FXUS61 KPHI 282021
AFDPHI
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ
321 PM EST SAT NOV 28 2009
.SYNOPSIS...
LOW PRESSURE WILL LIFT NORTHEAST AND HIGH PRESSURE WILL BUILD IN
FOR SUNDAY. A COLD FRONT WILL APPROACH SUNDAY NIGHT AND CROSS THE
AREA ON MONDAY. HIGH PRESSURE WILL BUILD IN FOR TUESDAY AND EARLY
WEDNESDAY. LOW PRESSURE MAY APPROACH BY EARLY THURSDAY.
&&
.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
WINDS TONIGHT WILL DIMINISH FAIRLY RAPIDLY, AND WE EVENTUALLY MAY
GET SOME DECOUPLING UNDER MAINLY CLEAR SKIES, BUT FOR NOW WE HAVE
STAYED CLOSE TO A GUIDANCE AND CONTINUITY COMPROMISE IN THE EVENT
THAT WE DON'T RADIATE STRONGLY AFTER MIDNIGHT. THIS MAY END UP
BEING AN ERROR, SINCE WE ARE APPROACHING THE TIME OF YEAR WHEN
NIGHTS ARE ABOUT AS LONG AS THEY GET AROUND HERE. SOME HIGH CLOUDS
MAY DRIFT ACROSS MAINLY THE NORTHERN PART OF THE AREA, BUT SKIES
SHOULD GENERALLY BE CLEAR.
&&
.SHORT TERM /6 AM SUNDAY MORNING THROUGH TUESDAY/...
WE SHOULD START SUNDAY WITH DECENT SUNSHINE WITH THE SHALLOW MID
LEVEL RIDGE AXIS TO THE WEST, BUT HIGH AND THEN MID LEVEL CLOUDS
SHOULD START TO INCREASE IN ADVANCE OF THE APPROACHING COLD FRONT.
WE STAYED ON THE COOL SIDE OF GUIDANCE WITH THE RETURN FLOW JUST
DEVELOPING.
THE SURFACE FRONT IS ASSOCIATED WITH A POSITIVELY TILTED MID LEVEL
TROF, WHILE ITS ADVANCING SPEED OF 20 TO 25 KT IS A BIT SLOWER
THAN THE H925 TO H8 FLOW BEHIND THE FRONT. SOUNDINGS BEHIND THE
FRONT TAKE A WHILE TO GO DRY ADIABATIC OR TO DEVELOP A SUBSIDENCE
INVERSION ALOFT. THIS SENDS MIXED SIGNALS WITH REGARD TO ANAFRONT
POTENTIAL. STILL, THE BEST Q-VECTOR CONVERGENCE ALOFT AND
FRONTOGENESIS AT LOWER LEVELS ARE PROGGED OVER THE AREA DURING THE
DAY ON MONDAY, WHICH SEEMS TO BE AFTER THE SURFACE FROPA. THERE IS
NO APPRECIABLE ISENTROPIC LIFT AHEAD OF THE FRONT AND NO
INSTABILITY THAT WE CAN FIND, SO WE ARE GOING WITH JUST CHANCE
POPS NEAR THE FRONT ON SUNDAY NIGHT AND THEN HIGH CHANCE TO LIKELY
POPS BEHIND THE FRONT ON MONDAY. QPF VALUES ARE ON THE ORDER OF A
FEW TENTHS.
THE H925 FLOW STIFFENS FROM THE SOUTHWEST IN ADVANCE OF THE FRONT,
AND THAT MADE US RESIDE ON THE HIGH SIDE OF GUIDANCE WITH REGARD
TO SUNDAY NIGHT MINS. EARLY RADIATING COULD CAUSE THIS TO BE IN
ERROR, BUT THE FORECAST HAS THE VIRTUE OF MAINTAINING CONTINUITY.
ON MONDAY, WE THOUGHT IT WOULD BE FAIRLY DREARY FOR AT LEAST PART
OF THE DAY AND SO WE STAYED CLOSER TO THE LOW SIDE OF GUIDANCE.
THERE APPEARS TO BE ANOTHER SHORT WAVE IN THE MID LEVEL FLOW THAT
MOVES ACROSS THE AREA EARLY TUESDAY, SO AFTER MOVING OUT THE PCPN
FROM THE FRONT WE BRING BACK SOME MODEST CHANCE POPS OVER THE
NORTHWEST FOR LATE MONDAY NIGHT AND EARLY TUESDAY. TEMPERATURES ON
MONDAY NIGHT AND TUESDAY ARE A COMBINATION OF THE CONSISTENT STAT
GUIDANCE AND CONTINUITY.
