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000
FXUS66 KSEW 181527
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
827 AM PDT Thu Apr 18 2024

.SYNOPSIS...Dry upper level trough over the area through tonight.
Upper level ridge offshore Friday moving inland Friday night. Slow
moving frontal system approaching the coast Saturday moving inland
Saturday night. Upper level trough behind the front Sunday morning
moving east Sunday afternoon. Weak upper level ridge over Western
Washington Monday into Tuesday replaced by a weak upper level
trough Tuesday night and Wednesday.

&&

.SHORT TERM /TODAY THROUGH SATURDAY/...Clear skies continue across
the region with temperatures beginning to warm up into the 30s and
low 40s. No updates to the forecast this morning. LH

Dry upper level trough over the area today with northerly flow
aloft. Frost advisory for portions of the area has expired. With
the upper level trough overhead temperatures aloft are not
particularly warm so even with sunny skies all day highs will only
be a couple of degrees above normal, in the upper 50s to mid 60s.

Trough shifts east tonight with a weak upper level ridge off the
coast Friday. Low level flow turning offshore with the cross
Cascade gradient down to -3 or 4 mb by Friday afternoon. This
combined with the warming temperatures aloft will result in about
5 degrees of warming putting highs in the 60s and lower 70s. Lows
tonight still on the cool side in the mid 30s to lower 40s with
the colder locations getting down to near freezing.

Weak upper level ridge out ahead of the next frontal system moving
into Western Washington Friday night and into Eastern Washington
Saturday morning. Frontal system offshore becoming negatively
tilted Saturday morning. This will slow down the eastward movement
of the front. The front will be approaching from the southwest so
there is a good chance that the Northwest Interior will be dry
Saturday. For the remainder of the area rain likely along the
coast with a chance of rain over the interior Saturday afternoon.
Low level flow staying offshore ahead of the front but the
increase in cloud cover will knock a couple of degrees off the
high temperatures Saturday with upper 50s to mid 60s in the
forecast. Lows Saturday morning with some high clouds will be
warmer, in the upper 30s to mid 40s.

Front moving through Saturday night with rain spreading over the
area. Snow levels start out near 5000 feet dropping to around
3500 feet behind the front so little snow expected in the higher
passes. There is a small pool of cool air aloft right behind the
front, 500 mb temperatures -25 to -28C, so have included a chance
of post frontal thunderstorms along the coast Sunday morning.

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...Extended models in good
agreement with upper level trough behind the front moving through
Western Washington Sunday morning making for a cool and showery
day. Highs only in the lower to mid 50s. Weak upper level ridge
Monday for a dry day. Northwesterly onshore flow Monday will keep
highs near normal, mostly in the lower 60s.

Operational models in good agreement with a weak upper level
trough approaching the coast Tuesday moving over the area Wednesday.
Model blend has some low pops Tuesday into Wednesday. Ensembles
show just a few slightly wet solutions for Tuesday. Will back off
the slight chance pops Tuesday and go with a dry forecast. With
the trough over the area Wednesday will stay with the blend chance
pops solution. Highs in the lower to mid 60s Tuesday cooling a
little for Wednesday.

Felton

&&

.AVIATION...Northwesterly flow will continue as an upper level ridge
builds to our west, with thermal troughing strengthening today along
the coast. Winds at the surface will remain northerly through the
TAF period, with flow turning easterly along the coast as the ridge
moves over today. Inland terminals will remain northerly with some
gusts in the afternoon. Wind gusts from 14 to 20 kt possible 20-03Z
this afternoon.

KSEA...VFR and clear skies with northwesterly flow aloft and
northeasterly flow at the surface between 6-8 kts. Winds will remain
northerly through the TAF period, with gusty winds up to 15 to 20 kt
Thursday afternoon, lasting potentially as long as 06Z.

Kristell/HPR

&&

.MARINE...An upper level ridge will continue to build offshore over
the Pacific, and will move inland today. High pressure further
inland combined with a surface trough along the coast will promote
offshore flow with gusty east winds of 10 to 20 kt through the
Strait of Juan de Fuca, into the northern offshore waters, and
through the central Puget Sound. Winds in these areas look largely
under SCA criteria, but a few gusts over 20 kt are possible through
late tonight. The next frontal system is expected to cross the area
waters Saturday with the next chances for widespread marine
headlines.

Expect seas to remain around 3 to 6 ft through Saturday, then rising
to 8 to 10 feet into Sunday.

Kristell/HPR

&&

.HYDROLOGY...No river flooding is expected in the next seven days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...None.
&&

$$