Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 170034
Area Forecast Discussion
National Weather Service Seattle WA
434 PM PST Mon Jan 16 2017
.SYNOPSIS...A frontal system over the offshore waters will move
into Western Washington Tuesday morning and will weaken as it
encounters a strong ridge of high pressure to the east. A wetter
and warmer frontal system will move through the region later
Tuesday afternoon through Wednesday bringing wet and windy
conditions to the area along with freezing rain in the passes. A
cool upper level trough will settle over the area from the second
half of the week into next weekend bringing cool and showery
weather along with breezy to locally windy conditions at times.
.SHORT TERM...Clouds are increasing this afternoon as the frontal
system that is still out over the offshore waters approaches
western Washington. This front will come through the area in two
pieces, the first weakening piece will move through the area
midday Tuesday bringing precipitation, windy conditions over the
north interior and along the coast, and some mountain snow to the
Cascades above 3500 to 5000 feet. More significantly, the second
wave on the front, now centered about 35N 150W, will move
northeast across the area Tuesday night and Wednesday. This second
wave will contain subtropical moisture originating around Hawaii,
and will be accompanied by southwest winds 50 to 65 knot at 5000
feet that will enhance orographic flow over the southwestern
Olympic mountains and the Cascades of Whatcom and Skagit counties.
The strong and moist flow will give snow levels over 7000 feet,
heavy rain with 5-7 inches in the Olympics and 3-5 inches in the
North Cascades and windy conditions, especially along the coast
and north interior. Cold air remains deep over Eastern Washington,
so easterly flow in the passes later Tuesday into Wednesday will
result in significant accumulations of Freezing rain. At this
time it appears that the passes will see one half to over one inch
ice accumulations Tuesday night and Wednesday, and the summit area
at Snoqualmie Pass may see well over one inch.
The Winter Storm Watch in the Cascades of Whatcom and Skagit
counties was upgraded to a winter weather advisory for a period of
snow late tonight and Tuesday Morning. The Winter Storm Watch
elsewhere was converted to a warning in the passes for first some
snow, then a significant accumulation of ice Tuesday afternoon
A Wind Advisory has been issued for the coastal zones and the
north interior zones north of Everett to the Canadian Border from
midnight tonight through Wednesday. Winds will be 25 to 35 mph
with gusts to 50 mph and will likely come in two bursts, one with
each passing front, late tonight and Tuesday morning then again
Tuesday night through Wednesday.
A Flood Watch has been issued for the Olympic Peninsula and
Western Whatcom County Tuesday evening through Wednesday evening.
See the hydrology section below for details.
Once the second wave passes late Wednesday, a cool and showery
pattern will develop. Temperatures will return to normal from the
balmy ones we see later Tuesday and Wednesday. Albrecht
.LONG TERM...Friday through next Monday will generally see cool
and showery conditions prevail. More significant model differences
start to take place Saturday night and Sunday with the GFS taking
a developing low eastward in a strong jet toward southern
California and the ECMWF and Canadian solutions taking the low
into the Washington offshore waters. With the uncertainty in the
details of the solutions, a model blend was used for pops and
temperatures. Generally, we expect temperatures to gradually drop
below average by 3-4 degrees by the end of the period and shower
chances to gradually decrease. Albrecht
.AVIATION...Clouds will increase, thicken up, and cigs will lower
late tonight as the first in a series of frontal systems moves
ashore. High pressure is inland with low pressure offshore. The air
is stable and fairly still this afternoon but will become moist
Tuesday as the rain develops. Westerly flow aloft will become
southwest and increase tonight.
KSEA...Mid and high clouds and then ceilings lowering later tonight
or Tuesday morning as rain arrives. Southeast breeze.
.MARINE...A series of fronts will move through the waters this
week. Gale force winds are likely at times Tuesday and Wednesday in
a typical southeast sucker wind pattern that favors most waters
with windy SSE wind, but not so much on Puget Sound.
.HYDROLOGY...A warm front will move through Western Washington
Tuesday morning. A second front will bring heavier rainfall with
high snow levels 7000-8000. A 24 hour period of strong southwest
flow aloft (50 to 65 kt at 5000 feet), along with a plume of
moisture originating around Hawaii, will result in heavy
precipitation on the southwest facing Olympics and in the North
Cascades around Mt Baker. Peak 24 hour precipitation amounts
Tuesday night and Wednesday in the Olympics will be near 6
inches. 4 inch rainfall amounts are likely around Mt Baker. A
flood watch was issued for the Olympic Peninsula from Grays Harbor
and Mason counties northward to Clallam County and for Whatcom
County in the north interior of Western Washington. At this time,
it appears that the rivers most likely to flood are the Skokomish
in Mason County and possibly the Nooksack in Whatcom County. Other
rivers such as the Elwha and Satsop may approach flood stage at
some point late Tuesday night or Wednesday. The GFS and University
of Washington WRF models continue to be on the high side of
forecast precipitation amounts. Albrecht
WA...Winter Storm Warning from 6 AM Tuesday to 6 PM PST Wednesday for
Cascades of Pierce and Lewis Counties-Cascades of Snohomish
and King Counties.
Flood Watch from Tuesday evening through Wednesday evening for
Grays Harbor, Mason, Jefferson, Clallam, and Whatcom Counties.
Winter Weather Advisory from midnight tonight to noon PST
Tuesday for Cascades of Whatcom and Skagit Counties.
Wind Advisory from midnight tonight to 3 PM PST Wednesday for
Admiralty Inlet Area-Central Coast-North Coast-San Juan
County-Western Skagit County-Western Whatcom County.
PZ...Small Craft Advisory for rough bar until 2 AM PST Wednesday for
Grays Harbor Bar.
Gale Warning until 2 AM PST Wednesday for Admiralty Inlet-
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-West Entrance U.S. Waters
Strait Of Juan De Fuca.
Small Craft Advisory until 2 AM PST Wednesday for Central U.S.
Waters Strait Of Juan De Fuca.
You can see an illustrated version of this discussion at