Weather Service Report


156 
FXUS66 KSEW 230341
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
841 PM PDT Wed Sep 22 2021

.UPDATE...No changes were needed to the forecast this evening.

A shortwave trough continues to depart to the east this evening
and is currently located near the ID/OR border, per water vapor
imagery. Ridging will build over W WA in its wake overnight and 
through Thursday, contributing to height rises. At the surface, 
high pressure will remain in control following this afternoon's 
frontal passage. Extensive lower level cloud cover will continue 
through the remainder of the evening and will then gradually lower
more overnight, with many locations waking up to continued mostly
cloudy to overcast conditions. Some additional mid level moisture
may affect the northwest coast and San Juans/north interior 
towards Thursday afternoon, but still expecting abundant sunshine 
the second half of the day. Lows tonight in the low to mid 50s 
with highs tomorrow in the upper 60s to low 70s.

Previous discussion is included below with an updated aviation 
section.

Kovacik

&&

.PREV DISCUSSION... /issued 234 PM PDT Wed Sep 22 2021/ 

.SYNOPSIS...Drier and warmer conditions return Thursday into 
Friday as high pressure builds into the area. Another weak frontal 
system is expected to move through late Saturday. Wetter, cooler, 
and unsettled conditions are in the offing for early next week as
a series of weather systems affect the region.

&&

.SHORT TERM /TONIGHT THROUGH SATURDAY/...Just a few showers 
remain this afternoon as a weak upper trough exits the region. 
Upper level ridging rebuilds into Western Washington on Thursday. 
That combined with increasing northerly flow near the surface will
aid to dry out the air mass and bring mostly sunny skies to most 
areas by Thursday afternoon. High temperatures on Thursday will be
modestly warmer than today. The upper ridge axis will shift 
overhead on Friday. At the same time, thermally induced low 
pressure near the surface will lift northward along the coast. 
This will turn the low level flow weakly offshore. Plentiful 
sunshine and offshore flow will give high temperatures on Friday 
afternoon a significant boost...with much of the lowlands getting 
into the 70s...including coastal areas. 

The warming trend will be short-lived as the upper level ridge
flattens Friday night into Saturday and the low level flow turns
back onshore. A weak frontal system will approach the area by
later in the day Saturday. Clouds will begin to increase by 
afternoon with a few showers reaching the north coast and north 
interior by the evening. High temperatures on Saturday will be a 
little cooler but still seasonable for late September.

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...Saturday's weak front
merely opens the door for a series of early autumn systems to
affect the area. Models are beginning to latch onto a stronger 
frontal system reaching the area on Sunday. While not particularly
noteworthy, it'll bring a better chance of widespread light rains
and high mountain snow for the conclusion of the weekend. Model
ensembles are starting to come into better agreement with another
frontal system arriving on Monday with more significant rainfall
and breezy to locally windy conditions...perhaps much like late
last week. Unsettled, cool conditions then look to linger into at
least the middle part of next week. Autumn is right on schedule.

27

&&

.AVIATION...NW flow aloft, with a moist and is generally stable 
atmosphere. Conditions beginning to transition MVFR levels over the 
interior with ceilings lowering to IFR levels along the coast and 
strait. MVFR to IFR conditions will linger into Thursday morning 
then lift through the late morning and early afternoon to VFR levels.

KSEA...Ceilings have been lowing to MVFR levels - at or around 3000 
feet already this evening. Expect ceilings to lower by around 10Z to 
1000 to 1500 feet then lift through the late morning to VFR levels. 
Southerly wind around 5 kts this evening will veer to the N to NE 
late tonight.

&&

.MARINE...Lower pressure inland and high pressure offshore in the
wake of an exiting front will remain over the waters through 
Sunday. This looks like it may result in SCA winds in the Strait 
this evening and early tonight before easing, so will post 
headline to that effect with afternoon forecast. A thermal trough 
is expected to develop along the Oregon Coast late Thursday, 
causing northerly winds offshore to increase to near advisory 
strength Thursday and Friday afternoon and evening. Model guidance
shows speeds remaining just under SCA criteria for now, so will 
leave any potential headlines for future shifts. Another weak 
front will cross area waters over the weekend, reinforcing 
westerly flow down the Strait over the weekend. A more organized 
system is expected to approach early next week, likely yielding 
stronger winds across area waters Monday and Tuesday.

Offshore, seas generally 6 to 8 feet before decreasing to 4 to 5 
feet by Friday afternoon. Seas will increase slightly again this 
weekend before increasing substantially early next week. Seas over 
10 feet are possible with the passage of the system early next week. 

18

&&

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

&&

$$

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Western Regional Climate Center, wrcc@dri.edu