Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 171046

Area Forecast Discussion
National Weather Service Seattle WA
346 AM PDT Tue Apr 17 2018

.SYNOPSIS...A weak upper ridge combined with moist low level
onshore flow will produce scattered showers and sun breaks across
Western Washington today. Showers will become isolated on
Wednesday, with dry mild weather Thursday into Friday. A frontal
system will move quickly across the area Friday night and
Saturday, with dry mild weather again Sunday and Monday.


.SHORT TERM...The upper low that moved across Western Washington
on Monday is over Alberta early this morning, with northwest flow
aloft over the region ahead of an offshore upper ridge. A minor
trough or vorticity lobe extending southwest into the region from
the upper low sustained showers into the night, but that feature
is moving east of the Cascades and weakening now, and showers are
decreasing. Moist onshore flow continues at low levels, with the
Puget Sound convergence zone producing enhanced showers from
Whidbey Island across Snohomish and Skagit Counties. Temperatures
at 3 am were in the 40s to lower 50s.

The transitory offshore upper ridge will move to the Washington
coast this afternoon and then east of the Cascades tonight. Moist
low level onshore flow will continue today, along with shallow
instability, for scattered showers especially in the convergence
zone and along the west slopes of the mountains. Areas on the
north and south side of the convergence zone will have partly
sunny skies and just spotty light showers. Highs today will only
be around 50.

The snow level will be around 2500 to 3000 ft today and tonight,
with another 3 to 7 inches of snow above that elevation in the
mountains during the next 24 hours. There should be a little snow
at both Stevens and Snoqualmie Passes.

An upper low will dig southeast into the offshore waters of
northern California on Wednesday. A weak upper trough extending
north from the low will move slowly east across our offshore
waters on Wednesday and then inland Wednesday night. As a result,
the upper flow will turn southerly and bring some high level
moisture into the area, but the low levels will dry as onshore
flow ends. Wednesday should be a mostly cloudy day, but spotty
light showers will be limited mainly to the mountains. It will be
a little warmer with highs in the 50s.

An upper ridge will tilt positively into Western Washington on
Thursday, extending into the region from an upper low well west of
California. The ridge will bring ... wait for it ... a day of
mostly sunny weather with highs near to slightly above normal.
Much of the interior lowlands from around Seattle southward should
break the 60 degree mark. McDonnal

.LONG TERM...The upper ridge will move east of the region Thursday
night, and the next Pacific frontal system will approach Western
Washington from the west on Friday. The leading edge of
precipitation will move inland Friday afternoon or evening, with
most GFS runs continuing to be slightly faster than the ECMWF. The
front and its upper trough will move through quickly on Friday
night, with showers and probably another Puget Sound convergence
zone on Saturday. Models agree that another upper ridge will move
over the area for dry and milder weather again Sunday and Monday.


.AVIATION...West to northwest flow aloft over Western Washington
this morning as an upper level trough axis now over northern Idaho
shifts into Montana. The flow aloft will back to southerly tonight
as a broad trough offshore digs southward. The air mass is moist and
somewhat unstable. Near the surface, strong low level onshore flow
will gradually weaken through the day as a surface ridge builds into
the area. MVFR ceilings throughout much of the interior of Western
Washington will gradually improve to VFR by this afternoon with the
exception of areas in the convergence zone over Skagit/Snohomish
counties where MVFR in showers may linger into this evening.

KSEA...Discussion above applies. MVFR with showers in the vicinity
to continue through much of the morning. Should see a gradual
improvement to VFR ceilings toward afternoon. Daytime heating may
increase shower coverage a little this afternoon, but convergence
zone activity is expected to remain north of the terminal. Low level
onshore flow will keep gusty S-SW winds 12 to 20 knots going through
mid-day then they are expected to gradually relax to 6 to 12 knots
by this evening.      27


.MARINE...Winds remain worthy of headlines this morning as a fair
number of obs still fall withing small craft or gale criteria.
Thankfully there have been no gales have remained
confined to areas under a gale warning and SCA winds
elsewhere...with some locations under the SCA starting to fall below
criteria. As it stands...will leave headlines intact for morning
issuance...the only change being the issuance of a SCA to follow up
the gales in the strait as winds will taper there far more gradually
than the other waters.

That being said...winds will gradually ease through the day
today...with most headlines expected to fall by the wayside by this
afternoon as high pressure over the coastal waters begins to assert
some influence over the area. This onshore flow will become light
tonight through Thursday as a high builds over the inland waters
then tracks slowly northeast.

Southerly flow will increase on Friday as a cold front approaches
the waters from the west. The front will move across the region on
Saturday. SMR


PZ...Small Craft Advisory until 11 AM PDT this morning for Admiralty
     Inlet-Coastal Waters From Cape Flattery To James Island 10
     To 60 Nm-Coastal Waters From Cape Flattery To James Island
     Out 10 Nm-Coastal Waters From James Island To Point
     Grenville 10 To 60 Nm-Coastal Waters From James Island To
     Point Grenville Out 10 Nm-Coastal Waters From Point
     Grenville To Cape Shoalwater 10 To 60 Nm-Coastal Waters
     From Point Grenville To Cape Shoalwater Out 10 Nm-Puget
     Sound and Hood Canal-West Entrance U.S. Waters Strait Of
     Juan De Fuca.

     Small Craft Advisory for rough bar until 3 PM PDT this afternoon
     for Grays Harbor Bar.

     Small Craft Advisory from 7 AM this morning to 11 PM PDT this
     evening for Central U.S. Waters Strait Of Juan De Fuca-East
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Gale Warning until 7 AM PDT this morning for Central U.S. Waters
     Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of
     Juan De Fuca.

     Small Craft Advisory until 3 PM PDT this afternoon for Northern
     Inland Waters Including The San Juan Islands.


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