Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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509
FXUS61 KPHI 251457
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
957 AM EST Sat Feb 25 2017

.SYNOPSIS...
A strong cold front will move across the region from late this
afternoon through early this evening. High pressure then builds
south of the region on Sunday before moving offshore on Monday.
Low pressure approaching from the west will lift a warm front
through the region on Tuesday. The aforementioned low will track
from the Great Lakes into northern New York, and then into
eastern Canada on Wednesday, and this will drag a cold front
through the region. Several weak disturbances may impact the
Northeast for the end of the work week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
930 am update: Boundary layer mixing has successfully eroded
most of the lower clouds and fog in far southeast PA and much
of inland NJ; however, patchy dense fog continues across coastal
NJ. Coastal webcams showing Seaside Heights shrouded in fog,
with at least patchy fog also visible near Atlantic City,
Belmar, Tom`s River, and Cape May. Though improvement is
expected through the morning hours, nudged temperatures downward in
coastal areas just a touch through the day as the consistent
southeast fetch will likely advect cooler air over the ocean
waters inland and may keep low clouds in place through much of
the day.

Temperatures were running warmer than forecast in areas where
clouds have eroded, but suspect the warming trend will be brief
given the increasing clouds moving in from the west. Tweaked
hourly temperature grids to account for the current trends, but
only modified max temps a degree or two in most locations, as
the expected slowing trend of the warming will likely preclude
temperatures from exceeding guidance excessively, as has been
observed several times in the past week.

Regarding the convective potential this afternoon, not much has
changed with the forecast thinking. High-resolution guidance has
been quite consistent with the timing of the main line of
convection generally from 20Z to 01Z from west to east.
Instability continues to look marginal but sufficient for
updraft maintenance as the cold front approaches the region
this afternoon. Within an environment of favorable deep-layer
shear and unidirectional wind profiles with directions mostly
parallel to the front, a line of convection will likely progress
through the area. The line may contain little lightning given
the marginal/thin CAPE profile, but with steep low-level lapse
rates underneath fast midlevel flow, downward momentum transfer
of these strong winds should occur with relative ease during the
afternoon hours. With favorable low-level helicity in vicinity
of the front, bowing structures and even an embedded rotating
storm/mesovortex cannot be ruled out.

Previous discussion below...

Another rather mild early morning across the area. Several
items to discuss today as we have more warmth ahead of a strong
cold front. Some varying amounts of low clouds with patchy fog
this morning, with much of this (at least low clouds) across
portions of eastern Pennsylvania and northern New Jersey due to
more of an upslope component of the low-level flow. Some thicker
fog/stratus has been noted along portions of New Jersey coast
and near the far upper portion of Delaware Bay near Wilmington.
Some guidance keeps low clouds across much of the area all day,
however the thinking is that enough mixing and boundary layer
heating occurs to erode this by early afternoon. Some fog was
kept from northern Delaware eastward to the New Jersey coast.
We now switch to the convective potential with an incoming
strong cold front this afternoon.

A strong upper-level trough will continue to move from the
Midwest to across the Great Lakes region today. This trough is
forecast to start taking on a negative tilt as it reaches the
eastern Great Lakes later today. This will drive strong height
falls eastward this afternoon along with a strengthening wind
field. The forecast soundings indicate deep unidirectional flow
with some veering below about 900 mb. There should be enough
instability to produce some thunder this afternoon, however
strong forcing along the leading edge of the height falls along
with robust short wave energy should result in a convective
line. This line will probably be more low-topped, however
enough instability is expected inland to produce some lightning.

Given the deep unidirectional flow contributing to
strengthening shear, some bowing segments are possible within
the line. Momentum transfer of stronger winds from aloft down to
the surface can result in locally damaging gusts, especially
with any bowing segments along with stronger cores due to water
loading of the downdrafts. While some smaller hail cannot be
ruled out, instability should be limited as soundings show more
of a thin CAPE profile. We will continue to carry gusty winds in
the forecast with thunder (may need to add damaging winds
later), with the convective line generally moving from west to
east across the area from about 20-00z. The intensity of the
convection should lessen as it reaches the coast due to the
cooler ocean influence.

Southerly flow ahead of the cold front will result in another
very warm day, with several areas already starting rather warm.
Highs will be able to over achieve expectations if more clearing
occurs ahead of the cold front during peak heating. High
temperatures were a MOS/continuity blend but then raised some
away from the coast given a warmer start.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/...
A negative tilt upper-level trough will be lifting across New
England tonight, with low pressure tracking across eastern
Canada. This will push a strong cold front offshore early in the
evening with and showers and thunder quickly coming to an end.
Strong cold air advection occurs immediately behind the cold
front through about the first half of the night. This combined
with stronger flow should create a gusty west to northwest for a
time. As the colder air advects into the area, it will also be
drying out however some forecast soundings indicate that as a
subsidence inversion develops, there will be some lingering
moisture trapped for a time. This may result in bands or areas
of stratocumulus for awhile especially across the northern
zones.

