Heavy Snow in Washington, January 25 to 26, 2018

Area Forecast Discussion
National Weather Service Seattle WA
915 AM PST Thu Jan 25 2018

.SYNOPSIS...Low pressure aloft will keep conditions cool and
unsettled through Friday. Expect an occluded front to approach the
region Friday night before moving across the area on Saturday.
Significant rainfall is possible early next week due to a milder
and wetter pattern setting up over the region.

&&

.SHORT TERM...
There were numerous showers over the CWA at this time.
Temperatures across the lowlands were mostly in the 30s this
morning; thus, the precipitation has fallen as a rain/snow mix or
wet snow in some areas. Temperatures will warm into mostly the
40s this afternoon across the lowlands and this will end the
threat of snow. However, expect (possibly a large quantity of)
small hail to accompany the heavier showers this afternoon. The
threat of snow will continue at the higher elevations (above 1000
feet) in the Black Hills, Cascade foothills, and Olympic foothills,
where there could be a couple of inches of snow in some spots.

Meanwhile, a chilly upper level trough will remain over the region
through tonight for showery conditions. The air mass may
destabilize enough this afternoon for a slight risk of
thunderstorms from about the coast westward. Strong zonal flow
aloft will prevail on Friday. Weak disturbances embedded in the
flow will keep conditions unsettled.

An occluded front will approach the area Friday night for the
development of widespread steady precipitation. The front is
expected to move across the CWA on Saturday. Locally heavy snow is
possible in the mountains with this system.

.LONG TERM...
Still looking at an atmospheric river or firehouse to become aimed
at western WA during the early part of this period. Confidence of
this occurring was higher due to the medium range solutions being
in better agreement. The main difference was in the details, and
this was where confidence was lower.

A warm front will lift north across the CWA late Saturday night
into Sunday morning, resulting in rising snow levels. Precipitation
rates will increase during the day Sunday. At the onset, the
precipitation will be in the form of snow in the mountains but
will change over to rain by Sunday afternoon. There may be a
brief period of freezing rain in the Cascade passes on Sunday
before the precip changes over to regular rain.

The upper level ridge will amplify Sunday night, causing the focus
of the heavy rain to shift mostly north of the CWA. However, this
will be brief. The baroclinic band will sag back into the CWA
Monday morning for periods of locally heavy rain during the day
Monday.

Total rainfall amounts are still anticipated to be significant,
particularly in the mountains. Heavy rain on top of what has
already fallen could result in river flooding and urban/small
stream flooding. The heavy rains will also increase the risk of
shallow landslides.

Beyond Monday, a cold upper level trough will move across the
Pacific Northwest Monday night and Tuesday for plummeting snow
levels. The medium range solutions have backed off on the idea
of a strong upper level ridge building into the region from the
west on Day 7. Thus, expect continued cool and unsettled
conditions to persist into Day 7.

&&

.AVIATION...Showery as a trough moves through. Mountains mostly
obscured. The cooler more unstable air with a chance of
thunderstorms is just off the coast--some of that unstable air will
work in over southwest WA this afternoon.

KSEA...Showery today with light and variable winds this morning and
a southerly breeze developing later this afternoon after the trough
moves through. Cigs will be around 035, perhaps dipping for awhile
to 025 with the precip through early afternoon.

&&

.MARINE...A trough will move through Western Washington
today. A low pressure system will approach the area and the
associated front will reach the area Friday night. A slow moving
front will affect the area Sunday and Monday.

&&

.HYDROLOGY...
The Skokomish River will continue to slowly recede and should be
below flood stage by late this afternoon.

Meanwhile, still looking at an atmospheric river to become aimed
at the CWA late this weekend through Monday. Confidence of this
occurring was higher due to the medium range solutions being in
better agreement. The main difference remained in the details,
and this was where confidence was lower.

A warm front will lift north across the CWA late Saturday night
into Sunday morning, resulting in rising snow levels. Snow levels
are anticipated to be above 6000 feet across much of the area by
Monday morning. Precipitation rates will increase during the day
Sunday.

Anticipate the upper level ridge to amplify or strengthen Sunday
night. This will cause the focus of the heavy rain to shift
mostly north of the CWA or into Canada. However, this will be
short-lived. The front will sag back into the CWA Monday morning
for periods of locally heavy rain during the day.

Significant rainfall is still anticipated, especially in the
mountains. Heavy rain on top of what has already fallen the past
several days could result in flooding on many rivers early next
week. Urban and/or small stream flooding is also a possibility.
The additional rains will also increase the risk of shallow
landslides.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Warning remains in effect for the Skokomish River in
	 Mason County.

PZ...Small Craft Advisory until 9 AM PST Friday 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 for rough bar until 9 AM PST Friday for
     Grays Harbor Bar.

     Small Craft Advisory until 3 PM PST this afternoon for Admiralty
     Inlet-East Entrance U.S. Waters Strait Of Juan De Fuca.

&&

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