FXUS61 KPHI 251701
AFDPHI
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ
1201 PM EST FRI DEC 25 2009
.SYNOPSIS...
LOW PRESSURE WILL INTENSIFY OVER THE CENTRAL PLAINS TODAY 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 TONIGHT 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 TONIGHT/...
LOW CLOUDS HAD ABOUT COMPLETED THEIR COVERAGE OF OUR AREA...AND WARM
ADVECTION HIGH CLOUDS WERE SWEEPING OVER THE AREA AS WELL EARLY THIS
MORNING AS THE IMPENDING WEATHER-MAKER MAKES ITS INITIAL MOVE TOWARD
OUR AREA. MANY SECTIONS ESPECIALLY OVER EASTERN AREAS WERE ACTUALLY
ABOVE FREEZING BEFORE SUNRISE...AS WARMER AIR WAS ARRIVING ON
DEVELOPING NORTHEASTERLY WINDS. ACCORDING TO MODEL FIELDS SUCH AS
PRECIPITATION, OMEGA, AND MID LEVEL RELATIVE HUMIDITY, AS WELL AS
RADAR TRENDS, PRECIPITATION LOOKS ON TARGET FOR LATE MORNING OR
MIDDAY SOUTH AND LATER THIS AFTERNOON NORTH...SO A TREND OF
INCREASING PRECIPITATION PROBABILITIES NORTHWARD MAKES SENSE DURING
TODAY. HIGHER ELEVATIONS OF THE NORTHWESTERN ZONES...MAINLY CARBON
AND MONROE...LOOK LIKE THEY WILL RETAIN COLD AIR NEAR THE SURFACE,
AND PERHAPS AT THE SURFACE TOO. THE INHERITED FREEZING RAIN ADVISORY
WAS RETAINED, BUT BACKED UP FOR A START TIME OF 17Z (FOR THIS
AFTERNOON). THERE APPEARS TO BE ENOUGH OF A WARM NOSE ALOFT TO MELT
MOST PRECIPITATION TO RAIN BEFORE IT REACHES THE GROUND, ESPECIALLY
IF IT DEVELOPS LATER IN THE AFTERNOON AS PLANNED. HOWEVER, THERE
ALWAYS SEEMS TO BE A LAYER THAT CAN CAUSE SOME ICE PELLET FORMATION,
SO SLEET WAS ADDED IN FAR NORTH AS WELL, BUT THIS SHOULD BE MINIMAL;
IF IT WASN'T WE WOULD CHANGE THE ADVISORY TO A WINTER WEATHER TYPE.
REGARDING HIGH TEMPERATURES AND TEMPERATURE TRENDS, NAM AND GFS MOS
WERE SIMILAR FOR SOUTHERN AND CENTRAL SECTIONS, BUT IT APPEARS THE
GFS MOS IS TOO WARM NORTH, ESPECIALLY WHEN MODEL SOUNDINGS ARE
OBSERVED FOR COMPARISON TO THE LOW LEVELS.
&&
.SHORT TERM /SATURDAY THROUGH SUNDAY/...
THE FREEZING RAIN ADVISORY WAS EXPANDED SOUTHWARD AND EASTWARD A BIT
TO LEHIGH, NORTHAMPTON, WARREN, SUSSEX, AND MORRIS COUNTIES FOR
TONIGHT. TEMPERATURES SHOULD BE JUST WARM ENOUGH TODAY AT THE
VULNERABLE LOCATIONS TO KEEP FREEZING RAIN FROM OCCURRING, BUT
SLIGHT COOLING TONIGHT ESPECIALLY WITH ANY HEAVIER PRECIPITATION
MAY DROP TEMPERATURES TO THE FREEZING LEVEL, HENCE THE ADVISORY
ISSUANCE. THE FREEZING RAIN ADVISORIES WERE EXTENDED JUST INTO
SATURDAY MORNING WHEN TEMPERATURES ARE EXPECTED TO RISE ENOUGH TO
PRECLUDE ANY FURTHER PROBLEMS.
