Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 160429
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
930 PM PDT Sun Oct 15 2017

.SYNOPSIS...The flat upper level ridge over Western Washington this
evening will weaken overnight. This will allow a frontal system over
southern British Columbia to bring rain and breezy winds as it moves
south across the area starting Monday afternoon and continuing
through Tuesday. A second wet and windy frontal system will affect
the area Tuesday night through Thursday. A third frontal system is
expected to arrive around Saturday.

&&

.SHORT TERM...The flat weak upper level ridge over W WA will slowly
give way to the frontal system developing over southern B.C.
tonight. In fact, a narrow band of light rain slid south to roughly
a Neah Bay to Blaine line this evening before falling apart. This
was probably mostly virga since RADAR had bases up above 5000 feet.

Models remain pretty much on track for tonight and Monday. Skies
will remain mostly clear tonight except for the far north part. Lows
should reach the upper 30s and 40s with some patchy fog forming over
the SW interior and S Puget Sound. Monday will be a nice sunny day
for about the southern two thirds of W WA. Highs should reach the
lower to mid 60s.

Frontal clouds and light rain will spread steadily S over the N part
of W WA through Monday, reaching the N coast and Bellingham area by
late morning then reaching the central coast across the Olympics to
Mount Vernon and the N Cascades by late afternoon.

The B.C. frontal system will continue sliding slowly S Monday night,
with light rain reaching the Chehalis area by early Tuesday morning.
Monday night is also when the heaviest rain is expected over the
Olympics, generated as WSW winds at 850 MB rise to around 45 kt. The
far N Cascades will also begin picking up heavy rain late Monday
night.

The frontal system will continue S on Tuesday, but should finally be
S of Chehalis late Tuesday afternoon. Heavy rain is expected to
continue during the day over the Olympics and the N half of the
Cascades. Westerly low level winds will maintain a rain shadow over
the Puget Sound region through the entire event, so heavy rain is
not expected. Rainfall accumulations should reach 1-2 inches over
the Olympics and N Cascades, 0.5 to 1.5 inches along the coast, and
0.25 to 0.50 for most of the interior lowlands, although spots close
to the Cascades could see near an inch of rain. So, that`s the first
wave.

After a brief lull for most of Tuesday night the next frontal system
will begin spreading rain inland before sunrise Wednesday. This
atmospheric river event will be much wetter and longer lasting than
the first system. SW winds aloft could reach 60 kt on Wednesday with
the Olympics and N Cascades once again receiving the heaviest rain.
The front will slow to a crawl late Wednesday night, thus extending
the heavy rain into Thursday. See the Hydrology section below for
more specifics.

Surface winds with both systems will become windy with some spots
reach the 20-30 mph range, but advisory level winds are looking
unlikely at this point. Kam

.LONG TERM...Previous discussion from the 355 PM AFD...Locally heavy
rainfall may continue into Thursday as the cold front shifts very
slowly to the southeast across the area. A cooler and showery
pattern with snow levels dropping to around 4000 feet or so is
expected Thursday night through Friday night.

The latest long term models continue to show the redevelopment of
an atmospheric river pattern over the area starting Saturday of
next weekend and continuing through Sunday. The 18Z GFS shows
impressive rainfall amounts on the west slopes of the Olympics
and in the North Cascades. The ECWMF has lower totals but shows an
impressive pattern that typically results in heavy rainfall. It
is too early for details at this point, but it is looking quite
wet and there is the potential for more significant river flooding
over portions of the area next weekend. Albrecht

&&

.AVIATION...Weak high pressure aloft will break down Monday
afternoon. A front will approach the area Monday evening and push
across Wrn Washington later Monday light into early Tuesday morning.
Light southwest flow aloft becoming moderate to strong Monday
afternoon and night. Air mass stable and somewhat moist in the upper
levels, then becoming moist at all levels Monday into Monday night.

Areas of low level moisture and IFR conditions along the coast and
into the SW interior and south Sound associated with the developing
onshore flow Monday morning. Light rain will reach the coast Monday
morning, spread into the north interior by afternoon and then the
rest of the area by evening.

KSEA...VFR through tonight with some higher clouds well above 10K ft
passing over the terminal at times. Low stratus associated with the
developing onshore flow overnight will get close to the terminal
after 12z Monday. Thick mid clouds by midday, lowering into a VFR
stratus OVC layer by evening. Variable wind to 5 kt, then light
southerly by 12z Monday. dtm

&&

.MARINE...A front will approach the waters tonight. Anticipate small
craft advisory winds to develop along the coast Monday. The front is
expected to sag southeast across the inland waters Monday night and
early Tuesday.

The next stronger Pacific frontal system is set to arrive Wednesday
and anticipate gale force winds over the coastal waters. Solid small
craft advisory winds are likely over the inland waters. Forecasts
should be monitored closely as the precise track of the front could
bring stronger or weaker winds than forecast, especially over the
inland waters.

These Pacific frontal system will also help generate rising ocean
swells, building above 10 feet by Monday night for the coastal
waters, and likely close to 20 ft by late Thursday. dtm

&&

.HYDROLOGY...An atmospheric river event is expected Tuesday night
through perhaps Thursday. On average, the focus of heaviest
precipitation is aimed toward the Olympics and North Cascades.
Forecast models started the more northward focus with last nights
runs, but the latest 18Z GFS solution is slightly farther to the
south. Rainfall amounts in the Olympics are expected to be 7 to 10
inches over a 36 hour period from late Tuesday night through
Thursday morning. The North Cascades will likely see 3-6 inches,
and the lowlands will likely see between 0.75 and 1.5 inches.

All area rivers will see rises. Since rivers are currently rather
low, the river most likely to flood will be the particularly
flood-prone Skokomish River in Mason County. A few other rivers
may approach flood stage. More widespread flooding is possible if
the atmospheric river hangs over the area without moving to the
north of the area on Wednesday afternoon. In the lowlands, a
period of heavy rainfall could result in some urban flooding due
to autumn leaf litter clogging drains.

Another period of heavy rainfall is increasingly likely next
weekend. With rivers starting at a higher level, the weekend event
could result in more widespread and significant flooding if it
occurs. Albrecht,

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small craft advisory coastal waters from 6 AM Monday through
      5 PM Tuesday.

&&

$$

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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