174
FXUS66 KSEW 270320
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
820 PM PDT Wed Mar 26 2025

.SYNOPSIS...Cold front moving through the area tonight with
thunderstorms, possibly severe, out ahead of the front. Upper
level low remaining off the coast into Friday keeping showers in
the forecast. Low weakening to an upper level trough and moving
inland Friday night. Next system arriving late in the weekend.
Another upper level low off the coast for the first part of next
week.

&&

.SHORT TERM /TONIGHT THROUGH SATURDAY/...Satellite imagery shows
storms developing over Northern Oregon moving north at 4 pm/23z.
Middle level clouds out ahead of the storms spreading over the
southern portion of the area. Middle level cloud deck also
exiting the state to the north. Temperatures in the mid 60s to
lower 70s Seattle southward and in the 50s north of Seattle.

Severe thunderstorm watch 65 issued for King, Pierce, Thurston and
Lewis counties until 9 pm tonight. Near record warm air mass over
the southern portion of the area. Steep lapse rates in place along
with CAPE values of 1000 to as high as 2000 j/kg indicative of a
very unstable environment. Dew points in the mid 50s to mid 60s
so plenty of moisture to work with as cold front approaches the
area from the southwest. Wind profiles over the area supportive of
possible supercell development. Note the definition of a severe
thunderstorm is hail 1 inch or larger and/or winds gusting 58 mph
or higher. The amount of lightning strikes is not a factor in
determining if a thunderstorms is severe. The storms will reach
Lewis and Thurston county 5 to 6 pm and into Pierce and King
county around 7 pm. With the cooler surface temperatures and the
end of the daytime heating right now it looks like the storms will
weaken as they move north of Seattle. Front moving inland behind
the convection keeping showers going into the early morning hours.

Upper level low remaining off the coast Thursday with another
short wave spinning out of the low moving to the coast by late
afternoon. Showers increasing over the area in the afternoon.
There is a slight chance for thunderstorms but with the air mass
much cooler Thursday if a thunderstorm does form it will be much
weaker than the storms tonight. High temperatures cooling into the
mid and upper 50s.

Low drifting north and weakening Thursday night into Friday. Low
still close enough to send showers into Western Washington with
the heaviest showers in the afternoon Friday. Highs will continue
to cool into the lower to mid 50s.

Low weakening into a trough and moving inland Friday night. Weak
upper level ridge trying to build Saturday but with the air mass
still slightly unstable will have to keep showers in the
forecast.

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...Upper level ridge
strengthening Sunday as another negatively tilted trough
approaches from the southwest. Sunday looks to be the dry day in
the next week. Negatively tilted trough moving inland for another
round of rain Sunday night. Upper level low remaining off the
coast Monday keeping showers in the forecast. Low moving inland
and weakening Tuesday and Wednesday. Temperatures will remain near
normal through the period with highs in the 50s and lows in the
upper 30s to mid 40s. Felton

&&

.AVIATION...Southerly/southwesterly flow aloft continuing into at
least Thursday morning. Frontal passage occuring at the time of this
writing, with western terminals and those south of SEA showing
mostly SWerly winds while east Sound terminals to the north
remaining northerly. Widespread southerly winds expected after
FROPA. Speeds mostly 6-12 kts although terminals closest to the
front are seeing gusts up to 25 kts. Speeds will diminish a little
as threat of thunder ends and a transition to more uniform showers
commences overnight, with speeds ranging 5-10 kts.

Even with showers and thunderstorms in the area, cigs over W WA are
largely VFR although terminals that see heavier rainfall have seen
cigs sink into MVFR to IFR. As showers persist tonight and into the
overnight hours, these periodic deterioration in conditions will
still be possible, however the only terminals expected to dip into
and remain in MVFR to IFR conditions are those that either saw the
heaviest rainfall during FROPA or are prone to lower cigs
anyway...with emphasis on OLM and CLM. Widespread VFR conditions
expected to return mid to late Thursday morning, although showers in
the afternoon may bring cigs down a little...resulting in some
locations seeing cigs dip down into high-end MVFR at times.

KSEA...Starting to see thunderstorm activity over the terminal at
the time of this writing and this threat will persist through around
06Z tonight. While gusts up to 25 kts remain possible also through
06Z, winds are expected to remain south/southwesterly with speeds
easing from 10-15 kts down to 5-10 kts for remainder of TAF period.
Showers will persist during the overnight and into Thursday morning
before second system starts to impact terminal late morning/early
afternoon.

18

&&

.MARINE...980 mb low well west of the area will approach the
coastal waters Thursday with gale force winds over the coastal
waters into Thursday night. Low far enough offshore for only small
craft advisory winds at the east entrance to the Strait Thursday.
Low weakening and moving off to the north Thursday night into Friday
with small craft advisory winds continuing over the coastal waters
and possible small craft advisory winds over the northern portion of
the inland waters.

Weak upper level ridge building over the waters Saturday with winds
easing. Next surface low associated with the negatively tilted
trough late in the weekend staying well offshore.

Seas building into the 12 to 15 foot range Thursday will subside to
near 10 feet Friday night and Saturday. Seas continuing to subside
Sunday into Monday. Felton

&&

.HYDROLOGY...Showers and thunderstorms tonight with potential for
locally heavy rainfall at times in stronger activity. However,
due to the convective and spotty nature of this precipitation,
most area rivers are expected to stay below flood stage.
Precipitation will continue into the first part of next week with
Sunday looking like the only dry day at this time. The
precipitation is not expected to cause significant rises to area
rivers.

The exception is the Skokomish River, where heavy showers are
favored to bring the Skokomish into flood stage by late Thursday
evening. The river will crest near moderate flood stage Friday and
remain over flood stage into Saturday. Large uncertainty exists
over the amount of precipitation over the basin. Conditions will
continue to be closely monitored. Felton

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Watch from Thursday afternoon through Saturday afternoon
     for Foothills of the Western and Southern Olympic Peninsula-
     Middle Chehalis River Valley-Olympia and Southern Puget
     Sound-Olympics-Southern Hood Canal-Willapa and Black Hills.

PZ...Small Craft Advisory from noon Thursday to 6 AM PDT Friday for
     Grays Harbor Bar.

     Small Craft Advisory from 6 AM Thursday to 6 AM PDT Friday for
     East Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 11 PM this evening to 6 AM PDT
     Thursday 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.

     Gale Warning from 6 AM Thursday to 2 AM PDT Friday 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.

&&

$$