FXUS66 KSEW 262140

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

.SYNOPSIS...After a showery end to the weekend, an overall warming
and drying trend is expected through the middle of the week as
offshore develops and eventually a period of brief high pressure.
A return to wetter and cooler weather will then follow the latter
half of the week and through next weekend as low pressure returns
and seasonally strong frontal system arrives.


.SHORT TERM /TONIGHT THROUGH WEDNESDAY/...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

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.


.LONG TERM /THURSDAY THROUGH SUNDAY/...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.



.AVIATION...Southerly flow aloft will increase tonight as a surface
low slides south well offshore. Mainly VFR this afternoon with areas
of MVFR mainly along the Olympic Peninsula and Southwest Interior. A
few showers across the coast and Southwest Interior will slide
northward into the central Puget Sound and weaken into this evening
for VFR/MVFR cigs. Cigs will scatter out a bit more tonight with
generally a VFR mid level deck and increasing high clouds. Areas of
patchy fog will be possible into early Monday, mainly for portions
of the southern Puget Sound, although additional areas cannot be
ruled out with clearing. Light southerly winds will shift more
easterly tonight and northerly into Monday. JD

KSEA...VFR cigs into tonight. A brief period of showers later this
afternoon into this evening (generally 23-03z). Cigs will scatter
further tonight into Monday with VFR mid and high clouds. Patchy fog
cannot be ruled out into early Monday, but expected to be south of
the terminal. Southerly winds will shift more easterly tonight and
increase northerly later Monday morning. JD


.MARINE...Light southerly winds will continue through tonight. A
surface low will slide southward across the Offshore Waters on
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 have been issued 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 on Wednesday and southwesterly later Thursday.

Seas 6 to 8 feet this afternoon will subside to 3 to 5 feet tonight
into Monday. Seas will build again 6 to 9 feet on Tuesday. JD


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


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.



NWS SEW Office Area Forecast Discussion