130
FXUS66 KSEW 190313
AFDSEW
Area Forecast Discussion
National Weather Service Seattle WA
810 PM PDT Wed Oct 18 2017
.SYNOPSIS...A rather wet front is over Western Washington this
evening. Heavy rain will turn to showers behind the front Thursday
and the snow level will fall to 3500 feet Friday. Another strong
front will reach the area for the weekend--with another round of
heavy rain. Drier weather is expected to develop early next week as
a ridge of high pressure strengthens.
&&
.SHORT TERM...The OLM-BLI gradient peaked just under 7mb at 4pm and
has since come down considerably for most of Western Washington.
Will keep wind advisories up for Everett south til 11pm, but ended
the high wind warnings in the north. The rain will only change over
to showers slowly--with the steady precip probably staying hung up
over southwest WA til daybreak. There is a flood watch in effect for
portions of the area as the rivers rise overnight.
Precipitation will taper to showers Thursday through Friday or
Friday night with snow levels gradually falling to 3000-3500 feet
by Friday. Snowfall amounts in the mountains may require the
issuance of a Winter Weather Advisory at times from later Thursday
through Friday.
The air mass is unstable behind the front, and there is a risk of
isolated thunderstorms over the coastal waters and the coastal
zones Thursday and Thursday night. Albrecht
.LONG TERM...Another strong warm front will lift northward into
Western Washington on Saturday. An atmospheric river type event
is possible in the warm sector, especially across the southern
half of the forecast area later Saturday through Sunday. It will
also be windy across the region with strong south to southeast
pressure gradients being amplified as the winds aloft interact
with the terrain. While the exact location of the heavy rainfall
axis is still uncertain, model trends appear to be focusing on the
area from Everett or Seattle southward into extreme northern
Oregon. Rainfall will gradually ease on Sunday night and Monday.
An upper ridge is expected to build northward into the Pacific
Northwest Monday into the middle of next week bringing a change to
dry and mild weather. Albrecht
&&
.AVIATION...A frontal system will sag southeast through the area
tonight with southwest flow aloft and low level onshore flow
behind the front into Thursday. The air mass is moist and stable.
It will become unstable near the coast Thursday.
KSEA...Rain will taper tonight as a front moves inland but low
clouds will remain prevalent through Thursday. South wind will
ease tonight. Schneider
&&
.MARINE...A front will move inland tonight. Gale force winds have
ended. Small craft advisory winds will gradually ease tonight and
Thursday morning. Onshore flow will prevail into Friday. A second
vigorous frontal system will move through the area Saturday and
Sunday. Onshore flow behind the front will ease and eventually
turn weakly offshore later Monday. A 20-24 foot swell train will
affect the Coastal Waters later Thursday through Friday morning
before easing below 20 feet Friday. Schneider
&&
.HYDROLOGY...The rain is still hung up over the area and there is
no change in the flood watches. Rainfall rates in the Cascades are
generally 0.2 to 0.4 an hour and in the Olympics are 0.3 to 0.6 an
hour under the plume. The rivers to watch are the Skokomish,
Nooksack, Stillaguamish, and Skagit and so the Flood Watches
will continue through midday Thursday for Snohomish, Skagit,
Whatcom and Mason counties.
Lowland locations from about Seattle southward may see urban and
small stream flooding due to the combination of high rain rates
and drainage issues due to leaves clogging drains. This will be
a middle of the night problem and the rain will have turned to
scattered showers in time for the morning commute.
Rivers will recede later Thursday through Friday night as snow
levels drop to 3000-3500 feet and precipitation becomes more
showery.
Hydrologically significant precipitation appears increasingly
likely Saturday through Sunday across the southern half of the
forecast area (especially from a Stevens Pass-Everett-southern
Olympics line southward). Many rivers may reach flood stage if
precipitation develops as expected. At this time it appears that
any flooding that occurs will be minor.
Conditions will dry out starting later Sunday or Monday of next
week allowing all rivers to recede.
&&
.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Wind Advisory until 11 PM PDT this evening for Bellevue and
Vicinity-Bremerton and Vicinity-Hood Canal Area-Seattle and
Vicinity-Southwest Interior-Tacoma Area--Everett and vcnty.
Flood Watch through Thursday morning for Bellevue and Vicinity-
Cascades of Snohomish and King Counties-Cascades of Whatcom
and Skagit Counties-East Puget Sound Lowlands-Everett and
Vicinity-Hood Canal Area-Lower Chehalis Valley Area-
Olympics-Western Skagit County-Western Whatcom County.
High Surf Advisory from 11 AM Thursday to 6 AM PDT Friday for
Central Coast-North Coast.
PZ...Small Craft Advisory for rough bar until 5 PM PDT Friday for
Grays Harbor Bar.
Small Craft Advisory until 11 AM PDT 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.
Gale Warning until 9 PM PDT this evening 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-Puget Sound and Hood Canal.
Small Craft Advisory until 5 AM PDT Thursday for Admiralty Inlet-
East Entrance U.S. Waters Strait Of Juan De Fuca-Northern
Inland Waters Including The San Juan Islands.
Small Craft Advisory until 6 AM PDT Thursday for West Entrance
U.S. Waters Strait Of Juan De Fuca.
Small Craft Advisory until 6 AM PDT Thursday for Puget Sound and
Hood Canal.
&&
$$
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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