Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 241110
AFDSEW

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

.SYNOPSIS...A flat upper ridge will bring relatively dry weather
to much of Western Washington today through Saturday morning.
Another frontal system will arrive Saturday afternoon, with wet
and locally windy weather through Sunday. After a brief showery
break on Monday, another front will move through Monday night and
Tuesday. There is a chance of dry weather Wednesday and Thursday.

&&

.SHORT TERM...Moist westerly flow aloft continues over Western
Washington early this morning, with a flat upper ridge over the
region. A weak warm front extends southeast into the forecast
area from Vancouver Island/coastal southern British Columbia, and
our coastal radar is showing some light precipitation over the
Olympics; the Camano Island radar remains out of service at this
time, but observational data show that there is some light
precipitation over the west slopes of the Cascades as well. Aside
from spotty light rain, skies are generally cloudy across the
lowlands at 3 am with temperatures in the 40s.

The upper ridge will amplify slightly today and shift east of the
Cascades. The warm front will continue to bring some light
precipitation to the forecast area, mainly the coast and
mountains. The air mass will dry somewhat tonight and Saturday
morning as the ridge builds a bit more, and the area of warm
advection precipitation will be pushed northward. Therefore the
Western Washington interior lowlands from around Seattle southward
will have little or no rainfall tonight and Saturday morning.
And, really, the precipitation across all of Western Washington
will be relatively light through at least Saturday morning,
especially by recent standards.

Another warm front - this one associated with a deep, developing
system approaching from the southwest - will lift north into
Western Washington Saturday afternoon and evening, and its cold
front will move inland from the west later Saturday night and
Sunday. This will be a fairly wet period, though certainly not as
wet as the last few days.

Rainfall amounts of up to 1 inch are likely in the lowlands,
heaviest at the coast. Precipitation amounts of 1 to 3 inches
should be common in the Olympics, with 1 to 2 inches in the
Cascades; the snow level will be around 4000 feet as the system
arrives, rise to around 6000 feet late Saturday night behind the
warm front, and then fall to around 3500 feet late Sunday or
Sunday night as precipitation turns to showers behind the cold
front. Paradise could end up with a half foot of snow, but the
passes - especially Snoqualmie Pass - will get little to none, as
most of the precipitation will fall when the snow level is high.

The system`s surface low is forecast to deepen fairly dramatically
at it tracks from well offshore of California early Saturday
morning to a position off Vancouver Island early Sunday morning.
The GFS actually shows the system bombing (by definition,
deepening at least 24 millibars in 24 hours) from around 1002
millibars early Saturday to 974 millibars early Sunday. Even
though the low won`t get much inside of 130W over our offshore
waters, it will produce windy conditions especially at the coast
and in the north interior and perhaps near the west slopes of the
Cascades. High winds look unlikely, but it will be worth watching
and the formation of a more consequential frontal wave isn`t out
of the question. McDonnal

.LONG TERM...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

&&

.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 3 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 still rising and will crest midday
today with minor flooding. The upper reaches of the Nooksack have
receded below flood stage.

The Skagit River is experiencing major flooding along its lower
reaches. At Concrete, the river was just falling below moderate
flood stage, and won`t fall below flood stage till midday.
Downstream at Mount Vernon, the river is above major flood stage
and still rising; it will crest midday and flooding will continue
through tonight or Saturday morning.


On the Stillaguamish River, flooding has ended.

The Skykomish River near Gold Bar fell below flood stage around
midnight. The Snoqualmie River at Snoqualmie Falls fell below
flood stage around midnight too, and the Snoqualmie near Carnation
was near its crest just above moderate flood stage. The Snohomish
River - which is formed by the confluence of the Skykomish and
Snoqualmie - has moderate flooding, but it`s starting to recede.

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.

Flooding has ended on the Nisqually River near National.

The Cowlitz River at Randle crested around midnight, but remains
above moderate flood stage as it falls slowly. It should drop
below flood stage this afternoon.

The Satsop River, which flows off the south side of the Olympics,
fell below flood stage late Thursday. 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, and the remaining flood watches will be
allowed to expire this morning. McDonnal

&&

.AVIATION...A frontal system will stall and dissipate over
southern British Columbia today. Showers across Western WA are
isolated to scattered and will mainly affect the coast and
mountains. The flow aloft is westerly. Ceilings are mainly in MVFR
range this morning but should see VFR conditions this afternoon.
Expect gusty south winds to 20-25 kt this afternoon in the
interior. 33

KSEA...MVFR conditions this morning with VFR conditions this
afternoon. S winds at the surface with gusts to 20 kt this
afternoon. 33

&&

.MARINE...A frontal system will stall and dissipate over southern
British Columbia today. However, will still see moderate southerly
flow across Western WA with Small Craft Advisory winds over most
waters. A stronger system will track north over the Offshore
Waters Saturday night with gales possible - a Gale Watch is in
effect. Strong onshore flow will follow on Sunday. Another strong
frontal system will arrive Monday or Monday night.

&&

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Watch until 6 AM PST early this morning for Admiralty
     Inlet Area-Bellevue and Vicinity-Cascades of Pierce and
     Lewis Counties-Cascades of Snohomish and King Counties-
     Cascades of Whatcom and Skagit Counties-Central Coast-East
     Puget Sound Lowlands-Eastern Strait of Juan de Fuca-Everett
     and Vicinity-Hood Canal Area-Lower Chehalis Valley Area-
     North Coast-Olympics-Seattle and Vicinity-Southwest
     Interior-Tacoma Area-Western Strait of Juan De Fuca-Western
     Whatcom County.

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

     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 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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