Heavy Snow in Washington, December 12 to 13, 2018

Area Forecast Discussion
National Weather Service Seattle WA
1026 AM PST Wed Dec 12 2018

.SYNOPSIS...After a brief lull today, expect more rain and heavy
mountain snow tonight in to Thursday. Expect windy conditions,
especially along the coast and north interior including Admiralty
Inlet. Another wet and potentially windy system will reach the
area Friday night, then a break on Saturday. Additional fronts
will affect the area early next week keeping the pattern active.

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.SHORT TERM...Latest radar loop is indicating scattered showers
streaming into Western Washington this morning. A few heavier
showers still offshore may reach the coast over the next couple
hours with brief moderate to heavy rain possible, mainly south and
west of Puget Sound. IR and WV satellite imagery shows the next
approaching low pressure system developing outside 135w. Models
indicate steady rain will develop at the coast late this afternoon
and spread inland during the evening. The main low will pass well
north into B.C. but almost all models are showing a significant
meso low forming northeast of the Olympics due to the strong
southwest flow aloft. This should result in advisory level winds
from at least Admiralty inlet to the east entrance, and also
exposed coastlines of the north interior. The GFS/NAM differ on
the strength of gradient, but the GFS and ECMWF and fairly close
in showing around +10 mb PDX-BLI and +6 mb OLM-BLI. This suggests
portions of Puget Sound could get advisory strength winds as well.
Will consider adding areas from Tacoma northward to the advisory.
Winds should peak around midnight tonight. Forecasts should be
monitored closely for potential forecast changes when all the 12z
meso models and ECMWF are available.

Snow will pile up in the mountains with this next system. Heavy
snow will develop by this evening and continue into Thursday. Snow
levels may not rise much as cross cascade gradient increases and
keeps passes cold enough for snow through the event, even at
Snoqualmie pass. Snow levels may rise slightly or not at all,
remaining in the 2500 to 3000 foot level through Thursday. No
changes made to the mountain snow forecast with a couple feet of
snow expected. The lighter amounts will fall in the passes,
perhaps 6 to 12 inches, highest at Stevens and White pass.

Models are having a difficult time resolving the next strong but
complex frontal system approaching Thursday night. Most models
show different tracks and forecast gradients vary widely. There is
a chance this system could be windier than currently forecast, if
the bent-back lifts across the area. Low pressure and high tides
will need to be evaluated the next couple days for potential
minor coastal flooding during high tide.

Mercer

.LONG TERM...The weather pattern will remain active through early
next week due to strong, moist SW flow over the Pac NW. This
means more rain and mountain snow through the period. The ECMWF
hints at a break late next week but for now will lean toward
persistence. 33

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.AVIATION...A brief lull in between systems will continue this
morning before the next storm system approaches later in the day.
Radar depicts some light showers across portions of western
Washington and this trend will continue over the next few hours
before more widespread rain approaches from the west. Before the
steadier rain arrives, VFR conditions should generally be the rule.
Thereafter, with the rain will see a lowering of ceilings to at
least MVFR. Deep moisture should hold on thru much of tonight and
Thursday. Winds should also begin to gust over 20kts with the
arrival of the next system, with guidance also indicating strong
winds off of the sfc so will likely have to add wind shear to the
forecast this afternoon.

KSEA...VFR conditions expected into the early afternoon with
ceilings lowering to MVFR as rain approaches the area later this
afternoon into the early evening. Lower level moisture then looks to
linger for most if not all of Thursday with little improvements in
ceilings expected. Winds are expected to become gusty by late in the
day/evening and are expected to remain southerly. With hints of
strong winds off the sfc, wind shear may be added in the coming
TAF updates.

Kovacik

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.MARINE....The local area finds itself in between storm systems this
morning but this will be short lived as the next frontal system and
associated Pacific energy move into the area this afternoon and
tonight. Current headlines as of 9am feature a Gale Warning for the
offshore waters and the west entrance, along with Rough Bar for
Grays Harbor. Gale force winds across the central and eastern Strait
appear to have diminished according to trends in recent
observational data. As the next storm system approaches later today,
winds will pick up across all waters, with Gale Warning continuing
across the offshore water and west entrance. SCA for wind will be in
effect this afternoon and into early Thursday for Puget Sound and
central Strait. Gale force winds are expected for this same time
period for Admiralty Inlet and the Northern Inland Waters. As the
system departs on Thursday, we will see a very brief relief in the
winds during the morning, however by Thursday afternoon and evening
another strong system will move in from the Pacific, assuring the
need for the next round of marine headlines.

Kovacik

&&

.HYDROLOGY...The Skokomish river in Mason county crested in minor
flood stage overnight and will continue receding today. It will
start rising again tonight and Thursday due to another round of
heavy rain over the region. Minor to moderate flooding is
forecastfor the Skokomish River with this next wave. Other rivers
flowing of the Olympics will be monitored. Otherwise, additional
flooding is not expected through the end of the week.

The stormy pattern continues into next week and still show a potent
storm mid-week, but current model runs are trending to push some of
the heaviest rain just north into Vancouver Island as the upper
ridge amplifies, leaving western WA with less precipitation than
earlier runs. Still, river models are showing there could be more
widespread flooding with that system.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Storm Warning from 6 PM this evening to noon PST Thursday
     for Cascades of Pierce and Lewis Counties-Cascades of
     Whatcom and Skagit Counties.

     Winter Storm Warning until noon PST Thursday for Cascades of
     Snohomish and King Counties.

     Wind Advisory from 6 PM this evening to noon PST Thursday for
     Admiralty Inlet Area-San Juan County-Western Skagit County-
     Western Whatcom County.

     Wind Advisory from 6 PM this evening to 6 AM PST Thursday for
     Central Coast-North Coast.

PZ...Small Craft Advisory for rough bar until 4 PM PST Thursday for
     Grays Harbor Bar.

     Gale Warning until 7 AM PST Thursday 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.

     Gale Warning from 4 PM this afternoon to 7 AM PST Thursday for
     Admiralty Inlet-East Entrance U.S. Waters Strait Of Juan De
     Fuca-Northern Inland Waters Including The San Juan Islands.

     Small Craft Advisory from 1 PM this afternoon to 7 AM PST
     Thursday for Central U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 4 PM this afternoon to 7 AM PST
     Thursday for Puget Sound and Hood Canal.

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