000
FXUS61 KPHI 171953
AFDPHI
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ
353 PM EDT SAT MAY 17 2008
.SYNOPSIS...
A MID LEVEL VORTEX OVER SOUTHEASTERN CANADA WILL DROP INTO NEW
ENGLAND BY EARLY NEXT WEEK, THEN MOVE VERY SLOWLY TO THE WESTERN
ATLANTIC AND BECOME PART OF A BLOCKING PATTERN BY THE END OF THE
WEEK. THIS SHOULD RESULT IN UNSETTLED WEATHER WITH SOMEWHAT BELOW
NORMAL TEMPERATURES THROUGH WEDNESDAY AS A SERIES OF SURFACE FRONTS
AND LOW PRESSURE AREAS MOVE ACROSS OUR REGION. WEATHER MORE
TYPICAL OF MAY MIGHT DEVELOP BY THE END OF THE WEEK IF THE VORTEX
ESTABLISHES ITSELF FAR ENOUGH TO OUR EAST.
&&
.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
THE FIRST IN A SERIES OF MID LEVEL IMPULSES ROTATING AROUND A
SOUTHEASTERN CANADIAN LOW WILL MOVE ACROSS THE AREA IN THE FIRST
PERIOD. THERE INITIALLY IS SOME UPPER JET SUPPORT WITH THIS ALONG
WITH A LITTLE BIT OF MID LEVEL DIFLUENCE. WE DO SEE SOME LOWER LEVEL
FRONTOGENESIS AS WELL. BECAUSE TEMPERATURES ALOFT ARE COOL, THERE IS
SOME ELEVATED CAPE FOR THE DYNAMICS TO TAP, BUT THERE ISN`T A LOT
AND THAT SHOULD DISAPPEAR FAIRLY EARLY IN THE EVENING. WET BULB ZERO
VALUES ARE CONDUCIVE TO SOME HAIL, ALTHOUGH AS IMPLIED ABOVE THE
0 TO -20 DEG C CAPES AREN`T VERY HIGH. THERE IS DIRECTIONAL AND
SPEED SHEAR IN THE ENVIRONMENT AND THE WIND FIELD IS FAIRLY STRONG
UNTIL THE SURFACE COLD FRONT MOVES THROUGH. MOISTURE IS LIMITED, AND
MOST OF THE ABOVE FACTORS ARE MORE FAVORABLE FOR ANY CONVECTION THE
FURTHER NORTHWEST IN THE FORECAST AREA ONE GOES. THE STORM
PREDICTION CENTER PUTS THE NORTHWESTERN THIRD OF THE FORECAST AREA
UNDER A RISK OF GENERAL THUNDERSTORMS, AND MENTIONS ONLY THE
POSSIBILITY OF ISOLATED STORMS WITH STRONG WIND GUSTS AND SMALL
HAIL. THE OVERALL INDICATION OF HIGHEST CHANCES NORTHWEST AND LOWEST
CHANCES SOUTH, TARGETED MOSTLY FOR THE EVENING, ARE ACCEPTED. WE`LL
MAINTAIN THE MILDLY ENHANCED WORDING EARLY NORTHWEST. WINDS TONIGHT
SHOULD GO LIGHT WITH TIME AFTER THE IMPULSE IS THROUGH, AND THERE
MAY BE A BIT OF RADIATING DESPITE HIGH CLOUDS ADVANCING AHEAD OF
THE NEXT IMPULSE. TEMPERATURES ON THIS FIRST DAY BACK ARE A BLEND
OF CONTINUITY AND GUIDANCE. THE POSSIBILITY OF RADIATING AND THIS
OVERALL PATTERN SEEM TO ARGUE AGAINST RAISING GUIDANCE. THERE MAY
BE SOME PATCHY FOG AROUND THE AREA TOWARD DAYBREAK, BUT WE`LL NOT
MENTION IT IN THE PUBLIC FORECAST.
&&
.SHORT TERM /6 AM SUNDAY MORNING THROUGH MONDAY NIGHT/...
