FXUS61 KPHI 240417
AFDPHI
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ
1117 PM EST WED DEC 23 2009
.SYNOPSIS...
A RIDGE OF HIGH PRESSURE FROM SOUTHERN CANADA THROUGH THE EASTERN
GREAT LAKES TO THE CENTRAL MID ATLANTIC COAST TONIGHT WILL CONTINUE
THURSDAY BUT MOVE EASTWARD FRIDAY. LOW PRESSURE WILL INTENSIFY OVER
THE CENTRAL PLAINS THURSDAY NIGHT AND MOVE SLOWLY TO THE MID AND
UPPER MISSISSIPPI VALLEY BY SATURDAY. SECONDARY LOW PRESSURE IS
EXPECTED TO FORM OVER THE CENTRAL MID ATLANTIC REGION FRIDAY NIGHT
AND SATURDAY AND MOVE THROUGH OUR AREA BY SUNDAY MORNING. HIGH
PRESSURE WILL RETURN BRIEFLY LATER SUNDAY THROUGH MONDAY WHILE
ANOTHER COLD FRONT IS EXPECTED TO SWING THROUGH THE AREA TUESDAY.
HIGH PRESSURE SHOULD FOLLOW FOR WEDNESDAY.
&&
.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
COLD AIR STRATOCUMULUS WAS CONTINUING TO BRUSH OUR NORTHERN
ZONES LATE THIS EVENING...BUT THESE SHOULD MOVE OUT OVERNIGHT
ACCORDING TO SATELLITE TRENDS. OTHERWISE HIGH PRESSURE WILL
PROVIDE FAIR AND COLD WEATHER TO OUR AREA OVERNIGHT. WITH MOSTLY
CLEAR SKIES...OR BECOMING CLEAR...AND LIGHT WINDS, TEMPERATURES
ARE EXPECTED TO ONCE AGAIN DROP PRECIPITOUSLY. LOWS WILL AVERAGE
IN THE 10 TO 20 DEGREE RANGE. BUT WITH THE SNOW COVER, SOME LOW
TEMPERATURES COULD DROP INTO THE SINGLE DIGITS, MUCH LIKE LAST
NIGHT.
&&
.SHORT TERM /6 AM THURSDAY MORNING THROUGH SATURDAY/...
THE HIGH PRESSURE RIDGE WILL BE OVER US THURSDAY AND THEN BEGIN
TO MOVE OFF TO THE EAST THURSDAY NIGHT. EXPECT TO SEE ANOTHER
BRIGHT SUNNY DAY THURSDAY WITH HIGH TEMPERATURES MAINLY IN THE
30S, SLIGHTLY HIGHER THAN TODAY. AS THE WINDS SWING AROUND TO AN
ONSHORE DIRECTION THURSDAY NIGHT, CLOUDS WILL BEGIN TO INCREASE.
LOW TEMPERATURES THURSDAY NIGHT WILL BE MAINLY IN THE 20S. OUR DRY
WEATHER WILL COME TO AN END ON CHRISTMAS AS A COLD FRONT
APPROACHES FROM THE WEST. LOW PRESSURE IS FORECAST TO DEVELOP ON
THE TRIPLE POINT OVER VIRGINIA CHRISTMAS NIGHT AND MOVE NORTH
ACROSS OUR AREA SATURDAY. MOST OF THE PRECIPITATION WITH THIS
SYSTEM WILL FALL AS RAIN, SPREADING FROM SOUTHWEST TO NORTHEAST
ACROSS THE REGION DURING THE DAY FRIDAY. IT IS EXPECTED TO ENTER
OUR FORECAST AREA FRIDAY MORNING, REACH THE PHILLY METRO AREA BY
EARLY AFTERNOON AND COVER THE ENTIRE AREA BY EVENING. WITH THE
SNOW COVER AND LIGHT EASTERLY WINDS, COLD AIR WILL BE TRAPPED AT
THE SURFACE IN OUR NORTHERN VALLEYS. THIS COULD MEAN AN EXTENDED
PERIOD OF FREEZING RAIN WITH SOME ICE ACCUMULATION, PROBABLY IN
THE 1/10 TO 1/4 INCH RANGE. LATER FRIDAY NIGHT INTO SATURDAY
MORNING, WINDS WILL SWING TO THE SE AND INCREASE AS A WARM FRONT
MOVES ACROSS. THIS WILL HELP PUSH TEMPERATURES WELL INTO THE 40S
AND POSSIBLY LOWER 50S IN SOME AREAS. COMBINE THESE TEMPERATURES
WITH RAINFALL EXPECTED TO AVERAGE 3/4 TO 1 INCH, AND THE
POSSIBILITY OF FLOODING WILL INCREASE DRAMATICALLY. OUR SNOW PACK
SHOULD BE ABLE TO HOLD UP TO 1 1/2 INCHES BEFORE LOSING ITS ENTIRE
WATER EQUIVALENT. EVEN IF THE SNOW PACK REMAINS INTACT, WE CAN
EXPECT SOME URBAN FLOODING LATE FRIDAY NIGHT AND SATURDAY. STREAM
FLOODING MAY ALSO OCCUR IF ENOUGH WATER COMES OUT OF THE SNOW PACK
AND RAINFALL AMOUNTS AVERAGE HIGHER THAN CURRENTLY EXPECTED. WE
WILL CONTINUE TO MONITOR THIS SITUATION AND HAVE MUCH MORE
DETAILED INFORMATION AS THE EVENT GETS CLOSER.
