Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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195
FXUS66 KSEW 220525
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
900 PM PST Sun Jan 21 2018

.SYNOPSIS...A cool upper trough will maintain showers tonight with
snow at times in the mountains. After a lull on Monday, another
front will cross Western Washington Tuesday night and Wednesday. A
cool showery upper trough will reach the area Thursday.

&&

.SHORT TERM...An upper trough with -34C cold pool at 500 mb
will slowly shift east across the region through Monday morning.
Radar indicates showers still streaming into Wrn WA, although
partial shadowing is limiting showers east/northeast of the
Olympics. The airmass is somewhat unstable with some occasional
lightning strikes with cells, mainly near the coast. Another
embedded short wave is shown by models to move across tonight,
enhancing showers and producing periods of heavy snow in the
Cascades. Cold top cumulus on IR imagery extend well offshore so
expect precipitation to increase and persist through Monday morning.
A couple feet of snow has fallen at Mount Baker with generally less
6 inches in the passes. The increase in showers tonight with the
coldest air aloft could easily drop up to 6 inches including the
passes tonight. The winter storm warning for the north
Cascades/Olympics and advisory for remaining Cascades will remain in
effect through Monday morning. Showers will decrease in the
afternoon as cyclonic flow shifts east of the region.

Besides decreasing showers and mountain snow, no other significant
weather expected Monday. A lull or break in precipitation is
expected Monday afternoon and night. Some partial sunshine is
possible with temperatures in the 40s.

The next system arrives Tuesday and may affect the region through at
least Wednesday. Another round of wind and mountain snow/lowland
rain appear likely. Gradients will have a strong easterly component
which will limit the stronger winds to portions of the north
interior and possibly the coast. High wind criteria will be harder
to hit with this next front as it stretches out. Winds will probably
warrant an advisory at some point around Tuesday/Tuesday night. Most
of Puget Sound southward will be breezy, much like this past system.
Mercer

.LONG TERM...Previous discussion...A cool upper level trough will
cross the Pac NW Wednesday night into Thursday. This system is deep
with 500 mb heights in the 520s and temps -30C. The cooler air mass
will knock snow levels down to around 1,000-1,500 feet with a chance
of snow showers on the higher hills. The surface flow is southerly,
though, which should moderate temps. Heavy snow is expected in the
mountains. Toward the weekend, a strong warm front may bring heavy
rain to the coast and mountains. However, this front is wavering
between southern B.C. and Western WA in the models for a wide range
in QPF. 33

&&

.AVIATION...A broad upper level trough will move eastward across
the area tonight and will be followed by a flat upper level ridge
Monday into Monday night. Moderate westerly flow. The air mass is
somewhat unstable this evening and will stabilize later tonight
through Monday evening.

An band of showers sits from the Olympic Peninsula southwestward
into the Oregon offshore waters this evening. This shower band is
expected to shift eastward into the interior early Monday morning.
Cigs across the area are a mixed bag with most of the major
terminals seeing MVFR conditions. Expect ceilings to lower somewhat
early Monday morning as the shower band passes through, then
conditions will improve Monday as showers become confined to the
terrain. Isolated thunderstorms have been observed on the
northwestern Olympic Peninsula this evening. Expect the threat of
additional tstm activity to decrease rapidly as the air mass
stabilizes. Albrecht

KSEA...Expect a general break in showers until about 10Z when the
next bend now over the Olympic Peninsula moves into the area. Cigs
will generally be in the MVFR category (FL025-030) through Monday
morning, with a possible drop to FL015-020 12Z-16Z with the showers.
South flow 6-10 kt through Monday evening. Albrecht

&&

.MARINE...Moderate southerly onshore flow tonight will gradually
decrease on Monday. Small craft advisories continue across most of
the waters tonight into Monday morning with the breezy onshore flow.
Small craft advisories will continue over the coastal waters into
Monday evening as seas only slowly subside and remain hazardous to
small craft.

A strong warm front will lift northeast across the waters on
Tuesday. Expect possible gale southeasterly flow to develop late
Monday night over the coastal waters -so a gale watch is being
issued for the coastal waters late Monday night into Tuesday. A gale
watch may also be needed for the eastern entrance to the Strait of
Juan de Fuca, but confidence is lower in that area.

A weakening low center will reach the waters off Vancouver Island on
Thursday. While gales are a possibility over the coastal waters with
this system, there is better threat that seas to the south of this
low will reach the coastal waters at 20-25 ft on Thursday.

Another strong warm front will increase southeast flow over the
waters Friday night and Saturday. Albrecht

&&

.HYDROLOGY...The Skokomish river in Mason county rose above flood
stage around midday today with minor flooding occuring. The river
appears to be cresting around 9 pm pst, and should slowly fall
through Monday morning. See the latest flood statement for current
information.

Rainfall through tonight will probably push the USGS landslide
indices close to thresholds for shallow landslides.

Models are showing heavy rain possible in the Olympics and
Cascades next weekend from a strong warm front. Exact impacts are
unclear as the front may waver between southern B.C. and Western
WA.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory until 9 AM PST Monday for Cascades of
     Pierce and Lewis Counties-Cascades of Snohomish and King
     Counties.

     Winter Storm Warning until 9 AM PST Monday for Cascades of
     Whatcom and Skagit Counties-Olympics.

PZ...Gale Watch from late Monday night through Tuesday afternoon for
     Coastal Waters From Cape Flattery To James Island 10 To 60
     Nm-Coastal Waters From Cape Flattery To James Island Out 10
     Nm-Coastal Waters From James Island To Point Grenville 10
     To 60 Nm-Coastal Waters From James Island To Point
     Grenville Out 10 Nm-Coastal Waters From Point Grenville To
     Cape Shoalwater 10 To 60 Nm-Coastal Waters From Point
     Grenville To Cape Shoalwater Out 10 Nm-West Entrance U.S.
     Waters Strait Of Juan De Fuca.

     Small Craft Advisory until midnight PST Monday night for Coastal
     Waters From Cape Flattery To James Island 10 To 60 Nm-
     Coastal Waters From Cape Flattery To James Island Out 10 Nm-
     Coastal Waters From James Island To Point Grenville 10 To
     60 Nm-Coastal Waters From James Island To Point Grenville
     Out 10 Nm-Coastal Waters From Point Grenville To Cape
     Shoalwater 10 To 60 Nm-Coastal Waters From Point Grenville
     To Cape Shoalwater Out 10 Nm-West Entrance U.S. Waters
     Strait Of Juan De Fuca.

     Small Craft Advisory for rough bar until 4 AM PST Tuesday for
     Grays Harbor Bar.

     Small Craft Advisory until 3 AM PST Monday for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca.

     Small Craft Advisory until 9 AM PST Monday for Admiralty Inlet-
     Northern Inland Waters Including The San Juan Islands-Puget
     Sound and Hood Canal.

&&

$$

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