THE SECOND IMPULSE APPROACHES TOMORROW WITH ANOTHER ATTENDANT COLD
FRONT. WE AGAIN SEE SOME UPPER JET SUPPORT AND SOME MID LEVEL
DIFLUENCE ALONG WITH SOME LOWER LEVEL FRONTOGENESIS. INSTABILITY
WITH THIS SYSTEM MAY BE A BIT STRONGER THAN WITH THE FIRST ONE,
ALTHOUGH THE WORST OF THE CAPES /ELEVATED CAPES AND/OR 1 KM MUCAPES/
WE CAN FIND STILL ARE MAINLY BELOW 500 J/KG. THE EXCEPTION IS OVER
THE SOUTHWESTERN ZONES DURING THE AFTERNOON ACCORDING TO THE NAM,
BUT THE NAM MUCAPES AREN`T MUCH HIGHER THERE EITHER. WET BULB ZERO
VALUES AGAIN ARE CONDUCIVE TO SOME HAIL, BUT AGAIN THERE ISN`T TOO
MUCH CAPE IN THE 0 TO -20 DEG C LAYER. THERE IS MORE MOISTURE
AVAILABLE WITH THIS IMPULSE. THE WIND ENVIRONMENT IS SIMILAR TO
THE ONE WE`LL SEE EARLY THIS EVENING. THE TIMING OF THE SURFACE
FRONT, OVER WHICH THERE SEEMS TO BE DECENT AGREEMENT, WOULD HAVE
IT THROUGH THE NORTHWESTERN HALF OF THE AREA BY 00Z MONDAY AND
THEN TAKE IT WELL THROUGH BY 06Z. WE`LL CONTINUE TO CARRY THE
HIGHER POPS WE`VE BEEN CARRYING, THEN BACK OFF ON THE EVENING POPS
IN THE NORTHWEST TO THE SLIGHT CHANCE REALM. BECAUSE OF THE WEAK
INSTABILITY, WE WON`T CARRY ANY ENHANCED WORDING AT THIS POINT,
ALTHOUGH WE WILL MENTION THUNDER IN THE SOUTHEASTERN TWO-THIRDS OF
THE AREA.
FAST ON THE HEELS OF THE SECOND IMPULSE COMES A THIRD. THERE`S
GENERAL AGREEMENT THAT IT SHOULD BE THROUGH MUCH OF THE AREA BY
00Z TUESDAY, THAT MOISTURE AVAILABLE TO THIS FEATURE EASILY IS
LESS THAN THAT AVAILABLE TO EITHER OF THE FIRST TWO, AND THAT
THERE IS VERY LIMITED INSTABILITY AS A MID LEVEL CAP TRIES TO
DEVELOP. WE WILL KEEP THE LOW CHANCE POPS IN THE NORTH AND PUT IN
SOME SLIGHT CHANCE POPS DOWN TO AROUND THE I95 CORRIDOR, BUT
THAT`S ABOUT IT. FOR NOW, WE`LL GO OPTIMISTIC FOR MONDAY NIGHT,
ALTHOUGH YET ANOTHER IMPULSE WILL BE APPROACHING US.
TEMPERATURES REMAIN A BLEND OF CONTINUITY AND GUIDANCE WITH A NOD
TOWARD COOLER VALUES IN THIS PATTERN. BY THE TIME WE GET TO MONDAY
AND MONDAY NIGHT, WE`RE FORECASTING VALUES THAT ARE AROUND TEN
DEGREES BELOW NORMAL.
&&
.LONG TERM /TUESDAY THROUGH SATURDAY/...
TUESDAY WILL BEGIN WITH A BROAD UPPER LEVEL TROUGH OVER THE EASTERN
STATES AND A RIDGE OVER THE WEST. THE EASTERN TROUGH SHOULD REMAIN
IN PLACE THROUGH WEDNESDAY AND THURSDAY AS ANOTHER TROUGH DIGS OVER
THE WESTERN STATES. A FAIRLY SHARP RIDGE WILL BECOME ESTABLISHED
OVER THE MISSISSIPPI RIVER VALLEY AS A RESULT. THE ENTIRE UPPER
PATTERN IS EXPECTED TO SHIFT SLIGHTLY TO THE EAST DURING FRIDAY AND
SATURDAY.
AT THE SURFACE, WEAK LOW PRESSURE IS FORECAST TO PASS WELL TO OUR
SOUTH ON TUESDAY, WITH HIGH PRESSURE GRADUALLY BUILDING DOWN FROM
THE NORTHWEST DURING THE MIDDLE AND LATE PARTS OF THE NEW WEEK.
&&
.AVIATION /20Z SATURDAY THROUGH THURSDAY/...
