000
FXUS66 KSEW 190412
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
812 PM PST Fri Jan 18 2019

.UPDATE...Forecasts are on track this evening. A strong occluded
front associated with a deep low over the offshore waters will
lift northeast across the area late tonight. Winds are increasing
over the northern interior from about Whidbey Island northward and
along the coast. Wind advisories are in effect for those areas.
Elsewhere, light northerly winds will shift to the south and
increase to 15 to 30 mph later tonight. The aviation and marine
sections are updated; otherwise, the afternoon discussion applies
and is attached below. Albrecht

&&

.AVIATION...Southwest flow tonight will become more westerly on
Saturday. A strong occluded front will lift northeast across the
area overnight. Low level winds will be east to northeast ahead of
the front and will turn to the south or southwest late tonight
through Saturday. The air mass is moist and stable, and will
become somewhat unstable behind the front late tonight through
Saturday.

VFR conditions dominate over the northern portions of the area
early this evening with MVFR conditions to the south. Expect
conditions to deteriorate from the south to MVFR with local IFR as
the front approaches, then remain mainly MVFR with pockets of VFR
on Saturday. Albrecht

KSEA...Cigs will trend down to BKN025 08Z-09Z as the front to the
south approaches and vis will average 3-5 sm -ra br. Post frontal
onshore flow will give mainly MVFR cigs with showers at times on
Saturday, though some clearing at times as flow aloft interacts
with the Olympics. Winds 13012kt will turn to the south 15g25kt
after about 09Z then will turn to 20010-12kt 14Z onward. Albrecht

&&

.MARINE...A deep 972 mb low near 50N 134W will lift north into
Haida Gwaii overnight as it slowly fills. An occluded front
extending ESE from the low will lift NE across the area overnight.
Southeasterly gales ahead of this occlusion will become south or
southwesterly behind the front late tonight into Saturday morning,
then winds will slowly diminish across the region on Saturday.
Gale warnings remain over all of the waters except Puget Sound and
Hood Canal and the west entrance to the strait -where small craft
advisories are in effect.

A coastal flood advisory continues for the shoreline of Western
Whatcom County late tonight through Saturday. Minor flooding
appears likely around the two high tides -with most of the impacts
felt along the coast at Birch Bay and at Sandy Point. While
astronomical high tides plus storm surge appears about a half to
three-quarters of a foot below what will normally produces
flooding, a fetch of southwest winds 15 to 30 knots over the
Strait of Georgia all the way from NW of San Juan Island to the
coastline will produce 4-6 foot waves around the high tides. This
will result in waves crashing against and overtopping seawalls and
other shoreline protections resulting in some minor flooding in
places.

The weather will be considerably calmer Sunday and Monday, with
another system possibly arriving Tuesday. Albrecht

&&

.PREV DISCUSSION... /issued 300 PM PST Fri Jan 18 2019/

SYNOPSIS...A vigorous storm system will affect the area through
Saturday morning with winds and moderate precipitation. Conditions
will improve during the day Saturday with generally dry conditions
expected Sunday and Monday. Active weather will return Tuesday into
Wednesday, followed by a shift to a dry, benign pattern later in the
week.

SHORT TERM...Stratiform precipitation has begun to shift
onto much of Washington coast this afternoon ahead of the
approaching system over the eastern Pacific. This system will bring
a swath of deep moisture  and moderate precipitation to much of the
area this evening into Saturday morning. Expected QPF with this
system will range from a half inch over the lowlands to 1-2 inches
across the Cascades and 3 to 4.5 inches over the Olympics. The
heavier accumulations will result in possible flooding on the
Skokomish river - a flood watch remains in effect for Mason County
through late Saturday night. Snow levels ranging from 3500 to 4000
feet this evening will rise rather significantly to 4500 to 6000ft
overnight as the warm Pacific air mass moves inland. It does look
like the west slopes of the northern Cascades - in Whatcom and
Skagit Counties - will pick up snow accumulations in the
neighborhood of 5-10 inches this evening before the snow levels rise
with the warm air. Have issues a winter weather advisory to cover
the impacts associated with the snowfall there. For the rest of the
Cascades, there will likely be snow accumulations of 3 to 5 inches
this evening before snow levels and precipitation changes to rain at
all but the highest elevations.

This storm system will also bring another period of enhanced winds
to the area - particularly for the coast and interior from Admiralty
Inlet northward - late this evening into the early morning hours
Saturday. Winds tonight continue to look to be a bit stronger than
Thursday`s system, with sustained winds of 20 to 35 mph with gusts
to 45 mph along the coast and north interior. Areas across Puget
Sound could experience gusts to 40 mph - windy, but just under
advisory criteria. The windy conditions, coupled with the very high
tides expected Saturday morning could result in some coastal
flooding at some locations along the shorelines of Western Whatcom
County around Sandy Point and Birch Bay. For more information please
see the marine section below.

