FXUS61 KPHI 222045
AFDPHI
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ
345 PM EST TUE DEC 22 2009
.SYNOPSIS...
THE SOUTHERN END OF HIGH PRESSURE CENTERED OVER SOUTHERN CANADA WILL
CONTINUE TO NOSE ITS WAY ACROSS THE AREA THROUGH AT LEAST
THURSDAY. AN AREA OF LOW PRESSURE WILL BE LIFTING THROUGH THE
CENTRAL STATES AND THROUGH THE GREAT LAKES LATE IN THE WEEK AND
INTO THE WEEKEND. THIS WILL DRAG A FRONTAL BOUNDARY ACROSS OUR
AREA OVER THE WEEKEND. A STEADY NORTHWEST FLOW WILL DEVELOP ACROSS
THE AREA EARLY NEXT WEEK.
&&
.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
A SMALL AREA OF VERY LIGHT SNOW SHOWERS/FLURRIES MOVED OUT OF
CENTRAL PENNSYLVANIA INTO SOUTHEAST PENNSYLVANIA AND ARE MOVING
INTO THE PHILADELPHIA METRO AREA LATE THIS AFTERNOON. THEY ARE IN
RESPONSE TO A WEAK SHORT WAVE/VORT MAX MOVING ACROSS THE AREA.
THIS SHORT WAVE SHOULD MOVE THROUGH RATHER QUICKLY, AND ANY
PRECIPITATION SHOULD MOVE OUT AND/OR DISSIPATE BY THROUGH DUSK.
OTHERWISE, WE EXPECT WINDS TO DIMINISH AS THE EVENING PROGRESSES
WITH CLEARING SKIES, SO WE EXPECT TO END UP WITH A COLD NIGHT
TONIGHT. WITH THE SNOW PACK AND CLEARING SKIES/WEAK WINDS, WE WENT
A COUPLE OF DEGREES BELOW GUIDANCE.
&&
.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH FRIDAY/...
WEDNESDAY THROUGH THURSDAY SHOULD BE RELATIVELY QUIET DAYS AS HIGH
PRESSURE ACROSS SOUTHERN CANADA NOSES ITS WAY DOWN INTO OUR AREA.
A MID/UPPER LOW SLIDES TO OUR NORTH TONIGHT INTO WEDNESDAY, BEFORE
RIDGING TRIES TO TAKE PLACE FROM THURSDAY INTO FRIDAY. FOR HIGH
TEMPERATURES ON WEDNESDAY AND THURSDAY, WE DECIDED TO GO A COUPLE OF
DEGREES ABOVE MOS SINCE THEY HAVE BEEN UNDER DOING THE HIGHS THE
LAST COUPLE OF DAYS EVEN WITH THE SNOW PACK. CONVERSELY, WE WENT A
COUPLE OF DEGREES BELOW GUIDANCE FOR THE LOW TEMPERATURES DUE TO THE
SNOW PACK AND RELATIVELY MINIMALLY CLOUDY SKIES.
BY THURSDAY NIGHT INTO FRIDAY HOWEVER, THE HIGH MOVES INTO EASTERN
CANADA, SO WE WILL BE GETTING MORE OF AN ONSHORE FLOW, AND BUILDING
THICKNESSES, WHICH SHOULD HELP MODERATE OUR TEMPERATURES. AN AREA OF
LOW PRESSURE WILL BE LIFTING THROUGH THE CENTRAL STATES AND INTO THE
GREAT LAKES REGION LATE IN THE WEEK, WITH A FRONTAL BOUNDARY
APPROACHING THE AREA FROM THE SOUTH AND SOUTHWEST. WARM AIR
ADVECTION PRECIPITATION COULD START DURING THE DAY FRIDAY, AND IF IT
STARTS EARLY ENOUGH, IT MAY LEAD TO SOME WINTRY PRECIPITATION,
BEFORE CHANGING TO MOSTLY LIQUID RAIN. PRECIPITATION IS EXPECTED TO
BEGIN IN OUR SOUTHWEST AREAS, THEN SPREAD TO THE NORTHEAST LATE IN
THE DAY.