A COLD FRONT MOVING THROUGH THE EASTERN GREAT LAKES THIS AFTERNOON
WILL CONTINUE TO TRACK EASTWARD THROUGH TONIGHT. THIS FEATURE IS
EXPECTED TO CROSS ALL OUR TERMINALS TONIGHT, HOWEVER NOT MUCH OF A
WIND SHIFT IS EXPECTED AS THE FRONT ITSELF TENDS TO DAMPEN OUT SOME
AND PERHAPS STALL IN THE VICINITY. A SCATTERED TO BROKEN DECK OF
CUMULUS AND EVEN SOME ALTOCUMULUS WILL BE AROUND THIS AFTERNOON AND
FOR AWHILE TONIGHT, HOWEVER BASES ARE EXPECTED TO REMAIN IN THE VFR
CATEGORY. THERE WILL PROBABLY BE TIMES WHERE THE CEILINGS GO
OVERCAST, THEN THIN OUT SOME LATER TONIGHT AS HEATING IS LOST. WE
DID DISSIPATE THE CUMULUS AND THEN CARRIED SOME MIDLEVEL CLOUDS
OVERNIGHT. SURFACE WINDS THIS AFTERNOON AND EARLY EVENING WILL
GENERALLY BE FROM THE SOUTHWEST AND A BIT GUSTY AT TIMES, THEN
DIMINISH SOME DURING THE EVENING WITH THE LOSS OF MECHANICAL MIXING.
A SOUTHWESTERLY FLOW OF ABOUT 10 TO 15 KNOTS SHOULD PRECLUDE A TRUE
SEA BREEZE FROM FORMING. MOST TERMINALS SHOULD HOLD ONTO SOME BREEZE
THROUGH THE NIGHT, HOWEVER MAINLY KRDG, KABE AND KMIV SHOULD HAVE
WINDS GO LIGHT AND VARIABLE. A BIT OF RIVER FOG MAY IMPACT KRDG, SO
SOME FOG WAS ONLY ADDED TO THIS TERMINAL FOR NOW. REGARDING
PRECIPITATION, WE ADDED A TEMPO GROUP TO ONLY KRDG AND KABE LATE
THIS AFTERNOON AND THROUGH EARLY THIS EVENING AS SHOWERS AND
POSSIBLE THUNDER ACROSS CENTRAL AND WESTERN PENNSYLVANIA TRACK
EASTWARD. SOME THUNDER IS POSSIBLE NEAR OUR WESTERN TERMINALS
ESPECIALLY, HOWEVER DID NOT INCLUDE A MENTION FOR NOW AS AS
INSTABILITY WILL BE DIMINISHING THROUGH THE EVENING.
FOR SUNDAY, ENERGY ROTATING AROUND AN UPPER-LEVEL TROUGH SHOULD
SPAWN A WEAK AREA OF LOW PRESSURE. THIS FEATURE IS EXPECTED TO
CRUISE ACROSS THE REGION ON SUNDAY, BRINGING SHOWERS. FORECAST
SOUNDINGS INDICATE THE CEILINGS LOWERING THROUGH THE MORNING BUT
REMAINING MOSTLY VFR. WE LOWERING CEILINGS TO NEAR MARGINAL VFR
STARTING AT 14-15Z. SHOWERS MAY QUICKLY ARRIVE TO, THEREFORE WE
INDICATED THIS ALSO AT 14-15Z BUT DID NOT INDICATE ANY VISIBILITY
RESTRICTIONS. MVFR CONDITIONS MAY HOLD OFF UNTIL 18Z OR THEREAFTER.
FORECAST SOUNDINGS ALSO INDICATE SOME INSTABILITY DEVELOPING SUNDAY
AFTERNOON, THEREFORE SOME THUNDER /OR A CB DESCRIPTOR/ MAY NEED TO
BE ADDED WITH SUBSEQUENT TAF ISSUANCES. SURFACE WINDS ON SUNDAY ARE
EXPECTED TO BE FROM THE SOUTH AND SOUTHWEST LESS THAN 15 KNOTS.
OUTLOOK...
