Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 250316
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
816 PM PDT FRI JUN 24 2016

.SYNOPSIS...Weak instability and onshore flow will give some
showers to mainly the central Puget Sound area and the
Cascades tonight. A weak ridge of high pressure aloft will give
drier conditions on Saturday along with warming conditions. The
upper ridge will shift slightly inland over the interior Pacific
Northwest and Great Basin by Tuesday. This pattern will favor
mostly dry weather and above average temperatures Sunday through
next week.

&&

.SHORT TERM...Weak instability combined with onshore flow is
producing a weak convergence zone from the northeastern Olympic
Peninsula ESE through the Seattle area and into the central
Cascade mountains. Scattered showers are also seen along the west
slopes of the Cascades while some sprinkles are seen along the
coast and through the south interior. Conditions will gradually
dry late tonight as the upper low that gave cool conditions and
precipitation to the region over the past couple of days shifts
off to the east and is replaced by a short wave upper ridge that
is now along 130W longitude.

Satellite imagery and short range models show a weak front along
135W this evening. All models weaken this front as it moves into
the approaching short wave ridge. Model output statistics from the
NAM12 and GFS do show 20 percent POPS across the region on
Saturday, but given the incoming ridge and the drier ECMWF
solution...believe that nothing will be left of the offshore front
by the time it reaches the coast.

The main upper level ridge now centered from Texas into the
western Great Plains is forecast to retrograde into the Four
Corners region of the Desert Southwest over the weekend. In
response to the pattern shift, upper level heights over Washington
will rise to just over 580 DAM Saturday night through Monday, but
low level flow will generally remain onshore as temperatures rise
over the Great Basin and the Interior Northwest. The expected
pattern is typically one of dry conditions with temperatures a few
degrees above normal, but not overly hot. A weak upper level
disturbance may produce a shower or thunderstorm in the North
Cascades by Monday afternoon, but timing of the feature that may
produce the isolated tstms is uncertain enough that the forecast
can remain dry for now.

The short term forecast was updated in the mountains for snow
levels about 1000 feet lower tonight than what was previously
depicted. But the remainder of the forecast looks good and no
further changes are anticipated. Albrecht

.LONG TERM...From the previous long term discussion: Global
models show little change in the general pattern through next
week. High pressure aloft will remain anchored over the Great
Basin and Rockies with a series of weak upper lows brushing by
just north of Washington. This will cause some variation in the
strength of onshore flow which will affect temperatures a bit.
Went with a model blend in the extended which generally indicates
slightly above average temperatures and dry weather. Morning low
clouds could develop at the coast at times and may seep into the
western Strait or lower Chehalis gap, but nothing significant.
Mercer

&&

.AVIATION...An upper ridge over the offshore waters will weaken
and move over Western Washington tonight and Saturday with
westerly flow aloft. At the surface, onshore flow will prevail
with high pressure offshore and lower pressure inland. The air
mass is moist and slightly unstable. The air mass will become
stable later tonight and then drying will occur on Saturday with
clouds scattering most areas in the afternoon or early evening.

A Puget Sound Convergence Zone over northern King county should
weaken tonight and dissipate Saturday morning.

KSEA...Low clouds, mainly VFR this evening then likely MVFR early
Saturday morning, will lift and breakup Saturday afternoon. Clouds
will probably scatter entirely late in the afternoon. Southerly
wind 4-8 knots will becoming northerly Saturday afternoon.
Schneider

&&

.MARINE...Onshore flow will prevail for the next several days with
high pressure offshore and lower pressure inland. Small craft
advisory west winds are likely each evening in the Central and
Eastern Strait of Juan De Fuca. There will also be small craft
advisory northwest winds at times for the Coastal waters, mainly
late afternoons and evenings. Schneider

&&

.SEW Watches/Warnings/Advisories...
WA...None.
PZ...Small Craft Advisory until 5 AM PDT Saturday for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca.

&&

$$

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You can see an illustrated version of this discussion at
www.weather.gov/seattle/gafd/latest_webafd.html



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