043
FXUS66 KSEW 180928
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
228 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/...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...A weak upper level trough will cross western WA today
for a chance of showers. The flow aloft is westerly. The low
level air mass remains moist with MVFR ceilings likely in the
central sound (where convergence zone showers are also expected).
Showers will decrease in coverage tonight as the trough exits
east. With onshore flow, MVFR cigs will likely reform during the
overnight hours tonight. 33

KSEA...MVFR conditions this morning with light showers in the
vicinity. Winds shifting to N around 21z then back to S by 06z
tonight. 33

&&

.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. 33


&&

.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.

&&

$$