SATURDAY. THIS SYSTEM COULD BRING OUR
NEXT ROUND OF ADVISORY OR WARNING CONDITIONS.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...NONE.
NJ...NONE.
DE...NONE.
MD...NONE.
MARINE...SMALL CRAFT ADVISORY UNTIL 6 AM EST WEDNESDAY FOR ANZ450>455.
     SMALL CRAFT ADVISORY UNTIL 6 AM EST TUESDAY FOR ANZ431.

&&

$$

SYNOPSIS...MIKETTA
NEAR TERM...IOVINO
SHORT TERM...MIKETTA
LONG TERM...GORSE
AVIATION...GORSE
MARINE...GORSE/RPW