FXUS61 KPHI 232039
AFDPHI
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ
339 PM EST WED DEC 23 2009
.SYNOPSIS...
HIGH PRESSURE PUSHING DOWN FROM EASTERN CANADA WILL MOVE SLOWLY EAST
THROUGH FRIDAY. LOW PRESSURE WILL INTENSIFY OVER THE CENTRAL PLAINS
THURSDAY NIGHT AND MOVE SLOWLY TOWARD THE WESTERN GREAT LAKES BY
SATURDAY. SECONDARY LOW PRESSURE IS EXPECTED TO FORM OVER THE
CENTRAL MID ATLANTIC REGION FRIDAY NIGHT AND MOVE NORTH ACROSS OUR
AREA SATURDAY, HEADING OFF TO THE NORTHEAST SATURDAY NIGHT. HIGH
PRESSURE WILL RETURN FOR THE FIRST HALF OF NEXT WEEK.
&&
.NEAR TERM /UNTIL 6 AM THURSDAY/...
THE HIGH PRESSURE SYSTEM PUSHING DOWN FROM THE NORTH WILL CONTINUE
TO PROVIDE FAIR AND COLD WEATHER TO OUR AREA TONIGHT. WITH MOSTLY
CLEAR SKIES AND LIGHT WINDS, TEMPERATURES ARE EXPECTED TO ONCE
AGAIN DROP QUICKLY TONIGHT. 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 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 /21Z WEDNESDAY 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.
HOWEVER, WITH THE SNOW COVER AND LIGHTER WINDS EXPECTED THIS
EVENING, KACY AND KMIV VISIBILITIES SHOULD DROP TO MVFR OVERNIGHT.
OTHERWISE, WINDS WILL REMAIN OUT OF THE WEST OR NORTHWEST WITH
SPEEDS DIMINISHING THROUGH THIS EVENING.
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...
THE SCA HAS BEEN CANCELLED FOR THE REST OF THE EVENING AS GUSTS HAVE
BECOME LESS FREQUENT ACROSS THE NORTHERN WATERS. HOWEVER, AN
OCCASIONAL GUST NEAR 25 KT IS POSSIBLE TONIGHT AS THE GRADIENT
INCREASES WITH AN UPPER LEVEL LOW PUSHING SOUTH AND THEN EAST.
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...RPW
NEAR TERM...RPW
SHORT TERM...RPW
LONG TERM...IOVINO
AVIATION...MEOLA
MARINE...MEOLA
