Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 221535
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
835 AM PDT Tue Aug 22 2017

.SYNOPSIS...High pressure will give dry weather and warmer than
average temperatures today. An upper level system will approach
the region Wednesday bringing cooler onshore flow and a few
showers to the coast. The upper trough will move eastward across
Western Washington Wednesday night and Thursday giving spotty
light showers. Highs on Thursday will struggle to reach the lower
70s over the interior lowlands. Another upper level ridge will
build into the region Friday through Monday giving dry weather and
above normal temperatures.

&&

.SHORT TERM...Morning satellite imagery shows stratus limited to
the immediate Pacific beaches while fog is limited to the lower
Chehalis river valley in Grays Harbor county. The rest of the area
is clear this morning with only a thin layer of smoke seen over
the central portions of the Washington Cascades north of
Snoqualmie Pass.

The upper level ridge responsible for the dry and warm weather
over the past couple of days is now well to the east of the
region. Expect onshore pressure gradients to ramp up this
afternoon and evening with the ridge continuing to move off to the
east and daytime heating in the interior. The increase in onshore
flow appears that it will be sufficient to produce gale force
westerly winds in the Strait of Juan de Fuca this evening and to
force stratus now over the coastal waters to push inland covering
most of the interior lowlands Wednesday morning.

As morning stratus burns off midday on Wednesday, clouds with an
approaching weak frontal system will move into the area. Showers
are possible along the northwest portion of the Olympic Peninsula
later Wednesday afternoon, then there will be a small threat of
light shower activity Wednesday night across much of the area as
the front pushes inland.

Clouds and the small threat of showers will continue, especially
over the Cascades and in a Puget Sound Convergence Zone, during
the day Thursday. Forecast models continue to show the trough that
will be passing by to the north of the area to be quite weak, and
now show clouds breaking up during the afternoon hours. So, the
current streak of consecutive 70 degree days at SeaTac airport may
continue, and the record of 62 days could be in jeopardy later
next week.

A forecast update was made for less clouds over the coastal areas
this morning. Otherwise forecasts appear to be in good shape this
morning. Albrecht

.LONG TERM...From the previous long term discussion: High
pressure builds Friday through early next week. The pattern
appears dry with a strengthening ridge. High temperatures were
trended warmer in the forecast with widespread 80s around interior
Western Washington over the weekend and possibly into Monday. A
few of the warmer locations in the southwest interior could get
close to 90 by Sunday or Monday but that`s a ways out still.
Mercer

&&

.AVIATION...Southwest flow aloft over Western Washington this
morning will continue through tonight as an upper level trough
approaches the Pacific Northwest from the offshore waters. At the
surface, high pressure remains offshore with lower pressure east
of the Cascades. Onshore flow will increase late today and tonight,
with the marine layer deepening and the air mass becoming moist
below 5000 ft.

Low stratus/fog over parts of the immediate coast and in the
lower Chehalis River valley will burn off by mid to late morning.
Otherwise there will be no significant cloud cover below 12,000 ft
today. Stratus will push inland tonight, and widespread low-end
MVFR and local IFR conditions are likely by 12Z.

KSEA...Light variable wind becoming southwest 4-10 kt midday and
eventually veering to northwest late afternoon. Wind should back
to southerly again after sunset. There is high confidence that
stratus will move into KSEA late tonight, probably around 12Z,
with ceilings OVC009-015. McDonnal

&&

.MARINE...A typical summer pattern of high pressure off the coast
and lower pressure east of the Cascades will continue through the
weekend, maintaining varying degrees of onshore flow across
Western Washington.

A weak upper level trough will approach British Columbia and the
Pacific Northwest tonight and Wednesday, then continue inland
Thursday, resulting in stronger onshore flow tonight through
Wednesday night. Gale westerlies are likely both nights in the
central and eastern Strait of Juan de Fuca, with small craft
advisory winds in the adjacent Admiralty Inlet and Northern
Inland Waters. McDonnal

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Gale Warning until 6 AM PDT Wednesday 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 6 AM PDT
     Wednesday 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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