&&
.LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/...
AN EXPANSIVE CLOSED LOW IS FORECAST TO BE IN THE PLAINS AND MIDWEST
DURING THE START OF THIS TIME FRAME. THE MODEL GUIDANCE OVERALL HAS
CONTINUED TO BE IN DECENT AGREEMENT ON THE PATTERN ALOFT, WITH
RIDGING ACROSS THE WEST COAST AND ALSO ALONG THE EAST COAST. THERE
HAS BEEN A TENDENCY IN THE MODEL GUIDANCE TO SLOW DOWN THE EASTWARD
PROGRESSION OF THIS LARGE SYSTEM AND ITS ASSOCIATED SURFACE FRONT.
THE DEEP SURFACE LOW WILL LIKELY OCCLUDE GIVEN THE ADVERTISED
UPPER-LEVEL PATTERN, WHICH WILL DRIVE THE BOUNDARY OUR WAY ALONG
WITH A WARM FRONT OR EVEN A COASTAL FRONT LIFTING NORTHWARD.
MEANWHILE, A STRONG SURFACE HIGH IS FORECAST TO BE IN SOUTHEASTERN
CANADA BUT TRENDING TO BE TRANSIENT. HPC WENT WITH THE ECMWF
ENSEMBLE MEAN. WE INITIALIZED THE DATABASE WITH HPC GUIDANCE,
HOWEVER SOME MOSTLY MINOR ADJUSTMENTS WERE MADE BASED ON ADDITIONAL
COLLABORATION WITH OUR SURROUNDING OFFICES.
BASED ON THE ABOVE, HIGH PRESSURE IS FORECAST TO BE LEAVING THE AREA
FRIDAY NIGHT AS THE OCCLUDED FRONT ARRIVES FROM THE WEST. PLENTY OF
ENERGY ROUNDING THE STRONG UPPER-LEVEL TROUGH TO OUR WEST,
THEREFORE A SURFACE LOW MAY FORM ALONG THE TRIPLE POINT. THERE
ARE HINTS THAT A WARM FRONT OR EVEN A COASTAL FRONT WILL BE WORKING
NORTHWARD, HOWEVER JUST HOW FAR NORTH THIS FEATURE GETS IS IN
QUESTION. IT APPEARED THAT THIS SETUP WAS MORE CHARACTERISTIC OF
COLD AIR DAMMING, HOWEVER THE TREND OF SLOWING THE ARRIVAL OF THE
SYSTEM DOWN SOME MORE WILL GIVE THE LINGERING COLD AIR AT THE
SURFACE TIME TO BE ERODED. THERE IS STRONG WAA FORECAST TO BE
OCCURRING AHEAD OF THIS SYSTEM, AND THAT WILL TRANSPORT MILDER AND
MORE MOIST AIR ACROSS OUR AREA FROM SOUTH TO NORTH. THIS COULD ALLOW
THE PRECIPITATION TO ARRIVE SOONER AND ALSO SOME FOG TO BECOME AN
ISSUE IF ENOUGH SNOW COVER EXISTS. SINCE THERE IS SOME SNOW COVER
STILL EXPECTED TO BE AROUND, THIS MAY KEEP THE SURFACE TEMPERATURES
COLDER A LITTLE LONGER THAN THE MODELS ARE PORTRAYING. IN ADDITION,
SOME ICING COULD OCCUR FOR A TIME DESPITE THE AIR TEMPERATURES BEING
ABOVE FREEZING. BASED ON THE FORECAST SOUNDINGS, A MOSTLY PLAIN RAIN
EVENT WOULD BE IN STORE FOR OUR AREA, HOWEVER WE WILL STILL MENTION
A MOSTLY BRIEF PERIOD OF ICE/MIX ACROSS OUR FAR NORTHWESTERN ZONES.
