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000
FXUS61 KPHI 201044
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
644 AM EDT Tue Apr 20 2021

.SYNOPSIS...
As a surface low lifts northeastward through eastern Canada, a
cold front will approach the region today. Another wave of low
pressure will develop along the front in the middle Mississippi
Valley, lifting northeastward through the Northeast on
Wednesday. The cold front will sweep off the coast by Wednesday
afternoon, with a strong high building into much of the central
and eastern U.S. through the end of the week. Another system
will affect the region this weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
Objective 00z upper-air analysis indicates continued troughing
across much of North America, with fast 250-mb flow confined to
the far southern/southeastern U.S. Two midlevel vorticity maxima
were noteworthy at 500 mb: one in southeastern Canada and the
Great Lakes region and another now entering the Great Plains. An
850-mb baroclinic zone was oriented northeast-to-southwest from
southeastern Ontario to the central Rockies, with strong cold
advection in the northern/central Plains in advance of a
developing surface high that will bring unusually cold air to
this region the next few days. This will also aid in the
development and intensification of a surface low along the
baroclinic zone today, which will be our main player for
Wednesday and Thursday.

Meanwhile, as the central U.S. storm evolution plays out today,
downstream ridge amplification will occur in the eastern U.S.
Aiding in this process will be increasing warm advection on the
upstream side of a weak high in the western Atlantic.
Considerable surface warming will occur across the region as a
result, with southwesterly flow enhancing low-level mixing.
Temperatures will soar in this pattern, with highs likely in the
70s for much of the area. Skies should be mostly sunny this
morning, but there may be some increasing cloudiness later today
as the system out west increases its influence on our region,
especially for the northwest CWA. The only issue of concern
today is fire weather; see the discussion below for more
information.

The central U.S. shortwave trough will pivot to a neutral
orientation tonight in the Midwest, as a kicker perturbation
digs into the northern/central Plains and aided by the continued
ridge amplification along the East Coast. A surface low will
intensify along the baroclinic zone stretching northeastward to
the Northeast. Southerly flow will continue overnight as a
response to the developing low, and warm-advection precipitation
should develop to our northwest, perhaps affecting areas
generally northwest of the Fall Line overnight. Was fairly
skittish with PoPs, as models tend to be too aggressive with the
southeastward extent of precipitation into the developing warm
sector. Nevertheless, think there will be some showers in the
Poconos and Lehigh Valley at least during the overnight hours.
Temperatures will be mild, with lows in the 50s for all but the
Poconos and far northwest New Jersey.

&&

.SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
Main concern remains with severe potential on Wednesday and
winds Wednesday night and Thursday.

The shortwave trough advancing into the Northeast on Wednesday
will begin to take on a negative tilt as a 500-mb jet streak
surges eastward into the central/southern Appalachians and
adjacent Mid-Atlantic. Models are converging on the timing of
the low and attendant cold front, with the GFS trending slower
and ECMWF trending faster. The timing of the front will be
critical, as its current consensus is generally in the 17z-21z
time window across the CWA. If the timing is a little faster,
sufficient instability may not develop to produce vigorous
convection for at least western portions of the area. However,
convection-allowing models seem to suggest favorable timing for
a strongly-forced squall line (or at least a broken convective
line) to develop along the surging front. Model soundings
indicate a strongly sheared environment, with deep-layer bulk
shear of 40-50 kt and 0-1 km of 150-250 J/kg. CAPE will be
modest at best (probably around or below 750 J/kg), but this has
often proved sufficient in the past for strong/damaging gusts
given similar-looking wind profiles. Given the thermodynamic
profile, think this event would be similar to 28 March in terms
of lightning. There may be very little to speak of given the
shallow nature of CAPE and its overall thin appearance through
the profile.

One caveat to this potential is extent of clouds and
precipitation in the morning, especially in the northern CWA. It
is quite possible temperatures struggle to reach the 60s in the
Poconos and far northern New Jersey. Should precipitation be
more extensive, these cooler temperatures may linger toward the
I-78 corridor as well. For this reason, have sharpened the
high-temperature gradient on Thursday, with temperatures near or
above 70 southeast of the I-95 corridor (slightly above
consensus) and in the mid 50s in the Poconos (slightly below
consensus).

Wednesday will be breezy, as well, as mixing in advance of the
front will allow for speeds of 10-20 mph and gusts to 25+ mph.
Then, in the immediate post-frontal environment, a surge of
stronger gusts may occur as winds become west or northwest.
Conditions will be breezy on Wednesday night (especially during
the evening, with gusts of 25-35+ mph possible), but winds will
likely reach a nocturnal minimum. By Thursday, speeds will pick
up again, with model soundings indicating mixing up to the
800-700 mb layer. Widespread gusts of 30-40+ mph are likely, and
an advisory may be needed if models trend more aggressively.

