Area Forecast Discussion
Issued by NWS Mt. Holly, NJ-- Remove Highlighting --
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FXUS61 KPHI 251441
Area Forecast Discussion
National Weather Service Mount Holly NJ
1041 AM EDT Sun Sep 25 2016
High pressure centered north of the Great Lakes will build
southward along the eastern U.S. seaboard today and tonight. The
high then moves off the New England coast Monday as a cold front
approaches from the west. The front is expected to pass through
the area Monday night and then stall off the coast Tuesday. A wave
of low pressure may develop to our south and move up the coast
sometime during the middle to late week. The low eventually
departs to our northeast and high pressure builds in either late
in the week or next weekend.
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
No changes needed to the forecast today. High pressure will
continue to build across the northeast states, keeping fair
conditions across the area. RH values may fall into the upper 20s
to low 30 this afternoon. However, winds will be light with the
high pressure system in place over the region.
.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM MONDAY/...
High pressure will continued to be anchored across the region
tonight. This will allow for one more night of clear skies and
efficient radiational cooling. Temperatures should fall into the
40`s overnight for many of us. However, some of the higher
elevations of the southern Poconos and NW NJ may see lows in the
upper 30`s. With light winds, some patchy frost looks possibile.
However, 850 mb temperatures look several degrees warmer than
ideal coupled with dry ground conditions should limit the coverage
of any frost to patchy.
.LONG TERM /MONDAY THROUGH SATURDAY/...
On Monday, high pressure centered over New England in the morning
will retreat offshore in the afternoon while a cold front advances
eastward toward the western side of the Appalachians. Low-level
southerly flow in between these two systems will develop along the
eastern seaboard. Forecast high temperatures on Monday are only a
degree or two warmer than today since ensuing WAA pattern over the
region looks to be modest. Mid and high clouds are expected to
stream in ahead of the front during the day, resulting in periods
of clouds and sun.
The cold front is expected to move through the area Monday night.
The juxtaposition of a strong (40 kt S-SWly) low-level jet, a
well-defined atmospheric river (PWAT values peak between 1.75-2.00
inches) and low-level theta-e ridge provides support of an
organized band of showers ahead of the front Monday night and
Tuesday morning. There will be additional forcing for ascent
higher aloft that arrives when the forecast area becomes
positioned in the right-entrance region of a 100+ kt upper-level
jet streak that is located on the eastern side of the Great Lakes
low. With minimal spread noted in the guidance for the fropa
timing, felt it was time to raise PoPs to categorical. The
progressive nature of the front will limit our rainfall potential.
QPF amounts range between one-half and one inch, but there could
be locally heavier totals if deeper convection (embedded
thunderstorms) develop in the warm sector. We`ll take any rain
we can get at this point, especially across the north, to help
narrow the rainfall deficit...even if it`s by a small fraction.
The front may slow down as it moves off the coast Tuesday in
response to the primary low that hangs back to our northwest
becoming cutoff over the Great Lakes region. Therefore, showers
will likely be slow to exit the coastal plain and may even linger
well into the afternoon or evening near the coast and lower
There continues to be sizable uncertainty in the forecast for the
second half of the week with models having a difficult time
capturing the evolution of the cutoff low over the eastern CONUS
and secondary cyclogenesis along the East Coast. There is
increasing support for a coastal low to form to our south
Wednesday and slowly track up the mid-Atlantic coast into the end
of the week. This would result in two or more days of below normal
temperatures and rainy conditions across much of the region. The
last three runs of the ECMWF (including the operational run and
about one-third of its ensemble members) and now the UKMET
operational keeps the unsettled pattern right into next weekend
with the low stalling nearby to our south for several more days.
Trended toward a more pessimistic (cool/damp) scenario for the
second half of the week but changes to the previous forecast were
conservative given the relatively limited skill inherent with
forecasting cutoff lows (i.e., nothing is set in stone yet).
.AVIATION /14Z SUNDAY THROUGH THURSDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.
VFR today and tonight with northerly winds of 10 knots or less
becoming lighter and variable this evening.
Monday...VFR. Southerly winds around 10 kt. Gusts to 20 kt develop
late in day.
Monday night...Lowering to MVFR from west to east as showers move in
ahead of a cold front. IFR possible late in heavier showers. A 40-kt
southwesterly low-level jet moves overhead, but model forecast
soundings do not indicate it meeting criteria to warrant LLWS in the
Tuesday...Showers and MVFR restrictions to start the day until the
front moves through. Fropa will be marked by a wind shift from
southerly to westerly. Conditions will first improve to VFR in
western terminals (e.g., ABE/RDG) sometime in the morning and then
eastern terminals (e.g., MIV/ACY) during the afternoon.
Tuesday night and Wednesday....Predominately VFR.
Wednesday night and Thursday...Sub-VFR conditions and showers may
return but forecast confidence is lower than normal.
Small Craft Advisory will continue as seas remain 4-6 feet across
the Atlantic waters. WaveWatch guidance shows a slow decrease of
the waveheights today to around three feet tonight. Winds will be
northerly around 10-15 knots at times, then become lighter and
Monday...No marine headlines anticipated.
Monday night and Tuesday morning...SCA possible. Southerly winds
ahead of a cold front increase to around 20 kt with gusts near 25
kt. Seas build to 4-5 ft in our coastal waters.
Tuesday afternoon through Thursday...Winds and seas below SCA
RIP CURRENTS...A long period swell coupled with 3-5 ft seas are
both still factors that the rip current risk will be best
represented by the moderate risk category today. The probable risk
for the formation of dangerous rip currents for Monday is low.
MARINE...Small Craft Advisory until noon EDT today for ANZ450>455.