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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS66 KSEW 270355
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
855 PM PDT Sun Mar 26 2023

.SYNOPSIS...
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Lingering showers tonight before an overall warming
and drying trend is expected Monday through the middle of the week
as offshore flow develops and eventually a period of brief high
pressure sets up. A return to wetter and cooler weather for the
latter half of the week and into next weekend as low pressure
returns and a seasonally strong frontal system arrives.
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&&

.SHORT TERM /TONIGHT THROUGH WEDNESDAY/...
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Still some lingering
showers on radar, but activity is extremely isolated as the sun has
set. Temps over the area have already dipped into the upper 30s to
lower 40s for much of the CWA...and with dewpoint temps in the
lower to mid 30s, looks like another cold overnight period can be
expected as skies over W WA clear out. This falls in line very well
with inherited forecast and as such, foresee no need for an evening
update. For additional forecast details, please see the Previous
Discussion section below.  18

From Previous Discussion...Water vapor imagery this afternoon shows
a vort max centered near Grays Harbor bar in WA, with a ridge of
high pressure noted immediately to its NE over NE WA and southern
BC. On both the upstream and downstream flanks of these features
exist 2 broad areas of closed low pressure, one immediately west of
the BC Coast and the other over the central and southern Rockies,
the former of which is rather robust. Near the surface, an area of
low pressure just west of Haida Gwaii coincides nicely with the
upper close low just mentioned, while weak troughing continues along
the WA/OR Coast and high pressure funnels down the Plains.

MRMS radar imagery over the PNW looks very similar this afternoon
as compared to 24 hours ago. The vort max near Grays Harbor is
responsible for contributing to most of the lift needed to
maintain and further initiate activity, with daytime heating also
helping to contribute in tandem by helping fuel an unstable
atmosphere. Most of the activity as of 2PM is near the Coast and
towards southern Puget Sound, closest to the shortwave, and with
this wave expected to pivot towards the Sound this evening,
activity may blossom across central and northern portions of the
Sound this evening. SPC mesoanalysis shows 100-250 J/kg MUCAPE,
which could help support an isolated lightning strike through
sunset.

Most of the shower activity will wind down after sunset as the
shortwave weakens and what`s left of its energy begins to pull N
of the area. Snow levels will tumble back towards 500-700ft into
early Monday, but with the absence of appreciable moisture and
favorable location for lift, do not anticipate much of a threat
for a rain/snow mix in the lowlands. The cold airmass will still
linger, with low temps expected to dip near or just below freezing
for most locations. As the shortwave moves N/NE overnight, the
aforementioned robust closed low off the BC Coast will sink S/SW
well west of the WA/OR Coast. With its sfc reflection remaining
displaced just to its east, the associated frontal moisture could
bring showers or areas of light rain to the coastal water zones
and perhaps the coast overnight. This sfc low is then expected to
merge with a low tracking towards S OR, creating a much, much
stronger sfc cyclone and will help intensify/deepen the upper
level low. Most of the moisture associated with the system will
remain to the south, making for a dry but partly to mostly cloudy
day as some mid level cloud cover spreads in. Offshore flow will
increase in the low levels and with enhanced upslope flow on the
east side of the Cascades, a few showers may develop and drift
west of the crest. Offshore flow will help boost lowland high
temps into the mid to upper 50s Monday.

Monday night the upper level low will begin to move SE towards the
S OR/N CA Coasts, bringing with it the deep layer moisture inland
across these areas. Moisture will also begin to creep northward
from OR into WA and may clip Lewis County into Grays Harbor and
Thurston Counties. Moisture will have a very difficult time making
it much further north than that given strengthening offshore flow
off the Cascades in response to the tightening pressure gradient
between the sfc cyclone and higher pressure over BC. Breezy
conditions are likely across the area, with locally windy
conditions near the Cascade gaps and in the usual spots, like
Enumclaw and North Bend. A wind headline may be needed.

The upper level pattern will not change much through Tuesday
afternoon, with some light rain/areas of showers continuing to
affect Lewis/Grays Harbor, perhaps Thurston Counties. Continued
strong offshore flow should help boost highs towards 60 despite
better advection and coverage of mid and high level clouds.

