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000
FXUS66 KSEW 190300
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
800 PM PDT Thu Apr 18 2024

.SYNOPSIS...Pleasant conditions tonight and Friday as an upper
level ridge offshore moves across the area Friday. Slow moving
frontal system brings the next chance for showers Saturday night
into Sunday. Weak upper level ridge over Western Washington
Monday into Tuesday replaced by a weak upper level trough Tuesday
night and Wednesday.

&&

.SHORT TERM /TONIGHT THROUGH SUNDAY/...Weak upper ridge axis
remains offshore this evening as it gradually shifts eastward toward
the region. With rising heights and a little more offshore component
to the low level flow, high temperatures on Friday will warm a few
degrees over todays highs. A short term update was made to the
forecast this evening to edge low temperatures tonight downward over
the Southwest Interior and Lower Chehalis Valley. Otherwise, the
forecast remains on track.  Previous discussion follows. 27

A very pleasant day across western Washington today with temps in
the upper 50s to low 60s and clear skies. Dewpoints are a bit higher
and with a little bit more wind overnight expected across the area,
currently not expecting widespread frost, with low temperatures a
touch warmer, in the upper 30s to low 40s, except in the low to mid
30s through the Southwest Interior.

Clear skies expected again tomorrow as a weak upper-level ridge
moves into the region. With a thermal surface trough, winds will
be primarily easterly, and may be a bit breezy through the gaps in
the terrain. These conditions set up for a warmer day on Friday,
with highs reaching into the upper 60s to lower 70s, especially
through the Southwest interior up into the Cascade foothills south
and west of Seattle (50-70% probabilities). Low temperatures will
also be warmer, into the low to mid 40s.

An upper level trough will swing into the area Saturday into
Sunday, bringing with it a surface low pressure system. With
trough strongly negatively tilted, rain will work its way slowly
from southwest to northeast. Rain will begin on the southwest
coast Saturday morning with areas Seattle and north staying mostly
dry until Saturday evening. Temperatures still above average, in
the mid 60s, with offshore flow becoming southerly and breezy
ahead of the front Saturday night into Sunday. Lows in the low to
mid 40s.

Scattered showers continue across the region into Sunday. A few
lightning strikes are possible late Saturday night into Sunday
behind the frontal passage, but instability should be fairly
limited and restricted to the Pacific coast. Sunday will be much
closer to normal with high temperatures in the mid to low 50s.

With snow levels ranging from 5000 ft on Saturday down
to around 3500 ft on Saturday, not expecting significant
accumulations on the passes, with an inch or two possible on
Stevens Pass and along the soon- to-be-open Route 20. Three-
quarters to an inch of rain is possible along southwest facing
portions of the Olympics and the North Cascades, with a tenth of
an inch possible in the lowlands and up to a quarter of an inch
along the coast.

.LONG TERM /MONDAY THROUGH THURSDAY/...Deterministic and ensemble
guidance both indicate an upper level ridge developing over the
area on Monday for another warmer and drier period with highs back
in the 60s. Tuesday should also be on the warm side mostly dry
across the area. There is also good consensus on an upper level
trough developing and moving into the area Tuesday night into
Wednesday. Moisture looks to be be very limited, with only a
handful of ensemble members showing the potential for rain making
it into the interior Wednesday and beyond. Temperatures
accordingly cool back closer to normal.

LH

&&

.AVIATION...Flow aloft will remain out of the northwest through the
TAF forecast as an upper level ridge continues to build offshore. A
thermal trough also remains positioned off the coast. Weather
remains dry as well with VFR conditions/clear skies expected. A
couple pockets of frost are possible overnight (although overnight
temperatures are slightly warmer than the previous night) - but no
fog is expected for Friday morning with dry grounds/upper level
ridging. Gusty north winds up to 20 kt remain possible through
Thursday afternoon - which will decrease to under 5 kt and become
northeasterly for Friday - picking up again in the afternoon to 20
kt.

KSEA...VFR/clear skies through the TAF period. Gusty winds out of
the north Thursday afternoon at 20 kt will diminish overnight to
around 5 to 8 kt northeasterlies, and pick up out of the north again
Friday.

HPR

&&

.MARINE...An upper level ridge will begin to move inland
tonight/Friday. A thermal trough remains positioned off the coast of
Washington. Northerly winds (with easterlies in the Strait of Juan
de Fuca) will continue through Friday. Gusty winds to 20 kt have
been observed in Puget Sound and central Strait of Juan de Fuca
waters this afternoon, but are expected to remain largely under
criteria needed for a small craft advisory. However, the next
approaching system for this weekend will increase gradients in the
central Strait of Juan de Fuca Friday (with stronger easterly winds
to 25 kt). A small craft advisory was issued Friday morning through
the evening. Additional gusty winds are possible Saturday and Sunday
with the next system.

Seas remain around 3 to 6 ft through Saturday, before rising to 8 to
10 feet Sunday.

HPR

&&

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

&&

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

$$