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369
FXUS66 KSEW 020255
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
800 PM PDT Wed May 1 2024

.SYNOPSIS...Another weak system will slide to the south tonight
bringing in light shower activity. Drier and warmer conditions are
expected Thursday. More precipitation arrives in time for Friday,
the weekend, and beyond.

&&

.SHORT TERM /TONIGHT THROUGH SATURDAY/...
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Current satellite showing a
mix of high and mid level clouds this evening. Zooming out to the
entire Pac NW, IR loop shows the system passing to the south of the
area, however the aforementioned clouds are the northern edge of it.
This segues nicely to current radar, showing echoes mainly over the
coastal waters with maybe just the slightest of spillover onto the
far Southwestern corner of the CWA. While there may be a little more
slight eastward seepage to this precip, it is generally expected to
be pretty limited as this system continues its way southward.

That said, forecast on track for a dry overnight period and upper
level ridging for Thursday. As such, no evening updates needed at
this time. For additional forecast details, please refer to the
Previous Discussion section below.

18

From Previous Discussion...A weak frontal system will clip SW WA
tonight for a chance of rain (mainly coast and south of Olympia).
We`re seeing a mix of mid and high clouds on satellite associated
with this system with sunnier skies toward the north. Temperatures
will be chilly again overnight and in the mid to upper 30s over the
south sound. Patchy frost is possible although should be brief.

A weak ridge will build offshore on Thursday and we`re looking for
mainly dry weather for western WA. We could see a few pop-up
showers in the Cascades otherwise dry in the lowlands. The air
mass will be a little warmer with temperatures in the 60s. The
ridge axis moves inland Thursday night with dry weather
continuing.

The pattern changes as we move into Friday. By then the ridge
shifts farther east and inland while a deep upper low settles down
over the NE Pac. Attached to this low is a frontal boundary with
rain reaching the coast in the morning then spreading inland
during the afternoon.

The upper low shifts south off the coast on Saturday but we`re
still under moist/diffluent flow over western WA. This looks like
more rain spreading over the region through much of the day (and
not much of a break). With cooler and cloudier conditions we`re
only expecting highs in the lower to mid 50s. 33

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...From Previous
Discussion...The broad upper level low shifts inland over the
weekend and is still meandering over the West as we move into
Sunday. There`s still plenty of wrap-around moisture moving into
western WA with additional showers expected. Snow levels are around
4500 ft for rain at the Cascade passes. Moving on, we`re under solid
onshore flow early next week keeping us cool and wet through the
period. Onshore flow eases by Wednesday and drier N/NW flow may give
us a brief break in the action. 33
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&&

.AVIATION...
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North to northwesterly flow aloft tonight and Thursday
as an upper level trough continues to move eastward to make way for
an incoming ridge. VFR conditions in place over W WA this evening
with some high to mid level clouds overhead due to passing system to
the south. Majority of terminal sites likely to remain this way
through the overnight before additional clearing is expected during
the day Thursday. Mix of high to mid clouds should keep lower cigs
or fog out of OLM, however timing on clearing might just be early
enough to allow for IFR conditions to emerge at PWT. Will evaluate
this further for 06Z TAF issuance. Widespread VFR conditions
expected late Thursday morning to the end of the TAF period. Low
level winds starting to see some transition to northerly for
northern half of the CWA and should start to see remainder of sites
from SEA south transition to this direction by 06Z tonight and
remain that way for much of Thursday. Wind speeds generally staying
around 4 to 8 knots.

KSEA...VFR conditions persist with mid to high level clouds
lingering this evening and overnight. Clearing skies expected by mid
morning Thursday and remaining that way for the remainder of the TAF
period. Lingering S/SW winds as of 02Z at 4 to 8 knots will shift to
the north by 06Z and generally maintain the same speeds into
Thursday afternoon.

18
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&&

.MARINE...
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Onshore flow will persist with surface high pressure over
the coastal waters. A weak low pressure system will continue to move
south into the Oregon border tonight, allowing for winds over the
coastal waters to increase and turn southerly. Latest guidance keeps
winds over the coastal waters around 10 to 20 kts, remaining under
any headline thresholds.

Weak high pressure will move through the waters Thursday into
Friday, before a stronger system moves in Friday into Saturday
with small craft southerlies. Another system looks to move
through during the weekend.

Combined seas this afternoon are around 5 to 7 feet and will
subside to around 3 to 5 feet tonight. Seas will remain 3 to 5
feet before increasing on Friday.

Maz/18
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&&

.HYDROLOGY...No river flooding is expected over the next 7 days.

&&

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

$$