NWS Seattle, WA wrote its last area forecast discussion before the first event at Dec 29, 4:21 PM PST, 2 hours 39 minutes earlier.
Area forecast discussion, Dec 29, 4:21 PM PST, 2 hours 39 minutes before
Area Forecast Discussion
National Weather Service Seattle WA
421 PM PST Tue Dec 29 2020
.SYNOPSIS...A series of wet systems will bring lowland rain,
impactful mountain snow, and gusty winds to Western Washington
tonight and again this weekend. Unsettled weather may persist into
the start of the first full week of 2021.
&&
.SHORT TERM /TONIGHT THROUGH FRIDAY/...The first in a series of systems
is moving inland this evening, bringing widespread rain, mountain
snow and breezy to gusty winds to the region tonight through
Wednesday night. The following hazards can be expected, listed
below from most impactful to least impactful area-wide:
Snow: Periods of heavy snow are expected across the Cascades and
Olympics tonight, with impactful accumulations expected in the
Cascade passes and near Hurricane Ridge through Wednesday morning.
Snow levels will gradually rise Wednesday morning as the system
moves across the region, with levels briefly rising above 5000 feet
in the Olympics Wednesday afternoon before falling back below by
Wednesday evening. Snow levels will briefly rise to around pass
level at Snoqualmie Pass Wednesday morning - thus rain may briefly
mix with snow. However, this will not curtail snow totals here, as
snow levels will likely be lowered below pass level once again as a
second shot of heavy snow is expected to push through the Cascades
Wednesday afternoon and evening. Those with travel plans across the
passes should be prepared for winter weather driving conditions. Be
sure to check WSDOT for the latest information on road conditions.
Rain: Periods of moderate to briefly heavy rain are expected tonight
and again Wednesday as this system moves through the area. The
highest rainfall totals will be along the outer coast as a
widespread 1 to 2 inches with isolated amounts to 3 inches can be
expected. Lowland locations east of Puget Sound will generally see
rainfall amounts up to 1 inch, with amounts up to 1.5 inches in the
Cascade foothills. Lighter amounts in rain shadowed areas. Minor
flooding will be possible in low lying and poor drainage areas,
especially in areas along the coast and in the Peninsula. The
afternoon period will be one to keep an eye on as convective showers
could increase rain rates across much of the region, potentially
yielding isolated areas of urban flooding before rain pushes out of
the area.
Winds: Southeasterly winds will increase late tonight and peak
Wednesday morning, with the strongest winds likely along the coast,
Admiralty Inlet, the San Juans and into the interior coastal areas.
Gusts up to 50 mph will be possible along the outer coast, with 40-
45 mph possible elsewhere. Power outages may be possible in these
areas before winds gradually decrease Wednesday afternoon through
Wednesday night.
Thunder: There will be a brief period Wednesday afternoon where
showers could become convective, tapping into the limited
instability in place along the coast as 500 mb temps cool to -25
to - 28 degrees C. Increased bulk shear as the secondary upper
shortwave trough ejects across the region will provide the support
for an isolated robust updraft. Looking at forecast soundings
along the coast, vertical moisture profiles looks mostly
saturated, which will be a large limiting factor for thunder
potential. If it does occur, expect it to be limited to a few
strikes along coastal locales.
Behind this system, a shortwave ridge will move translate across the
northwest ahead of an amplifying upper level shortwave trough
ejecting out of the Gulf of Alaska. Quiet conditions will be short-
lived, as the aforementioned upper low will eject several shortwaves
that will bring wet and active weather to the region. A quick shot
of lowland rain and mountain snow is expected on Friday before a
more substantial system approaches for Saturday and Sunday.
Pullin
.LONG TERM /SATURDAY THROUGH TUESDAY/...Few changes in the anticipated
resultant weather for this weekend. A second, stronger and wetter
wave is expected to push across Western Washington on Saturday,
bringing additional periods of heavy rain, impactful mountain
snow, including mountain passes, and potentially impactful winds.
Those with travel plans across the Cascade passes will want to
keep an eye on how the forecast evolves.
