FXUS61 KPHI 240830
AFDPHI

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ
330 AM EST THU DEC 24 2009

.SYNOPSIS...
A RIDGE OF HIGH PRESSURE FROM SOUTHERN CANADA THROUGH THE EASTERN 
GREAT LAKES TO THE CENTRAL MID ATLANTIC COAST TODAY WILL MOVE 
EASTWARD FRIDAY. LOW PRESSURE WILL INTENSIFY OVER THE CENTRAL PLAINS 
TONIGHT 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. A WESTERLY FLOW WITH WEAK HIGH 
PRESSURE WILL PREVAIL LATER SUNDAY THROUGH MONDAY WHILE ANOTHER COLD 
FRONT IS EXPECTED TO SWING THROUGH OUR AREA EARLY TUESDAY. HIGH 
PRESSURE SHOULD BUILD TOWARD OUR REGION FROM THE WEST WEDNESDAY.

&&

.NEAR TERM /THROUGH TODAY/...
THERE IS NO REAL WEATHER STORY FOR TODAY AS THE WEATHER PROBLEMS
WILL BE IN THE SHORT TERM PERIOD AND BLEEDING OVER A BIT INTO THE
FIRST PART OF THE LONG TERM...LATE FRIDAY INTO SUNDAY.

COLD AIR STRATOCUMULUS THAT WAS OVER OUR NORTHWESTERN ZONES
EARLIER IN THE OVERNIGHT PERIOD WAS ERODING WESTWARD BEFORE
SUNRISE. AS THE RIDGE OF HIGH PRESSURE SURFACE AND ALOFT
CONTINUES TODAY...SKIES ARE FORECAST TO BE MOSTLY SUNNY, WITH
MINIMAL CUMULUS. SOME CIRRUS MAY SPILL OVER THE UPPER RIDGE THIS
AFTERNOON, BUT ANY THAT DOES IS EXPECTED TO BE MOSTLY THIN.

GFS MOS HAS BEEN SUPERIOR IN MAXIMUM TEMPERATURE FORECASTING AS
OF LATE, AND THE NAM MOS SEEMS PRETTY COLD GIVEN AIRMASS
MODIFICATION AND FULL, ALBEIT WEAK, SUNSHINE.

&&

.SHORT TERM /TONIGHT THROUGH SATURDAY/...
THE HIGH PRESSURE RIDGE WILL BEGIN TO MOVE OFF TO THE EAST
TONIGHT. AS THE WINDS SWING AROUND TO AN ONSHORE DIRECTION
TONIGHT, CLOUDS WILL BEGIN TO INCREASE. LOW TEMPERATURES TONIGHT
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 AND SATURDAY NIGHT.
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, REACHING THE PHILADELPHIA METROPOLITAN AREA BY EARLY
AFTERNOON, AND COVERING 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. ALSO, 1000-850 HPA THICKNESSES
WILL BE AROUND OR A LITTLE BELOW 1300 METERS. THIS IS LIKELY TO
MEAN A PERIOD OF FREEZING RAIN WITH SOME ICE ACCUMULATION,
PROBABLY AROUND 1/10 OF AN INCH OR IN THE 1/10 TO 1/4 INCH RANGE.
ALSO, SOME SLEET COULD OCCUR AT THE START OF THE EVENT OVER THE
NORTHWESTERN ZONES LATER IN THE DAY FRIDAY. MODEL SOUNDINGS APPEAR
TO HAVE A DRY LAYER, WHICH MAY DELAY THE ONSET OF PRECIPITATION
UNTIL LATER IN THE DAY. EVEN SO, THERE STILL COULD BE A FAIRLY
BRIEF PERIOD OF FREEZING DRIZZLE OVER OUR NORTHERN ZONES FRIDAY
MORNING AS THE INITIAL UPWARD VERTICAL MOTION PASSES OVERHEAD.
THIS WILL HAVE TO BE MONITORED.

LATER FRIDAY NIGHT INTO SATURDAY MORNING, WINDS WILL SWING TO THE
SOUTHEAST 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 (PROBABLY0 MINOR
FLOODING WILL INCREASE. 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 AS THE EVENT NEARS.

&&

.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. THE ONLY
PRECIPITATION MENTIONED FOR SUNDAY WILL BE A SLIGHT CHANCE OF
SNOW SHOWERS NORTHWEST.

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 DROPPING 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 /08Z THURSDAY THROUGH MONDAY/...
THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG, 
KMIV, KACY AND SURROUNDING AREAS.

