Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000
FXUS61 KPHI 212049
AFDPHI

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ
349 PM EST SUN DEC 21 2014

.SYNOPSIS...
WEAK HIGH PRESSURE ACROSS THE REGION THIS EVENING WILL REMAIN
THROUGH MONDAY. WEAK LOW PRESSURE WILL MOVE NORTHWARD UP ALONG THE
EAST COAST MONDAY AFTERNOON THROUGH TUESDAY NIGHT. THIS LOW WILL BE
FOLLOWED BY ANOTHER STRONGER SYSTEM FOR WEDNESDAY AND WEDNESDAY
NIGHT. HIGH PRESSURE WILL BUILD IN FOR THE END OF THE WEEK. A COLD
FRONT WILL CROSS THE AREA NEXT WEEKEND.

&&

.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
THE CENTER OF A 1030 MB SURFACE HIGH OVER EASTERN CANADA WILL
CONTINUE TO RIDGE SOUTHWARD DOWN THE EASTERN SEABOARD THROUGH
TONIGHT. A MID- TO UPPER-LEVEL RIDGE AXIS WILL PROGRESS TOWARD THE
EAST COAST THIS EVENING.

SOME HI CLOUDS WILL SPILL INTO THE AREA OVERNIGHT AS THE RIDGE
SHIFTS DOWNSTREAM OF OUR AREA AND IS QUICKLY REPLACED BY A LEADING
SHORTWAVE TROUGH THAT IS EXPECTED TO MOVE IN FROM THE SOUTHWEST.

THE SOUTH TO NORTH LOW-LEVEL THERMAL GRADIENT ASSOCIATED WITH A WARM
FRONT IS FORECAST TO STRENGTHEN AS IT MOVES TOWARD SOUTHEAST VA LATE
TONIGHT. ISENTROPIC LIFT WILL DEVELOP ACROSS THE BOUNDARY AND EXPAND
NORTHWARD INTO THE DELMARVA BEFORE DAYBREAK. THE AIRMASS ON COLD
SIDE OF THIS BAROCLINIC ZONE WILL BE INITIALLY DRY SO EXPECT THE
THICKER LOW CLOUDS AND PRECIP TO REMAIN SOUTH OF DELAWARE AND THE
EASTERN SHORE OF MD TONIGHT.

&&

.SHORT TERM /6 AM MONDAY MORNING THROUGH 6 PM MONDAY/...
SOUTHERLY FLOW ALOFT AROUND THE BACKSIDE OF A WESTERN ATLANTIC RIDGE
AND AHEAD OF A TROUGH OVER THE NORTHERN PLAINS WILL BECOME
ESTABLISHED OVER THE EASTERN CONUS ON MONDAY. WAA DEVELOPS OVER THE
FORECAST AREA IN THIS PATTERN, ESPECIALLY IN THE LOW AND MID LEVELS.
AT THE SURFACE, HIGH PRESSURE IS FORECAST TO CONTINUE NOSING
SOUTHWARD DOWN THE EASTERN SIDE OF THE APPALACHIANS EVEN WITH THE
CENTER OF THE AFOREMENTIONED HIGH RETREATING TOWARD THE CANADIAN
MARITIMES. A CAD SETUP WILL KEEP THE COLDER AIR ANCHORED OVER OUR
REGION WITH A NORTHEASTERLY WIND NEAR THE SURFACE. THIS SHOULD
SUPPRESS THE ADVANCEMENT OF THE SURFACE WARM FRONT AS IT APPROACHES
FROM THE SOUTH ON MONDAY.

THE LATEST 12Z NAM CONTINUES TO BE QUICKER AND FARTHER NORTH WITH
THE DEVELOPMENT OF OVERRUNNING PRECIP COMPARED TO THE OTHER
OPERATIONAL MODELS. THE AIRMASS WILL BE VERY DRY INITIALLY, SO
PRECIP MAY NOT REACH THE GROUND AS QUICKLY AS THE NAM INDICATES.
HOWEVER, THERE APPEARS TO BE ENOUGH MOISTURE RETURN AND ISENTROPIC
LIFT FOR PRECIP TO EVENTUALLY OVERCOME THIS INITIAL DRY AIR,
ALLOWING THE PRECIP SHIELD TO EXPAND NORTHWARD THROUGH THE DAY.
CHANCE POPS FIRST ADVERTISED IN THE MORNING FOR THE DELMARVA, MIDDAY
FOR THE SOUTHERN PHILLY SUBURBS AND LATE AFTERNOON FOR THE I-80
CORRIDOR.

PTYPE WILL PREDOMINATELY BE RAIN ON MONDAY, BUT THERE COULD BE A
LIGHT WINTRY MIX (SNOW/SLEET) AWAY FROM THE COAST AT THE ONSET DUE
TO EVAPORATIONAL COOLING. THERE IS STILL NOT ENOUGH FORECAST
CERTAINTY TO PUBLISH A SNOWFALL MAP, BUT THE SETUP APPEARS LIMITED
FOR ANY ACCUMULATIONS EVEN IN THE HIGHER ELEVATIONS OF PA. WE
PREFERRED TO USE THE COOLER NAM/ECMWF FOR THERMAL PROFILES IN THIS
CAD SETUP TO DIAGNOSE POTENTIAL PTYPES AND FOR FORECASTING TEMPS.

