Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 182337
Area Forecast Discussion
National Weather Service Seattle WA
337 PM PST Wed Jan 18 2017
.SYNOPSIS...A broad trough of low pressure aloft will settle into
the offshore waters tonight through the weekend while higher
pressure remains east of the Cascades. Several low pressure
systems will rotate around the upper trough and through our
offshore waters bringing showers and breezy conditions to the area at
times. Temperatures will generally be around normal for late
January through the weekend. A large ridge of high pressure at the
surface and aloft will build into the region early next week; this
ridge promises to bring an extended period of dry weather
beginning Tuesday or Wednesday.
.SHORT TERM...The atmospheric river that brought a period of
heavy rain and windy conditions to the area Tuesday through this
morning has now moved east and south of the area. The air mass
across Western Washington has become more unstable giving more
showery conditions to the area, and temperatures aloft have cooled
somewhat resulting in precipitation in the passes to turn to light
snow showers. The high wind warnings have all ended, and the
winter storm warning for the Cascade Passes of Snohomish and King
counties will be allowed to expire at 4 PM. With the showery
nature of the precipitation and lower snow levels, the threat of
additional flooding is decreasing rapidly. The flood watch for
Whatcom county and the Olympic Peninsula has been cancelled, but
continues for King, Pierce and Lewis counties until midnight
A long wave trough will set up around 135W tonight and will
persist at about that longitude through the upcoming weekend.
Forecast models generally agree that one low will curl northward
into the offshore waters on Saturday and will be followed by a
second deeper low moving toward the Oregon or Washington offshore
waters late Saturday. An upper ridge will remain centered over the
inland northwest while surface high pressure remains east of the
Cascades. This combination will result in mainly low level
offshore flow that will result in modest precipitation amounts
through the period. Temperatures through Saturday will slowly
moderate cooler to around normal values for mid to late January.
It will be breezy at times, but stronger winds are expected to
remain offshore. Albrecht
.LONG TERM...A second and deeper low will curl northward into the
Washington offshore waters on Sunday then will move southeastward
and inland over Oregon Sunday night or Monday. The ECMWF model is
faster to move the low pressure and resulting precipitation well
south and east of the area - so Monday may end up being drier than
the current consensus forecast. There could be some east wind
issues in portions of the East Puget Sound Lowlands later in the
weekend, but it is far out to try to pin that down.
All long range models build a strong upper level ridge into the
western US while they build high pressure over or just to the east
of the forecast area. At this time the period starting Tuesday
looks pretty certain to be dry. What will be unclear until the
pattern develops is whether or not fog and stratus will develop
over the area (hinted at by the ECMWF) or offshore flow will be
stronger and conditions clearer (per the GFS). The ECMWF would
result in cooler and more stagnant conditions than the GFS.
.AVIATION...Southwesterly flow aloft tonight will become more
westerly Thursday as an upper level trough moves through the area.
At the surface moderate southeast gradients will continue. The air
mass is moist and slightly unstable with showers likely. Most
ceilings are VFR 4-6K ft. Conditions will deteriorate to MVFR
tonight, then improve to VFR Thursday afternoon.
KSEA...Discussion above applies. Easterly wind 8-12 KT this evening
will become more southeasterly after 03Z and diminish to 4-8 KT
.MARINE...A series of frontal systems will move through the area at
a rate of about one per day through Sunday. Small craft advisory
strength winds are likely over most waters as each system passes,
and gale force winds are not out of the question. For now have kept
all winds over the inland waters sub-gale, with gales indicated over
the outer coastal waters only on Friday. Next week high pressure is
forecast to build over the region, which will give lighter winds.
The combination of high astronomical tides and low atmospheric
pressure may cause minor tidal overflow Thursday morning at a few
spots around Puget Sound. Will leave any advisory to a later shift.
Westerly swell is forecast to reach as high as 20 feet Saturday.
.HYDROLOGY...The only river in flood at this time is the Skokomish
River in Mason county. Precipitation has become showery in nature
and snow levels are falling, so the threat of additional flooding
is decreasing rapidly with time. Flood watches have been
cancelled for the Olympic Peninsula and for Whatcom County. The
flood watch continues until midnight tonight for King and Pierce
counties where there still is a chance of flooding on the White
River at the R street Bridge near Auburn, and in Lewis County
where there remains a threat of flooding on the Newaukum River
After tonight, the threat of river flooding across the area will
become low. An extended period of dry weather appears likely to
develop early next week. Albrecht
WA...Flood watch in effect until midnight tonight for King,
Pierce, and Lewis counties.
PZ...Small Craft Advisory for rough bar until 4 PM PST Thursday for
Grays Harbor Bar.
Small Craft Advisory until 10 AM PST Thursday 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 PM PST Thursday for Admiralty Inlet-
East Entrance U.S. Waters Strait Of Juan De Fuca-Northern
Inland Waters Including The San Juan Islands.
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