Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 172217
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
315 PM PDT Fri Aug 17 2018

.SYNOPSIS...An upper level ridge will build over the area through
Saturday, with onshore surface flow continuing. On Sunday the
surface pattern will change, with offshore and northerly flow
developing. This pattern will continue through Tuesday, allowing
smoke from British Columbia fires to return to Western Washington
and possibly bringing unhealthy air quality. A marine push should
bring cleaner and cooler air around Tuesday night.

&&

.SHORT TERM...Skies are clearer this afternoon than they have been
for a week or more, though there is still some mid and upper level
smoke. Highs today will be in the 70s inland, 60s coast. Air quality
should remain mostly in the good category. Onshore gradients will
weaken somewhat tonight, resulting in less stratus coverage Saturday
morning. There will be earlier evaporation of the stratus as well.
Temperatures will be a few degrees higher on Saturday than today,
with some low 80s in the southwest interior.

The surface pattern will change Sunday, with offshore and northerly
flow developing over Western Washington. This will allow low level
smoke from fires in British Columbia to return. The smoke will
affect northern areas first, but seems likely to eventually cover
most parts of the interior, possibly even the coast. Air quality
concerns could return starting Sunday night or so. Have issued a
special weather statement to highlight the possible return of low
level smoke. There will be considerably less stratus on Sunday, with
high temperatures reaching well into the 80s over inland areas, 70s
coast.

Offshore and northerly flow will continue Monday and Tuesday.
Thermally induced low pressure will be over Western Washington.
Aloft there is likely to be a weak upper low over Oregon. This
pattern will allow smoke to continue at low levels, with continuing
air quality concerns. High temperatures will be warmer than normal,
with lots of 80s and perhaps some low 90s inland. The heat could
be enough to trigger a few thunderstorms in the Cascades and
adjacent areas. The coast could also be quite warmer than normal. If
there is a lot of smoke, temperatures will be lower than what
guidance gives, but it should be noted that MOS and other guidance
currently show upper 80s and low 90s over much of the area. Burke

.LONG TERM...Models show a marine push Tuesday night, with low level
onshore flow for the rest of next week. This should return
temperatures to near normal and alleviate most air quality concerns.
A weak system around Thursday has introduced low chance pops to the
forecast then. Burke

&&

.AVIATION...High pressure will build into Western Washington
tonight and Saturday for dry and stable conditions. The flow
aloft is NW to W. Near the surface, onshore flow prevails and low
level stratus clouds will likely develop along the coast
overnight. Stratus will push into the interior early Saturday
morning although coverage will be less compared to today. Low
clouds will burn back to the coast 18-21z.

KSEA...VFR conditions tonight with low clouds possibly reaching
the terminal by 12z. Confidence is not high as onshore pressure
gradients are weaker. Any low clouds will burn off early Sat
afternoon for VFR conditions. Light S/SW winds becoming N 12-15z
Sat. 33

&&

.MARINE...High pressure over the NE Pacific will maintain onshore
flow tonight. Highest wind and waves will be over the Outer
Coastal Waters and the Strait of Juan de Fuca - Small Craft
Advisories are in effect. Winds will gradually ease late tonight.
The flow will become northerly Saturday night and persist into
Sunday due to higher pres over British Columbia with lower pres
over Oregon. The next onshore push is Tuesday night into
Wednesday. 33

&&

.FIRE WEATHER...Have some concerns for critical fire weather from
Sunday through Wednesday, but there are also mitigating factors.

First, mid-level Haines 6 conditions are forecast from Sunday
through Tuesday. This indicates dry and unstable conditions in the
850-700 mb layer, or roughly 4,500-10,000 feet msl. This typically
leads to active burning conditions over the mountains. However,
possibly thick smoke is expected to spread over the area from
British Columbia on Sunday and Monday, persisting through Tuesday.
Thick smoke has a shading and therefore stabilizing effect on the
lower atmosphere, which would heavily mitigate the Mid-Level
Haines 6 conditions.

In addition, a few thunderstorms with lightning are possible from
Monday through Wednesday, mainly over the Cascades. However, if
southeast steering flow develop, some of the storms could venture
across the Southwest Interior or into the Southern Puget Sound
lowlands. Still much uncertainty with the pattern at this point,
which would affect coverage and location of lightning. Haner

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 2 AM PDT Saturday for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca.

     Small Craft Advisory until 6 AM PDT Saturday for Coastal Waters
     From Cape Flattery To James Island 10 To 60 Nm-Coastal
     Waters From James Island To Point Grenville 10 To 60 Nm.

&&

$$

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