Aside from the anticipated impactful snow across the Olympics and
Cascades, several forecast ensemble signals this afternoon point
toward impactful rainfall and wind with the Saturday system. Given
antecedent conditions, periods of short fused (urban & poor
drainage) flooding cannot be ruled out over the weekend. River rises
can also be expected, though lower snow levels will limit runoff.
Therefore, the Skokomish River in Mason County looks to remain the
only river flood threat at this time.
Longer range ensemble guidance into next week is trending at a mean
trough remaining across the northwest, keeping wet and active
weather in place into the first week of the new year.
Pullin
&&
.AVIATION...Clouds have already started to move into the area as a
frontal system offshore approaches the area. Stratus is expected
to continue to move into the area dropping ceilings down to MVFR
this evening. Additionally, as the front moves into the area their
is a possibility of some low level wind shear particularly for the
terminals near the Puget Sound from 0600-1200 UTC. Moreover, rain
will begin overnight and continue into Wednesday. During the
heaviest rain, visibility and ceilings will likely go down to IFR
conditions for much of the area, and LIFR for KOLM, KPWT and KPAE
which are a little more fog prone. Southerly winds will also be
increasing ahead of the front with sustained winds 10-15 knots
with gust 20-25 knots for most terminals. KHQM and KPAE will
likely experience gusts closer to 25-35 knots. Winds will begin
to shift westerly and rain will start to scatter out Wednesday
night.
KSEA...Clouds will continue to move into the area this afternoon
and evening, a return to MVFR is expected around 0300 UTC. Rain
is expected to begin shortly there after. There may be times where
conditions drop down to IFR as the rain moves through the area.
Southeasterly winds 8-12 knots will continue through the evening
before turning more southerly tonight. Starting around 0600 UTC we
expect moderate wind shear as winds at 2000 ft are expected to be
southwesterly. This shearing is expected to only last a couple of
hours as the front approaches.
Butwin
&&
.MARINE...The marine forecast is currently on track, with winds
over the coastal waters are just below Gale Force as of 1400
PST. Southerly winds will continue to increase this afternoon and
evening as the front approaches. Total wave height is expected to
increase from 8-12 ft to 15-20 ft overnight. Seas will remain
high through the rest of this week and into early next week as we
have a series of systems move through the area. The Northern
Inland Waters, Admiralty Inlet, and the East Entrance to the
Strait of Juan de Fuca are still expected to have Gale force winds
starting tonight. As a result waves 3-5 feet are expected through
this area until Wednesday afternoon. The Puget Sound and the
Central and West Entrance to the Strait are expected to remain in
Small Craft Advisory conditions. Conditions at Grays Harbor bar
will be rough to severe as waves will be building to 13-15 feet.
Butwin
&&
.HYDROLOGY...The first in a series of weather systems is
approaching the area tonight. Snow levels are expected to remain
above 3000 feet for each of these systems moving in. This should
prevent widespread runoff and river flooding for most of the
Western Washington area. The exception being the Skokomish River,
which is expected to reach minor flood stage Wednesday. The river
will then recede slightly before rising again due to the system
moving into the area this weekend. With the weekend system being
more potent, the Skokomish river is expected to reach moderate
flood stage.
Butwin
&&
.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory from midnight tonight to 4 AM PST
Thursday for Cascades of Pierce and Lewis Counties-Cascades
of Snohomish and King Counties-Cascades of Whatcom and
Skagit Counties.
Wind Advisory from 10 PM this evening to 10 AM PST Wednesday for
Admiralty Inlet Area-Central Coast-North Coast-San Juan
County-Western Skagit County-Western Whatcom County.
Winter Weather Advisory from 10 PM this evening to 10 PM PST
Wednesday for Olympics.
PZ...Small Craft Advisory until 10 PM PST Wednesday for Grays Harbor
Bar-Puget Sound and Hood Canal.
Gale Warning until 4 PM PST Wednesday 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.
Gale Warning until 6 PM PST Wednesday for Admiralty Inlet-East
Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland
Waters Including The San Juan Islands.
Small Craft Advisory until 3 PM PST Wednesday for Central U.S.
Waters Strait Of Juan De Fuca-West Entrance U.S. Waters
Strait Of Juan De Fuca.
&&
www.weather.gov/seattle