Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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725
FXUS66 KSEW 290433
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
933 PM PDT Sun May 28 2017

.SYNOPSIS...An upper ridge over Eastern Washington will continue
to shift slowly eastward through Memorial Day. Onshore flow will
increase tonight through Tuesday as the upper ridge shifts 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 next week.

&&

.SHORT TERM...Onshore flow is increasing this evening. Pressure
gradients through the Strait of Juan de Fuca are now up to 2.5 mb
and increasing, while the pressure gradient from Hoquiam to
Seattle is now 3.5 mb. The marine push appears to be on now with
stratus streaming inland through the Chehalis Gap and some fog
moving in through the Strait of Juan de Fuca. With the southerly
flow aloft, expect the push to initially be shallow and clouds to
burn off in the interior by midday on Memorial Day. Still,
temperatures will be 7-9 degrees cooler in the interior on
Memorial Day than they were today. With southerly flow aloft and
warm air persisting in the Cascades, expect a slight chance of
showers or thunderstorms during the late afternoon and evening
hours on Monday.

The strength of the onshore flow is expected to increase Monday
night, allowing marine air to penetrate all the way to the Cascade
crest by early Tuesday. At the same time, expect mid and upper
level moisture to increase and southerly flow aloft to become
somewhat diffluent on Tuesday as an upper level trough moves
northeast through the coastal and offshore waters. This will
result in the formation of some showers and possible
thunderstorms, particularly Tuesday afternoon and evening. With
the deeper moisture in place and the shower activity, temperatures
on Tuesday will be quite a bit cooler, in the upper 50s to mid
60s.

An evening update was issued to blend the forecast of cloud cover,
POPS, and precipitation amount with the model consensus. Albrecht

.LONG TERM...From the previous long term discussion: The rest of
the week will probably see Western WA in a regime of deep marine
layer clouds that break out in the afternoon and evening. There is
a chance or slight chance of showers each day but the forecast is
fairly meaningless and the differences in timing of weak
shortwaves makes it impossible to say which days have the best
chance of being decent and which will be gloomier. One way it
often turns out is with some areas of morning drizzle and then
late afternoon sunbreaks. The PSCZ area is obviously a likely
place to see drippy weather this time of year with weak shortwaves
and middling heights. In the GFS, upper level heights do recover
a bit around the end of the week, so perhaps Friday and Saturday
will be decent days before an upper trough moves into the
northeast Pacific early the following week. And then there is the
ECMWF which is cool and showery as an upper low moves over the
area by next Saturday. A period of sunny weather with above normal
temps like we have seen this weekend is not at all likely for
some time based on the global models I see today.

&&

.AVIATION...Wrn WA will remain btwn an upper level trof offshore
and a ridge over the interior west for SW flow aloft. Expect the
surface onshore pres gradient to continue strengthening overnight,
leading to fairly wdsprd MVFR CIGs across the lower elevations
Mon morning. There will also be areas of IFR CIGs/VSBYs over the
interior lowlands after 0900 UTC.

KSEA...Expect MVFR CIGs to arrive between 2 am and 5 AM, possibly
dipping into the IFR category for a couple of hours Monday
morning. Winds will back to sly 5-10 knots overnight.

&&

.MARINE...High pressure offshore coupled with lowering pressure
east of the Cascades will result in strengthening onshore flow
through Monday. The flow will weaken Tue as the offshore high
shifts east, eventually moving onto 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.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Gale Warning in effect until 6 AM PDT Monday for the central
 and eastern Strait of Juan de Fuca.

  Small Craft Advisory in effect until 6 AM PDT Monday for the
 Admiralty Inlet, Northern Inland Waters, and west entrance
     to the Strait of Juan de Fuca.

&&

$$

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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