&&
.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
HIGH PRESSURE WILL QUICKLY MOVE THROUGH THE AREA TUESDAY NIGHT INTO
WEDNESDAY MORNING. MEANWHILE, AN AREA OF LOW PRESSURE WILL BE
DEVELOPING AND STRENGTHENING ALONG THE GULF STATES, BEFORE MOVING
TOWARD OUR AREA DURING THE DAY WEDNESDAY. THE HIGH PRESSURE WILL
PUSH OUT OF THE AREA DURING THE DAY WEDNESDAY, POSSIBLY ALLOWING
SOME WARM AIR ADVECTION PRECIPITATION TO TAKE PLACE BEFORE THE REAL
SHOW WEDNESDAY NIGHT. THE STRENGTHENING LOW PRESSURE WILL MOVE ALONG
THE APPALACHIAN MOUNTAINS OVERNIGHT WEDNESDAY, LEADING TO A FAIRLY
STEADY PERIOD OF RAIN; THE HIGHER ELEVATIONS MAY SEE SOME FREEZING
RAIN IF TEMPERATURES GET COLD ENOUGH. SNOW DOES NOT SEEM LIKE A
HIGH POSSIBILITY AT THIS TIME AS THE SOUNDINGS INDICATE A WARM LAYER
ALOFT THAT WOULD MELT ANY FROZEN PRECIPITATION BEFORE FALLING TO THE
GROUND AND REFREEZING AGAIN. PRECIPITATION AMOUNTS ARE NOT EXPECTED
TO EXTRAORDINARILY HEAVY, BUT COULD AVERAGE AROUND A HALF AN INCH TO
AN INCH. THIS AREA OF LOW PRESSURE WILL CONTINUE TO MOVE NORTHEAST
INTO EASTERN CANADA FOR THURSDAY INTO FRIDAY. HIGH PRESSURE WILL THE
TRYING TO BUILD INTO THE REGION FROM THE SOUTHEAST, SO THIS WILL
HELP LEAD TO A BREEZY WESTERLY FLOW THURSDAY INTO FRIDAY. HIGH
PRESSURE IS THEN EXPECTED FOR THE EARLY PART OF THE WEEKEND, WHICH
SHOULD PROVIDE A MOSTLY DRY FORECAST THROUGH SATURDAY.
&&
.AVIATION /20Z SATURDAY THROUGH THURSDAY/...
THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY AND SURROUNDING AREAS.
AN INTENSE AREA OF LOW PRESSURE WILL CONTINUE TO MOVE UP THROUGH THE
CANADIAN MARITIMES TONIGHT. MEANWHILE HIGH PRESSURE, CENTERED OVER
THE SOUTHEASTERN UNITED STATES, WILL SLIDE EASTWARD AND ITS NORTHERN
EDGE WILL BUILD ACROSS OUR AREA TONIGHT INTO SUNDAY. THIS WILL ALLOW
THE PRESSURE GRADIENT TO CONTINUE TO RELAX AND THEREFORE THE GUSTY
NORTHWEST WINDS WILL DIMINISH CONSIDERABLY INTO THIS EVENING. WE
THEREFORE STEPPED THE WINDS DOWN THROUGH THE REMAINDER OF THE
AFTERNOON AND INTO THE EARLY EVENING. DESPITE THE WINDS DIMINISHING
UNDER 10 KNOTS TONIGHT /LIGHT AND VARIABLE AT SOME TERMINALS/, A
SHIFT TO THE SOUTHWEST IS EXPECTED. THIS SOUTHWESTERLY FLOW IS
EXPECTED TO INCREASE JUST A LITTLE BIT DURING SUNDAY WELL AHEAD OF A
COLD FRONT. THE AIR MASS IS RATHER DRY THEREFORE WE ARE NOT
EXPECTING MUCH IN THE WAY OF CLOUDS THROUGH THE PERIOD, AND ANY FEW
TO SCATTERED CLOUDS WILL BE VFR.
AS A WEAK LOW PRESSURE SYSTEM TRAVERSES THE GREAT LAKES REGION
SUNDAY AND A TRAILING COLD FRONT MOVES INTO THE OHIO VALLEY LATE IN
THE DAY, SOME HIGH TO PERHAPS EVEN MID LEVEL CLOUDS COULD MOVE
ACROSS OUR REGION.
OUTLOOK...
