Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 212314
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
714 PM EDT Tue Mar 21 2017

.SYNOPSIS...
A cold front will slide through the area tonight. A large area of
high pressure then builds in from the northwest and stays over our
region through Thursday before shifting off shore. A warm front is
forecast to lift through our region on Friday. Low pressure is
expected to approach from the west over the weekend.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
630 pm update: Next in the series of northwest-flow systems is
set to move through the Northeast during the next 24 hours with
a weak predecessor perturbation ejecting eastward through the
Ohio Valley into the Mid-Atlantic region by late this evening.
Associated large-scale lift should generally remain south of the
CWA, but high-resolution guidance is suggesting the potential
for a sprinkle or two in southern Delmarva around or after
midnight. Not nearly enough confidence at this point to raise
PoPs, nor would any precip amounts justify raising them anyway.
Main effect in our area is a noticeable increase in clouds this
evening, especially south of I-76. This will likely have some
impact on the temperature forecast, lowering confidence in
projections this evening somewhat. The main vort max digs
through the Great Lakes overnight into the Northeast on
Wednesday, with a strong cold front sweeping through the region
late tonight. Winds will pick up noticeably after passage, and
that will be the cue marking the end of our brief taste of
spring early this week.

Made some minor changes to the hourly temperature/dew point
forecast through the evening hours based on latest trends.
Otherwise, going forecast is in good shape.

Previous discussion...

Mostly quiet conditions much of tonight before a strong cold front
crosses the area. The front should cross the northern areas 3 - 5
a.m. and reach the southern parts of the forecast areas 5 - 7 a.m.
No precipitation and little in the way of cloudiness is expected.
Temperatures will drop into the 20s up across the north and dip into
the low/mid 30s over south NJ and se PA. The front will not reach
Delmarva before morning, so readings there will only drop into the
mid/upper 30s. Winds will be light before the front and then gust to
20-30 mph from the NW behind it.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/...
Strong high pressure will build towards the area Wednesday. A strong
and gusty NW wind with dry air will be across the area through the
day. Skies will be clear and temperatures will be below normal.
Highs will only be in the low/mid 40s over Delmarva and the 30s
elsewhere. Wind chill values will will only be in the 20s in many
areas for the afternoon. Winds will gust 30-40 mph at times.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/...
The long term will start off tranquil, but get complicated
quickly!

Thursday...With the center of the high passing almost directly
over the region, Thursday should be a quiet and tranquil day
albeit colder than normal. Temperatures will generally be 10 to
20 degrees below normal, with some record lows possible Thursday
morning (see climate section below).

Thursday night into Friday...This is where the forecast starts
getting complicated. A warm front is expected to lift through
the region during the day on Friday (a bit slower than what most
models were depicting yesterday). At the same time, a broad
upper level ridge will be approaching from the west. So we could
potentially have mesoscale lift (associated with the front) in
the lower levels, while some subsidence is possible in the upper
levels. At this point, do not expect that as the ridge appears
to be trailing far enough behind the front that the two will not
be juxtaposed. Further complicating matters is the fact that
the boundary layer will be quite dry leading into this event,
with dew points likely starting in the teens and lower 20s
across much of the region early Friday morning. If we see enough
lift out ahead of the front, we could see a brief period of
freezing rain (and possibly some sleet) generally north and west
of the I95 corridor. Having said that, there have been a few
events this winter where models were too fast with warm air
advection across the Poconos and NW NJ, so they have the highest
risk for multiple hours of freezing/frozen precipitation, but
still considerable uncertainty.

Saturday...Warm air advection continues. The one possible
negating factor could be persistent cloud cover through the day.
However, will continue to go on the high side of guidance as
most guidance has had a consistent cold bias with southerly flow
patterns.

