Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 220443

Area Forecast Discussion
National Weather Service Seattle WA
943 PM PDT THU JUL 21 2016

.SYNOPSIS...An upper trough over the coastal waters will move
into Western Washington tonight, bringing partly to mostly cloudy
skies and some showers. Thunderstorms are possible over the
Olympics, Cascades, and south interior this evening. Scattered
showers will decrease Friday. A weak upper ridge should bring
sunny weather Saturday through Thursday.


.SHORT TERM...Moist unstable southerly flow aloft ahead of an
upper low off the Oregon coast has brought some showers and a few
thunderstorms this evening. Most of the thunder has been over the
Cascades and the Olympic peninsula. The thunder threat appreas to
be about over now with a general shower threat continuing for all
areas overnight as the upper low moves overhead. The HRRR does
show an area of heavier showers and possible thunder forming again
over the Northeast Olympics and stretching across the Eastern
Strait into Snohomish county tonight...apparently in an area of
deformation as the upper low moves closer.

The upper low will shift into Eastern Washington Friday. Scattered
showers will gradually taper but could linger into the evening in
some spots like the Cascades and perhaps a Puget Sound Convergence
Zone north of Seattle. Much stronger onshore flow will develop
tonight resulting in a cloudy start to the day on Friday. Partial
sunshine is expected inland in the afternoon but highs will be 10
or more degrees cooler compared to today.

A weak upper ridge will move over the Pacific Northwest this
weekend. The weather will be dry with a typical summer pattern of
some night and morning clouds and afternoon sunshine with clouds
becoming more confined to the coast by Sunday. Highs will warm a
few degrees Saturday and then several more degrees on Sunday with
some spots probably back into the lower 80s or so. Schneider

.LONG TERM...Previous discussion...A fairly typical pattern for
late July will persist next week with light zonal flow aloft and
500 mb heights generally between 5820 and 5900 meters. Light
onshore flow will continue at low levels. The result will be sunny
day and mostly clear nights, except some late night and morning
marine stratus mainly at the coast. Heights will probably climb
and onshore flow weaken during the week for a gradual warming
trend. Some parts of the lowlands should reach 90 degrees
Wednesday and especially Thursday. McDonnal


.AVIATION...Upper level low pressure just off the mouth of the
Columbia tracking NE. Moderate south to southwest flow aloft
becoming light northwest flow aloft by Friday morning. Mid-level
air mass is moist and unstable, as evidenced by Accas clouds and
lightning over the Olympic Peninsula this evening. The 03z HRRR
shows possibility of lightning developing near the coast in the
next couple hours, then spreading eastward across the peninsula
overnight and early Friday morning. Otherwise, low marine stratus
expected to surge inland overnight with widespread IFR ceilings
over on Fri morning. Cigs lifting over the interior on Friday
afternoon, but having a hard time with scattering.

KSEA...Low marine stratus will quickly spread across the terminal
around dawn on Friday. Lifting will begin around 18z, but actual
scattering will be difficult on Fri afternoon. Haner


.MARINE...Gale force winds are still possible in the Central and
Eastern Strait, but odds are now going down as more time lapses
with no Gale obs. Otherwise, light to moderate onshore flow will
continue through the middle of next week. Haner


.SEW Watches/Warnings/Advisories...
PZ...Gale Warning until 5 AM PDT Friday for Central U.S. Waters
     Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of
     Juan De Fuca.

     Small Craft Advisory until 9 AM PDT Friday for Northern Inland
     Waters Including The San Juan Islands.

     Small Craft Advisory until 1 AM PDT Friday for Admiralty Inlet.



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