Washington State Forecast Discussion

Western Washington


044
FXUS66 KSEW 190356
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
856 PM PDT Fri Apr 18 2025

.SYNOPSIS...Upper level ridge axis east of the area tonight. Fast
moving weak upper level trough moving through Saturday morning.
Cooler stronger trough arriving Sunday for more widespread shower
activity. Post trough convergence zone could give Stevens or
Snoqualmie Pass a couple inches of snow Sunday evening.
Northwesterly flow aloft Monday. Dry trough moving into the area
from the north Tuesday. Brief upper level ridge Wednesday with
another trough approaching by Friday.

&&

.SHORT TERM /TONIGHT THROUGH MONDAY/...No updates tonight with the
weak disturbance approaching the far northwest portions of the
area this evening with some light showers and increasing cloud
cover moving in. Previous short/long term sections follow.

Not much going on Saturday with Western Washington in between
troughs. Air mass slightly unstable by afternoon combined with
weak convergence over the Central Puget Sound could produce a
light shower. The main weather story Saturday will be more cloud
cover and much cooler temperatures with highs only in the mid 50s
to lower 60s.

Another upper level trough sagging south along the British
Columbia coast Saturday night will arrive in Western Washington
Sunday. Shower activity increasing late Saturday night into Sunday
morning with the showers continuing into the afternoon hours.
Northwesterly flow aloft will rain shadow the Central and Southern
Puget Sound initially. Cooling trend remaining intact with highs
only in the 50s. Lows Sunday morning in the lower to mid 40s.

Post trough convergence zone forming Sunday evening. With the
northwesterly flow aloft, the zone will be a little further south
than its favored location, settling up right over King county.
Snow levels down to at least 3500 feet by this point. Could see a
couple of inches of snow at Stevens Pass and maybe Snoqualmie
Pass if the precipitation rates drive the snow level down a little
further. Folks coming back to Western Washington late Sunday
evening over the passes should be prepared for winter driving
conditions. The convergence zone will dissipate early Monday
morning. Lows in the mid 30s to lower 40s. Extended models in good
agreement with little in the way of weather Monday. Northwesterly
flow aloft over the area with the air mass beginning to dry out.
Could see isolated showers lingering in the morning hours
especially in the Cascades otherwise just a mostly cloudy day.
Highs in the 50s.

.LONG TERM /TUESDAY THROUGH FRIDAY/...Dry upper level trough
moving down from the north Tuesday, we saw one of these earlier
this week, will actually dry the air mass out over the area. Even
with the sunny skies highs will just be near normal, mid 50s to
lower 60s. Lows on the cool side Tuesday morning with colder
locations in the Southwest Interior having the potential to drop
to freezing. For the remainder of the area mid 30s to lower 40s.

Upper level ridge building Wednesday with northwesterly surface
gradients. Highs a little bit warmer, in the upper 50s to mid 60s.
Another cool morning with lows mostly in the 30s.

Models having trouble with the Thursday and Friday time frame.
Solutions varying between and upper level ridge over the area and
a negatively tilted trough moving into the area from the south
Friday. The negatively tilted trough scenario was in the models a
few runs ago, disappeared for a couple of runs and now it's back
again on the 12z run. Ensemble solutions trending towards a wetter
solution for Friday, especially the GFS. There is a small, less
than 10 percent, number of the ensemble solutions indicating
heavier precipitation Friday hinting at possible thunderstorm
activity. At this point with much uncertainty will go with the
old mostly cloudy chance of showers forecast Friday and keep
Thursday dry with highs in the 60s. Felton

&&

.AVIATION...A departing upper level ridge will be followed quickly
by a subtle shortwave trough moving through tonight into Saturday
morning. Winds aloft primarily W to NW. VFR conditions will prevail
through this evening for all locations.

MVFR conditions is developing along the coast this evening,
currently at KUIL and later KHQM. For the interior terminals,
ceilings will slowly lower overnight, becoming MVFR as the front
moves through. VFR conditions look to quickly return to the coast
behind the front (around 12-15Z Saturday), with low clouds hanging
on through the interior through 18Z Saturday before conditions
return to VFR. Overall, this front will be mostly dry with no
significant precipitation expected.

Most terminals seeing light southwesterly winds. A westerly push
down the Strait of Juan de Fuca is allowing for winds reaching 10-15
kt with gusts up to 20 kt possible at CLM. Winds look to remain west
to southwesterly going into Saturday, but will increase to 8 to 12
kt.

KSEA...VFR conditions through this evening. High-end MVFR cigs
develop early Saturday morning with the frontal passage, slowly
breaking up and lifting to VFR after 18Z Saturday morning. Light
southwesterly winds look to switch to light northerly to
northeasterly this evening. Winds return to west/southwesterly
tomorrow, increasing to around 5 to 10 kt.

62/41

&&

.MARINE...A weak front across the northeast Pacific will move
southeastward across the coastal waters tonight into early Saturday
morning. A westerly push down the Strait of Juan de Fuca this
afternoon and evening ahead of the front, which will spill into
Admiralty Inlet as well. Northwesterly winds will begin to develop
tonight across the coastal waters, as well as in the Strait of
Georgia north of the San Juans. Winds will peak in these
aforementioned locations Saturday morning and will ease Saturday
evening. Another westerly push, behind the front, down the Strait of
Juan de Fuca looks possible again Saturday afternoon.

Small Craft Advisories have been issued for these winds through
Saturday evening.

Another weaker system looks to traverse the area waters on Sunday,
which may prompt another round of headlines for the coastal waters
and Strait of Juan de Fuca. Following, broad high pressure looks to
regain control, maintaining light north-northwest winds across the
waters going into next week.

Seas 5 to 7 ft this afternoon will build to 10 to 13 ft early
Saturday. Seas will be steep with a dominant period of 8 to 9
seconds. Seas ease to around 6 to 7 ft early Sunday. The front on
Sunday afternoon may push seas back to around 8 to 10 ft. Seas will
ease into next week, remaining around 4 to 6 ft.

62

&&

.HYDROLOGY...No river flooding is expected through the next 7 days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until midnight PDT tonight for Central U.S.
Waters Strait Of Juan De Fuca.

Small Craft Advisory until 2 AM PDT Saturday for Admiralty Inlet-
East Entrance U.S. Waters Strait Of Juan De Fuca.

Small Craft Advisory until 8 AM PDT Saturday for Northern Inland
Waters Including The San Juan Islands.

Small Craft Advisory until 11 PM PDT Saturday for 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.

&&