IT APPEARS THAT ANY WINTRY PRECIPITATION WOULD BE SLEET OR FREEZING
RAIN INSTEAD OF SNOW GIVEN THE STRENGTH OF THE WAA. FOR NOW, WE
SLOWED DOWN THE INCREASING POPS SOME FROM WEST TO EAST AND DID NOT
GO ANY HIGHER THAN HIGH LIKELY ATTM. TEMPERATURES MAY ACTUALLY START
TO RISE FRIDAY NIGHT INTO SATURDAY MORNING AS THE WAA AND BOUNDARY
LAYER FLOW STRENGTHENS. WITH THE EXPECTED RAINFALL AND SNOW MELT, WE
WILL MONITOR FOR THE POTENTIAL OF SOME FLOODING.
AS THE MAIN SURFACE LOW TRACKS ACROSS THE GREAT LAKES LATE SATURDAY
INTO SUNDAY, THE POSSIBLE SECONDARY LOW SHOULD BE JUST TO OUR EAST
WITH THE OCCLUDED FRONT CLEARING OUR AREA EARLY SUNDAY. THIS ENTIRE
SYSTEM THEN SLIDES ACROSS NEW ENGLAND AND SOUTHEASTERN CANADA
MONDAY, HOWEVER ADDITIONAL ENERGY LOOKS TO MAINTAIN AN AMPLIFIED
UPPER-LEVEL TROUGH FROM THE GREAT LAKES TO THE NORTHEAST MONDAY.
THIS COULD PRODUCE SOME FLURRIES OR SNOW SHOWERS GIVEN THE COLDER
NORTHWESTERLY FLOW ALOFT, HOWEVER FOR NOW WE WILL JUST HOLD FLURRIES
TO THE FAR NORTHWESTERN ZONES SUNDAY NIGHT THROUGH MONDAY NIGHT. WE
THEN SHOULD REMAIN UNDER A CYCLONIC FLOW AS LOW PRESSURE MOVES INTO
THE CANADIAN MARITIMES TUESDAY AND HIGH PRESSURE SLIDES INTO THE
OHIO VALLEY. AN ARCTIC FRONT MAY THEN BE WAITING JUST NORTH OF THE
GREAT LAKES REGION.
&&
.AVIATION /21Z TUESDAY THROUGH SUNDAY/...
OUR SC FLUCTUATIONS CONTINUE AS COLD AIR FLOWS OVER THE SNOW PACK.
AFTER DIMINISHING THIS MORNING, IT IS ONCE AGAIN ON THE INCREASE
THIS AFTERNOON. SO MUCH SO, THAT ANOTHER STREAMER BAND HAS DEVELOPED.
TODAY'S IS SLIGHTLY SOUTH OF YESTERDAYS AND IS RUNNING IN THE W TO NW
FLOW FROM NORTHERN BERKS COUNTY TO BETWEEN QUAKERTOWN AND POTTSTOWN
ON THE MONTGOMERY BUCKS BORDER TO JUST NORTH OF PHILLY AND INTO
WESTERN BURLINGTON COUNTY IN NJ. IT IS POSSIBLE THAT A FEW FLURRIES
COULD FALL OUT OF THESE CLOUDS THIS AFTERNOON. ONCE THE DAYTIME
HEATING ENDS THIS EVENING, THE CLOUD COVER SHOULD DECREASE. DRY AIR
IS ACTUALLY FORECAST TO DROP DOWN FROM THE NORTH OVERNIGHT, ALLOWING
SKIES TO CLEAR. WINDS THAT ARE GUSTING TO 20 TO 25 KT THIS AFTERNOON
WILL LOSE THEIR GUSTINESS THIS EVENING AND DROP DOWN TO 5 TO 8 KT
OVERNIGHT. AS HIGH PRESSURE BUILDS IN WEDNESDAY, WE SHOULD SEE
ANOTHER VFR DAY VERY SIMILAR TO TODAY, SCT TO OCCASIONALLY BKN SC
WITH NW WINDS 10 TO 15 KT WITH GUSTS TO AROUND 20 KT.
OUTLOOK...
EXPECT GOOD FLYING WEATHER WITH VFR CONDITIONS TO CONTINUE THROUGH
THURSDAY NIGHT AS THE HIGH PRESSURE SYSTEM MOVES SLOWLY ACROSS.
