Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 162336
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
636 PM EST Sat Dec 16 2017

.SYNOPSIS...
A cold front will move southeast into our region tonight, then
it will return northeast of the region as a warm front late
Sunday night. Another cold front will then move through the Mid-
Atlantic states Tuesday night. High pressure follows Wednesday
night. Thereafter, to end the week, an area of low pressure will
track northeast from the Great Lakes and advance a warm front
into our region.

&&

.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
As of 630 PM, the area of light snow across central New York to
just northwest of the Poconos continues to weaken. Looks like
time is running out for a more organized area of snow showers
for our northwest zones, with just some flurries looking more
likely. For now, kept some 20-30 PoPs for a few snow showers
through early this evening.

Otherwise, a cold front continues to push southeastward early
this evening. The model suite has been advertising some snow
showers across portions of the area north of I-80. Any snow will
be very light.

The cold front will slowly cross our region overnight. Expect
considerable cloudiness to continue especially invof and north
of the PA Turnpike to I-195 corridor. South of this area,
partial clearing is anticipated. With winds becoming light
overnight, dew points higher compared to last night, and the
moist low-level airmass, patchy fog may develop across portions
of southern NJ, southeast PA, and Delmarva.

Our min temperature forecast remains along the colder envelope of
the guidance, given the warm bias exhibited recently, including the
fresh snow cover around and east of I-95. But with cloud cover
across portions of the region overnight (PA Turnpike to I-195 and
points north), the forecast may need fine tuning.

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH 6 PM SUNDAY/...
Weak high pressure will be across the region on Sunday, with the
cold front just to our south. This front will begin to lift north as
a warm front, and cloudiness will increase during the day. For now,
precip is expected to hold off until later in the evening. (see the
long term section for more details on this system). Max temps will
be a few degrees below normal with light winds.

&&

.LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/...
500 MB: A warm ridge centered over the southern Appalachians will
weaken Wednesday with the passage of a substantial short wave through
se Canada. Thereafter...warmer that normal heights prevail over
the se USA with a resurgence of the ridge over the se coast next
weekend. Cooler than normal heights are near Hudson Bay, resulting
in a fairly strong wsw jet from the southern Plains to the mid
and north Atlantic coast.

Temperatures: The first 15 days have averaged below normal, ranging
from 3.5F below normal at KMPO to near 1 degree below normal elsewhere.
(Monthly melted w.e. pcpn has been below normal but snowfall,
at least in our area of the I95 corridor is already well above
the monthly normal at both PHL and NYC!)

In any case calendar day average temps are modeled near normal
around 5F above Monday, around 10 above Tuesday, maybe as much
as 5 above Wednesday, near normal Thursday then probably
warming again next Thursday and Friday to at least several
degrees above normal (the EC suggests 15F above normal average
temps next Saturday-we used the cooler WPC/MEX blend).

Forecast basis: Unless otherwise noted Sunday night-Monday
night is a 50 50 blend of the 12z/16 GFS-NAM MOS, Tuesday the
12z/16 GFS MEXMOS and next Wednesday-Saturday (D4-8) is the
15z/16 WPC 12 hr elements of the max-min temps/pops and 6 hrly
td/wind/sky.

Sunday night...our grids have a 25 to 40 pct chance for a
period(s) of light snow or mixed freezing-frozen pcpn in E PA
and the n2/3rds of NJ in weak waa. There is considerable
uncertainty regarding the extent and timing of any light pcpn so
snow and ice amounts are not officially posted in storm total
grids (less than 55% chance). Also, the forecast wording is also
therefore fraught with uncertainty. Near 1/2" snow is possible
near and north of I80 and spotty trace to .03 radial freezing
rain icing is possible in the I78-I276 region. Subfreezing
ground temps will need to be considered for any possible
advisory. Otherwise mostly cloudy and milder. Light south to
southwest wind. PWAT 0.55" Confidence: average.

Monday and Monday night...Variable cloudiness or P/S and milder.
PWAT dries out slightly from nw to se to 0.45". West wind.
Confidence: average

Tuesday and Tuesday night...a very very nice mid December day-
warmup...southwest wind may gust 20-25 MPH during the afternoon,
then shifts northwest by midnight. Above normal temps by 10 to
14 degrees. A CFP during the evening may generate a few rain
showers-sprinkles far north (end as snow showers or flurries
Poconos?) but PWAT is further drying out from nw to se so
Confidence: average or below average on whether it precipitates.

Wednesday...High pressure building in from the west. P/S cooler
with a nw wind gust 20-25 mph during the morning diminishing mid
afternoon.

Thursday...Variable, mostly high clouds. Seasonable colder.
Light wind.

Saturday...conservative and much below average confidence on
what may happen. Pretty much WPC grids.

Longer term:
A quick look into Christmas week does still provide the chance
of a white Christmas with an active weather pattern that looks
to turn colder thanks to a EPO which is falling several standard
deviations below normal. The - EPO will discharge cold from AK
southeast into the United States just before Christmas. However,
the colder air can dive into the western United States first
leading to warmer conditions and a typical ridge in La-Ninas
over the Southeastern United States. Exactly how this plays out
is still outside the forecast window and a lot of possibilities
are in play. Be mindful of any weather information you see about
Christmas can still change quite a bit over the next few days.

The only additional note to Mitchell`s discussion from this
morning is a concern about warming aloft. Should the se USA
ridge build more than currently modeled... snow risks would
tend to be limited to more of a transition into mixed phase
(Ice is already in some of the long range member guidance)

&&

.AVIATION /00Z SUNDAY THROUGH THURSDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Tonight...VFR, with a ceiling mainly between 4000-8000 feet at most
terminals. Southwest winds, becoming northerly overnight at or below
5 knots as a dry cold front moves through.

Sunday...VFR overall. Some cloud bases around 2500 feet may develop
especially north and west of KPHL. Northeast winds around 5 knots or
light and variable.

Outlook...
Sunday night: Brief Sub-VFR possible with patchy light rain,
mixed precipitation possible at KABE, KTTN and KRDG. South to
southwest winds under 10 knots. Confidence: Average or below.

Monday and Monday night: Mainly VFR. Westerly sustained winds
generally under 10 knots. Confidence: Above average.

Tuesday through Wednesday: VFR. An isolated shower cannot be
ruled out Tuesday evening north. Southwest wind gusts around
15-20 knots then shifting to northwest and increasing to around
20-27 knots Wednesday morning. Confidence: Above average.

Thursday...VFR.  Light wind. Confidence: Above average.

&&

.MARINE...
The conditions continue to diminish enough early this evening,
therefore the Small Craft Advisory was expired/cancelled south of
Little Egg Inlet, NJ and Delaware Bay. North of Little Egg Inlet,
the advisory continues through 6Z Sunday mainly for offshore wind
gusts of 25-30 knots. As a cold front crosses the region overnight,
winds will veer from southwest to north-northwest, then to the north-
east on Sunday (expect southeast on DE Bay), but winds are not
expected to return to Small Craft Advisory levels late tonight
into Sunday.

Outlook...
Sunday night-Thursday...No headlines anticipated except for
Wednesday morning when a NW flow SCA of 25-30 kt wind gusts is
anticipated. Confidence: Above average.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory until 1 AM EST Sunday for ANZ450-451.

&&

$$
Synopsis...Drag
Near Term...Franck/Gorse
Short Term...Franck
Long Term...Drag
Aviation...Drag/Gorse
Marine...Drag/Franck/Gorse



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