Forecast Discussion for SEW NWS Office
FXUS66 KSEW 071124

Area Forecast Discussion
National Weather Service Seattle WA
324 AM PST Thu Dec 7 2023

.SYNOPSIS...A weak system traversing the region will bring lowland
rain and mountain snow to western Washington today into Friday
morning. A break in the weather is then expected Friday afternoon
and evening before the next system moves into the region on
Saturday and brings lowland rain, mountain snow, and gusty winds
to the area. Another break in the weather is expected by early
next week.


.SHORT TERM /TODAY THROUGH SATURDAY/...A weak system will continue
to make its way across the area today, bringing lowland rain and
mountain snow to western Washington. The system will be cooler and
accompanied by lowering snow levels, with snow levels looking to
drop down and hover between 2000-2500 feet by this afternoon.
Showers look to continue into early Friday, with hi-res guidance
indicating the likely development of a Puget Sound Convergence
Zone across the central Sound. Snow amounts look to be roughly
6-8 inches for Snoqualmie Pass and near 12 inches for Stevens Pass
should convergence zone banding set up within pass vicinity. Minor
impacts will be possible at times across the passes, so have issued
a Winter Weather Advisory for the Cascades through Friday morning.
Lowland areas look to receive a couple hundredths to half an inch
of rain, with a few spots expected to see half an inch to an inch.
Despite the extra precipitation, do not expect the amounts to have
much of an impact on area rivers - and expect to see them continue
to recede throughout the day today.

Except for some lingering convergence zone activity and a few
lingering showers in the Cascades, expect Friday to generally
trend drier as a shortwave ridge moves into the region. The break
in the wet weather will be relatively short-lived, however, as
the next system moves into the area on Saturday and brings with
it another round of lowland rain and mountain snow. Winds will be
gusty as the system traverses the area and look to increase Saturday
morning and afternoon. Gusts to 40-45 mph will be possible along
the Pacific Coast and for areas from Whidbey Island northward. This
system will be progressive, but will bring a round of moderate to
heavy precipitation to the area - with interior lowland locations
looking to receive 0.5-1 inches of rain and areas along the coast
looking to receive 1.5-2.5 inches. Snow levels will be low, hovering
between 1500-2000 ft on Saturday, so expect to see 8-15 inches of
new snow for the Cascades.

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...Precipitation looks to
gradually taper Sunday with drier conditions expected Monday and
Tuesday as ensembles and deterministic forecasts remain indicative
of an upper level ridge building back into the region from the
northeastern Pacific. Precipitation chances look to increase again
near midweek, however, as the upper ridge axis starts to push
inland across the interior West. This will then open the door for
additional systems to move across the Pacific Northwest. 14


.AVIATION...Light west-southwesterly flow aloft as a shortwave
trough moves over the Pacific Northwest today. At the surface will
drive a weak surface low and attendant cold front across Western
Washington. This is pushing a narrow band of rain northward through
the Puget Sound region currently with brief lowering of CIGs and
VSBYs for several hours. A return to MVFR thresholds by 15Z as the
band of steady rain lifts north. VCSH otherwise through much of the
balance of the day. Winds will be somewhat tricky today given the
track of the weak surface low, shifting from a south-southwesterly
direction to more of northwest to even northerly direction briefly
after 18Z at BLI and after 22Z at PAE and BFI. A return to a light
southerly wind is expected after 02Z this evening.

KSEA...South-southwesterly flow aloft as a shortwave trough moves
over the region. MVFR conditions through at least the early
afternoon with VCSH and periods of light rain, which may bring brief
IFR conditions. Southerly winds from 8 to 12 kt should shift to the
southwest then quickly shift to the north-northwest around 21 to 22Z
through around 00Z before returning to a southerly direction and
continuing through the rest of the night. A gradual improvement to
VFR conditions tonight with mid-level clouds from 3000 to 4000 ft.
CIGs look to lower closer to MVFR levels later into Friday morning.



.MARINE...A compact surface low pressure system will work its way
over the coastal waters this morning. Winds around the center of the
low are reaching 15-25 kt and will remain elevated until tonight.
Additionally, a push of westerlies through the Strait of Juan de
Fuca is expected as the system moves onshore, which will develop
this afternoon into early Friday morning. Therefore, Small Craft
Advisories have been issued for the coastal waters and for the
Strait of Juan de Fuca.

A strong low pressure system over the central Pacific will move
northwestward towards Haida Gwaii into the weekend, draping a cold
front over the region. Strong southerly winds reaching gale force
are expected, developing late Friday night and will last through
Saturday. Gusts may reach up to 50 kt for the outer coastal zone
north of James Island. Gales are also likely for the East Entrance
to the Strait of Juan de Fuca, the Northern Inland Waters, and
Admiralty Inlet.

Seas 11 to 13 ft at 13 s will ease to around 8 to 10 ft tomorrow,
before rising to around 12 to 15 ft though the weekend, reaching up
to 17 ft in the windiest regions. Seas will then plummet to 3 to 5
ft into the beginning of next week.



.HYDROLOGY...Area rivers continue to drop this morning across
Western Washington with only the Cowlitz, Snoqualmie, and Chehalis
Rivers remaining in Minor Flood Stage. These should drop back below
flood stage sometime later today and into Friday for the Chehalis
River at Porter.

Light rain and showers are currently moving over Western Washington
this morning in association with a weak surface low and attendant
cold front. This system is much weaker than the previous atmospheric
river frontal system, and is not expected to noticeably impact river

A slightly stronger frontal system will impact the region this
weekend with more widespread lowland rain and mountain snow, though
snow levels rise up above 6000 ft briefly Saturday evening into
Saturday night. Total forecast rainfall amounts will be much
lighter, generally ranging from half an inch to one inch in the
lowlands to 2-3 inches in the Cascades and Olympics. Given the
recent rain and flooding, rivers are running high and may be more
responsive to the rainfall than usual. Amounts will be highest in
the Olympics, so a Hydrologic Outlook was issued for Mason County
concerning the Skokomish River, which may rise back into Minor Flood
Stage late Saturday or Saturday night. No other rivers are forecast
to return to flood stage at this time.



WA...Flood Watch until 4 AM PST early this morning for Eastern Strait
     of Juan de Fuca-North Coast-Olympics-West Slopes North
     Cascades and Passes-Western Skagit County-Western Strait of
     Juan De Fuca-Western Whatcom County.

     Winter Weather Advisory until 10 AM PST Friday for West Slopes
     North Cascades and Passes-West Slopes North Central
     Cascades and Passes-West Slopes South Central Cascades and

PZ...Small Craft Advisory until 7 PM PST this evening for Grays
     Harbor Bar.

     Small Craft Advisory until 10 PM PST this evening 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-West Entrance U.S. Waters
     Strait Of Juan De Fuca.

     Small Craft Advisory from 1 PM this afternoon to 4 AM PST Friday
     for Central U.S. Waters Strait Of Juan De Fuca-East
     Entrance U.S. Waters Strait Of Juan De Fuca.



NWS SEW Office Area Forecast Discussion