Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 220526
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
900 PM PST Tue Nov 21 2017

.SYNOPSIS...A moist frontal system will bring rain to Western
Washington tonight. The heaviest rainfall will occur in the
mountains and should begin to gradually decrease from northwest
after midnight. The snow level will be near 8000 feet. After a lull
in the rain on Wednesday, another wet frontal system will move
through the region Wednesday night through Thanksgiving morning.
After another break on Friday, another system will reach the area
Saturday afternoon and night. This active weather pattern will
continue into early next week.

&&

.SHORT TERM...Water Vapor and IR imagery shows a long fetch of warm
moist air extending from just east of the Hawaiian Islands northeast
into the Pacific Northwest. This looks like a classic atmospheric
river pattern. The freezing level at Forks is over 10K feet so the
air mass is capable of holding a lot of moisture. Rain amounts the
past 24 hours range from roughly 3-5 inches in the Olympics to 2-4
inches in the Cascades. With the heaviest amounts over the Olympic
Peninsula it not surprising that a couple rivers are flooding.
Amounts are somewhat lighter in the Cascades, but still respectable.
See the hydrology section below and refer to the latest flood
bulletins for current and detailed river information.

Models generally show a gradually decrease in rainfall rates from
the northwest after midnight. The heavier rain has arrived in
batches and not the typically focused stream of heavy rain that
usually is observed. Pretty much all models show a lull in rain
across the area on Wednesday. It will still be a bit wet but rain
amounts will not be nearly as heavy.

The next wave of rain arrives Wednesday night and Thursday ahead of
a front approaching from the west-northwest. Models show this next
system to be more progressive in nature, so rain amounts should not
be quite as heavy. However, there will still be some flood risk on
area rivers and forecasts should be monitored through the week.
Another lull or break on some of the rain is expected on Friday.
Mercer


.LONG TERM...From previous discussion...The GFS and ECMWF are
actually in fair agreement with the large scale pattern through this
weekend. Another large upper level trough will deepen offshore along
roughly 140W Friday night and Saturday, which will re-invigorate the
SW flow over the PacNW. This will allow another shortwave trough
embedded in the SW flow to move NE across W WA Saturday afternoon
and evening. The weakening main upper level trough should follow on
Sunday. The models show a warm front arriving on Monday, but day 6
is a little far out for much confidence in this active pattern. Kam

&&

.AVIATION...Moist southwest flow aloft will continue through
Wednesday night. A warm front near the Canadian border will move
north of the area this evening, but rain will continue at times in
the moist southwest flow. The air mass will be stable. At the
surface, easterly gradients are becoming more southeasterly behind
the warm front. While low level wind shear has come to an end
behind the warm front, expect strong winds 19040-55 kt to continue
at times between 030 and 050 through Wednesday night.

There has been some improvement in conditions behind the warm
front, but the high levels of moisture are producing rain at times
and low ceilings in places. Overall, expect MVFR conditions with
pockets of IFR conditions to dominate later tonight through
Wednesday morning. Strong flow aloft interacting with the terrain
will give pockets of clearing at times to the lee of the Olympics,
especially midday Wednesday through Wednesday evening. Albrecht


KSEA...CIGS have improved to just over BKN030 this evening, but
expect them to lower to 015 08Z-20Z with some diurnal cooling and
a continued influx of tropically moist air from the southwest.
Expect periods of rain to reduce visibilities to 5SM at times
through tonight. Winds will be south 8-11kt through Wednesday
morning and S 12G20 kt late Wed afternoon through evening. Winds
aloft will generally be strong at 19035-55 kt between 2500 and
5000 ft MSL through Wednesday night. Albrecht

&&

.MARINE...A warm front near the Canadian border will lift north of
the waters this evening. Another frontal system approaching the
coastal waters from the west will give rather strong south to
southwest flow to the waters tonight through Wednesday night
behind the warm front. Small craft advisories are in effect for
the coastal waters, and Admiralty Inlet overnight. The Small craft
advisories will continue over the coastal waters through
Wednesday.

Winds are expected to develop ahead of the offshore front
Wednesday night as a wave of low pressure moves through the
offshore waters. Gale force winds are possible over the coastal
waters and at the entrances to the strait as the wave moves by, so
a Gale Watch continues for these areas for Wednesday night.

Windy southerly flow will continue on Thursday as a trough
brushes the area. Gales are likely on the coast and over the north
inland waters with this system. Another system will arrive around
Saturday, with gales again possible. Albrecht

&&

.HYDROLOGY...The axis of heaviest rain associated with today`s
warm front and its plume of subtropical moisture is moving east of
the area this evening, and precipitation is generally decreasing
across Western Washington. Already, though, 2 to almost 5 inches
inches of rain has fallen over the Olympic Mountains, with 1 to 4
inches at Mount Rainier and around 1 to 3 inches further north over
the Cascades. The snow level is around 9000 feet.

There are a number of rivers that will crest close to flood stage
tonight, and a flood watch remains in effect for most of the
forecast area (all but Island, San Juan, Kitsap, and Mason
Counties). With precipitation decreasing, the upper reaches of some
rivers are starting to crest - especially the Olympic Peninsula
rivers - and those crests will be moving downstream tonight.

The rivers that face the most imminent threat of reaching flood
stage tonight include the Nooksack, Stillaguamish, Skykomish, and
Snoqualmie; they are still too close to call at this point, but
warnings could be issued this evening. If flooding does occur on
these rivers, it will be minor.

On the Olympic Peninsula, the Skokomish River is flooding and the
Bogachiel is just reaching flood stage. Flood warnings are in effect
for both rivers.

With regard to rivers flowing off of Mount Rainier, the Northwest
River Forecast Center model shows reaches of the Nisqually and the
Cowlitz Rivers reaching flood stage, but the threat there does look
as imminent.

In addition, the threat of landslides will increase across the
lowlands of Western Washington. The USGS landslide guidance shows
one or two stations near or just above the threshold of concern, and
of course the additional rain through Thursday will drive that
higher. McDonnal

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Watch through Thursday afternoon for all of Western
     Washington from Grays Harbor and Lewis counties northward
     (Except for Mason, Kitsap, San Juan, Island counties).

PZ...Small Craft Advisory for rough bar until 4 PM PST Wednesday for
     Grays Harbor Bar.

     Gale Watch from Wednesday afternoon through late Wednesday 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-East Entrance U.S.
     Waters Strait Of Juan De Fuca-Northern Inland Waters
     Including The San Juan Islands.

     Small Craft Advisory until 4 PM PST Wednesday 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 midnight PST tonight for East
     Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland
     Waters Including The San Juan Islands.

     Small Craft Advisory until 4 AM PST Wednesday for Admiralty
     Inlet-West Entrance U.S. Waters Strait Of Juan De Fuca.

&&

$$

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


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