Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 291111

Area Forecast Discussion
National Weather Service Seattle WA
411 AM PDT Mon May 29 2017

.SYNOPSIS...Onshore flow will increase through Tuesday as upper
ridging exits to the east. Marine moisture, in the form of low
clouds will push further inland and persist longer each day. An
upper trough and southerly flow aloft will bring an increasing
chance of showers and a few thunderstorms, mainly Tuesday and
Tuesday night. Weak weather disturbances will keep clouds and a
chance of showers in the forecast through the end of the week


.SHORT TERM...This weekend`s warm and dry upper ridge will exit
east today toward the northern Rockies, with increasing southwest
flow aloft. This has already opened the door to cooler marine air
spreading inland overnight, along with low marine clouds. ASOS obs
and derived fog imagery show low clouds have already pushed inland
through the Strait to near PAE, and up from the southwest to PWT
and SEA. By sunrise, marine clouds should fill in further over the
lowlands. As a result of the well-defined albeit shallow marine
air, today`s afternoon temperatures will be down 10 or more
degrees over the Puget Sound lowlands from Sunday. Otherwise, a
few showers may pop up this afternoon over the Cascade crest and
the interior of the Olympic Mtns, but weak wind profiles and
plenty of convective inhibition suggests that any showers will be
short-lived, just a quick pulse.

On Tuesday, we will be eyeing a disturbance that will swing up
from the southwest. Mid-level lapse rates will steepen with the
approach of this feature, with large-scale lift helping showers to
blossom over Western Washington on Tuesday. The marine layer will
deepen, and marine clouds are likely to be stubborn, leading to a
cool day with below normal temperatures. Owing to a cool and
stable boundary layer but unstable mid-levels, convection on
Tuesday will be elevated, instead of surface-based. The mid-level
wind profile will have quite a bit of directional and speed shear,
so some organization into a few clusters of thunderstorms seems
plausible. The focus of any thunderstorms will shift from the
Olympic Peninsula late in the morning to the Cascades by early Tue
evening, passing across the Seattle metro area during the
afternoon. With low-hanging marine clouds, sky watchers will not
be able to see the convective clouds above.

An upper low off the B.C. coast and W-SW flow aloft will continue
to deepen the marine air on Wednesday. Precip chances go down on
Wed due to less large-scale lift, but orographic lift with the
onshore flow pattern and moist boundary layer could lead to a few
light showers. Haner

.LONG TERM...Thursday and Friday will feature onshore flow and
broad upper troughing over the Pac NW. This will favor near or
slightly below normal temperatures along with a few light showers,
mainly on windward mountain slopes and in a Puget Sound
Convergence Zone. The ECMWF basically maintains this pattern
Saturday and possibly into next Sunday, while the GFS becomes a
bit more zonal with 500 mb heights rising above 570 decameters.
This will mainly affect the inland extent of marine clouds and
would make a few degrees difference in high temperatures, but
nothing more impactful than that. Haner


.AVIATION...Wrn WA will remain btwn an upper level trof offshore and
a ridge of high pres over the interior W. SW flow aloft will become
more sly with time.

Meanwhile, expect wdsprd IFR CIGs over the coast, Puget Sound region
swd, and areas adjacent to the Strait of Juan de Fuca this morning,
with areas of IFR CIGs from Whidbey Island nwd. There will be areas
of LIFR CIGs/VSBYs over the interior lowlands. CIGs will lift into
the MVFR category range and the fog will dissipate by 1900 UTC.
Expect the clouds to scatter out over the interior btwn 1800 and
2000 UTC; however, stratus will linger over the coast all day.

KSEA...The stratus was moving in from the south at this time. Expect
IFR CIGs to lift into the MVFR category range (1-2K ft) by late this
morning. There will likely be fog reducing the surface visibility to
5 SM, possibly as low as 2SM, this morning. Expect the clouds to
scatter out between 11 AM and noon. Winds will be southerly 5-10
knots, becoming westerly this afternoon.


High pressure offshore coupled with lowering pressure east of the
Cascades will result in strengthening onshore flow through tonight.
The flow will weaken Tuesday as the offshore high shifts east,
eventually reaching the WA coast Tue evening. A 1014 mb high on the
WA coast with lower pressure east of the Cascades will result in
light onshore flow on Wednesday.


PZ...Gale Warning until 5 AM PDT Tuesday for Central U.S. Waters
     Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of
     Juan De Fuca.

     Small Craft Advisory until 5 AM PDT Tuesday for Northern Inland
     Waters Including The San Juan Islands-West Entrance U.S.
     Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 3 AM PDT Tuesday for Admiralty Inlet.



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