Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 190958
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
458 AM EST Sat Jan 19 2019

.SYNOPSIS...
A ridge of high pressure will be across the area today before low
pressure approaches from the Tennessee Valley. This low will
strengthen and move across the Delaware Valley early Sunday and up
across New England Sunday night. Arctic high pressure will build in
from Canada across the Middle Atlantic for the first half of next
week. Another low and its associated fronts will arrive for
Wednesday and persist into Thursday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
No surprise, the main focus through out the forecast period is on
the storm that will be affecting our region today into Sunday.
During the day, we should be only getting the leading edge of
precip. The bulk of the precip, and in fact impacts, will come
tonight into early Sunday morning (more on that in the short term
section).

The warm air advection may begin to set up across Delmarva by this
afternoon, but most of the region won`t see an elevated warm layer
through the day. Thus, expect rain or snow through the day today.

With mostly an onshore wind through the day, and increasing clouds,
highs are expected to be in the 30s across most of the region.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/...
As mentioned above, the main impacts of the storm are expected
overnight through early Sunday morning.

What has changed: many of the models trended further NW with the
track, with the GFS trending so far NW, that it depicts almost all
rain for the vast majority of our region. That appears to be an
outlier though, so have trended close to the NAM/ECMWF and
short range guidance. However, even with these models there has
been a slight trend NW, which means more ice along the I78
corridor, and less snow. The I-95 corridor isn`t completely out
of the woods yet in terms of winter weather hazards, especially
from Mercer County northward where temperatures will be very
close to freezing for hours, but it is looking more likely that
freezing rain should stay NW of this area.

Timing: No major changes in the timing of the precipitation or the
cold front with the 00Z models. The cold front will likely move
through our area between 12 and 18Z on Sunday.

Precipitation type: The northwest shift in the storm track has some
big implications for precipitation type. It looks like the main
transition zone between rain and freezing/frozen precipitation will
be near the I78 corridor by late this evening (during the day on
Saturday, precip could start as all snow even as far SE as
Delmarva/S Jersey). Thanks to a southerly low level jet and
resulting elevated warm layer, in the transition zone, the
precipitation is likely to be a mix of freezing rain and sleet. It
no longer looks like any area within our region will see all snow
with this event. The wintry mix line could move back southeast on
Sunday with the cold front, but any precip behind the front will be
brief as dry air advection is expected. The winter storm watch
and winter weather advisory cover most of the area that is
expected to get freezing rain. However, in Mercer County and the
western suburbs of Philly, it will be very close, so will
continue watching these locations closely if the advisory may
need to be expanded another row further southeast. Additionally,
there could be some areas that reach advisory criteria with the
post frontal precipitation, but as mentioned above it should be
brief, so have held off on expanding the advisory to include
these areas as well.

Ice and snow amounts: With the increasing ice threat, ice amounts
increased slightly, while snow amounts were mostly decreased.

Hydrology: System still looks to deliver 1-2+ inches of QPF with
models in overall good agreement on this. For areas near as well as
S/E of I-95 that should see a change all rain, the rain could be
heavy enough to lead to urban/small stream flooding issues Saturday
night into early Sunday. However, still don`t have enough confidence
for a flood watch.

Winds: Late tonight into tomorrow morning, a southerly low level
jet is expected to develop over Delmarva, S Jersey, and far
southeastern PA. This will lead to some very strong winds just a
few thousand feet above the surface. It is pretty hard to have
efficient mixing of southerly winds this time of year, but with
ongoing heavy rain, it isn`t completely out of the question. The
bigger concern is winds behind the cold front. At this point, it
looks like the northwesterly winds should stay just below wind
advisory criteria, even at the coast, but it will be something
to watch closely on Sunday.

&&

.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
The beginning of the long term will feature deep winter arctic cold,
but dry conditions. Temperatures will continue the fall (from
Sunday) and drop into the single digits and low teens across most
areas. Mercury readings across the southern Poconos may reach as low
as -5 by dawn Monday. As bad as the cold is, wind chills will be
brutal with sub-zero readings in most areas and readings around -20
across the Poconos. Proper precautions against the cold will be
required to be safely outdoors Monday. Dry weather is expected
Monday and into Tuesday as high pressure builds in. Readings Tuesday
morning will also be very cold, but temperatures will moderate by
days end back into the low 30s S/E and low/mid 20s N/W.

