FXUS61 KPHI 250412
AFDPHI

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ
1112 PM EST THU DEC 24 2009

.SYNOPSIS...
A RIDGE OF HIGH PRESSURE OVER THE AREA WILL MOVE EASTWARD
OVERNIGHT. LOW PRESSURE WILL INTENSIFY OVER THE CENTRAL PLAINS
OVERNIGHT 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 /UNTIL 6 AM FRIDAY MORNING/...
THE AREA WAS UNDERGOING A SQUEEZE PLAY BY HIGH CLOUDS FROM THE
SOUTHWEST AND OCEANIC STRATOCUMULUS INVADING FROM THE EAST. SKIES
WILL BE CLOUDY EAST AND BECOME CLOUDY WEST FOR THE OVERNIGHT
PERIOD AS SURFACE AND UPPER RIDGING WERE BEGINNING TO BREAK DOWN
AS THE NEXT DISTURBANCE WAS APPROACHING. IT WILL STILL BE COLD
OVERNIGHT...ALTHOUGH NOT AS COLD AS THE PREVIOUS COUPLE OF NIGHT/S
PAST. LOWS WILL BE IN THE LOWER AND MID 20S IN MOST AREAS WITH
SHORE AND METRO PHILADELPHIA REMAINING IN THE UPPER 20S. WINDS
WILL BE LIGHT.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH SUNDAY/...
THE STORM SYSTEM OVER THE MIDDLE PART OF THE COUNTRY BEGINS TO 
AFFECT OUR AREA DURING THE SHORT TERM. A LEADING SHORT WAVE BEGINS 
TO RIDE UP THE BACK SIDE OF THE RIDGE AND IT BRINGS CLOUDS AND THE 
FIRST CHC FOR PRECIPITATION TO THE WESTERN COUNTIES FRIDAY MORNING.
SRLY FLOW ALOFT AND A COLD NE WIND AND THE SFC REMAIN ACROSS THE
AREA FRIDAY AND FRIDAY NIGHT. THIS WILL BRING HIGHER CHC FOR
PRECIP FRIDAY NIGHT AND WITH THE SLOW MOVEMENT OF THE STORM...THE
CHCS SHOULD REMAIN FAIRLY HIGH INTO SAT AND SAT NIGHT. THE ONSET
OF THE PRECIP COULD BE ICY ACROSS THE POCONOS AND A FREEZING RAIN
ADVISORY IS BEING ISSUED WITH THIS PACKAGE TO ADDRESS THIS. THE
FREEZING RAIN ADVISORY WILL BEGIN AT 10 AM FRI AND LAST INTO FRI
NGT.

THE WARMER TEMPERATURES WILL BE ACROSS THE REGION SAT AND SAT NIGHT 
WITH HIGHS IN THE UPPER 40S TO LOW 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 HYDRO ISSUES. 

THE STEADY PRECIPITATION SHOULD END EARLY SUNDAY MORNING ONCE A
COLD FRONT CROSSES THE AREA. THERE WILL STILL BE SMALL CHC FOR
SHOWERS 9RAIN 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 
SLOWLY PROGRESSES EASTWARD AND FINALLY MOVES 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 SOME COLDER AIR INTO THE AREA AND AS A RESULT THERE REMAINS 
THE POSSIBILITY FOR SOME SNOW TO FALL ACROSS THE AREA. THEREFORE WE 
HAVE INCLUDED A SLIGHT CHANCE/CHANCE FOR SNOW SHOWERS ACROSS THE 
CWA. 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 /04Z FRIDAY THROUGH TUESDAY/...
SKIES WERE CLEAR AROUND THE AREA EARLY THIS AFTERNOON ON A LIGHT 
NORTH WIND, BUT A DECK OF EITHER STRATUS OR STRATOCUMULUS WAS SLOWLY 
TRYING TO MOVE INLAND FROM THE SOUTHERN NJ COAST. WITH THE PROXIMITY 
OF THIS DECK, WE DID NOT SLOW DOWN GUIDANCE INDICATIONS AND 
THEREFORE FORECAST CEILINGS AT OR A BIT BELOW 1000 FT BY AROUND 
MIDNIGHT MOST LOCATIONS AND SOMEWHAT EARLIER AT ACY. VISIBILITIES 
WERE EXPECTED TO DETERIORATE MORE SLOWLY, BUT SHOULD FOLLOW DURING 
THE DAY ON FRIDAY AT THE LATEST. WINDS WERE EXPECTED TO VEER FROM 
NORTH TO NORTHEAST OR EAST DURING THE TAF PERIOD. THEY SHOULD BE 
LESS THAN 10 KT OVERNIGHT BUT PICK UP SOME ON FRIDAY. 

