Washington State Forecast Discussion

Western Washington


289
FXUS66 KSEW 181612
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
912 AM PDT Sat May 18 2024

.SYNOPSIS...An upper level trough will produce cool and
occasionally showery conditions across Western Washington today
into early Sunday. The trough will move east of the region by
Sunday afternoon and a weak upper level ridge will give drier and
somewhat warmer conditions on Monday. Another trough will arrive
on Tuesday for a return to cool and unsettled conditions for much
of next week.

&&

.SHORT TERM /TODAY THROUGH MONDAY/...Radar shows scattered shower
activity across the region this morning. Expect shower coverage
to increase over the next few hours and for conditions to remain
rather showery and cool today. The forecast remains on track. The
previous discussion follows below, with updates made to the
aviation and marine sections. 14

Early morning satellite imagery shows the next in a series of
upper level troughs slipping down the British Columbia coast
toward the region. Onshore flow will increase through the day as
this system digs southward through the area. Shower coverage will
increase later this morning...especially across the northern half
of the CWA. A convergence zone is likely to form by late morning
then gradually drag southward across Puget Sound through the
afternoon before dissipating in the Cascades late this evening.
High temperatures will be a good 5 to 10 degrees below normal.

The upper trough axis will shift east of the Cascades tonight and
only a few remnant showers in the Cascades are expected by Sunday
morning. Drier northerly flow aloft takes over Sunday afternoon
and most lowland locations should see some afternoon
sunshine...allowing high temperatures to nudge upward a few
degrees. Weak upper ridging will move across the area on Monday
and low level onshore flow will weaken. That spells drier and
marginally warmer conditions...if only briefly.

.LONG TERM /TUESDAY THROUGH FRIDAY/...By early Tuesday, the ridge
gives way to yet another upper trough digging southward over
British Columbia. We'll see a return to cool and wetter conditions
on Tuesday. The QPF totals with this system look fairly robust by
late May standards. Cool and showery conditions look to continue
into Wednesday as the upper trough continues to progress
southeastward across the Pacific Northwest. The forecast details
become more uncertain late next week. In general, ensembles
maintain negative height anomalies over the western third of the
lower 48 with strong ridging centered over the Gulf of Alaska.
But there are considerable differences amongst ensemble members
with the placement and depth of the trough over the Western US.
Current forecast from the NBM have mostly chance PoPs and temps a
little below average late next week. This seems like a reasonable
forecast until the overall picture becomes clearer. 27

&&

.AVIATION...West/northwesterly flow aloft today as an upper level
trough moves through the region. Ceilings between VFR and MVFR this
morning with a broken low-level cloud deck. Ceilings look to rise
for most to low-end VFR as cloud cover remains broken with scattered
showers developing this afternoon. Ceilings will be locally lower
at times, especially around the convergence zone. A lightning strike
or two cannot be ruled out this afternoon. Showers will then taper
off this evening. MVFR ceilings look to redevelop overnight tonight
as the low-level airmass remains moist and light onshore flow
persists.

KSEA...MVFR conditions this morning with light showers in the
vicinity. Breaks to low-end VFR possible this afternoon and tonight,
however locally lower under scattered showers. Better chance at
redeveloping MVFR CIGs after 12Z Sunday. S winds shifting to light
N/ around 21z then back to S by 06z tonight.

LH

&&

.MARINE...High pressure over the NE Pacific will maintain onshore
flow across western WA through the weekend. Strongest winds will
be over the Strait of Juan de Fuca with Small Craft Advisory winds
extending through late tonight. The next frontal system will reach
the region early next week.

Seas around 6 ft through most of the forecast period, through the
frontal system on Tuesday may raise seas and be steeper.

33/LH

&&

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

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 5 AM PDT Sunday for Central U.S.
Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
Strait Of Juan De Fuca.

&&