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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KPHI 190236
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
936 PM EST Fri Jan 18 2019

.SYNOPSIS...
Low pressure will continue moving away from the coast tonight and
high pressure builds toward the north across southeastern Canada
yielding a brief period of quiet and dry weather. An expansive area
of low pressure will approach the region from the west on Saturday
and pass over our area overnight Saturday night and early Sunday
morning bringing the first significant winter storm to the region.
Strong Arctic high pressure will build quickly into the Mid-Atlantic
through the early part of next week. Another area of low pressure
will move across the Great Lakes and into southeastern Canada late
Tuesday into Wednesday pushing a cold front across the region
Wednesday.

&&

.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
The tail end of a cold front moves through this evening
followed by ridging from high pressure to our north. We can
expect a period of at least partial clearing this evening.

However, with skies partially clearing, winds dropping off below
5 mph, and plenty of low level moisture around, patchy fog will
be prevalent across the area tonight.

These partly cloudy skies early will degrade to cloudy by
morning as the next system approaches. Lows will be mainly in
the 20s with upper teens across the southern Poconos and low 30s
across southern portions of the Delmarva.

&&

.SHORT TERM /6 AM SATURDAY MORNING THROUGH SUNDAY NIGHT/...
Still expecting a significant storm to affect our region Saturday
into Sunday. Confidence is increasing our northern zones will reach
Winter Storm Warning criteria so we have upgraded to a Winter Storm
Warning as far south as a line from Berks County PA to Sussex/Warren
Counties in NJ. Farther south towards the I-95 corridor confidence
is lower so no headlines yet but we have trended the forecast a bit
colder based on arctic high to the north which should help low level
cold hang on a bit even here.


Here are the details:

Timing: No major changes in timing with 12z Models. Most of the
precipitation should be in the period from Saturday afternoon
through Sunday morning (heaviest Saturday night), with the
precipitation quickly ending behind the system`s sharp cold front
Sunday.

Storm track: From a distance, the differences in the track of the
center of the low look rather minor between the models, but even
these small differences (generally within 25-50 miles of each other)
will have major implications for impacts within our area.
Of the major operational models, the NAM appears to be the furthest
SE, while the GFS is the farthest north. Thinking track will lean
towards the middle of this spread, close to the I-95 corridor, but
that temps north of here will lean towards the colder models (NAM)
which will have implications on P-type...more snow, sleet, and
freezing rain.

Precipitation type: This still remains the most challenging aspect
of the forecast. Even if the models were spot on with the track,
there could still be discrepancies related to cold air damming
(which may be one of the issues with the GFS being too fast to warm
up especially in the NW). Also, there will be a very sharp
temperature gradient right over the area with the I-95 corridor
right in the middle of this with temps both at the surface and aloft
expected to be close to freezing as heavy precip moves in. For these
reasons, any small changes could significantly affect P-types. If
models trend closer to the NAM, then the southern Poconos, Lehigh
Valley, and far NW NJ could see all snow, otherwise, a mix of snow,
freezing rain, and sleet is likely for these areas. It is likely
that regardless of solution, southern Delaware and coastal NJ will
see mostly rain, with possibly a brief period of a rain snow mix. In
between these two areas it where it gets tricky. For the in between
areas, the most likely scenario is that the precip will begin on
Saturday as Snow or rain/snow mix, eventually changing to heavy rain
at some point Saturday night, changing back to all snow for a brief
period before it ends. During these transitions a period of sleet
and freezing rain will be possible.

Ice and snow amounts: ice and snow amounts will be highly dependent
on the duration of the precipitation types so confidence still only
moderate at best. Trended higher on ice/snow amounts for most areas
near and north of I-95. Up to a couple tenths of an inch or more of
ice look possible with with 7+ inches of snow likely over the
southern Poconos and portions of NW NJ.

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 lots of uncertainty here given P-
type challenges and overall threat looks to have lowered some over
portions of northern NJ due to more snow/ice looking likely. For
this reason, no hydro headlines.


Sunday afternoon and Sunday night: In the wake of the sharp cold
front that moves through by late Sunday morning into the afternoon
expect rapidly falling temps and strong winds. This will lead to a
rapid freezing wet/slushy surfaces and could lead to power outages
for areas that see significant icing. Northwesterly winds could gust
as high as 40 mph along the coast, up to 35 mph elsewhere. This will
lead to subzero wind chills by Monday morning.