PRECIPITATION CONTINUES TO LOOK LIKE IT WILL HOLD OFF UNTIL AFTER
18Z, ALTHOUGH SOME GUIDANCE INDICATES A LITTLE MAY FALL BEFORE
THEN. WE BRING IT IN AS PLAIN RAIN AT ALL TAF SITES, AS
TEMPERATURES AFTER 18Z SHOULD BE WARM ENOUGH FOR PLAIN RAIN TO
FALL. HOWEVER, IF TEMPERATURES DO NOT WARM AS QUICKLY OR SHOULD
PRECIPITATION BEGIN SOONER, ESPECIALLY IN THE NORTHERN AREAS, A
BRIEF PERIOD OF SOME FREEZING RAIN REMAINS POSSIBLE.

OUTLOOK...
WE EXPECT IFR CONDITIONS TO BE THE RULE INTO SATURDAY, AFTER WHICH A 
RETURN TO VFR CONDITIONS IS EXPECTED. THERE IS A CHANCE THAT 
PRECIPITATION COULD BEGIN AS A PERIOD OF FREEZING RAIN MAINLY AT ABE 
AND RDG.

&&

.MARINE...
THE STRENGTHENING GRADIENT BETWEEN WHAT INITIALLY SHOULD BE FAIRLY 
WEAK SECONDARY LOW PRESSURE APPROACHING FROM THE SOUTHERN 
MID-ATLANTIC AND STRONG HIGH PRESSURE RIDGING DOWN FROM SOUTHEAST 
CANADA WILL BE ENOUGH TO CAUSE SEAS TO BUILD TO ADVISORY LEVELS BY 
AROUND DAYBREAK FRIDAY OVER THE OCEAN WATERS WITH WINDS FOLLOWING 
DURING THE DAY. OVER THE BAY, WINDS ALSO ARE EXPECTED TO REACH 
ADVISORY LEVELS DURING THE DAY ON FRIDAY. THAT GRADIENT MAY WELL 
TIGHTEN SUFFICIENTLY TO PRODUCE GALES BEGINNING SOMETIME IN THE 
FRIDAY NIGHT/EARLY SATURDAY PERIOD. WE'VE GONE WITH A GALE WATCH FOR 
ALL WATERS EXCEPT THE UPPER BAY BEGINNING FRIDAY NIGHT. THE ONSET OF 
ANY GALES LIKELY WOULD HAPPEN LAST OVER THE NORTHERN ATLANTIC 
WATERS.  

CONDITIONS ARE EXPECTED TO IMPROVE BEGINNING SATURDAY NIGHT, 
ALTHOUGH THERE IS A SUGGESTION IN THE MODELS THAT CYCLOGENESIS WILL 
BEGIN OFF THE ATLANTIC COAST AGAIN ON MONDAY. 

REGARDING TIDES, A LOCAL PROGRAM SUGGESTS TIDAL ANOMALIES IN EXCESS 
OF WHAT THE SSE PREDICTS FOR THE EARLY SATURDAY, LATE SATURDAY AND 
EARLY SUNDAY HIGH TIDES, BUT WE ARE NEAR NEAP TIDE AND EVEN THE 
LARGER DEPARTURES DO NOT RESULT IN TIDAL FLOODING.

&&

.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.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...FREEZING RAIN ADVISORY FROM 10 AM FRIDAY TO 6 AM EST SATURDAY 
     FOR PAZ054-055.
NJ...NONE.
DE...NONE.
MD...NONE.
MARINE...GALE WATCH FROM FRIDAY EVENING THROUGH SATURDAY AFTERNOON FOR 
     ANZ431-450>455.
     SMALL CRAFT ADVISORY FROM 6 AM TO 6 PM EST FRIDAY FOR 
     ANZ450>455.
     SMALL CRAFT ADVISORY FROM NOON FRIDAY TO 6 PM EST SATURDAY FOR 
     ANZ430.
     SMALL CRAFT ADVISORY FROM NOON TO 6 PM EST FRIDAY FOR ANZ431.

&&

$$

SYNOPSIS...O'HARA
NEAR TERM.../ O'HARA
SHORT TERM...O'HARA
LONG TERM...MEOLA
AVIATION...DELISI / MEOLA
MARINE...DELISI
HYDROLOGY...KRUZDLO