Dense Smoke in Washington, September 11 to 15, 2020

Area Forecast Discussion
National Weather Service Seattle WA
811 PM PDT Thu Sep 10 2020

.UPDATE...Mostly clear skies this evening with smoke across much
of the area. Marine clouds along the coast may advance further
across the Southwest Interior tonight into Friday morning for
morning clouds in spots, mainly around the southern Puget Sound
and Strait of Juan de Fuca. Smoke will increase tonight into
Friday for diminished air quality into the weekend.

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.SYNOPSIS...The upper ridge over the region will gradually weaken
through the weekend. Smoke from wildfires is expected to result in
air quality issues across Western Washington into the weekend.
Light onshore flow will bring marine layer fog and low clouds to
the coast tonight and possibly into areas of the Puget Sound.
Marine layer clouds will deepen and spread further inland each
morning over the weekend. There is a good chance that some rain
will arrive next week as a weather system moves slowly over the
area.

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.SHORT TERM /TONIGHT THROUGH SUNDAY/...The heat and offshore flow
will give way to increasing marine air and a cooling trend through
the weekend. The switch is already occuring on the coast this
afternoon where the temp at Hoquiam was down 24 degrees from
yesterday at 2pm. Humidity is way up on the coast as well. Marine
clouds and fog will arrive on the coast tonight but the marine layer
will probably be too shallow for any stratus or fog to reach as far
as the Puget Sound basin Friday morning. But over the weekend marine
layer clouds should flood the lowlands late each night into the
morning hours. With an awful lot of smoke over Oregon and the
Oregon coastal waters, the switch to southerly flow will cause
air quality to get worse before it gets better over most of Western
Washington. An air quality alert is in effect. There's not a lot
you can do to escape the smoke, but closing the windows after it
cools off overnight and then keeping them closed on Friday will
help. Temps will cool through the weekend with the switch to
onshore flow so at least it will be easier to keep the windows
closed over the weekend if the smoke remains bad.

.LONG TERM /MONDAY THROUGH THURSDAY/...The prospect for a little
rain is looking good as a weak upper low gradually shifts into the
region. The particulars are not a done deal and the forecast is a
blend of the model guidance--but at least the windows is open for
some precip and varying degrees of onshore flow will prevent
temps from getting away from us again. Highs should range from the
60s on the coast to the 70s inland next week. 19

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.AVIATION...Light west to northwest flow aloft continues this
evening as strong upper ridging remains centered just offshore. The
air mass remains dry, but smoke aloft will gradually increase
tonight into Friday. Smoke may reduce surface visibility to around 5
to 7 miles...except locally lower across far SW Washington. Near the
surface, low level onshore flow is increasing and looks sufficiently
strong to bring a shallow marine into Puget Sound by around daybreak
Friday. A period of IFR ceilings is expected for the central/south
Puget Sound Friday morning, SW interior, and along the coast. Any
stratus and fog should burn off across the interior by around 16Z or
17Z, but IFR ceilings could hang on along the coast until early
afternoon.

KSEA...VFR tonight with persistent smoke bringing surface visibility
down to 5 miles at times. IFR ceilings in stratus expected by around
12Z and possibly persisting until 16Z or 17Z. Smoke will continue to
limit surface visibility through Friday as onshore flow brings more
of it into the area. Surface winds light northerly shifting to S-SW
4 to 7 knots after 08Z tonight. 27

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.MARINE...The flow will turn onshore tonight as low pressure
shifts inland. Smoke may reduce visibility over the waters. The
flow will remain onshore through the weekend with highest wind and
waves through the Strait of Juan de Fuca. A frontal system will
move inland early next week. 33

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.FIRE WEATHER...Easterly offshore flow will transition onshore
tonight. Temperatures will be cooling off over the course of the
next few days, with high temperatures leveling off starting on
Sunday and will be much closer to normal. Humidities will slowly
be increasing throughout the area over the course of the weekend,
with the Cascade region being the last to moisten up. RH
recoveries will also be improving over the weekend. By Monday
minimum RH values across W. WA will be at or above 50 percent.
Until then, conditions remain dry and unstable for much of the
area into Saturday. A front moving through the area Monday night
is expected to bring some precipitation into Western Washington. A
wetting rain is possible for the Olympics and maybe Southern
Cascade Area as rain will continue through Tuesday.

Butwin/JD

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.HYDROLOGY...The daily hydrology discussion has ended until the
start of the next rainy season; it will only be updated as needed
until then.

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.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Red Flag Warning until 11 PM PDT this evening for Black Hills
     and Southwest Interior Lowlands-Central Coastal Lowlands-
     East Portion of the Olympic Mountains-Northeast Puget Sound
     Lowlands Generally Below 1500 Feet-Southeast Puget Sound
     Lowlands Generally Below 1500 Feet-West Slopes of the
     Central Cascades Generally above 1500 Feet-West Slopes of
     the North Cascades Generally above 1500 Feet.

PZ...Small Craft Advisory until 5 AM PDT Friday for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca.

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