668
FXUS61 KPHI 232314
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
614 PM EST Thu Nov 23 2017

.SYNOPSIS...
High pressure currently over the region will move slowly
offshore on Friday as a cold front approaches the region from
the west. This cold front will pass through the region on
Saturday. Another area of high pressure will build into the
region for the early part of next week then move offshore with
the approach of another cold front by the middle of next week.

&&

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
A Very Happy Thanksgiving to all who read these discussions.

Tonight...clear to partly cloudy. Good to excellent radiational
cooling. Calm or light wind. A 2-3 hour wide band of multi deck
broken layers between 8000 and 15000 feet crosses from west to
east this evening then drying out-clearing. Lows 4 to 12F below
normal...greatest negative departures in the radiative
countryside. Moderate to heavy frost likely Delmarva, s NJ, nw
NJ where excellent radiation should occur and temps were lowered
below guidance.

Forecast basis was a 50 50 blend of the 12z/23 GFS/NAM MOS
guidance and then lowering that blend by 2-3F over the typical
countryside radiators. Dewpoints in the upper teens allow for
some lows in the upper teens tonight, for the typically cold
spots of NJ and PA.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/...
Sunny. Milder in the afternoon after a chilly start. Light wind
trending south to southwest in the afternoon. Max temps
generally within 2f of normal. Due to poor mixing despite a
sunny day and after early morning chill...think PHL max will be
51.

There may be a signal developing in some of the guidance with a
long dry spell apparently developing. High pressure and
radiation cooling options at night may verify temps lower than
guidance. GGEM seems to have that idea. This is due to lack of
mixing except a day before a frontal passage (gradient sw flow).
Daytime max`s may not be as high GFS guidance suggests...in
part because of short days.

&&

.LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/...
Friday night through Saturday night:

High pressure will continue to move offshore Friday night
giving way to an approaching cold frontal boundary. This cold
front will move through the area on Saturday. Overall, timing
remains consistent on various models and ensembles for an
afternoon/ evening frontal passage. Enough lift and moisture
should be present for a few spotty rain showers, primarily in
the afternoon hours. Temperatures Saturday morning may be close
enough to freezing in the southern Poconos and NW NJ for a
freezing rain concern. Right now any precipitation is expected
to hold off till mid-morning when it warms well above freezing.
Precipitation looks very light, only a few hundredths at best. A
few southerly wind gusts in the afternoon around 15 mph are
possible before the frontal passage. Northwest flow on the
backside of the front could also kick off some sprinkles and
flurries toward sunrise that could be enhanced where the
elevation is highest (southern Poconos and NW NJ).

Sunday through Thursday:

A fairly strong push of cold air advection will occur behind
the front. This will lead to another period of more winter like
temperatures across the region with stronger northwest winds,
gusting perhaps to 25 mph on Sunday. The northwest flow may also
allow for some moisture to transport southeast off of the Great
Lakes. The moisture could be enhanced by the higher terrain.
Highs Sunday and Monday will generally be in the 40`s with
overnight lows will be in the 20`s and 30`s. Conditions are
expected to clear later Sunday and Monday as the northwest flow
weakens and high pressure builds into the region.

We should see a fairly quick rebound Tuesday and Wednesday with
temperatures into the 50`s, maybe 60 Wednesday? Right now the
forecast stays close or a little above to the mean of the
ensemble guidance on Tuesday and Wednesday. A majority of
ensemble guidance does indicate a pattern favorable for a period
of warm weather. However, the 12z OP EC 11/23 does indicate a
stronger push of cold air advection from the northwest Monday
into Tuesday which would lead to lower temperatures. The OP EC
looks like an outlier solution at this time. Some uncertainty is
also present with the speed of a cold frontal boundary moving
toward the area sometime Wednesday or more likely Thursday of
next week. For now, went middle of the road between the GFS and
ECMWF and brought in a chance of showers on Thursday. This has
trended a little slower the last couple of model runs. After a
warmer Wednesday, temperatures Thursday should cool a few
degrees back closer to normal.

Happy Thanksgiving from all of us here at the National Weather
Service in Mount Holly!

&&

.AVIATION /23Z THURSDAY THROUGH TUESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Tonight...VFR conditions expected with probably only a 2-3 hour
wide band of multi layered bkn clouds 8000-15000 ft early in
the night. Winds will generally be westerly at 5 KT or less, but
could become light and variable. High confidence.

Friday...VFR clear. Light west wind becoming southwest to south
during the afternoon. High confidence.

OUTLOOK...

Friday night: VFR, westerly winds becoming southwesterly but
staying at 10 knots or less. High confidence.

Saturday and Saturday night: Sub-VFR possible with some
scattered showers primarily from 15 to 23Z. Southerly wind gusts
10-15 knots, becoming northwest Saturday night. Medium to high
confidence.

Sunday and Sunday night: VFR, a few sprinkles or flurries
possible at KABE Sunday morning. Northwest wind gusts from 20-25
knots. Medium confidence.

Monday through Tuesday night: VFR, West winds around 10-15
knots becoming more southerly on Tuesday. High confidence.

&&

.MARINE...
No marine headlines anticipated through Friday. A 3 foot se
swell at 10 seconds is the primary driver of the Atlantic
coastal waters forecast.

OUTLOOK...

Friday night: Sub-SCA southwesterly wino below SCA levels under
15 knots and seas around three feet. High confidence.

Saturday and Saturday night: Seas building late Saturday but
staying under 5 feet. Southwesterly wind gusts 15-20 knots in
the afternoon. Winds becoming northwesterly Saturday night and
increasing to 20- 25 knots. Medium confidence.

Sunday and Sunday night: Seas steady just under five feet with
some northwest wind gusts around 25 knots. An SCA currently
looks like it will be needed in this period. Medium confidence.

Monday through Tuesday night: Westerly wind gusts generally
around 15 knots but increasing on Tuesday closer to 25 knots.
Seas decrease Monday but should begin to build up by Tuesday
close to five feet. Low to medium confidence.

&&

.CLIMATE...
As it stands with our 330PM forecast. It appears the month will
average about a degree or 2 below normal vcnty I-80, near or
slightly below normal remainder of the area except about a
degree above normal near GED. Short waves passing through
eastern Canada have not dug quite as far south as indicated 8
days ago when it appeared a below normal month was probable.
This would end up being about degree warmer than our
deterministic solution from the 15th.

There is still uncertainty regarding how temperatures verify
from Tuesday the 28th onward. Todays 12z/23 GGEM and ECMWF
operational cycles for 2m temps are significantly cooler for a
day or two between Tuesday and Thursday (28-30th). For now we
lean with the warmer GFS per the GEFS and even the ECEFS.

&&

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

&&

$$

Synopsis...Gaines
Near Term...Drag
Short Term...Drag
Long Term...Gaines
Aviation...Drag/Gaines
Marine...Drag/Gaines
Climate...Drag

NWS PHI Office Area Forecast Discussion