AN AREA OF LOW PRESSURE AND ITS ASSOCIATED COLD FRONT WILL MOVE
ACROSS THE AREA DURING SUNDAY AFTERNOON. THIS WILL PRODUCE SOME
SHOWERS AND POSSIBLE THUNDER INTO THE EVENING. A WIND SHIFT TO THE
WEST IS EXPECTED AS THE FRONT MOVES THROUGH. A WEAK DISTURBANCE MAY
BRUSH THE AREA ON MONDAY WITH PERHAPS MORE CLOUDS, HOWEVER IT
APPEARS THE PRECIPITATION CHANCE IS LOW. OUR ATTENTION THEN TURNS TO
ANOTHER WEAK LOW PRESSURE SYSTEM POSSIBLY SCOOTING JUST TO OUR SOUTH
ON TUESDAY, THEN A COLD FRONT MAY MOVE THROUGH LATER WEDNESDAY.
&&
.MARINE...
A COLD FRONT WILL APPROACH THE AREA LATE THIS AFTERNOON, THEN MOVE
INTO THE MARINE AREA TONIGHT BEFORE POTENTIALLY STALLING. THIS
FRONT MAY KICK UP THE WINDS FOR A TIME THIS EVENING AS A LOW-LEVEL
JET MOVES THROUGH, HOWEVER IT DOES NOT APPEAR ROBUST OR LONG ENOUGH
TO HOIST AN ADVISORY. AS A RESULT WE WILL KEEP GUSTS JUST UNDER 25
KNOTS. THERE WILL PROBABLY NOT BE A WIND SHIFT EITHER WITH THIS
BOUNDARY AS IT EDGES INTO THE REGION. WE THEN LOOK TO OUR WEST AS
UPPER-LEVEL ENERGY SPAWNS A SURFACE LOW WHICH THEN TRACKS ACROSS OUR
AREA ON SUNDAY. A COLD FRONT WITH THIS LOW IS EXPECTED TO PRODUCE A
MORE NOTABLE WIND SHIFT SUNDAY NIGHT. BACKING UP A BIT, A LOW-LEVEL
JET OF AROUND 40 KNOTS IS FORECAST ON SUNDAY, HOWEVER FORECAST
SOUNDINGS ARE NOT REALLY SHOWING MUCH IN THE WAY OF THE STRONGER
WINDS MIXING DOWN TO THE SURFACE. THERE MAY BE SOME GUSTS TO ABOUT
25 KNOTS, BUT RIGHT NOW THE THINKING IS TO HOLD OFF ON AN ADVISORY
AS IT APPEARS MARGINAL. THE SOUTHERLY FLOW MAY BUILD SEAS UP TO 5
FEET BUT WAVEWATCH CAPS SEAS OFF AT 4 FEET AT THE MOMENT.
THE TIME FRAME WHEN AN ADVISORY MAY BE NEEDED SHOULD BE ON MONDAY AS
COOLING ALOFT MOVES OVER THE WATER, AND THE PRESSURE GRADIENT
TIGHTENS UP BEHIND SUNDAY`S DEPARTING SURFACE LOW. FORECAST
SOUNDINGS ON MONDAY DO INDICATE BETTER VERTICAL MIXING AS LOW-LEVEL
LAPSE RATES STEEPEN, ALTHOUGH THE GFS IS MORE ROBUST WITH THIS
SCENARIO COMPARED TO THE WRF-NMM. FOR NOW WE WILL GO WITH GUSTS TO
AROUND 25 KNOTS ON MONDAY. CONDITIONS THEN IMPROVE MONDAY NIGHT AS
THE PRESSURE GRADIENT WEAKENS AND A WEAK AREA OF HIGH PRESSURE MAY
SCOOT BY. WE THEN TURN OUR ATTENTION TO ANOTHER STORM SYSTEM THAT
MAY TRACK JUST TO OUR SOUTH ON TUESDAY. THIS SHOULD TURN THE WINDS
TO THE SOUTH, BUT ATTM THEY DO NOT LOOK ALL THAT STRONG AS THE
SYSTEM IS FORECAST TO BE WEAK. A COLD FRONT MAY THEN APPROACH LATE
ON WEDNESDAY, WITH HIGH PRESSURE TRYING TO BUILD TOWARD OUR REGION
THURSDAY FROM THE GREAT LAKES.
&&
.PHI WATCHES/WARNINGS/ADVISORIES...
PA...NONE.
NJ...NONE.
DE...NONE.
MD...NONE.
MARINE...NONE.
&&
$$
SYNOPSIS...DELISI
NEAR TERM...DELISI
SHORT TERM...DELISI
LONG TERM...IOVINO
AVIATION...GORSE
MARINE...GORSE
