Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 280426
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
926 PM PDT Sat May 27 2017

.SYNOPSIS...A strong upper level ridge over Western Washington
will move slowly east into Eastern Washington Sunday and Memorial
Day. It will remain clear and warm in the interior while shallow
marine moisture affects the coastal zones. Increasing onshore
flow will cool temperatures starting on Tuesday and southerly flow
aloft will bring a chance of showers to the area. Weak
disturbances may bring a chance of showers to the area into the
end of the week.

&&

.SHORT TERM...A strong upper level ridge is sitting over Western
Washington this evening. At the surface, high pressure is seen
over the offshore waters while a weak thermally induced trough
sits from the southern Cascades southward into northeastern
California. Satellite imagery shows clear skies over the interior
of Western Washington. Shallow and locally dense fog can be seen
from around Cape Flattery southward along the immediate coastline
to just east of Westport and down onto the Oregon coast. The fog
and stratus will develop inland about 25 miles or so late tonight
into early Sunday morning then burn back to the coastline during
the afternoon hours. High temperatures in the interior on Sunday
will be up to a couple degrees higher than today in the interior
and a few degrees cooler on the coast.

The incoming GFS looks stronger with the onshore flow for Sunday
night and Monday. This is somewhat at odds with the earlier GFS
solutions and with the ECMWF, though with the increasing thermal
gradients between the coast and the interior make the forecast
pressure gradients plausible. But the slower ECMWF may also be
correct. Forecast models always have a very hard time forecasting
the details of a shallow marine layer offshore and its progression
inland, especially when upper level forcing is weak with a slowly
eastward moving ridge. At this time the forecast shows stratus
and fog spreading further inland, into the Strait of Juan de Fuca
and into the Chehalis Gap, late Sunday night into Monday as
pressure gradients become increasingly onshore over the next day
and a half. If the GFS is correct, it could spread further inland
and be a bit deeper. The forecast shows high temperatures on
Memorial Day are now about 5 degrees cooler than Sunday. The
incoming GFS, if correct, would indicate a bigger cool down near
the Puget Sound westward, but still rather warm to hot near the
Cascades and in the Cascade valleys.

Expect strong inflow through the gaps in the terrain and the
Strait of Juan de Fuca by Monday evening. This will result in a
deep marine layer and good cooling for Tuesday. Southerly flow
aloft and height falls associated with an approaching shortwave
trough will give an increasing chance of showers to the area
Tuesday or Tuesday night, and there could be a few thunderstorms
as well.

Gridded forecasts were massaged to account for the fog and low
clouds through the Strait of Juan de Fuca and onto the coast this
evening. Otherwise the short term forecasts are reasonable for
now. Albrecht

.LONG TERM...From the previous long term discussion: A chance of
showers will linger with cooler temps and marine air mass. The
best chance of showers may turn out to be Tuesday night and
Wednesday as a shortwave trough moves through the area--the 12z
GFS is wetter than the ECMWF. Then, for the rest of the week the
showers look pretty few and far between except perhaps on Friday,
but pops are still pretty low for now and the extended forecast
hasn`t been changed much.

&&

.AVIATION...An upper level ridge will remain over the region for sw
flow aloft. Weak low level onshore flow will become flat overnight.
Expect areas of LIFR CIGs and VSBYs at the coast to become a little
more wdsprd overnight.

KSEA...VFR. Winds will generally be light nly.

&&

.MARINE...
Weak onshore flow will become nearly flat or neutral overnight.
This will allow areas of dense fog on the coastal waters and west
entrance to the Strait of Juan de Fuca to persist through at least
Sunday morning.

High pressure will strengthen some over the offshore waters during
the day Sunday. This combined with lowering pressure east of the
Cascades will result in strengthening onshore flow on Sunday. It
now appears that gale force winds may be a possibility over parts of
the Strait of Juan de Fuca Sunday night, especially with afternoon
temperatures remaining quite warm over the interior lowlands in
comparison to the cooler coast. Expect moderately strong onshore
flow to prevail Monday and Tuesday.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 1 AM PDT Sunday for Central 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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