High Wind Watch 2, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
322 AM PST Tue Jan 14 2020

.SYNOPSIS...Light snow showers will continue this morning. A warm
front moving north across portions of the area will bring another
round of lowland snow through Wednesday morning. The next system
moves in Wednesday PM into Thursday with a chance for lowland snow
and gusty easterly winds through the Cascade gaps and foothills.
A trend toward warmer conditions and lowland rain will develop
late in the week and into the weekend.

&&

.SHORT TERM /TODAY THROUGH THURSDAY/...Latest hi-res guidance
keeps snow shower activity going through the morning into the
early afternoon hours. As has been the case overnight, it's going
to be scattered in nature. This means light accumulations where
showers form and no snow for others. Pinpointing exact
accumulations then becomes tricky/futile. But even a dusting on
untreated roads is enough to cause some issues, especially for the
morning commute. Social media and spotter reports reveal that
there's a layer of ice under dusting of snow in a lot of spots so
expect slick conditions to continue. Morning commuters may want to
leave themselves some extra time and prepare for road conditions
that can change rapidly over small distances.

A bit of a break for most places outside the mountains later this
afternoon though a few showers will continue. The next item of focus
becomes a warm front lifting north from parent low out over the
Pacific. Latest model trends are concerning and have made some major
changes to the forecast this morning. Still some disagreement
between hi-res solutions and ensembles on starting point of warm
front as it lifts north but general consensus is from just south
of the Strait of Juan de Fuca over to somewhere in northern King
or Snohomish counties during the evening hours, then lifting north
into Skagit, Whatcom and eventually BC Canada by Wednesday
morning. 6Z model runs aren't all the way in, but a quick peek
suggests the trend is even a bit further south of the 00Z runs
which obviously has implications for the Seattle metro area. As
the band lifts north, locations could see a pretty good burst of
moderate to heavy snowfall. This would have substantial impacts to
travel. Have upped precipitation amounts and resulting snow
forecasts but amounts could be higher than currently indicated,
especially in favored upslope areas along the Strait and if the
band starts out further south. Also could see some enhancement as
moist southerly air meets northeasterly cold Fraser outflow up
north. As such, will be issuing a winter storm watch for a good
portions of the lowlands from about Seattle northward and along
the coast and Strait. Near-term trends will need to be monitored
very closely today as uncertainty in heaviest snow placement
remains high and changes to the forecast will likely occur.

As cold Fraser outflow continues across western Whatcom and San Juan
counties, very cold wind chills will persist. South of the warm
front, dry weather is expected overnight.

Low pressure system centered around 50N and 135W then moves NE
toward the northern tip of Vancouver Island Wednesday into Thursday.
As a result, offshore flow and surface pressure gradients across the
Cascades strengthen. Current model solutions put the SEA-EAT
gradient between -11 to -15 mb. This would point to a bout of strong
easterly winds through the Cascade gaps into the foothills later
Wednesday into Wednesday night. Still some uncertainty in the
details but current forecast suggests gusts of 45 to 55 mph. This
would likely lead to power outages and tree damage, especially with
heavy wet snow currently in place. Outside of the winds, another
round of precipitation will rotate through the area. On current
track, bulk of moisture moves into Vancouver Island with
precipitation more showery in nature over Western Washington
Wednesday afternoon into Thursday. Downslope winds may limit snow
chances for portions of the Puget Sound lowlands. On the flip
side, this type of pattern also enhances snow in ares such as the
Kitsap Peninsula, Hood Canal and eastern slopes of the Olympics.
East winds may also lead to localized banding of snow as well even
in the lowlands. Snow levels remain a bit tricky and could
fluctuate between the surface and 1000 feet on Thursday. All this
sums up to the main message that snowfall accumulations with this
next system will be highly variable and forecast will change as
mesoscale details evolve in the near term. Regardless, area
residents should prepare for snowy travel at times with possible
power outages and tree damage for Cascade gaps and foothills.
Overnight lows on Wednesday and Thursday nights will be a bit
warmer but still near the freezing mark which would be quite
chilly for those without power.

CEO

.LONG TERM /FRIDAY THROUGH MONDAY/...Showers linger into Friday
morning as associated trough swings through the area. Any break
between systems will be short lived as next system moves through
the area into the weekend. Temperatures will warm substantially so
looks like a rain-maker for the lowlands. Rising snow levels are
expected so will need to monitor the potential for flooding if
heavy rain falls on existing snowpack in foothill locations and
below. Details will be refined.

