Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 242249

Area Forecast Discussion
National Weather Service Seattle WA
349 PM PDT Sat Mar 24 2018

.SYNOPSIS...An upper low will move onto the Oregon coast this
evening, with a few showers lingering over Western Washington this
evening. A warm front will become established over the area late
Sunday and persist through at least Tuesday, leading to widespread
light to moderate rain. High pressure will nudge into the area on
Wednesday and Thursday for a warming and drying trend. Colder air
will try to spread back into Western Washington with a few showers
next Friday and Saturday.


.SHORT TERM...A large upper low off the Oregon coast is in the
process of weakening into an open trough as it gets ready to move
across the Oregon coast this evening. This feature is wrapping a
band of precip around its northern quadrant into Southwest
Washington (mainly Lewis County). This band of precip will
continue in some form over the far south forecast area tonight.

For the rest of the forecast area, there are no large-scale
features to enhance precip this afternoon and tonight. With an
unstable air mass due to very cold air aloft, showers are mainly
focusing over the mountains and along coastal seabreeze fronts.
These showers will dissipate after sunset due to the loss of
daytime heating. So the overnight period and Sunday morning will
be dry for all but Lewis County.

West and northwest flow aloft will increase on Sunday PM and on
Monday around the periphery of an offshore upper ridge. Western
Washington will be in the dirty, drippy warm advection zone on the
ridge`s periphery. The ensuing warm advection lift and deepening
moisture will support the development of light to moderate rain
areawide on Sunday PM which will continue on Monday and Tuesday.

.LONG TERM...The offshore upper ridge will sharpen modestly near
130W-135W on Wed, which should be enough to lighten or diminish
the stratiform rain. Thursday appears to have the best chance for
a dry day, but confidence was only high enough for a full-on dry
forecast over the southern half of the forecast area.

The ridge will flatten and dissipate on Thursday night with a
resumption of stronger west-northwest flow aloft. Upper height
falls will develop over B.C. and spread southward into the
Pacific NW next Saturday. Colder air will gradually filter back
into the region. A few convective showers are possible as the air
mass gets colder and a bit more unstable. Haner


.AVIATION...A broad upper level low remains over the coastal
waters this afternoon with a disturbance over Northeastern
Washington moving away, a low pressure system over the Oregon
Offshore waters moving SE into far southern Oregon, and a weak
disturbance over the Washington coastal waters dissipating. The
air mass remains somewhat moist and unstable, and will gradually
stabilize as the low to the south of the area moves onshore.

Observations and satellite imagery shows cumulus and some light
showers mainly over the higher terrain while clouds and showers
over the waters are suppressed. Cirrus and stratocumulus with
bases around 035 have spread into the southern portion of the area
from the low to our south. Conditions are mainly VFR with some
patches of MVFR in the showers. Expect the lower level clouds to
diminish somewhat with loss of insolation this evening leaving
just mid and high clouds with VFR conditions tonight. Patchy
stratus with bases around 015 is possible during the morning hours
on Sunday with nocturnal cooling.

It appears that instability Sunday afternoon will be sufficient to
allow cumulus to build into a ceiling around 040 Sunday afternoon
before mid and upper level moisture thickens ahead of an incoming
frontal system Sunday night. Albrecht

KSEA...VFR conditions are expected to prevail through tonight with
ceilings between 040 and 050. A ceiling around 015 may develop
around the terminal area during the Sunday morning hours with
nocturnal cooling, then will lift in the afternoon back into a
sct-bkn layer around 040. South winds 4-8 kt late this afternoon
are expected to shift to light NEly this evening then turn back to
the south 12Z Sunday and increase to 8-10 kt by 19Z Sunday.


.MARINE...A low pressure system over the Oregon offshore waters
will move into far southern Oregon tonight. A weak ridge of high
pressure will develop over the Western Washington interior
tonight. Light winds will prevail through Sunday morning across
the waters.

A weak cold front will move through the waters later Sunday and
Sunday night. This front will likely produce areas of small craft
advisory conditions as it passes, first on the coast late Sunday
afternoon and evening then in the inland waters later Sunday

Another frontal system will drop southeastward through the region
Monday through Tuesday. Stronger winds aloft associated with this
system will interact with the terrain and may produce small craft
advisory conditions at times, mainly in the Strait.

Moderate onshore flow will develop Tuesday night into Thursday as
high pressure builds offshore. Expect moderate to strong inflow
during the afternoon and evening hours in the Strait.  Albrecht


.HYDROLOGY...River flooding is not expected the next 7 days.



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