STRATO-CU DECK WAS MOVING SOUTHEAST INTO PENNSYLVANIA BUT WARM AIR 
ADVECTION IS MOVING INTO THE REGION AND THE CLOUDS ARE ERODING FROM 
THE LEADING EDGE AND NOW FROM ALLENTOWN NORTHWEST. 

CONTINUED VFR CONDITIONS ACROSS THE REGION TODAY INTO FRIDAY 
MORNING WITH WINDS LESS THAN 10 KT SHIFTING TO THE NORTHEAST BY 
MORNING FRIDAY. LARGE SYSTEM IN THE MID WEST WILL SPAWN A SECONDARY 
LOW ALONG THE DELMARVA THAT WILL CHANGE THE WINDS TO THE SOUTHEAST 
AND INCREASE TO 30 KT AT THE SURFACE AND AROUND 50 KT AT 5,000 FT. 
ALONG WITH THE WINDS WILL COME MVFR/IFR CONDITIONS FROM FRIDAY 
AFTERNOON INTO EARLY SATURDAY MORNING.

OUTLOOK...
HIGH PRESSURE WILL SLIDE TO THE EAST SATURDAY AND EXTEND INTO EARLY 
FRIDAY WITH A RETURN TO VFR CONDITIONS. 
   
&&

.MARINE...
AT 230 AM...NOAA BUOY 44009 REPORTS A NORTH NORTHWEST WINDS(340 
DEGREES) AT 19-27 KT AND A 4 FT SEA EVERY 5 SECONDS. AMBROSE LIGHT 
REPORTS 340 DEGREES AT 17-23 KT AND 3 FT SEA EVERY 4 SECONDS. THE 
HUDSON CANYON BUOY 320 DEGREES AT 23-33 KT AND A 9 FT SEA EVERY 7 
SECONDS. 

THE WINDS HAVE BEEN NEAR SMALL CRAFT SPEEDS EVERY ONCE IN A WHILE SO 
THE COASTAL WATERS ARE NOT UNDER ANY ADVISORIES AT THIS TIME.  LATER 
TODAY THE WINDS WILL DIMINISH EVEN MORE. HIGH PRESSURE WILL SPREAD 
OVER THE WATERS TODAY AND THEN MOVE OFFSHORE XMAS MORNING. THE DEEP 
LOW PRESSURE THAT HAS BEEN MAKING IT'S WAY ACROSS THE MID SECTION OF 
THE COUNTRY WILL SPAWN A SMALLER LOW ON THE DELMARVA COAST SUNDAY 
MORNING AND IT WILL RIDE NORTHEAST. THE LOW...AND THE DEPARTING HIGH 
PRESSURE WILL CREATE AN INCREASING WIND GRADIENT FROM THE SOUTHEAST 
BEGINNING DURING THE AFTERNOON FRIDAY. WINDS SPEEDS AT 5,000 FT WILL 
BE AROUND 50 TO 55 KTS BUT WE WILL SEE LOWER WINDS AT THE SURFACE AS 
WARM AIR IS MOVING IN ALOFT. IN ANY EVENT, SMALL CRAFT ADVISORIES 
WILL BE IN EFFECT FROM FRIDAY MORNING INTO SATURDAY MORNING AND 
THERE MAY BE SOME GALE FORCE(34 KT)GUSTS AT SOME TIME DURING THE 
EVENT.

IMPROVING CONDITIONS FOR SATURDAY NIGHT INTO SUNDAY AS HIGH PRESSURE 
BUILDS INTO THE REGION. ON SUNDAY NIGHT A FRONT WILL MOVE IN FROM 
THE WEST WITH WINDS SHIFTING TO THE NORTHEAST AND CONTINUING INTO 
TUESDAY.  

&&

.TIDES...
THE STRONG SOUTHEAST TO SOUTH WINDS WILL MOVE THE TIDE UP THE 
DELAWARE AND CHESAPEAKE BAY WITH DEPARTURES OF AROUND A FOOT. ALONG 
THE COAST DEPARTURES WILL ALSO BE AROUND A FOOT WITH LITTLE IF ANY 
TIDAL FLOODING. 

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...NONE.
NJ...NONE.
DE...NONE.
MD...NONE.
MARINE...NONE.

&&

$$

SYNOPSIS.../ 
NEAR TERM.../ 
SHORT TERM...RPW / 
LONG TERM...IOVINO
AVIATION...EBERWINE
MARINE...EBERWINE
TIDES...EBERWINE