THE NEXT BATCH OF INCLEMENT WEATHER WILL ARRIVE ON CHRISTMAS DAY
AS LOW PRESSURE TREKS ACROSS THE GREAT LAKES. THE ATTENDANT COLD
FRONT WILL APPROACH FROM THE WEST LATE CHRISTMAS DAY AND NIGHT AND
CROSS THE AREA ON SATURDAY. PRECIPITATION IS FORECAST TO SPREAD
FROM SOUTHWEST TO NORTHEAST ACROSS THE REGION LATE CHRISTMAS DAY
INTO CHRISTMAS NIGHT. RAIN WILL BE THE PREDOMINANT PRECIPITATION
TYPE, ALTHOUGH THERE COULD BE A WINTRY MIX FOR A TIME IN OUR
NORTHERN AREAS BEFORE MILDER AIR MOVES IN LATER FRIDAY NIGHT AND
SATURDAY. THIS SYSTEM WILL BRING REDUCED CEILINGS AND VISIBILITIES.
THE MILDER AIR ARRIVING OVER THE REMAINING SNOW COVER ALSO LEAD TO
FOG DEVELOPMENT. AS THE FRONT MOVES OUT OF OUR AREA FROM SOUTHWEST
TO NORTHEAST FROM MIDDAY SATURDAY INTO SATURDAY NIGHT, CONDITIONS
WILL IMPROVE. HIGH PRESSURE WILL PROVIDE FAIR WEATHER AND VFR
CONDITIONS FOR SUNDAY.
&&
.MARINE...
THE STRONG GRADIENT THAT WE'VE BEEN EXPERIENCING THE PAST SEVERAL
DAYS WILL EASE ONLY SLIGHTLY THROUGH WEDNESDAY NIGHT. WIND GUSTS
DROPPED A BIT OVER DELAWARE BAY, ALTHOUGH GUSTS CONTINUE TO
APPROACH 25 KT NEAR THE MOUTH OF THE BAY. THE SMALL CRAFT ADVISORY
HAS BEEN EXTENDED ON OUR OCEAN WATERS, WHERE GUSTS OF 25 TO 30 KT
WILL CONTINUE, THROUGH WEDNESDAY NIGHT. A HIGH PRESSURE RIDGE WILL
MOVE ACROSS OUR REGION THURSDAY AND THURSDAY NIGHT BEFORE MOVING
OFF TO THE EAST CHRISTMAS DAY. LOW PRESSURE MOVING INTO THE WESTERN
GREAT LAKES AREA WILL PUSH A COLD FRONT TOWARD OUR REGION FRIDAY
NIGHT. SECONDARY LOW PRESSURE IS FORECAST TO DEVELOP ON THE FRONT
OVER VIRGINIA FRIDAY NIGHT AND TRACK NE ACROSS OUR AREA SATURDAY.
THIS SYSTEM WILL MORE THAN LIKELY BRING OUR NEXT ROUND OF ADVISORY
OR WARNING CONDITIONS. WITH A FAIRLY STRONG ONSHORE FLOW DEVELOPING
FRIDAY NIGHT INTO SATURDAY, TIDAL DEPARTURES WILL BE INCREASING.
FORTUNATELY NORMAL TIDE HEIGHTS WILL BE QUITE LOW AND THE EVENT
WILL BE RELATIVELY SHORT LIVED, SO THE THREAT FOR COASTAL TIDAL
FLOODING WILL BE MINIMAL.
&&
.PHI WATCHES/WARNINGS/ADVISORIES...
PA...NONE.
NJ...NONE.
DE...NONE.
MD...NONE.
MARINE...SMALL CRAFT ADVISORY UNTIL 6 AM EST THURSDAY FOR ANZ450>455.
&&
$$
SYNOPSIS...ROBERTSON
NEAR TERM...ROBERTSON
SHORT TERM...ROBERTSON
LONG TERM...GORSE
AVIATION...RPW
MARINE...RPW
