Heavy Snow in Washington, January 29 to 30, 2018

Area Forecast Discussion
National Weather Service Seattle WA
946 AM PST Mon Jan 29 2018

.SYNOPSIS...A frontal system draped across southern Vancouver
Island late this morning will sag slowly southward across Western
Washington through this evening. Heavy rainfall and breezy
conditions will spread southeast ahead of the front. Cooler and
showery weather is expected behind the front from late this
evening through Wednesday. A warm front will bring rain and
mountain snow to the area on Thursday. Cloudy and mild conditions
are expected from Friday through next weekend with a chance of
rain at times.

&&

.SHORT TERM...Heavy rain is underway over parts of the forecast
area due to an atmospheric river type setup. A cold front is
currently draped across southern Vancouver Island and is moving
slowly south-southeastward. In advance of the front, the 12z UIL
sounding showed a moist Precipitable Water (PW) value of 1.03",
and southwesterly 850 mb winds of 45-60 knots favor heavy rain as
well, especially over the windward side of the Olympics and North
Cascades. This raises concerns for possible minor flooding and
shallow landslides; see the Hydro section for more. The warm, wet,
windy conditions have prompted the Northwest Avalanche Center to
issue an Avalanche Warning for the North Cascades until 6 pm
today.

The front will slip southeast of the forecast area this evening.
Behind the front, windy conditions will ease. Snow levels will
also fall overnight to around 2000 feet, then to around 1000-1500
feet on Wed morning. Post-frontal showers will prevail with
onshore wind flow from late this evening through Wednesday. Will
be considering Winter Weather Advisories for the Cascades during
the cool, showery regime from tonight through Wednesday. Snow will
be enhanced where a Puget Sound Convergence Zone intersects the
Cascades. Haner

.LONG TERM...From Previous Discussion: A frontal system that is
currently sitting out near 170W will move eastward and reach the
area late Wednesday night or Thursday. This front will give a
period of rain and mountain snow to the area, especially Thursday
and Thursday night. The precipitation will be accompanied by a
rapid rise in snow levels in the mountains later Thursday night or
Friday.

Models show good consistency with one another tonight for the
Friday through next weekend period but have not shown much in the
way of day- to-day continuity. Models now show Western Washington
in an area of confluence and frontogensis aloft giving clouds and
possible precipitation to the area next weekend. Cold air appears
that it will remain north and east of the area, so conditions
could be rather mild. Temperatures were adjusted upward toward the
new model means early this morning. Albrecht

&&

.AVIATION...A cold front aligned southwest to northeast will move
southeast through Western Washington today. Strong southwest flow
aloft will become moderate westerly tonight. Strong southeast flow
will become onshore this afternoon and tonight. The air mass is
moist and stable.

Mostly VFR ceilings will become MVFR as the front passes for at
least a few hours this afternoon and evening, then return to VFR.
There are some pockets of IFR including KPWT. Widespread rain will
change to scattered showers by this evening. A convergence zone may
form this afternoon and evening in Snohomish County.

KSEA...Ceilings have been VFR under the rain so far this morning.
Still expecting at least a few hours of MVFR between 20Z and 03Z,
followed by VFR. Southerly wind 5-10 kt this morning becoming
southwest in the afternoon and increasing to 10g20 kt. CHB

&&

.MARINE...A cold front will move through the area today, and
strong southeast gradients will become southwest by 00Z. Small craft
advisory strength southerly winds are likely over all waters today.
Westerly winds will develop in the strait this morning and continue
into the evening. Strongest winds in Puget Sound will likely be over
the northern portion this morning then over all of the Sound this
afternoon as the front moves through. Southwesterly small craft
advisory winds likely persist in Puget Sound through this evening
before easing.

Westerly swell building 13 to 15 feet at 12 seconds today will
create hazardous seas coastal waters and west entrance to the Strait
through Tuesday.

Flow will remain onshore Tuesday, then ease Wednesday. Another front
will arrive Thursday, with onshore flow Friday. CHB

&&

.HYDROLOGY...The Skokomish River in Mason County is expected to
remain above flood stage for the next 2-3 days.

Heaviest rainfall will generally occur from now through this
afternoon, with the heaviest amounts over the Olympic and North
Cascade headwaters. Rainfall rates and snow levels will both fall
quickly this evening, meaning no additional major runoff
contributions over the mountain headwaters after about 4-6 pm.
After that, will mainly be watching water route down the rivers
over the following 1-3 days. There is a Flood Watch for the
rivers coming out of the Olympics and North Cascades, i.e. from
Snohomish County northward and the Olympic Peninsula.

Soils are rather saturated across much of Western Washington this
morning as a result of recent rainfall. Today's period of heavy
rainfall may result in the production of some shallow landslides
across the area. A landslide Special Weather Statement has been
issued and provides additional information and links.

Beyond flooding stemming from today's rainfall, flooding is not
expected on area rivers through this weekend. Haner

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Watch from 4 PM PST this afternoon through Tuesday
     afternoon for Clallam, Jefferson, Grays Harbor, Whatcom,
     Skagit, and Snohomish Counties.

PZ...Small Craft Advisory for rough bar until 6 AM PST Tuesday for
     Grays Harbor Bar.

     Small Craft Advisory until 4 PM PST Tuesday 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 until 4 AM PST Tuesday for East Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Gale Warning until 10 AM PST this morning for East Entrance U.S.
     Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 4 AM PST Tuesday for Northern Inland
     Waters Including The San Juan Islands-Puget Sound and Hood
     Canal.

     Small Craft Advisory until midnight PST tonight for Central U.S.
     Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 10 PM PST this evening for Admiralty
     Inlet.

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