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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS66 KSEW 260336
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
838 PM PDT Sat Mar 25 2023

.SYNOPSIS...
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Upper level troughing over the area through the
weekend will make for cool and showery weather before giving way
to drier conditions Monday. Temperatures will rebound through mid-
week back into the 60s with continued dry weather and sunshine. An
active pattern will return towards then end of the week with wet
weather, cool temperatures, and lowering snow levels likely.
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&&

.SHORT TERM /TONIGHT THROUGH TUESDAY/...
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Shower activity persists
mainly over the western half of the CWA, however both coverage and
intensity of these lingering cells have diminished significantly
since sundown. Temps slowly receding from their daytime highs,
mostly in the mid to upper 40s with a handful of locations,
including here at the WFO, reaching 50. Rate of cooling seems to
suggest that inherited low temps appear to be on track. That said,
see no reason for an evening forecast update. For more details on
the forecast, please see the Previous Discussion section below.  18

From Previous Discussion...Broad upper level troughing extends from
the Pacific Northwest southeastward through the Central Rockies this
afternoon, containing within it numerous shorter wave troughs/vort
maxes. Cyclonic flow then extends across much of the CONUS, with
another potent shortwave trough across the Great Lakes. At the
surface, troughing was noted along the WA/OR coast, with high
pressure over the Canadian Prairies and into the Northern Great
Plains, with strong low pressure over the Great Lakes.

Given upper level troughing over W WA this afternoon, the
atmosphere remains unstable and favorable for shower development.
As of 2PM, shower coverage is highest along the Coast, which
corresponds nicely with the sfc troughing and near the WA/Canada
border which corresponds to the area just downstream of a subtle
vort max embedded within the broad upper trough. For the rest of
the day, as the subtle vort max rotates towards the Olympic
Peninsula, should see coverage increase slightly, especially
during the remaining daylight hours as sunbreaks provide some
added instability (latest SPC mesoanalysis shows up to 500 J/kg
MUCAPE over portions of the area as of 2PM). Isolated lightning
strikes will also remain possible into the mid-evening hours
(especially near the Coast and in the vicinity of the Olympics),
before diminishing into the overnight period. Isolated to
scattered showers will then remain possible into Sunday morning
as the vort max swings across the area. There could be pockets of
a rain/snow mix or brief transition to snow in the lowlands given
temperatures in the low-mid 30s, however, recent model guidance
has suggested snow levels will remain slightly higher than
previously forecast, now more towards the 700-900ft range. Showers
are likely to become more isolated into the morning hours Sunday,
but with continued troughing aloft and favorable shortwave
energy, coverage will likely increase through the day, making for
a very similar day to today. High temperatures will remain nearly
identical in the upper 40s to right around 50.

The upper level trough will then exit to the SE Sunday evening and
Sunday night, with high pressure attempting to very briefly take
its place, however, its influence is likely to be minimal. After
midnight Sunday into very early Monday, a strong area of closed
low pressure will sink S/SW from Haida Gwaii over the open ocean,
west of the outer coastal water zones of WA and OR. Surface low
pressure associated with the closed low will quickly merge with a
separate area of low pressure off the coast of southern OR,
creating a strong Pacific storm system with plentiful moisture.
Confidence is high that the bulk of this moisture will remain to
the west and south of W WA, resulting in a mostly dry day through
Monday and high temps rebounding back into the low to mid 50s.
Increasing easterly flow from this system looks to contribute to
upslope flow on the eastern side of the Cascades, with the
potential for shower development and for some of this activity to
drift west of the Cascade crest.

The upper level closed low will then begin to head towards the N
CA Coast Monday night into Tuesday, with some of the moisture on
its northern periphery skirting Lewis County, and possibly
portions of Thurston and Pierce Counties. There could be some
snow mixed with rain if moisture does make it into the area, but
overall the amount of moisture is not impressive. Aside from
maybe some lingering showers across these counties on Tuesday, dry
conditions persist over the area, with more spring-like
temperatures returning as afternoon highs approach 60.

Kovacik

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...From Previous
Discussion...An E-W oriented upper level ridge will temporarily
build into the area on Wednesday as the closed upper low near the CA
Coast continues to drift south. This will make for a very nice
spring day across W WA, with added sunshine allowing for
temperatures in the low 60s, maybe even mid 60s. High clouds will
begin to spread in at some point during the day as the next upper
level trough approaches.

A shortwave trough is expected to move across the area on
Thursday, however, moisture appears limited. A few showers may
materialize, primarily near the Coast and across the higher
terrain before a more promising chance of rain arrives at the end
of the week, along with a return to colder temperatures. A deep
trough is expected to build into the area from the Gulf of Alaska
on Friday, carrying with it an organized frontal system. Showers
and areas of light rain are likely ahead of the front, with high
temps dropping back into the low 50s. Widespread rain is then
likely Friday night into Saturday, with widespread showers in the
front`s wake. Expect a return to temps in the upper 40s to around
50 on Saturday, with snow levels dropping to at or just below 2000
ft. Potentially heavy mountain snow is expected with this system,
and may affect the Passes.

Kovacik
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&&

.AVIATION...
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Light flow aloft and in the lower levels through
Sunday.

Scattered clouds for the most part through 10z. Ceilings 1000-2000
feet developing 10z-13z. Ceilings lifting into the 4000-6000 foot
range 17z-20z with VFR condition continuing into Sunday night.

KSEA...Scattered clouds until 10z. Ceilings near 2000 feet
developing 10z-13z. Ceilings improving by 19z to 4000 feet with
scattered to broken layer around 5000 feet Sunday afternoon and
evening. Southerly winds 4 to 8 knots. Felton
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&&

.MARINE...
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Small Craft Advisory winds continue for the Coastal Waters
overnight with seas around 10 feet. Seas subsiding below 10 feet
Sunday morning. Mainly light southwesterly winds Sunday into Sunday
night. A surface low will slide south along the Offshore Waters
Monday and linger south of the area Tuesday. Winds turning offshore
Monday and will remain offshore Tuesday. Small Craft Advisory winds
are likely for most waters, particularly for the Coastal Waters and
the Strait of Juan de Fuca. Offshore winds will ease and transition
a bit northerly on Wednesday. Felton/JD
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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...
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WA...None.
PZ...Small Craft Advisory until 4 AM PDT Sunday 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-Grays Harbor Bar.
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&&

$$