&&

.LONG TERM /MONDAY THROUGH FRIDAY/...
Upper level trough associated with this weekend`s storm will slowly
lift off to the northeast into the day Tuesday. Highs will be well
below average Monday with single digits forecast across the Poconos
to low 20s across Delmarva. Lows will be even colder Monday night
with below zero values possible across the Poconos and NW NJ.
Temperatures moderate to a degree Tuesday, though much the area will
likely stay below freezing. A short ridge builds into the area
Wednesday before a cold front moves across the region Wednesday
night. Wednesday will likely be the warmest day of period, ranging
from upper 30s to the north to low 50s across the southern portions
of the area.

Some uncertainty exists as to what happens after the front moves
through Wednesday night. The ECMWF develops another strong coastal
storm system, similar to this weekend`s storm, while the GFS
develops a much weaker solution as a storm moves more off to sea.
Another blast of cold air moves into the region as a deep trough
moves into the region. Went with a blended guidance approach to
accommodate this uncertainty. Will need to watch whether this system
cranks into a resemblance of this weekend`s storm.

&&

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

Tonight...Mainly VFR expected this evening. However, winds will
slacken again overnight, and patchy fog has developed.
Visibilities will generally be around 4-5SM in fog. Fog will
gradually dissipate towards Saturday morning. Moderate
confidence.

Saturday...Mainly VFR conditions in the morning with winds
becoming easterly or northeasterly around or under 10 kts.
However, conditions will deteriorate rapidly during the
afternoon with precipitation moving in from west to east.
Precip will likely be snow initially at the Philly terminals
northwestward, but may begin to mix with sleet, freezing rain,
and rain by evening. MIV/ACY may see mostly rain. Moderate
confidence with overall evolution, but low confidence with
timing and precipitation types.

Outlook...

Saturday night through Sunday...A long duration of restrictions,
mainly IFR or worse, through most of Sunday. A plethora of
precipitation types is possible, with mainly snow, sleet, and
freezing rain at RDG/ABE; a mixture possible at the Philly
terminals, though this period may be mainly rain; virtually all rain
at MIV/ACY. As a cold front moves through on Sunday, precipitation
may quickly transition to snow before tapering off. VFR conditions
will rapidly move in by Sunday evening.  Light northeast or east
winds Saturday night becoming strong/gusty northwest Sunday
afternoon and night. Low confidence.

Monday through Tuesday...VFR conditions are expected.
Northwesterly winds gusting to 25 to 35 kts on Monday diminishing to
5 to 15 kts on Tuesday. High confidence.

Tuesday night and Wednesday...Sub-VFR conditions possible with a
chance of rain or snow. Southwest winds 5 to 15 kts, with stronger
gusts possible, especially near the coast. Low confidence.

&&

.MARINE...
Tonight...A continuation of fair weather. NW winds mostly around 10
knots early tonight will veer around to NE by dawn Saturday.

Outlook...

Saturday through Saturday night...SCA conditions, with southerly or
southeasterly winds gusting to 25 kt are likely to develop by
Saturday evening and continue overnight. Also expect seas over the
ocean waters to build to 6-9 feet by Sunday morning.

Sunday and Monday...A cold front on Sunday will bring an abrupt
shift to northwesterly winds. With this wind shift, gale force gusts
are expected and this could continue into Monday. Freezing spray is
also likely.

Monday night...Gale force winds/gusts will continue through the
day into the late evening hours before diminishing to SCA
criteria through the overnight. Seas will also gradually
diminish through the overnight.

Tuesday and Tuesday night...Sub SCA expected during the day with
marginal SCA criteria winds and seas ramping up overnight.

Wednesday...Marginal SCA winds and seas are expected to persist
through the day before gradually diminishing into the evening hours.

&&

.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 on Saturday, causing tidal
departures to increase at that time. While the wind is anticipated
to become southerly on Saturday night, 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 Storm Warning from 1 PM Saturday to 1 PM EST Sunday for
     PAZ054-055-060>062.
     Winter Storm Watch from Saturday afternoon through Sunday
     afternoon for PAZ103-105.
NJ...Winter Storm Watch from Saturday afternoon through Sunday
     afternoon for NJZ009-010.
     Winter Storm Warning from 1 PM Saturday to 4 PM EST Sunday for
     NJZ001-007-008.
DE...None.
MD...None.
MARINE...Gale Warning from noon to 6 PM EST Sunday for ANZ430-431-
     450>455.

&&

$$
Synopsis...Staarmann
Near Term...Fitzsimmons/Meola
Short Term...Fitzsimmons/Johnson
Long Term...Davis
Aviation...CMS/Meola
Marine...Fitzsimmons/Staarmann
Tides/Coastal Flooding...Iovino