The other issue of concern for the Wednesday night through
Thursday night period will be the cold air seeping into the
area. Temperatures will plunge behind the front, likely reaching
near-freezing or below northwest of the urban corridor. Given
the elevated winds Wednesday night, this continues to look like
a freeze/no-freeze forecast, and confidence is not overly high
given that the winds may keep temperatures somewhat higher than
model consensus. However, we will likely need to issue freeze
products for portions of the area Wednesday night.

Highs on Thursday will likely be around 15 degrees colder than
Wednesday, and winds will diminish somewhat on Thursday night
(but probably not enough for frost development). Another night
of freeze potential, though, especially north of I-78.

&&

.LONG TERM /FRIDAY THROUGH MONDAY/...
As the low in the Northeast lifts into the Canadian Maritimes
by the end of the week, the pressure gradient across the Mid-
Atlantic will begin to relax. Though breezy northwest winds
appear likely to continue on Friday, the day should be much more
tolerable than its predecessor. This will be especially true
since midlevel ridging will approach the East Coast, so column
warming will greatly increase temperatures from the previous
day. Expect highs about 10 degrees warmer.

However, that midlevel ridging comes in response to another
strong perturbation moving through the central U.S.
Additionally, a northern-stream counterpart will aid in some
phasing into a more-amplified/stronger trough as it approaches
the East during the weekend. Deterministic output is fairly
agreeable given the setup, suggesting that a surface low will
trek along or near the coast Saturday night and Sunday, bringing
widespread precipitation to the region. Strong winds will likely
accompany the low to its north and west, making for a rather
unpleasant few hours as the heavy rain moves through. A soaking
0.5-1.5 inches QPF seems to be the consensus, which will likely
mostly fall in the beneficial range of precipitation totals.
However, we will need to watch the system given its healthy and
dynamic look.

Breezy northwest winds and cooler temperatures are expected
following the system into the beginning of next week.

&&

.AVIATION /12Z TUESDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Early this morning...Mainly VFR, but patchy/transient fog is
possible through 14z, especially at TTN/PNE/ILG. Light and
variable winds. Moderate confidence.

Rest of today...VFR with southwest winds increasing to 7 to 15
kt with higher gusts by this afternoon. High confidence.

Tonight...Mainly VFR, especially during the evening, but some
restrictions may creep into areas northwest of the Philly
terminals overnight as the potential for showers increases.
Winds generally becoming south up to 10 kt. There is a chance
for south to southwest LLWS overnight. Have not placed in the
TAFs yet, but confidence is beginning to increase in the need
for mention. Moderate confidence.

Outlook...

Wednesday...Showers and isolated storms are likely during the
late morning into the middle afternoon, as a convective line
will likely move through the region. Strong/erratic winds and
brief restrictions are possible with the passage of the line.
Otherwise, mainly VFR is expected, with south to southwest winds
10 to 15 kt with gusts to 25 kt becoming west to northwest 10 to
20 kt with gusts to 30+ kt by late afternoon into the evening.
Moderate confidence.

Wednesday night...Mainly VFR with northwest winds 10 to 20 kt
with higher gusts possible. Moderate confidence.

Thursday...Mainly VFR with northwest winds 15 to 25 kt with
gusts to 35+ kt. High confidence.

Thursday night and Friday...VFR with west to northwest winds 10
to 20 kt with higher gusts. High confidence.

Friday night and Saturday...Mainly VFR with west to southwest
winds 5 to 10 kt.

&&

.MARINE...
Sub-advisory conditions are expected through tonight. However,
southwest to south winds of 10 to 20 kt may occasionally gust
close to 25 kt at times. Seas will be increasing to 2 to 4 feet
by tonight.

Outlook...

Wednesday...Advisory conditions will likely develop as south to
southwest winds increase to 15 to 25 kt with higher gusts. Seas
nearing 5 feet. A small craft advisory is in effect beginning at
noon. Storms will likely move through during the afternoon, with
gusty/erratic winds and higher waves in their proximity.

Wednesday night and Thursday...Winds becoming west or northwest
and increasing to 20 to 30 kt with gusts to 40 kt. A gale watch
is in effect.

Thursday night...Lingering gales possible in the evening;
otherwise, mainly advisory conditions expected.

Friday...Lingering advisory conditions expected, though winds
and seas should be subsiding during this time.

Friday night and Saturday...Sub-advisory conditions expected.

&&

.FIRE WEATHER...
Winds will increase out of the southwest today, becoming around
10 to 15 mph with occasional gusts to 20 mph or so. Though dew
points are expected to be increasing with time, the strong
heating and increasing winds may act to mix out the higher
moisture-content of the air. Minimum relative humidity values
may approach or even fall below 30 percent this afternoon,
especially for areas southeast of Interstate 95. We will
coordinate with fire-weather partners later this morning to
determine if a special weather statement will be required for
portions of the area today.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Gale Watch from Wednesday evening through Thursday afternoon
     for ANZ430-431-450>455.
     Small Craft Advisory from noon to 6 PM EDT Wednesday for
     ANZ430-431-450>455.

&&

$$
Synopsis...CMS
Near Term...CMS
Short Term...CMS
Long Term...CMS
Aviation...CMS
Marine...CMS
Fire Weather...CMS