Winds will begin to subside Tuesday evening as the pressure
gradient relaxes and the low near OR/CA weakens and begins to move
south. A few areas of rain/mountain snow may linger across Lewis
County into and through the overnight period, with upper level
high pressure beginning to build in briefly. Clearing is expected
from N to S into Wednesday, with Wednesday looking like the
warmest and sunniest day of the week. Some high clouds may spread
in, but think enough sun and subsidence will allow temps to rise
into the low-mid 60s.

Kovacik

.LONG TERM /THURSDAY THROUGH SUNDAY/...From Previous
Discussion...After a warm and sunny Wednesday/end to the short term,
the pattern will begin to change through the long term, trending
back to cooler and wetter conditions. For Thursday, a weak trough in
the upper levels will skirt the coast. Its mostly continental
trajectory will prevent it from obtaining much moisture, so likely
looking like some increased cloud cover with isolated showers,
mainly along the coast and across the higher terrain. High temps
will fall a few degrees into the mid and upper 50s.

Model guidance is then in good agreement in bringing a deepening
upper level trough towards the area from the Gulf of Alaska on
Friday, with an organized sfc frontal system with it. Exact
timing of its arrival will be fine-tuned in the coming days, but
the next chance for widespread precip is highly likely late Friday
and into Saturday. Widespread showers will then follow through
the remainder of the weekend, with lowland high temps falling back
into the upper 40s/around 50 with lows in the mid-upper 30s. Snow
levels will not dip quite as low as the previous weekend but will
remain low enough through this event to make for a potentially
heavy mountain snow event. Travel impacts across the passes are
appearing likely if current trends in the estimated precip amounts
continue. All in all, confidence is high in at least a wet and
cool pattern through most of the extended.

Kovacik
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&&

.AVIATION...
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Upper level low well offshore moving south through
Monday. Southwesterly flow aloft. Light flow in the lower levels
becoming offshore Monday.

Scattered clouds over the southern portion of the area. Ceilings
aoa 6000 feet over the northern portion. Scattered clouds all
areas by 10z continuing through Monday. With the light flow in the
lower levels patchy fog over the Southwest Interior and Kitsap
peninsula 10z-16z with ceilings below 300 feet and visibility less
than 1 sm.

KSEA...Scattered clouds. Could see some fog over the water and
possibly in the Kent Valley Monday morning 12z-16z but it should
stay below the terminal. South wind 4 to 8 knots becoming
northeast 13z-16z. Felton
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&&

.MARINE...
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Light southerly winds will continue come to an end early
Monday morning. A surface low will slide southward across the
Offshore Waters Monday and move towards the OR/CA coast later
Tuesday into Tuesday night. Offshore flow will increase during
this period, beginning Monday afternoon and continuing through
Tuesday afternoon before tapering off. Small Craft Advisories
remain in effect for the Coastal Waters as well as the
Central/Western Strait of Juan de Fuca with the increasing
offshore winds. In addition, will have to monitor the potential
for brief gale force wind gusts mainly later Monday night into
Tuesday morning for the Coast and Strait of Juan de Fuca, but at
this time gusts are not expected to be widespread. Additional SCA
winds are likely for the Northern Inland Waters as well as the
Puget Sound later Monday night into Tuesday. Winds shift more
northerly and ease Wednesday with southwesterly winds Thursday.
JD/Felton
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&&

.HYDROLOGY...
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No river flooding expected in the next 7 days.
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&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 5 PM Monday to 5 PM PDT Tuesday for
     Central U.S. Waters Strait Of Juan De Fuca-West Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 5 PM Monday to 2 PM PDT Tuesday for
     Coastal Waters From Cape Flattery To James Island 10 To 60
     Nm-Coastal Waters From Cape Flattery To James Island Out 10
     Nm-Coastal Waters From James Island To Point Grenville 10
     To 60 Nm-Coastal Waters From James Island To Point
     Grenville Out 10 Nm-Coastal Waters From Point Grenville To
     Cape Shoalwater 10 To 60 Nm-Coastal Waters From Point
     Grenville To Cape Shoalwater Out 10 Nm.

&&

$$