Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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682
FXUS61 KPHI 181440
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
940 AM EST Mon Dec 18 2017

.SYNOPSIS...
A warm frontal boundary will continue to lift north of the region
today with high pressure building into the southeast United States.
A cold front will then move through the region late Tuesday into
Wednesday. High pressure will then build southeastward into the
region for the middle of the week. On Friday, an area of low
pressure will track northeast into the Great Lakes bringing a warm
front through our region. This will be followed by a cold frontal
passage at the end of the week or early next week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Patchy fog developed across portions of the CWA around sunrise,
but is starting to wane as the sun gets higher in the sky.
Patchy fog has been added to the weather grids for the next few
hours mainly north of the I-94/195 corridors.
Otherwise...improving sky conditions are still expected this
afternoon as the warm front continues to lift north, with highs
in the upper 30s in the far north and mid 50s in the far south.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM TUESDAY/...
I am expecting this period to be dry. A few sprinkles are possible
ahead of a surface trough that digs southeast into the Great Lakes
region, but through the overnight hours, the latest models keep this
trough well to our west. There will be hardly any directional shear,
as flow from the boundary layer through the upper levels will be
westerly.

Forecast lows are slightly below GFS MOS guidance given the light
winds which should promote radiational cooling. However, if low
clouds build in, as one model is depicting, lows may be several
degrees higher. This appears unlikely given the off shore flow.

&&

.LONG TERM /TUESDAY THROUGH SUNDAY/...
Tuesday through Tuesday night:

Our region will be between a high pressure off the southeast
coastline and an approaching cold front from the west. This will
result in southerly winds and warming temperatures. The signal for
precipitation on both the operational runs and ensembles continues
to be fairly limited and confined to locations north and west of
our CWA. Will maintain a precipitation free period. It looks more
like the increase in moisture may just lead to more in the way of
low stratus early Tuesday.

Used a MAV/ECMWF blend for temperatures, the MET guidance looked too
cool because of overdone cloud cover, maybe snow cover mentioned
in short term? Right now the temperature forecast highs on
Tuesday well into the 50`s. Nighttime temperatures Tuesday night
should fall back into the 30`s.

Wednesday through Thursday night:

After the frontal passage, we return to northwest flow with cold
air advection into the region. Also, with high pressure
building in, both days should be mostly sunny. Another period of
gusty winds is not out of the question. Bufkit analysis shows
the potential for some gusts from the west at around 20 mph
Wednesday afternoon. High temperatures both days should fall
back into the 40`s with lows in the 20`s, opted to go on the
cooler end of the ensemble guidance given the cold air advection
into the region.

Friday through Sunday night & longer term:

A warm frontal boundary lifts northward across the region as a low
pressure system tracks into the Great Lakes on Friday. Another surge
of southerly flow and increase in southerly winds will come into the
region leading toward a moderating trend with temperatures, likely
reaching well into the 50`s and 60`s by Saturday.

A cold frontal boundary will then approach the region from the west
this weekend. The approach of the front has trended slower on the
modeling over the past day or so. However, model and ensemble
solutions have been rather unstable from one run to the next on the
timing.

The speed of the this front depends on where the discharge of very
cold air from a -EPO goes, most likely into the plains or western
United States at first. A response to this would to increase high
pressure off the east coast which slows the approach of the cold
front and continues to stream warm air northward into the region.
This would result in a large temperature contrast along the frontal
boundary this weekend.

Even with a slower approach, rain chances would enter the picture
either Friday or Friday night and last through much of the weekend.
If the front moves through quicker than forecasted currently,
temperatures would be a good deal cooler with more wintry concerns
(00z GFS 12/18).

Depending on the frontal timing, a non-diurnal temperature is
possible but not included given we are quite a ways away. Modeling
is keying in on the potential for quite a bit of rainfall
potentially with this system, pops were raised to likely on Saturday
where rainfall confidence is highest. Right now, temperatures look
too warm enough to prevent mixing with snow/ice on the front
end of the system Friday afternoon and evening.

The large uncertainty with the weekend front would also carry into
Christmas as well.

&&

.AVIATION /15Z MONDAY THROUGH FRIDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Today...VSBY restrictions were introduced at RDG/ABE/TTN for the
remainder of the morning hours based on current observations. A
meandering warm front lifting north is producing patchy fog,
which is expected to dissipate the sun rises high enough in the
sky. Then VFR conditions are expected for the remainder of the
TAF period, with ceilings around or above 5000 ft AGL through
the day. Light and variable winds are expected through the day.
Below normal confidence on the dissipation of low clouds,
otherwise average confidence.

Tonight...VFR conditions are expected. One model (NAM) is
depicting low clouds developing late tonight, but this appears
very unlikely given the off shore flow. Winds will still be
light (less than 10 kt), but should settle out of the southwest
or west by early this evening. Moderate confidence.

Outlook...

Tuesday through Tuesday night:  Mainly VFR, A brief interval of
lower celings possible around sunrise. Wind gusts from the
southwest at 15 knots Tuesday afternoon, decreasing at night.
Moderate confidence.

Wednesday through Friday: VFR. West to northwest wind gusts around
20 knots Wednesday afternoon decreasing gradually into Thursday.
Winds shifting to southeast by Friday but staying near or under 10
knots. Ceilings may start lower later on Friday. High confidence.

&&

.MARINE...
Winds and seas are expected to stay below small craft advisory
conditions today and tonight. Wind direction is expected to be
variable, eventually settling out of the west this evening, when a
few gusts near or above 20 kt will be possible.


Outlook...

Southwesterly wind gusts of 20-25 knots Tuesday afternoon and
night from the southwest gradually becoming northwest on
Wednesday. For the coastal waters off of northern New Jersey,
gusts may be around 25 knots for several hours late Tuesday
night. Gusts decreasing by Wednesday night on all ocean and bay
waters. Data continues to indicate seas stay below five feet
throughout the outlook period. High confidence.

&&

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

&&

$$
Synopsis...Gaines
Near Term...Miketta
Short Term...Johnson
Long Term...Gaines
Aviation...Gaines/Johnson/Miketta
Marine...Gaines/Johnson/Miketta



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