Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 190416
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
915 PM PDT Fri Aug 18 2017

.SYNOPSIS...A few light showers were falling around Western
Washington this evening. Clouds will decrease on Saturday. Another
weak system will move by to the north of the area on Sunday. High
pressure at the surface and aloft will give dry and warmer weather
to the area Monday and Tuesday. Another weak system will move across
mainly the northern portions of the area later Wednesday through
Thursday night.

&&

.SHORT TERM...Onshore flow will continue as a weak system moves by
to the north of the area and higher pressure builds in from the
west. A few light showers were falling here and there around the
Puget Sound area. Ridging offshore will deflect another weakening
frontal system to the north of the area Sunday. Some light showers
could fall on the northwestern Olympic Peninsula but mainly there
will be more clouds in the north and on the coast. Sunday high
temperatures will likely be near or slightly higher than Saturday.

Ridging aloft will build offshore Sunday night and will move east
across the area later Monday and Monday night. The ridging will
result in clearing late Sunday night into Monday morning and a
warming trend. With the ridging and warming aloft, highs on Monday
may reach 80 in the interior from Seattle southward. On Tuesday, we
will see the offshore ridge shift eastward into Western Montana by
late in the afternoon. The eastward movement of the upper ridge will
induce an increase in onshore flow, so it will be cooler on the
coast.

.LONG TERM...The ECMWF is showing better run to run consistency
than the GFS, and the latest GFS solutions are trending toward the
ECMWF.

The extended period will feature the transit of an upper
level low or trough to the north of the area during the latter
half of next week. This will result in an increase in onshore flow
and clouds and will give cooler daytime high temperatures. Even
the weaker ECMWF shows a possibility of showers across the area
later Wednesday through Thursday night, so they were included in
today`s extended forecast. The GFS appears to be backing away from
its solution of a deep upper low moving across the region mid to
late week.

Some ridging is possible beginning next Friday or Saturday, for
dry and warmer weather. The GFS is more extreme with the ridging;
a model blend about 2/3 toward the ECMWF was used for the forecast
fields for next Friday and beyond. Albrecht

&&

.AVIATION...West to northwest flow aloft over Western Washington
will continue tonight and Saturday, as the upper trough over
British Columbia moves east and an upper ridge builds offshore.
Surface high pressure remains offshore with lower pressure east of
the Cascades, and moderately strong onshore flow will prevail.

Ceilings below 1000 feet this evening along the coast. over the
interior scattered to broken layer near 3500 feet with a broken
cloud layer near 6000 feet. Little change for the next few hours.
Low level onshore flow combined with some convergence will lower
ceilings in the central Puget Sound area down to 1000-2000 feet
after 11z Saturday morning. Ceilings will improve all areas
Saturday afternoon.

KSEA...Ceilings near 6000 feet with scattered layer near 4000
feet going broken at times through early Saturday morning. Lower
stratus moving into the terminal around 11z with ceilings
lowering down to near 1000 feet. Ceilings improving late morning
with clouds scattering out by noon. Northerly wind 6 to 10 knots.
Felton

&&

.MARINE...KUIL-KBLI gradient near plus 3 mb this evening. Winds
have not hit gale yet in the central and eastern Strait of Juan de
Fuca but with the gradient this high will keep the gales going
into the early morning hours. Small craft advisories for
Admiralty Inlet and the Northern Inland Waters will also remain
in effect.

After tonight, a typical August pattern will prevail through the
middle of next week; onshore flow of varying strength will persist
due to high pressure offshore and lower pressure east of the
Cascades. Small craft advisory westerlies are possible in the
central and eastern Strait of Juan de Fuca in the evening and
early morning hours beginning Saturday evening and continuing into
the middle of next week. Felton

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until noon 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-
     Coastal Waters From Point Grenville To Cape Shoalwater 10
     To 60 Nm.

     Gale Warning until 3 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 3 AM PDT Saturday for Admiralty Inlet-
     Northern Inland Waters Including The San Juan Islands.

&&

$$

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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