OTHERWISE, NAM QPF HAS BEEN CONSISTENT ON THE 00Z AND THE 06Z FOUS
DATA, WITH AMOUNTS AT OR JUST OVER 1 INCH. CATEGORICAL PRECIPITATION
CHANCES ARE FORECAST FOR TONIGHT, AND WITH THE SLOW MOVEMENT OF THE
STORM...THE CHANCES SHOULD REMAIN FAIRLY HIGH INTO SATURDAY UNTIL
DRY SLOTTING OCCURS AND DROPS PRECIPITATION CHANCES FOR OR DURING
THE AFTERNOON.
THE WARMER TEMPERATURES WILL BE ACROSS THE REGION SATURDAY AND
SATURDAY NIGHT, WITH HIGHS SATURDAY IN THE UPPER 40S TO LOWER 50S
EXPECTED. THESE READINGS, ALONG WITH THE RAINFALL (WHICH COULD TOTAL
1.0 TO 1.5 INCHES) WILL MAKE A SIGNIFICANT DENT IN THE SNOW PACK.
THE HYDROLOGY SECTION BELOW ADDRESSES ANY UPCOMING HYDROLOGIC
ISSUES.
ANY LINGERING PRECIPITATION SHOULD END EARLY SUNDAY MORNING ONCE
A COLD FRONT CROSSES THE AREA. THERE WILL STILL BE A SMALL CHANCE
FOR SHOWERS (RAIN AND/OR SNOW) MAINLY ACROSS THE NORTH.
&&
.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
AT THE START OF THE PERIOD, A RATHER LARGE UPPER LEVEL TROUGH WILL
BE SITUATED OVER THE EASTERN HALF OF THE UNITED STATES. THIS FEATURE
IS SEEN TO PROGRESS SLOWLY EASTWARD AND FINALLY MOVE OFF THE COAST
MONDAY.
MEANWHILE, A SURFACE LOW OVER EASTERN CANADA WILL DRAG A COLD FRONT
ACROSS THE REGION SUNDAY NIGHT INTO MONDAY, WITH A REINFORCING COLD
FRONT DROPPING THROUGH SOMETIME LATE MONDAY INTO TUESDAY. THIS WILL
BRING COLDER AIR BACK INTO THE AREA, AND AS A RESULT THERE REMAINS
THE POSSIBILITY FOR SOME SNOW SHOWERS ACROSS THE AREA AND WE HAVE
INCLUDED A SLIGHT CHANCE OR CHANCE FOR SNOW SHOWERS. IT LOOKS
LIKE ANY LINGERING SNOW SHOWERS WILL END BY TUESDAY MORNING.
AS THE UPPER TROUGH FINALLY EXITS THE REGION, SURFACE HIGH PRESSURE
WILL BUILD IN FROM THE WEST. THIS HIGH WILL REMAIN IN CONTROL OF OUR
WEATHER FROM TUESDAY THROUGH THURSDAY.
&&
.AVIATION /18Z FRIDAY THROUGH WEDNESDAY/...
THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY AND SURROUNDING AREAS.
CEILINGS GENERALLY WERE IN THE MVFR RANGE LATE THIS MORNING. SOME
IFR CEILINGS WERE BOTH NORTHWEST AND SOUTHEAST OF THE DEL VALLEY
SITES. VISIBILITIES WERE QUITE VARIABLE. MANY PLACES REMAINED VFR;
SOME PLACES WERE MVFR IN LIGHT FOG, AND A FEW LOCATIONS WERE IFR
SOUTHEAST AND OVER HIGHER ELEVATIONS NORTHWEST. ALL THIS IS STILL
EXPECTED TO TREND DOWN TO WIDESPREAD IFR CONDITIONS DURING THIS
AFTERNOON AND REMAIN THERE INTO SATURDAY MORNING. THE WEATHER WILL
THEN SLOWLY TREND TO VFR BY SATURDAY NIGHT AND REMAIN SO INTO
SUNDAY AFTERNOON.
THE FOLLOWING IS THE FREEZING LEVEL INFORMATION FOR A FEW SITES.
STA 2512Z 25/18 26/00 26/06 26/12 26/18 27/00 27/06 27/12Z
LGA 02085 05940 04075 04190 07895 08895 11005 10205 10020
PHL 07625 09220 05490 07595 08895 10705 10710 10410 09115
IPT 000/ 01395 02395 01395 02095 07195 07670 05855 00185
DCA 08430 08660 06895 08990 10605 10510 10110 09315 05140
THE FIRST THREE VALUES ARE THE FREEZING LEVEL ABOVE THE GROUND
LEVEL THE LAST TWO ARE THE RH AT THAT LEVEL.