&&
.LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/...
THE LONG TERM PERIOD SHOULD BEGIN WITH A LONG WAVE UPPER TROUGH
OVER THE EASTERN HALF OF THE COUNTRY. THE TROUGH IS FORECAST TO
PROGRESS TO THE EAST, PASSING OFF THE COAST IN THE SUNDAY NIGHT
INTO MONDAY TIME FRAME. AN UPPER LEVEL RIDGE IS EXPECTED TO WORK
ITS WAY TOWARD OUR REGION FROM THE WEST, WITH ITS AXIS ARRIVING
OVERHEAD AROUND WEDNESDAY NIGHT.
AT THE SURFACE, COMPLEX LOW PRESSURE SHOULD BE OVER THE GREAT
LAKES AND THE NORTHEASTERN STATES ON SATURDAY NIGHT. THE LOW IS
FORECAST TO LIFT TO THE NORTHEAST, WITH DRY AIR BUILDING INTO OUR
REGION ON THE RESULTING NORTHWEST FLOW. WE WILL CONTINUE TO CARRY
POPS FOR SATURDAY NIGHT IN NORTHERN AND CENTRAL NEW JERSEY, AS
WELL AS IN PARTS OF EASTERN PENNSYLVANIA. IT SHOULD REMAIN WARM
ENOUGH TO KEEP THE PRECIPITATION MAINLY IN LIQUID FORM. WE WILL
NOT MENTION ANY PRECIPITATION FOR SUNDAY.
IT APPEARS AS THROUGH THE PERIOD FROM SUNDAY NIGHT INTO TUESDAY
WILL BE MAINLY DRY, EVEN THROUGH WE SHOULD REMAIN IN THE CYCLONIC
FLOW ASSOCIATED WITH THE DEPARTING LOW. WE HAVE INCLUDED SLIGHT
CHANCE POPS FOR SNOW SHOWERS IN THE NORTH. BOTH THE 1200 UTC GFS
AND ECMWF ARE INDICATING A REINFORCING COLD FRONT DROOPING DOWN
FROM THE NORTHWEST ON MONDAY NIGHT OR EARLY ON TUESDAY.
SURFACE HIGH PRESSURE IS ANTICIPATED TO BUILD INTO OUR REGION
FROM THE WEST AND NORTHWEST FOR TUESDAY NIGHT AND WEDNESDAY.
&&
.AVIATION /04Z THURSDAY THROUGH MONDAY/...
THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY AND SURROUNDING AREAS.
VFR CONDITIONS WILL CONTINUE THROUGH TONIGHT AND INTO THURSDAY.
WINDS WILL REMAIN OUT OF THE NORTHWEST AND NORTH.
OUTLOOK...
HIGH PRESSURE WILL SLIDE TO THE EAST FRIDAY AS A WEAK LOW
DEVELOPS OFF THE SOUTHEASTERN UNITED STATES. MVFR/IFR CONDITIONS
ARE EXPECTED FOR LATE FRIDAY THROUGH SATURDAY.
&&
.MARINE...
HIGH PRESSURE WILL MOVE OFFSHORE FRIDAY AS LOW PRESSURE DEVELOPS
OVER THE MIDWESTERN UNITED STATES. THIS LOW WILL TRACK EASTWARD AS
A SECONDARY LOW DEVELOPS OFF THE SOUTHEASTERN UNITED STATES. THE
GRADIENT BETWEEN THESE SYSTEMS WILL TIGHTEN BY FRIDAY, INCREASING
WINDS WITH SMALL CRAFT ADVISORIES LIKELY AND NEAR GALE FORCE WINDS
INTO THE WEEKEND. ALSO, SEAS ARE EXPECTED TO RISE ABOVE 5 FT WITH
THIS STORM.
THE LOW FINALLY CLEARS OUT SUNDAY WITH DECREASING WINDS AND
ALLOWING THE SEAS TO STEADILY SUBSIDE.
&&
.PHI WATCHES/WARNINGS/ADVISORIES...
PA...NONE.
NJ...NONE.
DE...NONE.
MD...NONE.
MARINE...NONE.
&&
$$
SYNOPSIS.../
NEAR TERM... / RPW
SHORT TERM...RPW
LONG TERM...IOVINO
AVIATION...IOVINO / MEOLA
MARINE...IOVINO / MEOLA
