Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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110
FXUS66 KSEW 182252
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
345 PM PDT Sat Aug 18 2018

.SYNOPSIS...A weak upper trough will shift across Oregon and
southern Washington Sunday. Northerly flow will bring wildfire smoke
from British Columbia into Western Washington. Air quality will
worsen Monday and Tuesday and easterly offshore flow brings thicker
smoke into the area. Onshore flow will gradually increase Wednesday
with the smoke and unhealthy air quality starting to improve. A
trough Thursday will bring clouds, a few showers, and cooler
temperatures.

&&

.SHORT TERM...A weak and mostly dry upper trough will shift
southeast across Oregon and southern Washington Sunday. The flow at
the surface and aloft will be northerly, ushering in additional
wildfire smoke from British Columbia. The ridge builds Monday and
Tuesday with light easterly flow leading to worsening air quality.
The WRF4km shows poor ventilation those days, likely to result in
unhealthy air at the surface. Temperatures will warm up Monday and
Tuesday but smoke should reduce potential highs by around 5 degrees
depending on how much smoke is in place. Kept temperatures below MOS
in the low 80s Monday and mid to upper 80s Tuesday.

Models show some weak instability over the Cascades, mainly south of
around I-90. Smoke over the area will reduce diurnal heating so this
will limit the chance for thunderstorms. But with trough clipping
the area Sunday, it seems reasonable there is some risk for
thunderstorms in the Cascades of Pierce and Lewis counties including
Mount Rainier.

.LONG TERM...Models differ on the timing of the push Wednesday. The
thermal trough appears to still be over interior Western Washington
Wednesday morning, not shifting east of the Cascades until late in the
day. Smoke could hang on across some interior areas into the afternoon
then begin to mix out as onshore flow increases. The air mass over the
Cascades could become unstable during the afternoon just ahead of the
push, so thunderstorms could develop but push off to the east by
evening.

A trough arrives Thursday with some showers and cooler temperatures.
Onshore flow will continue into Friday, but any showers would be limited
to the mountains at best. Northwesterly flow may persist into Saturday
with some onshore flow but also ridging off to the west. Expect
temperatures to run near average by the end of the week. Mercer

&&

.FIRE WEATHER...A critical fire weather pattern is coming up for
Western Washington, but the shading and therefore stabilizing effect
of smoke will be a significant mitigating factor.

A warm and dry northeasterly flow of continental air from B.C. and
Eastern Washington will strengthen tonight and peak on Monday
morning. Meanwhile, a somewhat diffuse thermal pressure trough will
peak along the coast on Tuesday morning. Mid-level Haines 6
conditions are also expected to develop on Sunday at continue at
times until early Wednesday. This indicates a dry and unstable air
mass between roughly 4,500 and 10,000 ft msl. Meanwhile, Min RH
values will drop below 30 percent in places on Monday and Tuesday.
All this would normally trigger the issuance of fire weather
headlines, such as a Red Flag Warning.

However, an impressively thick and dense area of smoke will spread
across Western Washington tonight through at least Tuesday. This
will keep afternoon temperatures about 5-8 degrees cooler than would
otherwise be expected, thereby acting to stabilize the lower
atmosphere and counter the Mid-Level Haines 6. The smoke shading
will also hold fuel temperatures down a bit. Will therefore use
forecaster discretion and avoid going with any highlights. Bottom
line is that the burning environment will support active fire
behavior, but would not expect anything extreme in the days ahead.

Otherwise, a strike or two of lightning is possible in zone 659 on
Sunday, but nothing widespread. Haner

&&

.AVIATION...Upper level ridging over the Pac NW will bring mostly dry
and stable weather to Western Washington tonight and Sunday. The
flow aloft is westerly. Near the surface, light onshore flow
prevails with marine stratus clouds over the coast. Smoky/hazy skies
are likely, especially by Sunday, as northerly flow brings B.C.
wildfire smoke into Western WA. There is a slight chance of
thunderstorms along the Cascade crest Sunday afternoon and evening
where heating and instability is the greatest. 33

KSEA...VFR conditions will prevail tonight. Northerly wind 5-10
knots. A layer of smoke aloft may mix down to the surface Sunday
afternoon with 4-6SM in FU. 33

&&

.MARINE...The flow will become northerly tonight and persist into
Sunday due to higher pres over British Columbia with lower pres over
Oregon. The northerly flow will cause smoke from B.C. wildfires to
begin drifting across the waters. This may persist through Tuesday.
Models are showing a strong onshore push on Wednesday with gales
possible through the Strait of Juan de Fuca Wednesday night. 33

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Air Quality Alert Sunday through Wednesday.
PZ...None.

&&

$$

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