It will be noticeably colder tonight with an added wind chill
to the mix, reminding us that it is still winter. Low
temperatures were mostly a MOS/continuity blend.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
Surface high pressure settles over the Southeast U.S. on Sunday
as an upper trough builds across the Northeast U.S. Conditions
will be dry, and temperatures return to normal throughout the
forecast area, with highs topping off in the 30s in the Poconos,
and generally in the low to mid 40s for much of SE PA and NJ.
Highs climb into the upper 40s for the Delmarva.

Upper flow flattens out Sunday night as both the upper trough
and the surface high move offshore. Return flow sets up on
Monday, allowing for temperatures to climb back up to well above
normal levels, generally topping off in the 50s to low 60s.

From there, unsettled weather on tap for the rest of the new
week. Low pressure develops over the central U.S., and a warm
front will develop ahead of that low. That warm front will lift
north through the region on Tuesday, and isentropic lift
associated with that front will touch off some rain across the
area.

Low pressure over the central U.S. will move through the Great
Lakes and Ohio Valley on Wednesday, and deep SW flow ahead of
that low will usher an unseasonably warm and humid airmass into
the region. Highs may be around 70 for most of SE PA, SW NJ, and
the Delmarva. As that low tracks to the north and east, it will
drag a cold front to the east, and that front will pass through
the region on Thursday.

The best chances for precip across the region will be on
Wednesday afternoon as one area of low pressure passes well
north and east of the area, and then again on Thursday as a
second low passes north and east of the area, but this low will
drag a cold front through the region.

Temps cool off a bit on Thursday, with highs generally in the
40s and 50s, but then temps return to normal on Friday as much
cooler air spreads throughout the Northeast. Will carry slight
chance PoPs for Friday as several shortwaves pass through the
region. Some snow/rain possible in the Poconos, but temps will
be warm enough for all rain for the rest of the CWA.

&&

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

930 am update: Low clouds/fog generally dissipating early this
morning near/southeast of I-95, but could still see instances
of sub-VFR conditions at KILG and KACY for the next couple of
hours. General MVFR conditions expected to continue at KRDG and
KABE through the morning hours. Winds may become somewhat
stronger from the south through this afternoon, with occasional
gusts to 20 kts. A line of showers and embedded thunderstorms
is expected to arrive this afternoon from west to east after
18z. As this line moves through, west-northwest wind gusts to
around 35 knots are anticipated; however, some locally higher
gusts are possible. Brief heavy rain will lower the visibility
as well. A TEMPO group was used to highlight the thunder and
gusty winds.

Tonight...A line of showers and embedded thunderstorms quickly
shift east and off the coast early in the evening, with
conditions improving to VFR. West to northwest winds 10-15 knots
with gusts 20-30 knots at times.

Outlook...
Sunday...VFR. W-NW winds around 15 KT with 20-25 KT gusts.

Sunday night...VFR. Winds become SW 5 KT or less.

Monday...VFR. SW winds 5-10 KT.

Monday night through Wednesday...Periods of rain and occasional
showers with sub-VFR conditions. Best chances during the day
Tuesday, and then again on Wednesday. S-SW winds, generally
around 10 KT. Low confidence on forecast and timing.

&&

.MARINE...
930 am update: Webcams continue to show dense fog over the
coastal waters of New Jersey. Expanded the dense fog advisory to
cover all marine zones adjacent to the coast through noon.
Otherwise, forecast is in good shape.

Previous discussion...

A narrow area of stratus and/or fog continues right near the
coast of New Jersey with this mostly confined from portions of
Ocean County northward. Farther south, the low-level trajectory
suggests less development however some fog has been reported.
The dense fog advisory this morning remains in effect for just the
northern two Atlantic coastal waters zones. Otherwise, a
southerly flow will continue today and the increase may be
slowed due to much warmer air over cooler water. This will
increase especially later today as a strong cold front arrives,
with colder air quickly arriving tonight in its wake. It is
tonight when stronger winds are expected as mixing becomes much
more efficient. There could be some low-end gale force gusts
tonight, however it looks marginal and potentially short in
duration therefore we will maintain the Small Craft Advisory.
Seas will also respond to the increasing wind, however this was
slowed a bit for this morning.

A line of showers and embedded thunderstorms will move across
the area late this afternoon and especially early this evening.
These may weaken some as they encounter the marine influence,
however some local gusts of 30-40 knots are possible along
portions of the line.

Outlook...
Sunday...SCA conditions continue on the waters, subsiding to
sub-SCA criteria late.

Sunday night and Monday...Sub-SCA conditions likely. Above
average confidence.

Monday night through Wednesday...Generally sub-SCA expected,
though winds and seas may approach criteria after potential warm
frontal passage Tuesday night or Wednesday. Periods of
rain/showers likely, especially during the day Tuesday and again
Wednesday afternoon. Very low confidence.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Dense Fog Advisory until noon EST today for ANZ450>453.
     Small Craft Advisory until 6 PM EST Sunday for ANZ450>455.
     Small Craft Advisory from 6 PM this evening to 6 PM EST Sunday
     for ANZ430-431.

&&

$$
Synopsis...MPS
Near Term...CMS/Gorse
Short Term...Gorse
Long Term...MPS
Aviation...CMS/Gorse/MPS
Marine...CMS/Gorse/MPS



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