Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 171619
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
920 AM PDT Thu Aug 17 2017

.SYNOPSIS...Seasonable summer weather will affect the area today and
Friday with morning clouds and afternoon sunshine. A weak system
will brush the area late Friday into Saturday morning bringing
slightly cooler conditions and possibly areas of drizzle. High
pressure will rebuild over the region Sunday through Tuesday for a
dry and warmer weather.

&&

.SHORT TERM...Marine stratus this morning has spread into all
but north interior and San Juan Islands with a bit of light drizzle
occuring over the Central Puget Sound area north of King County. As
expected, stratus has already begun to thin in many areas. Expect
skies to scatter out to partly or mostly sunny skies through out the
interior through the late morning. Low clouds may linger along the
coast into the afternoon before clearing out there as well. Upper
level heights over the area will rise today as a trough digging into
the Gulf of Alaska produces some shortwave ridging downstream over
the Pacific Northwest. A weakening frontal system will pass to the
north of the area Friday with resulting in a slight chance of light
precipitation mainly for the North Coast and possibly the North
Interior/North Cascades during the afternoon. More than anything
this feature will likely act to increase mid and high level clouds
but do little to effect the status quo of seasonal, dry weather into
Saturday.

.LONG TERM...from previous discussion...An upper level ridge
building over the Pacific waters will start to flex its muscles over
W WA starting Sunday as 500mb heights increase...signaling some
warming. This ridge will pass overhead Monday...with ECMWF
increasing the amplitude of its solution to match better with the
GFS...so overall looks like Eclipse day should see mostly clear
skies. While the ridge axis passes over the Cascades Tuesday...temps
end up peaking Tuesday afternoon before an upper level trough...and
cooling temperatures...enter the area for Wednesday.  SMR

&&

.AVIATION...An upper level ridge will prevail over the region
today for westerly flow aloft. The low level flow will remain
onshore. Expect the MVFR CIGs (with areas of IFR CIGs/MVFR vsbys)
that currently blanket much of the lower elevations to scatter
out by 2000 UTC.

KSEA...VFR conditions will return by 2100 UTC (2 PM PDT). Expect
low clouds (MVFR CIGs) to return late tonight and will probably
be slower to clear out tomorrow. Light westerly winds will become
northerly late this afternoon. Expect winds to become light
southerly late tonight.

&&

.MARINE...
Onshore flow of varying strength will continue through early next
week due to persistent high pressure offshore with lower pressure
east of the Cascades. The onshore flow will be strong enough
Friday night for the possibility of gale force winds over the
central and eastern Strait of Juan de Fuca.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisories are in effect for the central and
 eastern Strait of Juan de Fuca.

&&

$$

www.weather.gov/seattle

An illustrated version of this forecast discussion can be seen at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html



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