OUTLOOK...
A COLD FRONT WILL APPROACH FROM THE WEST SUNDAY NIGHT WITH COLD AIR
ADVECTION RETURN WITH SNOW SHOWERS ACROSS THE TAF SITES NORTH OF
PHL-PNE MONDAY MORNING INTO THE EVENING HOURS. A COOL NORTHWEST
FLOW WILL CONTOUR INTO MONDAY NIGHT WITH IMPROVING FLYING
CONDITIONS.
&&
.MARINE...
CONDITIONS OVER THE WATERS SHOULD DETERIORATE THIS AFTERNOON AND
CONTINUE TO DO SO TONIGHT. SMALL CRAFT CONDITIONS ARE EXPECTED TO
GIVE WAY TO GALES. IT IS GOING TO BE VERY ROUGH IN THE DELAWARE
BAY AS THE WINDS BLOW UP THE BAY. VERY CHOPPY CONDITIONS IN THE
CONFINES OF THE BAY AND ALSO ON THE OCEAN. WAVE FORECASTS FOR BUOY
44009 SUGGEST ABOUT 14 FEET EVERY 4 TO 6 SECONDS AFTER MIDNIGHT.
THE ROUGHEST CONDITIONS WILL BE FROM LATER THIS AFTERNOON THROUGH
SATURDAY AFTERNOON. IMPROVEMENT LATE SATURDAY AS THE WIND GRADIENT
SHIFTS TO THE NORTHEAST AND A LOW PRESSURE MOVES INTO THE WATERS
NEAR LONG ISLAND. THIS WILL DEADEN THE WINDS AND SEAS FOR ABOUT A
36 HOUR PERIOD BEFORE A FRONT MOVES IN FROM THE WEST LATE SUNDAY
WITH SHIFTING COOLER NORTHWEST WINDS AND SMALL CRAFT ADVISORIES
WILL PROBABLY BE IN EFFECT.
FOR THE CANYONS...GALES ARE EXPECTED TONIGHT INTO SATURDAY AND AGAIN
MONDAY INTO TUESDAY. CHECK THE WEATHER BEFORE HEADING TO THE
OFFSHORE CANYONS.
&&
.HYDROLOGY...
CERTAINLY NOT OUT OF THE WOODS, BUT THE HSA IS BETTER OFF THAN 24
HOURS AGO. BOTTOM LINE, THE CAPACITY TO HOLD WATER IN THE CREEKS AND
STREAMS HAS INCREASED A BIT. THIS WAS POSSIBLE DUE TO THE COLD
TEMPERATURES WHICH WERE CAUSING LITTLE IF ANY MELTING AND
SUBSEQUENT RUNOFF. WITH LITTLE IF ANY WATER ENTERING INTO OUR
CHANNELS, WATER LEVELS HAVE BEEN ABLE TO DECREASE. EVEN IF JUST A
BIT, EVERY INCH IS IMPORTANT.
USING A BELL SHAPED CURVE, WE FEEL THAT 1.00 TO 1.25 INCHES OF RAIN
IS MOST LIKELY TO OCCUR THROUGH SATURDAY NIGHT. A LITTLE LESS OR A
LITTLE MORE IS POSSIBLE, BUT WE FEEL CHANCES OF THIS OCCURRING
AREN'T AS GREAT. THE 00Z NAM FOUS WAS INDICATING SLIGHTLY OVER AN
INCH /1.07/ FOR PHILADELPHIA FOR THE EVENT TOTAL.
WITH TEMPERATURES TOPPING OUT AROUND 50 ON SATURDAY, COUPLED WITH
HIGH DEW POINTS, SOME WIND, AND THE RAIN, MELTING SNOW IS MOST
DEFINITE. THE QUESTION IS HOW MUCH SNOW WILL MELT? WE FEEL THAT 75%
OF THE WATER LOCKED UP IN THE SNOW IS GOING TO TRANSLATE INTO RUNOFF.