Saturday night into Monday...Good agreement that a low pressure
system will be sliding northeast out of the southern Plains into
the Great Lakes region through this time. That is about the
extent of what I have high confidence in. Less confidence,
though there appears to be better model agreement today, that
the low will be weakening/filling as it propagates northeast.
What I have very low confidence in, a front is expected to slide
south as a cold front Saturday night, then stall, and lift north
as a warm front Sunday night. How far south this front will get
before stalling could have a big impact on temperatures
(especially with the highs on Sunday) and precip type Sunday
night into Monday morning. At this point, most model solutions
show the front stalling somewhere between Philly and the
Delaware Water Gap. For locations that stay south of the front
through the entire event, expect another very warm day on
Sunday, and all rain. Locations north of the front can expect
temperatures as much as 15 degrees below normal on Sunday, and
in the higher terrain, a wintry mix is possible Sunday night
into Monday morning. For now have stayed close to a mean of
model solutions given the high uncertainty.

Tuesday...Another low may develop along what is left of the
previously mentioned front. Models depict little upper level
support and it should fill and weaken as well as it propagates
east.

&&

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

For the 00Z TAFs...VFR through the period. BKN-OVC CIGs will
develop/lower to around 10 kft this evening through the early
overnight before a strong cold front pushes through the region
between 08Z and 12Z, after which skies will rapidly clear.
Winds will sharply increase and begin gusting after frontal
passage, with directions ranging from 300 to 350 most of the day
tomorrow. Speeds of 15-25 kts with gusts to 35 kts are possible
after frontal passage through late afternoon Wednesday.

Outlook...
Thursday...Mostly VFR conditions are expected.

Friday...MVFR or lower conditions are possible with
precipitation generally along and northwest of the Delaware
Valley (including PHL, ILG, PNE, TTN, ABE, and RDG). For ABE,
RDG, and TTN, there may be a brief period of a wintry mix Friday
morning.

Friday night through Saturday...mostly VFR conditions are
expected.

Saturday night and Sunday...MVFR or lower conditions are
possible with rain across the region.

&&

.MARINE...
Nice conditions on the waters this afternoon will continue through
much of the overnight. A sharp cold front will cross the waters well
after midnight and cause winds to shift to NW and increase. Gale
warnings are in effect for the period after this frontal passage as
winds will probably gust 35 to 40 knots at times. We expanded the
Gale to cover Delaware Bay and the Delaware coastal waters. Cold air
and decent flow aloft should have no problem mixing down. No
precipitation is expected tonight or Wednesday.

Outlook...

Wednesday night...winds and seas will diminish through the
overnight hours.

Thursday...Once wind gusts drop below 25 kt early in the day,
sub-SCA conditions are expected.

Friday...winds may approach 25 kt leading to SCA conditions

Saturday...sub-SCA conditions expected.

Sunday...there is a chance wind gusts could build above 25 kt
late in the day.

&&

.FIRE WEATHER...
Relative humidity levels will drop behind the front Wednesday. Gusty
winds will cause near Red Flag conditions during the afternoon.
After calling the fire weather partners, it seems that soil moisture
is still rather high in many areas. The Delmarva is the only
exception where conditions may be rather marginal for RFW. We will
probably issue an SPS overnight to highlight these dry windy
conditions.

&&

.CLIMATE...
More record events possible this week.

One or 2 of the following are vulnerable record equaling lows
Thursday morning March 23, primarily ABE. Others are listed for
reference

ACY-13 1875

PHL-14 1885, 1875

ILG-14 1934

ABE-13 1934

TTN-13 1934

GED-16 1959

RDG-14 1906

MPO- minus 8 1912

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Gale Warning from 6 AM to 11 PM EDT Wednesday for ANZ430-431-
     450>455.

&&

$$
Synopsis...Johnson
Near Term...CMS/O`Hara
Short Term...O`Hara
Long Term...Johnson
Aviation...CMS/Johnson/O`Hara
Marine...Johnson/O`Hara
Fire Weather...Johnson/O`Hara
Climate...Drag



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