Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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436
FXUS66 KSEW 212134
AFDSEW

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

.SYNOPSIS...
A ridge of high pressure and warmer than normal temperatures will start
the week. A passing upper level storm system in southwest BC will
bring cooler weather, clouds, and a few showers by mid-late week.
Then a ridge of high pressure returns by the weekend along with
temperatures climbing well above normal.

&&

.SHORT TERM...
As forecast, a solar eclipse occurred across the area today.
During the peak of the eclipse, temperatures fell 3-5 degrees.
However this didn`t do much to inhibit the overall warming today
as the readings quickly jumped back up as full sunshine returned.
Presently at about 230 PDT, readings from the 60s on the coast to
mid to upper 70s inland, with a few lower 80s in the warm spots.
All in all, a fairly normal day for the latter portion of August.
Tomorrow should be similar to today, minus of course the moon
covering up most of the sun. The weather will bring temperatures a
few degrees warmer than today with heights remaining at or above
585dm. They will fall along the coast however. At 500 mb a trough
of low pressure will be pushing towards the central BC coast and
help heights across the coastal areas to trend downwards on
Tuesday here as well. Heights will continue to fall on Wednesday
across the area but generally still above 574dm for everyone.
There will be increased cloud cover and a deep marine layer with
stronger onshore flow. This will bring a much cooler day by around
5 degrees or so to the area. Some suggestion of spotty precip on
Wednesday afternoon but overall not too impressive given how high
the 500 mb heights are, but could have a few showers here and
there but not expecting rain of significance. Johnson

.LONG TERM...
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...Light west flow aloft over Western Washington this
afternoon will back to southwest tonight and continue Tuesday, as
an upper ridge over the Pacific Northwest moves east. At the
surface, high pressure remains offshore with lower pressure east
of the Cascades for weak low level onshore flow.

Low stratus will bring areas of IFR ceilings to the coastal area
tonight and Tuesday morning, in the shallow surface-based marine
layer. Otherwise there will be no significant cloud cover below
12,000 ft.

KSEA...North to northwest wind 6-12 kt, becoming north to
northeast 4-10 kt around sunset. Winds should start light and
southerly Tuesday morning, then become west to northwest around
midday. The terminal should remain nearly cloud-free through
Tuesday. McDonnal

&&

.MARINE...High pressure off the coast with lower pressure east of
the Cascades will maintain varying degrees of low level onshore
flow across Western Washington into the weekend.

Small craft advisory west winds are likely in the central and
eastern Strait of Juan de Fuca tonight. Gale westerlies are
possible in the Strait Tuesday and Wednesday nights as a weak
upper level trough approaches British Columbia, and a gale watch
is now in effect for Tuesday night. The upper trough will move
inland Thursday with weak to moderate onshore flow Thursday
through Saturday. McDonnal

&&

.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



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