Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 110455
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
955 PM PDT Thu Aug 10 2017

.SYNOPSIS...Upper level ridge over Western Washington will move
east of the area tonight into Friday with the flow aloft becoming
southwesterly. In the lower levels onshore flow will increase
through Friday night. The change in the pattern will begin pushing
the smoke out of the area Friday with more improvement over the
weekend. High temperatures will cool on Friday with more cooling
over the weekend. An upper level trough will reach the area on
Sunday with an associated dissipating front bringing the best
chance for rain in almost 8 weeks Saturday night into Sunday.

&&

.SHORT TERM...Satellite imagery shows stratus along the coast,
through the Strait of Juan de Fuca and into the lower Chehalis
valley. The stratus is a little further east tonight versus the
last couple of evening with the stratus already into southwestern
Mason County, the temperature at Shelton has dropped from 86 at 6
pm to 67 at 9 pm. Temperatures at 9 pm/04z are in the 50s along
the coast and Strait as well as in the San Juans. For the
interior mid 60s to mid 70s are common.

The changes to the pattern that have been advertised for the last
few days are slowly taking shape tonight. The upper level ridge
axis has made its way into Eastern Washington. The flow aloft is
still light so no improvement in the smokey conditions yet. In
the lower levels the onshore flow is a little stronger this
evening versus the past few evenings as evidenced by the stratus
being further inland. Expect the stratus to make it a little
further to the east Friday morning with the leading edge just east
of the Puget Sound into the Seattle metro area. The layer will be
low and shallow so locations with a little elevation will be
foggy. With the layer being shallow it will not last long over
most of the interior with the stratus burning back to the coast by
midday. The weak marine push will drop high temperatures Friday
by 5 to 10 degrees over the interior with 70s to mid 80s common.
Along the coast where the stratus will remain all day along the
beaches highs will only be in the 60s.

Upper level trough approaching the area Friday night into
Saturday with southwesterly flow aloft increasing. At the surface
a ridge of high pressure out ahead of the trough will increase
the low level onshore flow. The combination of the two will
result in a big improvement in the smokey conditions over the area
on Saturday. Late night and morning clouds will hang around
longer Saturday with a deeper marine layer. Highs will cool by
another 5 to 10 degrees with most places in the 70s, the coolest
day in two weeks. Highs on the coast will remain in the 60s.

Evening model runs still show a very weak front and upper level
trough moving into Western Washington Saturday night into Sunday.
This will be the best chance of rain over the interior in almost
8 weeks. The front is weakening so rainfall amounts will be light
at best for the interior with less than a tenth of an inch
possible. Higher amounts are likely along the coast. Highs on
Sunday will only be in the 60s and lower 70s.

Current forecast has the trends covered. No update this evening.
The air quality alert will remain in effect through noon Friday.
Felton

.LONG TERM...Previous discussion follows. Nearly zonal flow aloft
will prevail over the region during much of this period for near
normal temperatures. Low level onshore flow of varying strength
will result in mainly night and morning low clouds. There is a
possibility that weak disturbances embedded in the zonal flow
aloft could produce some spotty light showers but trying to time
these features at this point would be folly.

&&

.AVIATION...An upper level high pressure ridge axis will slowly
move from Western Washington to the Idaho Panhandle by Friday
evening. Once the ridge axis moves east, light south-southwest flow
aloft will develop, and low-level onshore flow will strengthen. The
air mass is currently dry and stable, but look for mid-level
moisture and instability will increase on Friday afternoon as south
flow picks up. By Friday night, will be watching thunderstorm
potential as storms possibly spread up from southwest Washington or
the South Washington Cascades. Otherwise, visibility reductions from
wildfire smoke will improve on Friday in response to the arrival of
cleaner marine air.

KSEA...Little change in the smoke situation this evening and
tonight. Around sunrise on Friday, low marine stratus might spread
across the terminal, but it should have no problem burning off by
late Friday morning. Smoke situation should improve nicely by Friday
afternoon as cleaner marine air arrives. Will end smoke-related
visibility reductions from the TAF effective late tomorrow
afternoon. Northerly winds expected this evening before becoming SW
overnight and early Friday morning...then turning westerly in the
afternoon as onshore flow arrives. Wind speeds generally 5 knots or
less. Haner/SMR

&&

.MARINE...Moderate low-level onshore flow will lead to another
round of small craft advisory strength westerlies through the Strait
this evening. A marine push will further strengthen onshore flow
on Fri PM, with the 12z GFS model forecasting the UIL-BLI
gradient to peak near +3.4 mb on Fri evening. This should be strong
enough for westerly gales through the Strait at that time. Want to
evaluate one last round of data before pulling trigger...so any
upgrade to a gale warning will wait until morning forecast package.

A cold front will cross the waters on Sunday morning, followed by
another round of strong onshore gradients on Sunday evening. Haner

&&

.CLIMATE...The average high temperature for the first 10 days of
August in Seattle was 88.1 degrees. This is the warmest first 10
days of August in over 120 years of records.

Today was the 6th day this year with a high of 90 degrees or more
in Seattle. For the first time on record Seattle has had at least
5 90 degree plus days four years in a row ( 2014 5, 2015 12, 2016
7 and 2017 6 days so far ).

The record for the driest July and August at Sea-Tac airport is
0.03 inches in 1967 ( July 0.01", August 0.02"). So far this year
just a trace. Felton

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Gale Watch from Friday afternoon through late Friday 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 7 AM PDT Friday 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



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