CEO

&&

.AVIATION...Cold upper trough remains over the Pacific NW this
morning with strong west to northwest flow aloft. Surface flow
remains offshore as strong surface ridging remains centered over the
interior of British Columbia. The air mass is moist and stable with
scattered snow showers across the region. A stationary frontal
boundary near the Oregon border is expected to lift northward as a
warm front tonight. This is expected to enhance snowfall across the
Puget Sound terminals later this evening into the overnight hours.
Low MVFR or IFR ceilings are expected to persist across much of the
area into tonight.

KSEA...Scattered snow shower activity to continue this morning
with a mix of low MVFR or IFR ceilings. Accumulation this
morning expected to be light...under an inch. Forecast models
now have a warm front lifting north this evening. This could act to
enhance snowfall. As of now, best estimate is possible 1 to 3 inch
accumulation between 03Z and 10Z tonight. Surface winds N-NE 5 to 10
knots through late morning, then shifting to S-SW 5 to 10 knots and
continuing into this evening. 27

&&

.MARINE...Strong surface ridge remains over the interior of British
Columbia with a weak trough near the coast. A combination of the
swell and wind will keep small craft advisories in place for the
coastal waters through this evening. Offshore flow and Fraser
outflow will keep winds at advisory levels at the west entrance to
the strait and northern inland waters through this evening.

Forecast models have a rapidly deepening surface low developing over
the offshore waters late tonight through the day on Wednesday. It
could deepen from near 998 millibars this evening to under 975
millibars by the time it reaches the northern offshore waters late
Wednesday night. This rapid intensification is expected to lead to
gale force or possible storm force winds over the coastal waters
with gales across much the inland waters on Wednesday with possible
exception of Puget Sound. Seas are expected to build to 17 to 21
feet over the coastal waters Wednesday night into Thursday.

The active pattern continues into the weekend with additional system
bringing periods of small craft advisory or gale force winds to much
of the area waters.  27

&&

.HYDROLOGY...At this point, river flooding is not forecast
through at least Friday. Rising snow levels in combination with a
round of heavy lowland rain possibly moving into the region Friday
into Saturday may lead to sharp rises on some area rivers this
weekend. Continue to monitor the latest forecasts this week.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory until 10 AM PST this morning for
     Admiralty Inlet Area-Bellevue and Vicinity-Bremerton and
     Vicinity-East Puget Sound Lowlands-Eastern Strait of Juan
     de Fuca-Everett and Vicinity-Hood Canal Area-Lower Chehalis
     Valley Area-Seattle and Vicinity-Southwest Interior-Tacoma
     Area.

     Winter Weather Advisory until 7 AM PST Wednesday for Cascades of
     Pierce and Lewis Counties-Cascades of Snohomish and King
     Counties.

     Winter Weather Advisory from 7 PM this evening to 7 AM PST
     Wednesday for Cascades of Whatcom and Skagit Counties.

     Winter Storm Watch from this evening through Wednesday morning
     for Admiralty Inlet Area-Bellevue and Vicinity-Bremerton
     and Vicinity-East Puget Sound Lowlands-Eastern Strait of
     Juan de Fuca-Everett and Vicinity-Hood Canal Area-North
     Coast-San Juan County-Seattle and Vicinity-Western Skagit
     County-Western Strait of Juan De Fuca-Western Whatcom
     County.

     Wind Chill Advisory until 11 AM PST this morning for San Juan
     County-Western Whatcom County.

     Gale Watch from late tonight through Wednesday afternoon for San
     Juan County.

PZ...Small Craft Advisory until 4 AM PST Wednesday for Grays Harbor
     Bar-Northern Inland Waters Including The San Juan Islands-
     West Entrance U.S. Waters Strait Of Juan De Fuca.

     Gale Watch from this evening through Wednesday afternoon 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.

     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.

     Small Craft Advisory until 4 AM PST early this morning for
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Gale Watch from late tonight through Wednesday afternoon for
     Central U.S. Waters Strait Of Juan De Fuca-Northern Inland
     Waters Including The San Juan Islands-West Entrance U.S.
     Waters Strait Of Juan De Fuca.

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