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000
FXUS61 KPHI 220056
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
856 PM EDT Sat Apr 21 2018

.SYNOPSIS...
Large area of high pressure over the northeastern United States
will continue to slowly build east, building off the New
England coast by Monday. Meanwhile, a low in the Arklatx region
will lift into the Mid Atlantic states by mid to late week.
Another area of low pressure may come in right behind it by next
weekend.

&&

.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
A 1030 hPa high pressure center remains just east of the Virginia
Capes. Further west, a lee trough is situated across the coastal
plain. This trough has resulted in a southerly component of wind
in the coastal plain with a northerly component to the northwest,
leading to a dew point gradient across the region (higher south
and east). Meanwhile, the right entrance region of a 130 kt 250
hPa jet is situated over the Tri-state area, with cirrus clouds
streaming across the region. The upper level water vapor imagery
from GOES 16 indicates the cirrus is thickest from the Lehigh
Valley into northern NJ. Have adjusted the sky grids to account
for this.

Despite winds gradually subsiding as the boundary layer decouples,
and the very dry air in place, the thicker cirrus should inhibit
otherwise ideal longwave cooling conditions. Temperatures have
been running on the cooler side of the guidance this evening, and
have been adjusted slightly upward. As the right entrance region
of the upper level jet slowly moves south of our area overnight,
expect skies to clear and temperatures to drop at a faster rate.
With minimum shelter temperatures forecast in the mid to upper 30s
generally south of the I-95 to PA Turnpike corridors, this should
result in patchy frost at most.

Impacts to the growing season across the aforementioned area are
expected to be minimal, therefore no frost headlines are in place.

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH 6 PM SUNDAY/...
As an upper-level trough slides across the Northeast and northern
Mid-Atlantic regions during Sunday, its axis along with short wave
energy crosses our area during the afternoon. As this occurs, a 250
mb jet streak will shift to our east and the higher level cloudiness
should settle south and east with it. Meanwhile, a closed low slides
across the Gulf Coast states which is tracking under a ridge to its
north. This ridge will keep surface high pressure anchored over the
Mid-Atlantic and Northeast through the day Sunday.

Another dry day is expected and with a good amount of sunshine
resulting in ample boundary layer warming, temperatures are forecast
to be at least a few degrees warmer than what was observed today
(Saturday). The flow is on the lighter side especially in the
boundary layer, therefore a light wind combined with warming on the
land (increasing thermal gradient between land and water) is
expected to result in the development of an afternoon sea/bay
breeze. This will turn the winds for the coastal areas to south and
southeast and provide some afternoon cooling. High temperatures are
mainly a blend of MOS and continuity, although the gradient was
tightened up some along the coast due to the expected cooling sea
breeze.

&&

.LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/...
The long term period begins with tranquil period as the dominant
feature will be the large area of high pressure that continues
to slowly build east across the northeastern U.S.. This high
should build off shore by Monday, allowing the low level flow to
become onshore, resulting in increasing moisture advection ahead
of the next low.

The next low currently over TX, will very slowly progress east
and then northeast, reaching the Mid Atlantic by mid week. What
is remarkable about this low is that while I wouldn`t consider
it a true cutoff low, it will be making very slow progress east
through the week as compared to systems along the northern
stream. As mentioned by the previous shift, the main period of
rain with this system looks to be Tuesday night into Wednesday,
coincident with stronger lift associated with the warm front.
Due to the slow progression of the low and low level on shore
flow, there is some concern of heavy rain, primarily late
Tuesday night into Wednesday morning. However, there are a few
factors working against the heavy rain threat. First, the cloud
layer looks to be rather shallow as southwesterly flow in the
mid an upper levels may keep drier air at these levels.
Secondly, the warm cloud layer is relatively shallow (generally
8000 to 9000 ft), thanks to continued below normal temperatures.
Finally, depending on the track of the surface low, some models
are depicting a dry slot developing over our area by Wednesday
morning. Not sure if this is the start of a trend, or a
temporary change, but something we will be watching over the
next few days.

Once that low lifts northeast away from our region by Thursday,
we will likely have brief reprieve of weak high pressure, before
a mid and upper level open wave trough (this time from the
northern stream) approaches our region next weekend. The
associated cold front could slide through our region as early as
Saturday, though the trough may be in the process of becoming
negatively tilted which could also slow the progress of the
surface low and associated front.

Temperatures during this time will generally be near or below
normal levels for most of the week.

&&

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

Overnight...VFR. Clouds above 10,000 feet. Light and variable
winds. Forecast confidence: High.

Sunday...VFR with some lingering high clouds. Light and variable
winds, becoming west to southwest less than 10 knots. Winds are
anticipated to become southeast at ACY during the afternoon.
Forecast confidence: High.

Outlook...

Sunday night through Monday night...Mostly VFR conditions are
expected. There is a small chance of visibility reduced in fog at
KMIV and KACY on Monday morning. Winds will be light and variable
for much of the period. High confidence on most aspects of the
forecast except the development of fog, which is low confidence at
this time.

Tuesday through Thursday...MVFR and even localized IFR conditions
will be possible especially on Tuesday night and Wednesday as
periods of rain are likely across the region. Easterly wind gusts up
to 20 kt are possible Tuesday night into Wednesday. Otherwise,
generally light winds (less than 10 kt) are expected. Medium
confidence on flight categories, but low confidence on the timing of
any flight category changes.

&&

.MARINE...
The conditions are expected to remain below Small Craft Advisory
criteria through Sunday. High pressure over the area will result in
lighter winds, which will be southerly into tonight overall but tend
to turn northerly during the night. The lighter flow on Sunday along
with plenty of warming on land will result in the surface winds
turning from the south or southeast mostly in the afternoon. Some
increase in the winds is forecast in the afternoon as the wind shift
occurs.

Outlook...

Sunday night through Monday night...Winds and seas are expected to
remain below SCA criteria.

Tuesday and Wednesday...Easterly winds will increase resulting also
in an increase of wave heights late Tuesday through the day
Wednesday. Currently forecasting SCA conditions only on the Atlantic
coastal waters. However, we will continue to monitor model trends on
the Delaware Bay as models tend to underestimate ESE winds
especially on the lower bay.

Wednesday night and Thursday...Winds shift to northwesterly and
subsequently decrease. The main question will be how long seas
remain above 5 feet on the coastal waters. Current forecast is for
elevated seas into late Thursday, but there is still considerable
uncertainty with this.

&&

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

&&

$$
Synopsis...Johnson
Near Term...LF
Short Term...Gorse
Long Term...Johnson
Aviation...Gorse/Johnson/LF
Marine...Gorse/Johnson