Small Craft Advisory 308, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
216 PM PST Tue Dec 19 2017

.SYNOPSIS...Low pressure at the surface and aloft over Northwest
Washington will move into Eastern Washington tonight. A ridge of
high pressure aloft offshore will build into the area Wednesday
through Thursday brining drier and cold weather. A weak
disturbance will bring a chance of showers and lowland snow
Thursday night and Friday morning. Dry and cold weather will
return to the area for the weekend. It appears that Christmas Day
will see an increase in clouds and a chance of showers.

&&

.SHORT TERM...The low that brought windy conditions to the area,
heavy mountain snow, and some snow along the Strait of Juan de
Fuca last night and this morning has moved into Eastern
Washington. A 527 DAM 500 mb upper low is now left in its wake
over Vancouver Island; this upper level feature will move
into southeastern Washington by late tonight.

Precipitation is changing to snow over northern portions of
Whatcom County this afternoon as cold air aloft filters in from
the northwest. A majority of mesoscale models show 1-3 inches of
snow accumulation across Western Whatcom County through tonight. A
winter weather advisory is being issued.

Snow in the Olympics rapidly tapered off behind the front and is
expected to be light for the duration of this event. Snow is now
starting to taper off to showers in the Cascades. However, with
the amount of snow that as already fallen (13 to 24 inches), the
additional 3-6 inches tonight in the passes will add to travel
woes there. Will keep the winter storm warning for the Cascades
going through 06Z.

Will snow fall elsewhere in the lowlands tonight and early
Wednesday morning? Temperatures will be cold enough to support
precipitation in the form of snow, especially above about 500
feet, later this evening onward. However, any snow will be
confined to difficult to forecast convergence zones. Short term
models have been really inconsistent in the placement of the
convergence zones with each run. Will mention the snow on higher
hills in the forecast and keep any accumulation less than an inch
outside of the Western Whatcom county area.

Dry air will filter into the region Wednesday morning, and clear
and cold conditions will dominate the conditions through Thursday.

Models continue to have major disagreements on the track and
strength of a short-wave trough moving southeast to the east of
the offshore ridge. The GFS and NAM keep the area completely dry
and shove energy with the trough east of the Cascades. The ECMWF
and Canadian models both show precipitation moving through Western
Washington Thursday night and Friday morning with portions of the
lowlands seeing a possible inch or so of snow. The current
forecast that favors the ECMWF/Canadian solution will be
maintained for now. Snow levels with this feature, if it
materializes, will be low enough to produce some light
accumulations down to near sea level. Albrecht

.LONG TERM...Long term models show dry and cold conditions across
the area for later Friday through Saturday. The GFS now shows
energy moving through the offshore ridge reaching the area on
Sunday for clouds and a chance of precipitation. The ECMWF remains
dry. Will follow the ECMWF through Saturday.

Models starting on Sunday and through Christmas day become a real
inconsistent mess as they have trouble dealing with small scale
features moving through a slowly retrograding upper ridge that has
been centered near 130W. Models now all show clouds and some
chance of precipitation by Christmas day. Will use a model average
for Sunday onward, but confidence is not high. Albrecht

&&

.AVIATION...The low pressure system will weaken over the area
tonight. The air mass is moist and somewhat unstable with showers.
Variable wind aloft will become north on Wed. The air is cool and
snow showers are possible in the north--mainly for KCLM and KBLI.

KSEA...Showery with a breezy SW wind becoming northerly on
Wednesday.

&&

.MARINE...A 997mb low was near Bellingham early this afternoon. The
low pres system will weaken overnight and gradients over Western
Washington will relax as the low fills. Northeasterly flow will
develop Wed with high pres over B.C. that will weaken Thu as a weak
system moves thru B.C. and then the high pres will strengthen again
Fri.

&&

.HYDROLOGY...River flooding is not expected the next 7 days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Storm Warning until 10 PM PST this evening for Cascades
     of Pierce and Lewis Counties-Cascades of Snohomish and King
     Counties-Cascades of Whatcom and Skagit Counties.

     Winter Weather Advisory until 4 AM PST Wednesday for Western
     Whatcom County.

PZ...Small Craft Advisory for rough bar until 2 AM PST Wednesday for
     Grays Harbor Bar.

     Small Craft Advisory until 2 AM PST Wednesday for Admiralty
     Inlet-Central U.S. Waters Strait Of Juan De Fuca-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-East Entrance U.S. Waters
     Strait Of Juan De Fuca-Northern Inland Waters Including The
     San Juan Islands-Puget Sound and Hood Canal-West Entrance
     U.S. Waters Strait Of Juan De Fuca.

&&

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