This system continues to look very progressive - rain and winds will
decrease pretty quickly Saturday morning. Snow levels do come back
down to 3000-3500 feet Saturday afternoon, but so does the threat of
significant precipitation by this time. The only likely lingering
threat Saturday may be the hydrologic impacts to the Skokomish
River. A trailing upper level trough will move onshore Sunday
afternoon. Models remain in good agreement with this system, keeping
the bulk of the precipitation associated with this system to the
south over Oregon. Have kept a chance of showers in the forecast as
this system passes...though the biggest impact will likely be
enhanced cloudiness.

LONG TERM...The large scale pattern will begin to change
Monday as an upper level ridge begins to build over the Eastern
Pacific and into the Pacific Northwest. This will result in a bit of
a break ahead of a weak system that is expected to brush the area
Tuesday. Tuesday`s system is really little more than a weak system
with some moisture riding up and over the ridge into the Pacific
Northwest. Rain looks to continue through Wednesday before the upper
level ridge is finally able to establish itself by Thursday as a
deep trough develops across the eastern US. This looks to trend the
later part of the week towards dry and benign weather.

AVIATION...Westerly flow aloft will continue before becoming
southwesterly tonight. Low level offshore winds will continue
through tonight before becoming more southerly late tonight/early
Saturday morning as another frontal system moves into the area.

Cigs generally VFR over W WA this afternoon as easterly winds help
to keep the lower levels free of low clouds. Will start to see cigs
drop as rain from approaching front draws nearer...with cigs
generally bottoming out right around the MVFR/VFR threshold
..save for those locations prone to low clouds...such as HQM...PWT
and OLM. Those locations may dip into MVFR to IFR conditions at
times into Saturday.

KSEA...Cigs have improved to around 7000 ft and will remain that way
in the 5000-7000 ft range through the afternoon and into this
evening and tonight even as rain moves over the terminal after 00z
late this afternoon. Cigs look to drop into MVFR after 09z as gusty
winds enter the area along with rainfall. Southeasterly to easterly
winds 5-10 kts becoming southerly 10-15 kts with gusts to 20 kts
after 09z and continuing into Saturday morning before gradually
easing.  SMR

MARINE...An approaching low pressure system and associated front
will move from its current position out over the Pacific toward the
Canadian coast. The strong front associated with this low will pass
through area waters tonight. Gale warnings remain in effect for the
coastal waters, the eastern two-thirds of the Strait of Juan de
Fuca, Admiralty Inlet, and the Northern Inland waters. Small craft
advisories remain in effect for Puget Sound and Hood Canal and for
the west entrance to the strait.

In looking at the inherited coastal flood watch for W
Whatcom...current wind models along with tidal data continues to
show a risk for 4-6 ft waves near the area of concern...potentially
resulting in some coastal flooding around the two high tides on
Saturday. In short...current thinking remains consistent with
previous forecast and as such...will transition the inherited
Coastal Flood Watch into an advisory with the afternoon package.

The weather will be considerably calmer Sunday and Monday, with
another system possibly arriving Tuesday. SMR

HYDROLOGY...The next frontal system will bring rain to the area
late through the first part of Saturday. Rain should be heavier in
the Olympics than elsewhere. There is a flood watch for Mason county-
-which probably says more about the ease with which the
Skokomish river gets up to flood stage than anything else.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory until 4 AM PST Saturday for Cascades of
     Whatcom and Skagit Counties.

     Flood Watch through late Saturday night for Hood Canal Area-
     Lower Chehalis Valley Area-Olympics.

     Wind Advisory until 6 AM PST Saturday for Admiralty Inlet Area-
     Central Coast-Eastern Strait of Juan de Fuca-North Coast-
     San Juan County-Western Skagit County-Western Whatcom
     County.

     Coastal Flood Advisory from 3 AM to 5 PM PST Saturday for
     Western Whatcom County.

PZ...Small Craft Advisory for rough bar until 6 PM PST Saturday for
     Grays Harbor Bar.

     Gale Warning until 6 AM PST Saturday for Admiralty Inlet-Central
     U.S. Waters Strait Of Juan De Fuca-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-East Entrance U.S. Waters Strait Of Juan De Fuca-
     Northern Inland Waters Including The San Juan Islands.

     Small Craft Advisory until 3 AM PST Sunday for West Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 6 PM PST Saturday for Puget Sound and
     Hood Canal.

&&

$$

www.weather.gov/seattle

NWS SEW Office Area Forecast Discussion

Discussion Script by Ken True at Saratoga Weather - WFO Script by SE Lincoln Weather