THE AFOREMENTIONED LOW PRESSURE SYSTEM WILL PULL A COLD FRONT ACROSS
OUR AREA DURING MONDAY. AS A RESULT, THE CLOUDS WILL INCREASE AND
LOWER DURING SUNDAY NIGHT INTO MONDAY WITH A PERIOD OF RAIN EXPECTED
MONDAY. THIS MAY TEND TO BRING A PERIOD OF MVFR CEILINGS AND/OR
VISIBILITIES TO THE AREA. FOLLOWING THE FRONT, A WIND SHIFT TO THE
NORTHWEST IS EXPECTED WITH PERHAPS SOME GUSTINESS MONDAY AFTERNOON
INTO MONDAY NIGHT. AN AREA OF HIGH PRESSURE THEN BUILDS IN LATER
TUESDAY AND WEDNESDAY. A POTENTIALLY STRONG STORM SYSTEM WITH A
DECENT AMOUNT OF MOISTURE IS THEN FORECAST TO ARRIVE FROM THE
SOUTHWEST LATE WEDNESDAY AND THURSDAY.
&&
.MARINE...
THE PRESSURE GRADIENT CONTINUES TO GRADUALLY RELAX AS THE INTENSE
STORM SYSTEM MOVES FARTHER AWAY IN THE CANADIAN MARITIMES.
MEANWHILE, A WEAK AREA OF HIGH PRESSURE WILL EXTEND INTO OUR AREA
TONIGHT AND SUNDAY BEFORE SLIDING EASTWARD. AS THE PRESSURE GRADIENT
WANES, THE WIND FIELD ABOVE THE SURFACE WILL ALSO DECREASE, ALLOWING
THE GUSTY SURFACE WINDS TO DIMINISH. THIS TREND HAS BEEN GRADUALLY
OCCURRING THIS AFTERNOON AND IS EXPECTED TO CONTINUE THROUGH THIS
EVENING. THE AVAILABLE OBSERVATIONS INDICATE THAT WINDS HAVE BEEN
MAINLY BELOW GALE FORCE ACROSS THE WATERS FROM ABOUT MIDDAY ON, AND
WITH A FURTHER DECREASE EXPECTED, WE WILL DOWNGRADE THE GALE WARNING
TO A SMALL CRAFT ADVISORY. BASED ON THE FORECAST SOUNDINGS AND
WAVEWATCH GUIDANCE, WE WENT WITH 01Z ACROSS THE SOUTHERN WATERS AND
DELAWARE BAY AND THEN 04Z ACROSS THE NORTHERN WATERS. THE END TIME
MAY BE TO LONG, ESPECIALLY FOR DELAWARE BAY, AS MUCH OF THE GUIDANCE
INDICATES A FAIRLY QUICK DECREASE. HOWEVER, THESE TIMES WERE CHOSEN
MAINLY BASED ON THE WIND FIELD DIMINISHING UNDER 25 KNOTS BELOW
1,000 FEET. THE DOWNWARD TREND IN THE OFFSHORE WIND WILL ALSO ALLOW
THE SEAS TO SETTLE THROUGH TONIGHT.
WE THEN GET A PERIOD OF TRANQUIL CONDITIONS AS THE NORTHERN EDGE OF
HIGH PRESSURE SLIDES NEARBY LATER TONIGHT AND SUNDAY. THIS WILL
ALLOW THE SURFACE WINDS TO SHIFT TO THE SOUTHWEST WITH TIME,
ESPECIALLY AS A COLD FRONT MOVES INTO THE GREAT LAKES AND OHIO
VALLEY TOWARD SUNDAY EVENING. THE WINDS AND SEAS WILL TEND TO
INCREASE SOME SUNDAY NIGHT AND ESPECIALLY MONDAY AHEAD OF THE FRONT,
ALTHOUGH THE WAA WILL TEND TO KEEP THE STRONGER WINDS ABOVE THE
SURFACE PRIOR TO THE FRONTAL PASSAGE. ONCE THE WINDS SHIFT TO THE
WEST AND NORTHWEST BEHIND THE FRONT, ADVISORY GUSTS COULD OCCUR IN
THE CAA REGIME. THIS MAY CONTINUE TUESDAY BEFORE HIGH PRESSURE
BUILDS IN. THE AREA OF HIGH PRESSURE GRADUALLY MOVES ACROSS THE AREA
WEDNESDAY. A POTENTIALLY STRONG STORM SYSTEM WITH A DECENT AMOUNT OF
MOISTURE AND STRONGER WINDS IS THEN FORECAST TO ARRIVE FROM THE
SOUTHWEST LATE WEDNESDAY AND THURSDAY.
&&
.PHI WATCHES/WARNINGS/ADVISORIES...
PA...NONE.
NJ...NONE.
DE...NONE.
MD...NONE.
MARINE...SMALL CRAFT ADVISORY UNTIL 11 PM EST THIS EVENING FOR
ANZ450>452.
SMALL CRAFT ADVISORY UNTIL 8 PM EST THIS EVENING FOR ANZ430-
431-453>455.
&&
$$
SYNOPSIS...DELISI
NEAR TERM...DELISI
SHORT TERM...DELISI
LONG TERM...ROBERTSON
AVIATION...GORSE
MARINE...GORSE
