Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 300358
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
858 PM PDT Mon May 29 2017

.SYNOPSIS...A weak upper trough will bring a chance of showers
and possibly some thunderstorms to Western Washington on Tuesday.
Onshore flow will peak on Tuesday then gradually ease the rest of
the week. Weak weather disturbances will keep clouds and a chance of
some showers in the forecast through the end of the week. Dry and
somewhat warmer weather may arrive by the weekend.

&&

.SHORT TERM...Thunderstorms popped up over the Oregon Cascades
this afternoon with one cell drifting over The Dalles. This cell
should weaken and remain S/SE of the CWA. Otherwise, skies are
clear except for stratus clouds along the coast. An onshore push
will bring stratus clouds farther inland overnight and cover most
of the interior by morning.

The upper level ridge that brought warm, dry and stable weather
to the region will push east on Tuesday. Tuesday will be cooler
with the increase in low level stratus clouds - highs mainly in
the 60s. An upper level short wave trough will swing N/NE through
W WA in the afternoon which may trigger a few thunderstorms.
These storms will be high based and most likely affect the
Cascades where instability is the greatest. T-storm activity
should diminish over the late evening.

An upper level low will spin off central/southern B.C. on
Wednesday then send a weak front inland on Thursday for a chance
of showers. Temperatures will remain close to normal. 33

.LONG TERM...Previous discussion...The GFS brings one more
disturbance through the area around Friday. This will keep some
clouds and a few showers around and probably hold high
temperatures back from reaching the 70s.

Both the GFS and ECMWF show more ridging by the weekend. A system
will brush the area but precipitation mostly should stay north of
the area. Kept the lowlands generally dry and indicated a slight
warming trend into the low 70s. The mountain may see a few showers
as a trough just barely brushes the area but is not expected to
amount to much. Mercer

&&

.AVIATION...A strong upper level ridge over Eastern Washington will
slowly shift east tonight and Tuesday. Flow aloft will become
southerly and allow upper level moisture into the area Tuesday. An
upper level shortwave approaching from the SW will cause the air
mass to become slightly unstable 12Z-23Z Tuesday for isolated
thunderstorms. The convection is expected to be elevated, which
means it would be occuring above the stable surface-based marine
layer.

The air mass over the interior is dry and stable, but marine stratus
on the coast has intruded as far as KSHN at 03Z. Moderate to strong
onshore surface flow will bring marine stratus into the interior
tonight. Widespread MVFR ceilings below 2500 ft are likely to cover
the lowlands after 10Z. Local IFR ceilings are possible. Ceilings
are expected to become MVFR/Low VFR levels after 21Z but most areas
will probably not clear out.

KSEA...Discussion above applies. Stratus ceilings below 2k ft are
expected after 10Z. Some improvement is likely in the afternoon
with VFR ceilings and some breaks in the overcast. Surface winds
will remain southwesterly 7-10 KT. CHB

&&

.MARINE...Currently solid westerly gales are occurring in the
central and east strait. Outflow winds are keeping Admiralty Inlet
and the northern inland waters in small craft advisories. All these
winds will taper later tonight.

Onshore flow will continue through the week. Gale force winds are
possible in the strait at times, along with small craft advisory
level winds on the coast and near the strait. After tonight, the
strongest winds appear to be around Thursday. CHB

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
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 2 AM PDT Tuesday for Northern Inland
     Waters Including The San Juan Islands.

     Small Craft Advisory until 11 PM PDT this evening for Admiralty
     Inlet.

&&

$$

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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