Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 241735
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
935 AM PST Fri Nov 24 2017

.SYNOPSIS...A frontal system over Vancouver Island should bring a
little light rain to mainly the north part of Western Washington
today and tonight. After a mostly dry lull Saturday morning, another
frontal system will arrive Saturday afternoon, with wet and
locally windy weather through Sunday. After another brief lull on
Monday, another frontal system will move through Monday night and
Tuesday. There is a chance of dry weather Wednesday and Thursday.

&&

.SHORT TERM...An east-west oriented frontal system has become
quasi-stationary over Vancouver Island this morning. W WA is in
the warm sector and some light rain has been reaching the WA
coast. The front will continue to bring a little light rain to
mainly the N part of W WA through tonight. The rain area will
probably recede a bit farther north tonight as the flow aloft over
the NE Pacific amplifies.

Snow levels have lowered to between 3500 and 4000 feet today and
will rise slightly to 4000 to 4500 feet tonight. This could bring
a little snow to mainly the northern passes, but amounts will be
light.

A large longwave trough deepening offshore along roughly 145W
tonight will amplify the flow offshore allowing stronger SW flow
to develop downstream over W WA on Saturday. A shortwave trough
embedded in the SW flow will spread rain northward over W WA
Saturday afternoon through Saturday evening. At the same time an
associated deep surface low will move N offshore along 130W. This
low will generate windy conditions along the coast and over the
Cascade foothills. There is a chance the winds could reach
advisory levels, especially near the Cascades Foothills.

The pattern offshore becomes a little more complicated and tricky
Saturday night and Sunday morning. The models have a small cold
front moving E across the area Sunday morning. N-S pressure
gradients could become strong enough for at least windy conditions
over the interior lowlands. Advisory level S winds are also a
consideration. However, this is a small scale feature a few days
out, so it wouldn`t take much for the models to change it.

There is also potential for another period of heavy rain over the
mountains Saturday night. A number of the 12Z models show about 2
inches over the Olympics and a little bit less over the north
Cascades.

The remnant of the large offshore upper level trough will move E
across the area Sunday night into Monday morning for showery
weather. Kam

.LONG TERM...Previous discussion...An upper trough, probably with
an associated surface trough, will follow Sunday`s cold front for
cooler showery weather Sunday night and Monday. Another vigorous
front will probably follow closely Monday night and Tuesday. The
longer range models agree roughly that a large upper ridge will
follow that, and there is a decent chance of dry weather Wednesday
and Thursday. McDonnal

&&

.AVIATION...No significant changes to the previous forecast.

A weak upper level ridge over Eastern Washington combined with an
expansive trough over the Eastern Pacific will give strong west
to southwest flow aloft through Saturday morning. High pressure at
the surface centered over west- central Oregon combined with a
frontal system that extends from central British Columbia
southwestward into the Pacific will give low level southerly flow
to the area. Showers ahead of the offshore front will move into
the Olympic Peninsula while only isolated showers can be expected
in the interior. Conditions are mainly VFR across the area with
moisture generally above 3500 ft MSL - and conditions are expected
to change little over the next 24 hours. Albrecht

KSEA...Southerly winds 9-13 knots will persist through tonight and
will become more southeasterly Saturday morning. Expect cloud
layers 7000-10000 ft MSL with a layer around 3500 ft scattered to
broken at times. Albrecht

&&

.MARINE...No changes to the previous forecast.

A frontal system extending from the Queen Charlotte
Sound southwestward into the Pacific in combination with surface
high pressure over west-central Oregon will give southerly flow to
the waters through tonight. Flow will become more southeasterly on
Saturday as a deepening low pressure system moves northward along
130W longitude and a warm front lifts northward through the
waters. Southerly pressure gradients through the waters today will
be strong enough to produce small craft advisory conditions for
most of the waters.

Behind Saturday`s warm front, expect flow over the coastal waters
to become southerly and to possibly strengthen to gale force
Saturday night. A Gale Watch continues in effect for the coastal
waters Saturday night. Small craft advisory conditions are most
likely for waters prone to southeasterly gradients like the
entrances to the Strait, Admiralty Inlet and the Northern Inland
Waters. All waters may see small craft advisory conditions on
Sunday as a frontal system moves through the area.

Another strong frontal system will arrive Monday or Monday night.
Albrecht

&&

.HYDROLOGY...Hydrologically significant rainfall has ended.
Rivers that are flooding now are responding to rainfall over the
past three days, which was copious in the mountains. The North
Cascades received 4-9 inches, the Central and South Cascades got
3-6 inches, and the Olympic peninsula got 4-10 inches. Upstream
reaches of rivers are receding now, and most downstream reaches
are getting close to their crests. Cooler weather and less
precipitation through Saturday morning will allow rivers to
recede further.

Here is a summary of the flooding as of 9 am, from north to south
and then out to the Olympic Peninsula. For a list of current flood
watches and warnings, please check:
www.wrh.noaa.gov/sew/get.php?pil=RVA&sid=SEW

The Nooksack at Ferndale is near its crest now with minor flooding.

The Skagit River is experiencing major flooding along its lower
reaches. At Concrete, the river fell below flood stage this morning.
Downstream at Mount Vernon, the river just crested above major flood
stage and flooding will continue through tonight or Saturday morning.

The Snoqualmie near Carnation crested this morning and is back down
into minor flooding. The Snohomish River - which is formed by the
confluence of the Skykomish and Snoqualmie - has just fallen below
flood stage at Monroe and is at minor flooding at Snohomish. It will
fall below flood stage this afternoon.

The White River at R Street near Auburn - where flows depend on
the output from Mud Mountain Dam - will probably have minor flooding
through Saturday night as the stored flood waters are slowly
released.

The Cowlitz River at Randle dropped below moderate flood stage as
it falls slowly. It should drop below flood stage this afternoon.

The Skokomish River is above flood stage but falling, and it should
drop below flood stage late today.

Looking further out, more fronts will move through Western
Washington this weekend and early next week, but rainfall will
not be as heavy and snow levels will be in the 3000 to 5000 foot
range. This probably presents a threat of further flooding only on
the Skokomish River. JBB/McDonnal

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Gale Watch from Saturday evening through Sunday 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 6 PM PST Saturday 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.

     Small Craft Advisory for rough bar until 10 AM PST this morning
     for Grays Harbor Bar.

     Small Craft Advisory until 4 AM PST Saturday for Admiralty Inlet-
     East Entrance U.S. Waters Strait Of Juan De Fuca-Northern
     Inland Waters Including The San Juan Islands-West Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 10 PM PST this evening for Puget
     Sound and Hood Canal.

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html


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