Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 151120
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
420 AM PDT Sun Oct 15 2017

.SYNOPSIS...A weak upper level ridge will remain over Western
Washington through tonight with dry weather and warmer afternoon
temperatures. The ridge will start to retreat to the south on
Monday, allowing a wet cold front to drop southward through the
area on Tuesday. A strong warm front will then lift back northward
on Tuesday night and Wednesday, with stronger wind and heavier
rain. An upper trough will reach the offshore waters on Thursday,
preceded by another wet cold front over Western Washington. Cooler
and showery weather will follow on Friday. A wet and windy
atmospheric river pattern is possible next Saturday.

&&

.SHORT TERM...Two more quiet days before the weather transitions
to a stormy fall pattern on Monday night and Tuesday.

For today and tonight, a flat upper ridge is over the Pacific
Northwest. 500 mb heights of 5760 meters will be favorable for dry
weather and indicates warmer air aloft. This will lead to a
warmer afternoon temperatures through Monday, with lowland highs
reaching the 60s. This is also a stable pattern, which is
favorable for some fog development. Have included fog in the
forecast this morning and again on Monday morning for the fog-
prone Chehalis and Snohomish valleys, and a broader swath of the
Southwest Interior, including Olympia.

The upper ridge will start to retreat back to the Southwest U.S.
on Monday afternoon. This will allow the active storm track
currently pointed at the northern B.C. coast to slip southward.
Eventually, a cold front will drag south through Western
Washington on Tuesday. In the warm sector preceding the front, a
few hours of heavy rain are expected, along with breezy
conditions. While this will be the most significant front so far
this season, it should not require land-based advisories or
warnings. It will be a preview of an even stronger storm to come
on Wednesday. Haner

.LONG TERM...Looks like a stormy fall day on tap for Wednesday,
probably requiring the eventual issuance of wind-related and
possibly flood-related watches, warnings, or advisories. A jet
streak will develop in the base of an upper trough over the Gulf
of Alaska. As the jet noses into B.C., it will push a strong warm
front inland on Tuesday night. This will bring a new round of
heavy rain, though the ECMWF hold heavy rain off until Wed
morning. Southeast gradients will become tight on Wednesday,
supporting windy conditions. Windiest weather will be in
southeast-wind prone areas, such as the coast and areas from
Everett north through the islands and Bellingham. Statistical
guidance from the GFS and ECMWF show sustained winds at Whidbey
Island reaching 34 knots and 31 knots respectively. The first such
wind event of the autumn is commonly more impactful than a
similar event later in the season, after nature`s tree trimmer has
already been through a few times. As for rain, models are
developing a consensus in keeping heaviest rain over the windward
Olympic Mtns and North Cascades. The 06z GFS shows 24-hour rain
totals as high as 8 inches over the northwest Olympic Mtns as of
11 pm Wed. The official NWS forecast has 6.5" at this wettest
location during the same 24 hours. Currently looking for 3-4
inches over the North Cascades. The interior lowlands currently
forecast to range from 0.75" south of Olympia to near 2" around
Bellingham. See the hydrology section for more impacts.

An upper trough will make headway off the eastern Pacific, pushing
a cold front through Western Washington on Thursday. Once it
passes, the atmospheric river type weather should ease. Cool and
showery post-frontal conditions will prevail on Friday.

Yet another strong, consolidated jet will nose into the Pac NW and
B.C. next Saturday. Looks like this will be another atmospheric
river setup for heavy rain. With 850 mb southwest winds of 40-60
kt, should be windy too.  Haner

&&

.AVIATION...Weak high pressure aloft over the Pacific Northwest will
begin to flatten tonight. A front will approach Wrn Washington from
the northwest on Monday. Light west-southwest flow aloft becoming
moderate-strong southwesterly aloft late tonight and Monday. Air
mass stable and generally dry, except residual surface moisture with
patchy IFR fog in prone areas like the southwest interior, parts of
Hood Canal, and river valleys (KPWT/KOLM most likely). Any fog will
dissipate by 16-17Z with some increasing mid level
moisture/scattered clouds above 10K, mainly north of Puget Sound.

KSEA...Scattered VFR skies. Fog at the terminal appears unlikely
this morning but a slight risk (15 percent or less) cannot be ruled
out within the vicinity, mainly 13-17z. Variable wind less than 5 kt.

&&

.MARINE...Higher pressure will persist over the waters with
generally light flow through early Monday morning. A front will
approach on Monday with small craft winds likely to develop over the
coastal waters. Small craft winds could spread into parts of the
inland waters Monday night, especially north of Puget Sound.

A stronger trailing system is set to arrive Wednesday and anticipate
gale force winds over the coastal waters. Solid small craft 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 well above 10 feet, possibly close to 20 ft, for
the coastal waters by around Wednesday and Thursday.

&&

.HYDROLOGY...A few hours of heavy rain on Tuesday is not expected
to produce river flooding on Tuesday.

An atmospheric river type event is expected Wednesday, with the
focus of heaviest precip aiming at the North Cascades and the
Olympic Mtns. All area rivers will see rises. Will need to monitor
the potential for minor flooding on a couple of rivers,
especially rivers flowing off the Olympics and North Cascades.
This includes the uniquely flood-prone Skokomish River. In the
lowlands, 1-2 inches of rain and autumn leaf litter clogging
drains could lead to localized urban flooding.

Another period of heavy rainfall is possible next weekend,
renewing concerns for flooding. Haner

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...None.

&&

$$

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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