Text Products for AFD Issued by SEW

Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 290450
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
950 PM PDT Tue Mar 28 2023

.UPDATE...-- Changed Discussion --Mostly quiet this evening as an upper-low continues to
track southward tonight. Scattered reflectivity is showing on area
radars but with dry air present at the surface given offshore
flow, conditions should be mostly dry tonight aside from
sprinkles/drizzle for the southern half of the CWA. Overnight lows
are to be in the upper 30s to low 40s.-- End Changed Discussion --

.SYNOPSIS...Mostly cloudy and dry conditions will persist through
tonight as weather impacts from a strong area of closed low
pressure near OR/CA remain to the south. Wednesday will be the
warmest day of the week as weak high pressure sets in ahead of a
shift back to cooler and wetter conditions the latter half of the
week and through the weekend. A seasonally strong front will cross
the area late Friday into Saturday, with showers continuing
through the weekend. Additional rain chances and cool weather will
then persist through early next week.

.SHORT TERM /TONIGHT THROUGH FRIDAY/...-- Changed Discussion --
The upper level low and
its associated sfc reflection will weaken some and slide south
towards the N CA Coast tonight, allowing a weak ridge of upper
level high pressure to approach the area. This should help begin
to clear skies from the north tonight, with any moisture over the
southern half of the area pushing further south towards the WA/OR
border. Expect skies to then clear more from north to south
Wednesday morning as high pressure arrives, with lingering
moisture mostly confined to central and eastern Lewis County,
particularly the southern Cascades. Weak subsidence aloft and
clear skies should allow Wednesday to become the warmest day of
the week with high temps reaching the low 60s. Some cloud cover is
expected to advect in as a subtle shortwave approaches the
southern BC Coast.

Wednesday night will be dry with ridging slowly becoming replaced
by weak troughing in the mid/upper levels. More cloud cover and
the development of isolated showers (mostly along the coast and
higher terrain) will lead to an overall cooler day with highs in
the mid/upper 50s. Current forecast could still be a bit
aggressive with rain chances given the shortwave trough`s largely
continental trajectory and overall lack of impressive dynamics
and moisture.

Conditions will then noticeable change on Friday as the next
organized, seasonally strong frontal system approaches the area.
Pre-frontal showers should increase in coverage Thursday night
into Friday with rain arriving along the Coast sometime Friday
afternoon or evening. Precip will then spread inland overnight and
through Saturday morning, resulting in measurable lowland rain and
heavy mountain snow. Winter headlines appear likely to be issued
in future forecast updates for this system, with travel impacts
likely across the Passes.

Kovacik/McMillian

.LONG TERM /SATURDAY THROUGH TUESDAY/...The local area will be in
an unstable post-frontal environment in the wake of Friday night`s
frontal system to begin the long term. Upper level trough
influence will increase as the day progresses on Saturday,
resulting in widespread shower activity across the area. This
activity is likely to be convective, resulting in the potential
for a few lightning strikes and small hail. This general pattern
will hold strong through Sunday, likely enhanced at times by
subtle vort maxes within the broader trough, resulting in
widespread showers through the entire weekend. Colder air aloft
and the cooler post-frontal airmass will result in weekend high
temperatures in the upper 40s to around 50, with lows back in the
30s.

Ensemble guidance is in good agreement with upper level troughing
continuing to be the primary weather influence through early next
week. There is some discrepancy in the strength of the trough,
but despite this, it is a fair bet that the weather will remain
cool and wet through at least Tuesday.

Kovacik-- End Changed Discussion --

.AVIATION...-- Changed Discussion --
A broad upper trough centered over southern Oregon
and northern California offshore waters will gradually shift
southward into Wednesday with easterly flow continuing over Western
Washington. Low level easterly flow will continue to weaken.
Expect primarily VFR ceilings with mid and high level moisture
streaming from east to west across the region overnight through
early tomorrow. However, locally lower ceilings coupled with
isolated showers will remain mostly from KOLM southward tonight.

KSEA...VFR ceilings under mid and high clouds overnight into early
Wednesday. East surface winds gradually ease and become northeast
overnight, before becoming west/northwest by Wednesday afternoon.-- End Changed Discussion --

.MARINE...-- Changed Discussion --
Moderate offshore flow will gradually weaken overnight
as a surface low west of the OR/CA border continues to shift
southward and fill over the next 24 hours. The flow will turn
onshore Wednesday afternoon as surface ridging builds into the
coastal and offshore waters. This might be enough to produce small
craft advisory westerlies in the central/eastern Strait of Juan de
Fuca Wednesday evening. A weak surface trough over the interior
waters on Thursday will keep winds across the waters under
advisory levels. The next in the series of fronts emerging from
the Gulf of Alaska will reach the area by later Friday and is
likely to generate additional headlines for area waters. The
active pattern will persist into the coming weekend.

S/SW swell over the coastal waters will approach 10 feet over
southern areas this evening before subsiding later tonight and
Wednesday. Coastal seas are then expected to remain below 10 feet
before building again this weekend. 27-- End Changed Discussion --

.HYDROLOGY...-- Changed Discussion --
No river flooding is expected through the next seven days.-- End Changed Discussion --

.SEW WATCHES/WARNINGS/ADVISORIES...
-- Changed Discussion --WA...None.
PZ...Small Craft Advisory until 9 PM PDT this evening for Central
U.S. Waters Strait Of Juan De Fuca-Coastal Waters From Cape
Flattery To James Island Out 10 Nm-West Entrance U.S.
Waters Strait Of Juan De Fuca.

Small Craft Advisory until 5 AM PDT Wednesday for Northern
Inland Waters Including The San Juan Islands.

Small Craft Advisory until midnight PDT tonight for Coastal
Waters From Cape Flattery To James Island 10 To 60 Nm-
Coastal Waters From James Island To Point Grenville 10 To
60 Nm-Coastal Waters From Point Grenville To Cape
Shoalwater 10 To 60 Nm.-- End Changed Discussion --

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