The midweek period looks unsettled as of now since another H5 trough
will be advancing from the Midwest and low pressure is expected to
form along it. Another wet/white storm is envisioned presently with
mostly snow N/W and more rain S/E. Plenty of differences with the
operational models presently, so we will just mention chc pops mostly,
with some likely pops across Delmarva Wed night. There is some potential
for this to become another significant system, so watch the forecasts
during the upcoming week and plan accordingly. More arctic cold
looks to follow it for next weekend.

&&

.AVIATION /10Z SATURDAY THROUGH WEDNESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Today...Conditions will be starting VFR, but around 18Z,
precipitation and lower clouds will be moving in from the west,
resulting in deteriorating conditions to MVFR and eventually IFR.
The precipitation could begin as all snow or a rain snow mix at all
the TAF sites. Low confidence on the timing.

Tonight...IFR or lower conditions are expected overnight. Freezing
and frozen precipitation is likely to continue at KRDG and KABE.
However, at the other TAF sites, precipitation will likely change
over to rain by early evening, if not sooner. Winds will be easterly
or southerly through much of the night at 10 to 15 kt. However, a
cold front approaching late tonight, will bring an abrupt shift to
northwesterly winds, though it may only reach KABE and KRDG before
12Z. Ahead of this front, low level wind shear is likely to develop
after 06Z. Low confidence on timing of flight category and
precipitation type changes.

Sunday...The cold front should sweep through the region by 18Z,
bringing a quick end to the precipitation (although a changeover
back to snow is possible for a very brief period) and a return to
VFR conditions. Northwesterly winds may gust up to 35 kt behind the
front. High confidence in the overall pattern, but low confidence in
the timing of flight category changes.

Outlook...
Sunday night thru Tuesday...VFR expected. Very cold. Gusty winds
Monday/Monday evening.

Tuesday...VFR. Very cold.

Tue night thru Wednesday...Increasing clouds. Sub-VFR possible
Rains S/E and Snows N/W.

&&

.MARINE...
Southerly and southeasterly winds will be on the increase especially
overnight. Small craft advisory conditions may develop late tonight.
A cold front during the morning hours on Sunday will bring an abrupt
shift to northwesterly winds and gale force wind gusts.


Outlook...
Sunday night thru Monday night...Gales are expected across the ocean waters
along with bitter cold temperatures and freezing spray.

Tuesday...Winds and seas will diminish with SCA conditions early. Fair.

Tuesday night thru Thursday...SCA conditions will develop again
with the possibility for Gales if the storm deepens enough. Rain.

&&

.TIDES/COASTAL FLOODING...
Astronomical tides will be high over the weekend due to the full
moon. It will take a positive tidal departures of +0.7 to +0.9 to
begin causing minor flooding.

An onshore flow is expected to develop today, causing tidal
departures to increase at that time. While the wind is
anticipated to become southerly tonight, lowering atmospheric
pressure will help to keep water levels from decreasing quickly.
As a result, there will likely be widespread minor coastal
flooding with Sunday morning`s high tide along the oceanfront
and along parts of Delaware Bay. Spotty moderate flooding is
possible.

It should be a one tide cycle event. Water levels are expected to
decrease sufficiently by Sunday evening`s high tide to prevent
another round of coastal flooding.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Winter Weather Advisory from 1 PM this afternoon to 1 PM EST
     Sunday for PAZ101-103-105.
     Winter Storm Warning from 1 PM this afternoon to 1 PM EST
     Sunday for PAZ054-055-060>062.
NJ...Winter Storm Warning from 1 PM this afternoon to 4 PM EST
     Sunday for NJZ001-007-008.
     Winter Weather Advisory from 1 PM this afternoon to 1 PM EST
     Sunday for NJZ009-010-012.
DE...None.
MD...None.
MARINE...Gale Warning from 8 AM Sunday to 6 AM EST Monday for ANZ430-
     431-450>455.

&&

$$
Synopsis...O`Hara
Near Term...Johnson
Short Term...Johnson
Long Term...O`Hara
Aviation...Johnson/O`Hara
Marine...Johnson/O`Hara
Tides/Coastal Flooding...


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