Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 182140
AFDSEW

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

.SYNOPSIS...Onshore flow will increase this evening behind a weak
system moving by to the north of the region. Clouds will increase
this evening and there will be a chance of showers, mainly in the
Cascades and with a Puget Sound Convergence zone. 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...Changes were made to the short term forecast for
tonight to increase clouds in the interior and to include a chance
of showers in the Cascades and in the Central Puget Sound area due
to a potential convergence zone. Otherwise, only minor updates to
move toward the new model consensus were made to the short term
forecast.

Details: While clouds have diminished from about Everett down to
Tacoma, satellite imagery shows abundant cloud cover over the
Olympic Peninsula, the northern interior zones, the Willapa
Hills, and the northern portions of the Cascades. Onshore pressure
gradients are increasing at this time and will become rather
strong early this evening as a weak system moves by to the north
of the area and higher pressure builds in from the west. Short
term models now show the rapid increase in moisture below 6000 to
7000 feet this evening and the development of some showers and a
convergence zone around Everett that will push southward to about
Tacoma. With the increased consensus of the short term models,
Cloud cover and POPS were increased tonight for the Puget Sound
area and the west slopes of the Cascades.

Flat ridging offshore is expected to shunt another weakening
frontal system to the north of the area Sunday or so (a bit slower
in the more recent model solutions). While some light showers may
impact the northwestern Olympic Peninsula, it appears that the
main impact of this dying feature will be to bring some higher
stratus into the northern portions of area and to 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. At this time, it appears that skies will be nearly
clear in the interior for the viewing of the partial solar
eclipse in Western Washington. With the ridging and warming aloft,
highs on Monday may reach 80 in the interior from Seattle
southward (but temperatures will possibly be held down a couple of
degrees due to the reduced insolation that will occur due to the
eclipse).

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 and near the gaps in the terrain.
Albrecht

.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
this afternoon 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. The air mass is somewhat moist, mainly below
8000 ft.

There was a mix of cloud cover over the forecast area at 2 PM,
with the coast covered by marine stratus and generally VFR
conditions inland with ceilings mainly 035-070. Additional cloud
layers associated with the weak upper trough should remain over
the area this afternoon and evening, mainly between 4000 and
10,000 ft. Stronger onshore flow this evening and tonight will
probably push low stratus into most of the Western Washington
interior lowlands late tonight.

KSEA...Wind backed to southerly at 2 pm, but the trend should be
toward northwest winds 6-12 kt this afternoon, then north to
northeast after sunset, and then light and variable overnight.
Winds on Saturday should again begin southerly and veer to
northwest. A layer of clouds should move into the Puget Sound
region in the next hour or two with ceilings BKN-OVC070. Low
stratus with ceilings 010-014 is likely late tonight. McDonnal

&&

.MARINE...A weak system moving by to the north will result in
increasing onshore flow over Western Washington later this
afternoon and this evening. Gale warnings are in effect for the
central and eastern Strait of Juan de Fuca, along with small craft
advisories for Admiralty Inlet and the Northern Inland Waters.

Otherwise, 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. McDonnal

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from midnight tonight to 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 from 6 PM this evening to 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 from 6 PM this evening to 3 AM PDT Saturday
     for Admiralty Inlet-Northern Inland Waters Including The
     San Juan Islands.

     Small Craft Advisory until 6 PM PDT this evening 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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