Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 230230
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
930 PM EST Mon Jan 22 2018

.SYNOPSIS...
Low pressure will track northeastward through tonight and reach the
Saint Lawrence River Valley and northern New England late Tuesday.
An associated strong cold front will move through our region Tuesday
afternoon. High pressure is expected to build into our region during
the balance of the week, before moving out to sea Saturday. A cold
front then arrives on Sunday.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
For the 930 PM update, had to make some significant changes to
the hourly temperatures given trends thus far. Much less
(thicker) cloud cover across much of the area along with light
winds has allowed temperatures for many areas to drop even more.
Low temperatures were also lowered for several areas. The warm
front looks to be draped still across the northern areas with a
light east to northeast wind now occurring along and north of
it. Some stratus is also starting to develop across the far
northern areas along with patchy fog. Once the clouds fill in
and the flow starts to increase, temperatures should tend to
level off or even rise some overnight.

A potent closed low will move into the Great Lakes and Ohio Valley
late tonight. This will drive surface low pressure into the Great
Lakes region by daybreak. A surface warm front sitting over the
northern half of our area this evening, should shift northward
gradually through the night as a strengthening low-level jet
arrives. The arrival of the low-level jet late tonight should allow
for some showers to begin developing, however the more pronounced
showers look to arrive much closer to daybreak across the
western areas. The PoPs were updated to show a bit slower
increase from west to east.

As some higher surface dew points start to advect in overnight
over the colder waters, some marine fog may develop and spread
into the coastal areas. This however could be limited by
stronger flow developing especially just above the surface
across the southern areas. It appears that any thunder chances
are mostly after daybreak, therefore no mention of thunder for
tonight.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH 6 PM TUESDAY/...
Low pressure will track from Michigan across Ontario into Quebec by
late in the day Tuesday. This will drag a strong cold front across
our area with numerous showers. Showers are expected to quickly
overspread the area in the morning and persist into the afternoon.

The forecast models, especially the NAM, are depicting enough
elevated CAPE for at least a few thunderstorms. Also, worth noting,
the mid level lapse rates will be fairly steep (near 7 C/km) with a
very strong low-level jet ahead of the cold front (around 60 knots
at 925 mb). If convective elements are able to become surface-based,
there is more of a concern for locally damaging winds mixing down to
the surface. This is especially for southeast PA, southern NJ, and
Delmarva where the instability will be the greatest. The Storm
Prediction Center has these areas in a marginal risk for severe
storms. The limiting factor will be that the low levels will be
fairly stable so confidence of severe weather is not high. The most
likely window for this to occur looks to be mid morning through
early afternoon after which time the cold front will clear the area
with showers ending. Also, PWATs will be up to 1.25 inches so
locally heavy rain will occur which can lead to local poor drainage
flooding.

Highs Tuesday range from near 50 across the southern Poconos to the
upper 50s to mid/upper 60s across SE PA, southern NJ, and
Delmarva.

&&

.LONG TERM /TUESDAY NIGHT THROUGH MONDAY/...
Surface low pressure is forecast to be located in Maine on Tuesday
evening. The system is expected to strengthen as it moves across
Atlantic Canada on Tuesday night and Wednesday. The gradient between
the departing low and high pressure building into our region from
the west should result in brisk conditions developing for Wednesday.
A west northwest wind around 10 to 20 MPH is anticipated at that
time.

Surface high pressure should move from the south central states on
Wednesday to the Carolinas and Virginia late on Thursday. The center
of the high is anticipated to slide over the western North Atlantic
on Friday and Saturday. The air mass is forecast to bring dry
weather conditions for the period from Wednesday into Saturday.
Temperatures are expected to be seasonable on Wednesday and Thursday
with a warming trend as we head toward and into the weekend.

Low pressure and a cold front are forecast to approach from the west
over the weekend. There is a spread in the guidance solutions
regarding the timing of the arrival of the cold front in our region.
We will carry a chance of showers for Sunday and Monday.

&&

.AVIATION /03Z TUESDAY THROUGH SATURDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Tonight...VFR conditions for awhile, then ceilings lower to MVFR
mainly after 08z. Some showers are expected to develop mainly after
about 08z. A southwesterly low-level jet at 2,000 feet increases
late tonight, leading to low-level wind shear. Local MVFR
visibilities are possible due to fog. Winds are light this
evening with varying directions, however they should become
southerly 5-10 knots overnight especially from PHL south and
east.

Tuesday...A period of IFR conditions as showers and possibly a few
embedded thunderstorms move through, ending from west to east in the
afternoon. Downpours will locally reduce the visibility more.
Southwesterly low-level jet at 2,000 feet of 40-50 knots will
diminish from west to east in the afternoon. Southerly surface winds
10-20 knots with gusts up to 30 knots (highest near and south/east
of PHL), become west-northeast later in the afternoon. Some stronger
wind gusts are possible with heavier showers or a thunderstorm.
Since the threat of thunder is more isolated, it is not included in
the TAFs.

Outlook...
Tuesday night through Saturday...Mainly VFR.

&&

.MARINE...
For tonight, south winds should begin to increase, especially
after midnight, with speeds reaching small-craft advisory criteria a
couple of hours before daybreak. Also, areas of fog may develop over
the waters overnight with showers arriving west to east near
daybreak.

Tuesday...Strong Small Craft Advisory conditions over the waters
with a few gusts to Gale force possible around midday, especially in
any heavier showers/thunder. Since this looks to be brief, we opted
for SCA, not Gale Warning. Marine Weather Statements and possibly a
Special Marine Warning may be needed for these locally stronger
winds. SCA conditions expected for DE Bay Tuesday. South winds in
the morning shift to the west in the afternoon behind the passage of
a cold front.

Outlook...
Tuesday night...A Small Craft Advisory is in effect on our ocean waters
for westerly wind gusts around 25 knots.

Wednesday through Thursday...A west northwest wind could gust around
25 knots and the Small Craft Advisory may be extended in time.

Thursday night through Saturday...No marine headlines are
anticipated.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory from 4 AM Tuesday to 6 AM EST Wednesday
     for ANZ450>455.
     Small Craft Advisory from 4 AM to 6 PM EST Tuesday for ANZ430-
     431.

&&

$$
Synopsis...Gorse/Iovino
Near Term...Fitzsimmons/Gorse
Short Term...Fitzsimmons/Gorse
Long Term...Iovino
Aviation...Gorse/Iovino
Marine...Fitzsimmons/Iovino


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