SNOW WATER EQUIVALENTS RANGE FROM LESS THAN A .5 INCH ACROSS THE
NORTHERN HSA TO ABOUT 1.5 INCHES ACROSS THE CENTRAL AND SOUTHERN
SNOW-PACK.
SO IF YOU ADD THE NUMBERS, 1.00 TO 1.25 INCHES OF NEW RAINFALL,
COMBINED WITH UP TO 1.10 INCHES OF LIQUID WATER FROM THE SNOW, WE'RE
LOOKING AT POSSIBLY 2.35 INCHES. IF WE GET 1.50 INCHES OF RAIN, WE
COULD BE LOOKING AT 2.6 INCHES.
ALTHOUGH THE HSA'S CAPACITY TO HOLD WATER HAS INCREASED, FLOODING IS
STILL POSSIBLE, THAT IS FOR CREEKS AND STREAMS. NUISANCE FLOODING
IN LOW LYING AND IN POOR DRAINAGE AREAS IS GOING TO HAPPEN. FOR OUR
CREEKS AND STREAMS, WE THINK WE'LL BE O.K. IF WE ONLY RECEIVE 0.75
INCHES OF RAIN. IF WE GET 1.00 INCH OF RAIN, MOST OF THE WATER WILL
REMAIN IN THE CHANNELS. IF WE GET 1.25 INCHES, CREEK AND STREAM
FLOODING WILL BE LIKELY...THE FAST RESPONDERS. IF WE RECEIVE 1.50
INCHES, CREEK AND STREAM FLOODING IS ALMOST A DEFINITE.
THE QUICK RESPONDERS INCLUDE THOSE IN NORTHERN DELAWARE, SOUTHEAST
PENNSYLVANIA, AND THE ASSUNPINK AND MILLSTONE IN JERSEY.
IF THE SCHUYLKILL BASIN RECEIVES 1.50 INCHES, 3/4 BANKFULL TO
BANKFULL ISN'T OUT OF THE QUESTION. THE DELAWARE WOULD SEE 1/2 TO
3/4 BANKFULL.
&&
.TIDES/COASTAL FLOODING...
THE INCREASING SOUTHEAST WINDS WHICH WILL REACH GALE FORCE AT TIMES
WILL BRING WATER LEVELS UP ABOUT 1 FOOT ABOVE NORMAL ON THE OCEAN
FRONT AND UP TO 2 FEET UP THE DELAWARE RIVER. HOWEVER, TIDAL
FLOODING IS NOT EXPECTED SINCE WE ARE NOT IN THE STRONG
GRAVITATIONAL CYCLE OF THE MOON. HIGH TIDES IN THE UPPER DELAWARE
BAY WILL BE BETWEEN 5 AND 6 OCLOCK THIS EVENING AND MORNING SATURDAY
AND ALONG THE OCEAN FRONT BETWEEN 230 AND 330 PM AND AM SATURDAY.
&&
.PHI WATCHES/WARNINGS/ADVISORIES...
PA...FREEZING RAIN ADVISORY FROM 6 PM THIS EVENING TO 9 AM EST
SATURDAY FOR PAZ061-062.
FREEZING RAIN ADVISORY UNTIL 9 AM EST SATURDAY FOR PAZ054-055.
NJ...FREEZING RAIN ADVISORY FROM 6 PM THIS EVENING TO 9 AM EST
SATURDAY FOR NJZ001-007-008.
DE...NONE.
MD...NONE.
MARINE...SMALL CRAFT ADVISORY UNTIL 1 AM EST SATURDAY FOR ANZ450>452.
GALE WARNING FROM 1 AM TO 4 PM EST SATURDAY FOR ANZ450>452.
SMALL CRAFT ADVISORY UNTIL 7 PM EST THIS EVENING FOR ANZ430-
431-453>455.
GALE WARNING FROM 7 PM THIS EVENING TO 10 AM EST SATURDAY FOR
ANZ430-431-453>455.
&&
$$
SYNOPSIS...O'HARA/
NEAR TERM...O'HARA/
SHORT TERM...O'HARA/
LONG TERM...MEOLA
AVIATION...DELISI/EBERWINE
MARINE...DELISI/EBERWINE
HYDROLOGY...KRUZDLO
TIDES/COASTAL FLOODING...EBERWINE
