915
FXUS66 KSEW 120940
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
245 AM PDT Mon May 12 2025

.SYNOPSIS...Lingering moisture with an upper low passing to the south
will keep scattered showers and possibly an isolated afternoon
thunderstorm in the forecast today. Upper level ridging will start
to build into the area midweek for the return of warm and dry
conditions. A broad upper level trough will bring active weather
back to the area for the end of the week.

&&

.SHORT TERM /TODAY THROUGH WEDNESDAY/...Latest satellite imagery
showing center of circulation for the current upper low over the
coastal waters of the norther OR coast. As such, both on satellite
and radar, seeing a wave of moisture being rotated up into the
southern portions of the CWA with echoes moving to the west-
northwest.

As this feature pushes inland today, that will keep the chance of
showers in the forecast for W WA at least into the late afternoon
before drying starts moving slowly over the area from west to east.
The coast should see activity wrap up by late in the afternoon while
locations east of the Sound seeing showers linger into tonight.
Given the isolated thunderstorms that happened Sunday afternoon and
with models suggesting some instability...will leave model generated
ProbThunder alone...especially since it falls in a pretty narrow
window of the mid to late afternoon time frame.

Models remain consistent in bringing in an upper level ridge into W
WA late tonight resulting in largely dry conditions Tuesday. This is
not a strong ridge by any means, being largely ineffective in
scouring out any lingering moisture. Thus, though generally dry
conditions are expected both Tuesday and Wednesday, there will still
be a fair amount of clouds over the area and a persistent chance for
showers over the Cascades.

High temp forecasts remain consistent with previous runs with little
change throughout the near term period: lower to mid 60s for the
interior, mid to upper 50s for the coast and water adjacent
locations.

18

.LONG TERM /THURSDAY THROUGH SUNDAY/...Deterministic models starting
to fall in line with each other, bring the next system into the area
Thursday...their timing about 3-6 hours off from one another.
Ensembles provide some fine tuning for the return of chances for
precip to the area...generally landing in the late afternoon/early
evening. As has been the case with most of these recent systems,
neither PoPs nor QPFs look terribly impressive...the former running
30-50 pct mostly and the latter falling short of 0.05 of an inch.
Depending on where you look, there seems to be a more favorable
opinion of the Friday system when it comes to generating rainfall.
Deterministic models are not really doing a good job of handling it
but ensembles provide a better picture with a strong preference for
overnight Friday and into Saturday. QPF values still fall short of
meh, with the ensemble mean falling largely right around 0.05 of an
inch, but there are a fair number of perturbations that clime to
0.10 of an inch or higher. As the trough responsible for this sinks
southward for the remainder of the forecast period, some
disagreement as to whether or not a closed low emerges. Regardless
of that, ensembles show chances slowly eroding throughout the day
Saturday and while not entirely gone Sunday, majority of locations
see only a slight chance by then.

This generally troughy pattern will not allow for any significant
change to the temperatures over W WA, with the bulk of the long term
echoing those of the short term.

18

&&

.AVIATION...An upper level low will spin through the Pac NW today
for scattered showers. The flow aloft is northerly. The air mass is
moist and slightly unstable with a slight chance of afternoon
thunderstorms in the interior and Cascades. Pockets of low clouds
and MVFR cigs this morning with more widespread MVFR conditions
Tuesday morning. 33

KSEA...Pockets of low clouds and possible MVFR cigs this morning.
Light S/SW winds this morning, increasing to 4-8 kt this afternoon.
Showers in the vicinity from 19-02Z. 33

&&

.MARINE...A 1000 mb low will weaken and shift east into Oregon
today, with onshore flow increasing tonight. Small Craft Advisory
winds are likely through the Strait of Juan de Fuca. Seas will also
build to around 10 ft over the outer Coastal Waters. High pressure
sets up offshore with lower pressure inland on Tuesday. This pattern
will remain intact through Thursday with the next frontal system
moving inland on Friday. 33

&&

.HYDROLOGY...The daily hydrology discussion has ended until the
start of the next rainy season; it will only be included as needed.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 5 PM this afternoon to 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 from 5 PM this afternoon to 5 AM PDT
     Wednesday for Coastal Waters From Cape Flattery To James
     Island 10 To 60 Nm-Coastal Waters From James Island To
     Point Grenville 10 To 60 Nm-Coastal Waters From Point
     Grenville To Cape Shoalwater 10 To 60 Nm.

&&

$$