Small Craft Advisory 213, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
925 PM PDT Mon Aug 21 2017

.SYNOPSIS...
Dry weather with above normal temperatures will continue through
Tuesday. An upper level system will brush the area Wednesday
bringing cooler weather, clouds, and a few showers. Upper level
ridging will return by the weekend with dry weather and temperature
rising well above normal.

&&

.SHORT TERM...
The upper level ridge over the region will continue to shift east
tonight into Tuesday. Expect upper level heights to remain around
585dm with a bit less low cloud coverage over the interior during
the morning hours. This will result in high temperatures over the
interior tomorrow warming a bit over todays highs into the low to mid
80s. The coast will remain in the upper 60s to low 70s. An
approaching upper level trough will help heights fall a bit Tuesday
evening into Wednesday with increased cloud cover and a deep marine
layer with stronger onshore flow by Wednesday morning. This will
help keep Wednesday much cooler day...likely by around 5 degrees or
so especially over the interior. Spotty light rain is also possible
Wednesday afternoon, especially over the north coast and north
interior.

.LONG TERM...from previous discussion...
By Thursday the upper trough of low pressure will move onshore
just north of Vancouver Island. Heights will fall even more and a
cool day is expected. This could be the day that breaks the
consecutive days above 70 streak for Seattle, but the present
forecast shows highs around 71 degrees, so it will be close. The
models suggesting a drier outcome than previously, have decreased
chances of rain and cloud cover. At best a low end chance for rain
and amounts not significant at all. The greatest change to our
weather will be cooler and cloudier conditions on Thursday.

Quickly behind this departing upper trough, ridging at 500 mb will
build into the area and send heights climbing again. Friday will
be the transitional day as readings are near normal. Then for the
upcoming weekend, indications are that well above normal
temperatures will return to the area. Latest model guidance
showing readings in the low to mid 80s around the Seattle area and
perhaps near 90 south of Seattle for Saturday, Sunday and into
Monday. Will have to continue watching this trend towards warmer
conditions. This represents a forecaster several degrees warmer
than the previous one. Climatologically it would be late to have
highs around 90 in Seattle, but not unheard of. Johnson

&&

.AVIATION...Strong upper level ridge over the Pacific NW slowly
drifting inland through Tuesday. Light westerly flow aloft backing
to SW overnight and Tuesday. At the surface, high pressure remains
offshore with lower pressure east of the Cascades for weak low level
onshore flow.

Skies clear across Western Washington this evening with the
exception of areas of fog just off the beaches along the south coast
drifting southward. The marine layer is surface-based and quite
shallow. Expect low stratus to bring areas of IFR ceilings to the
coastal region overnight and Tuesday morning with little if any
inland spread.

KSEA...Northerly wind around 10 kt easing to 5-8 kt about 06z and
then become light by sunrise. Skies to remain clear. Buehner

&&

.MARINE...Higher pressure off the coast with lower pressure east of
the Cascades is expected to continue into the weekend. This pattern
will maintain varying degrees of low level onshore flow across
Western Washington.

Small craft advisory west winds are likely in the central and
eastern Strait of Juan de Fuca tonight. A weak upper level trough is
forecast to approach British Columbia Tuesday night and Wednesday,
and continue inland Thursday. Gale westerlies are possible in the
Strait Tuesday and Wednesday nights as this upper level system
approaches. Buehner

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Gale Watch from Tuesday evening through late Tuesday night for
   Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S.
Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 3 AM PDT Tuesday for Central U.S.
Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of
Juan De Fuca.

&&

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html