405
FXUS66 KSEW 300330
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
830 PM PDT Sun Sep 29 2024

.UPDATE...Mostly clear conditions this evening with lingering mid
clouds across central Puget Sound and Cascades. Dry conditions
into Monday. Patchy fog may develop tonight into early Monday,
mainly in river valleys and south of Olympia. No major forecast
updates this evening.

&&

.SYNOPSIS...A subtle trough will exit western Washington tonight
as upper level ridging starts the week dry. However a trough
moving to our north will influence the pattern Tuesday onward. A
more organized frontal system looks to arrive on Friday before
ridging rebounds over the weekend.

&&

.SHORT TERM /TONIGHT THROUGH WEDNESDAY/...We`re on the backside of
an upper-level trough here in western Washington while sandwiched
between two areas of high pressure. One of which is upstream over
the NE PAC and will influence conditions here as we start the
week. Showers have mainly been within the confines of the
mountains today and are in line to end this evening. Subsidence
produced by the incoming ridge will kick-start area wide dry
weather for at least the next 24-36 hours. Min temps are to
bottom out generally in the 40s. NBM is highlighting a 30%
probability for lowland locations to see 30s, mostly Olympia
southward and also the typical cool pockets like Arlington. Under
enough clearing this could be achieved.

The ridge axis will position overhead on Monday as high
temperatures rise a few degrees compared to the previous day`s.
However, ridging is slated to flatten by Tuesday as a deep low in
the Gulf of Alaska tracks into BC. With it, precipitation chances
will return also for Tuesday. PoPs aren`t too impressive, topping
out around 30-40% for most of the area but the northern half of
the CWA could see values along the 50-60% range. up to 0.10" of
rainfall is expected, some locations could see a trace at best.

Quasi-zonal flow into Wednesday as conditions trend mostly dry.
Guidance has weak ridging sitting offshore around this time.
Highs in the short term will range mainly between the 60s with
upper 50s along the coast. Locations in the SW interior are in
line to top out in the lower to mid 70s on Tuesday. Lows are to be
in the 40s.

.LONG TERM /THURSDAY THROUGH SUNDAY/...Transient ridging on
Thursday as it brings another dry day. However, a frontal system
is still on track to develop into Friday but lots of uncertainty
remains. The ECMWF deterministic solution has a front passing
through W WA with a decent shot of widespread rainfall. However,
the GFS has it slowing up and dying out on arrival. Ensembles are
consistent suggesting some precip for much of the area but
amounts are 0.10" or less. A return to upper level ridging will
wrap up the forecast period with dry conditions.

McMillian

&&

.AVIATION...West to southwesterly flow aloft will become
northwesterly this evening as an upper trough axis continues to
shift east of the Cascades. VFR cigs across western Washington
today, with streets of fair weather cumulus streaming into the
Cascades and across the Olympics. Light offshore flow on Monday
morning will negate the widespread presence of marine stratus,
with some fog potential in the southwest interior and near OLM.
Elsewhere, expect VFR conditions to continue throughout the TAF
period.

KSEA...VFR at the terminal this evening with N/NE winds, with
speeds of 8 to 9 kt until around 12Z, where the speeds will
relent to 3 to 6 kt. Light north to northeasterly flow is
expected into Monday morning, therefore, no marine stratus
intrusion is expected at the terminal in the morning due in part
to the offshore component of the wind.

Kristell/Mazurkiewicz

&&

.MARINE...The ridge will shift onshore this afternoon, turning the
low level flow more northerly. Thermally induced low pressure
expanding northward along the Oregon coast on Monday will turn the
flow weakly offshore at times. Onshore flow resumes on Tuesday as a
weak front dissipates over the coastal waters with little to no
impact. Some degree of onshore flow continues into midweek as
surface ridging remains poised offshore with lower pressure east of
the Cascades. Hazardous double digit seas are expected to reach the
coastal waters later Tuesday into Wednesday as the result of a deep
surface low passing well north of the area in the Gulf of Alaska.
Future shifts will evaluate possible headlines for the heightened
seas later in the week.

Kristell

&&

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

$$