&&

.LONG TERM /MONDAY NIGHT THROUGH SUNDAY/...
UNSETTLED WEATHER WILL BE ACROSS THE REGION AT THE BEGINNING OF THE
LONG TERM. A H5 TROUGH WILL BE DEEPENING ACROSS THE MIDDLE PART OF
THE COUNTRY...AND ITS EFFECTS WILL BE FOR CLOUDS AND MOSTLY RAIN
BEGINNING MONDAY NIGHT AND PERSISTING INTO WEDNESDAY NIGHT AND
THURSDAY MORNING. THE FIRST BATCH OF RAIN WILL COME AS WEAK LOW
PRESSURE DEVELOPS ALONG THE CAROLINA COAST MONDAY AND MOVES
NORTHWARD ALONG THE NJ/DE COAST TUESDAY. WE HAVE CHC POPS N/W AND
LIKELY POPS S/E WITH THIS FEATURE. ON TUESDAY...THE HIGHER POPS MOVE
TO THE NORTHERN AREAS AS ONSHORE FLOW AND RESIDUAL MOISTURE FROM THE
LOW REMAIN ACROSS THE AREA.

THERE WILL NOT BE MUCH OF A BREAK IN THE WET PATTERN TUESDAY
NIGHT...ALTHOUGH SOME WEAK RIDGING WILL BE ACROSS THE AREA. ANOTHER
STRONGER LOW PRESSURE SYSTEM WILL BE DEVELOPING ACROSS THE SOUTHERN
STATES AND IT WILL MOVE NORTHWARD TUESDAY NIGHT THROUGH WEDNESDAY
NIGHT. THE LOW WILL TRACK TO THE WEST...RESULTING IN A SURGE OF
WARMER AIR ACROSS THE AREA. WE HAVE GONE ALONG WITH THE HIGH POPS
OFFERED BY WPC...WHICH ARE IN THE CATEGORICAL RANGE TUE NIGHT THRU
WED NIGHT.

A STRONG COLD FRONT WILL CROSS THE AREA OVERNIGHT WED NGT INTO THU.
IT APPEARS THAT MOST OF THE SHOWERS WILL BE DONE BY 12Z...BUT WE
HAVE LEFT IN A CHC FOR A FEW SHOWERS THU MORNING...IN CASE THE
SYSTEM SLOWS UP AT ALL. IT APPEARS THAT THE P-TYPE WILL STILL BE
RAIN...SINCE THE COLDER AIR WILL NOT ARRIVE FAST ENOUGH FOR A
CHANGEOVER.

THE REST OF THE EXTENDED PERIOD WILL FEATURE DRIER WEATHER AS A SFC
HIGH EXTENDS ACROSS THE AREA. THE H5 TROUGH WILL STILL BE TO THE
WEST...SO TEMPERATURES WILL REMAIN ABOVE NORMAL BOTH FRI AND SAT.
ANOTHER LOW AND DEVELOPING FRONT MAY BEGIN TO AFFECT THE AREA SAT
NIGHT INTO NEXT SUNDAY...LOW CHC POPS ARE IN THE GRIDS FOR THIS.

&&

.AVIATION /21Z SUNDAY THROUGH FRIDAY/...
THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY AND SURROUNDING AREAS.

LEFTOVER OVC STRATOCU CONTINUES TO ERODE LATE THIS AFTN OVER MOST OF
THE TERMINALS. AT 20Z, TTN AND RDG LIE ON THE EDGE OF THESE CLOUDS,
WHICH ARE YIELDING MVFR CIGS BETWEEN 2500 AND 3000 FT. EXPECT THE
CLOUDS TO CLEAR OUT AT THESE TWO TERMINALS BY SUNSET AT THE LATEST.

VFR THIS EVE. VFR ANTICIPATED FOR MOST TERMINALS OVERNIGHT AND
MONDAY MORNING THOUGH THE 12Z NAM DEVELOPS STRATUS FROM PHL NORTH
AND EAST, RESULTING IN MVFR AND EVEN IFR CIGS BETWEEN BETWEEN
09-15Z. THIS MAY BE A BIT OVERDONE, BUT WILL STILL NEED TO MONITOR
THE POTENTIAL AS THE MARINE LAYER TRIES TO BECOME ESTABLISHED IN
CONCERT WITH A LIGHT NORTHEAST WIND IN THE BOUNDARY LAYER.
OTHERWISE, MID CLOUDS WILL QUICKLY OVERSPREAD THE AREA FROM
SOUTHWEST TO NORTHEAST IN THE MORNING. RAIN IS ALSO FORECAST TO
DEVELOP IN VICINITY OF ILG/MIV DURING THE MID TO LATE MORNING,
PHL/PNE/RDG DURING THE EARLY TO MID AFTERNOON AND ABE/TTN LATE IN
THE DAY.

OUTLOOK...

MON NIGHT THRU TUE...MOSTLY VFR. INCREASING CLOUDS LATE TUE NIGHT.
TUE NIGHT THRU THU MORNING...OCNL LOW CIGS/VSBYS IN RAIN AND FOG.
THU AFTERNOON THRU FRIDAY...MOSTLY VFR. GUSTY WEST/NORTHWEST WINDS.

&&

.MARINE...
LIGHT NORTHWEST WINDS 5-15 KT THIS AFTERNOON WILL BECOME NORTHERLY
TONIGHT AND NORTHEASTERLY ON MONDAY. SEAS GENERALLY 1-2 FT DURING
THIS TIME.

OUTLOOK...
MON NIGHT...SUB SCA CONDITIONS.
TUE/TUE NGT...SCA CONDITIONS ON THE OCEAN..SUB-SCA OVER DEL BAY.
WED THRU FRI...SCA CONDITIONS WITH LOW END GALE GUSTS WED NGT AND THU.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...NONE.
NJ...NONE.
DE...NONE.
MD...NONE.
MARINE...NONE.

&&

$$
SYNOPSIS...O`HARA
NEAR TERM...KLEIN
SHORT TERM...KLEIN
LONG TERM...O`HARA
AVIATION...KLEIN/O`HARA
MARINE...KLEIN/O`HARA




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