Small Craft Advisory 1, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
330 PM PST Sat Dec 31 2016

.SYNOPSIS...A cold front will continue moving south across the
area this evening. Look for the precipitation to change over to
snow across much of the lowlands overnight. A modified arctic
front will sweep across the region on Sunday, ushering in a drier
and colder air mass. Weak low pressure aloft will prevail on
Monday, with high pressure aloft anticipated on Tuesday.

&&

.SHORT TERM...
A cold front, associated with an upper level low over western
British Columbia, was moving south across the CWA at this time.
The leading edge of the precipitation shield was currently over
northern Jefferson and far southern Snohomish Counties. Most of
the precipitation was falling as rain or a rain/snow mix on the
lowlands. Farther north of the cold front but south of an arctic
front (presently over central British Columbia), the precipitation
was falling as snow. Expect colder air to filter into the area
from the north this evening, causing the precipitation to change
over to snow across much of the lowlands by late tonight.

Went on ahead and expanded the Winter Weather Advisory to include
the cities of Seattle, Tacoma, Bellevue plus the Kitsap peninsula
and Hood Canal. Used a blend of the NAM and WRF-GFS QPF, of which
the result showed snowfall amounts of up to 2 inches across these
areas by late Sunday morning. Will still need monitor Snohomish
County for the potential for quasi-stationary convective bands
tonight in the wake of the cold front and ahead of the arctic
front. Also, will need to keep an eye on the eastern Strait of
Juan de Fuca for northerly upslope flow enhancing precip rates.
Presently, the forecast has localized amounts of up to 1.5 inches
for this zone. In general, the models are not that great in
predicting the location of convective bands or the locations where
the heaviest convective precip will occur. This is the reason we
have to monitor the situation and make any adjustments to the
forecast as needed.

Look for the modified arctic front to sweep across the area
during the day Sunday, ushering in a drier and colder air mass.
Strong northerly winds will also develop Sunday over the far
northwest interior, where a Wind Advisory was issued. Any
lingering showers after Sunday morning will be light.

Expect the upper level trough to linger over the Pacific
Northwest through Monday before finally weakening and exiting the
region on Tuesday. Expect dry and chilly conditions over the CWA
Monday and Tuesday. Highs will struggle to reach freezing (if at
all) across the lowlands and the min temperatures will be in the
teens and 20s.

.LONG TERM...
Temperatures will remain below normal during this period. Dry,
generally northerly flow aloft will prevail during through
Thursday. The upper level flow is anticipated to back to westerly
on Friday as a low pressure system approaches from the northwest.
This system will bring a threat of precipitation to the area late
in the period. Temperatures will also moderate as the low level
and upper level flow becomes more southerly, thereby advecting in
a milder air mass.

&&

.AVIATION...An upper trough will move over the area this evening.
Westerly flow aloft will ease and become variable tonight as a
weak upper low moves overhead Sunday morning. At the surface, a
front and an associated low will move south through the area
tonight. Fraser outflow will develop Sunday. The air mass is
moist and stable.

Areas of low clouds will become widespread this evening as a front
moves through. Ceilings should improve from north to south Sunday as
drier air moves in behind the front. Rain showers will switch to
snow from north to south tonight as colder air moves into the
region.

KSEA...Low clouds tonight. Rain showers should change to snow
showers a little after midnight tonight with about inch of
accumulation possible by Sunday morning. Snow showers will taper
late Sunday morning and ceilings should improve in the afternoon.
Schneider

&&

.MARINE...A surface low and an associated cold front will drop
south through the waters tonight. Strong west to northwest flow
will develop behind the front over the Coastal Waters and Strait
of Juan De Fuca. The current forecast is for high end small craft
advisory winds but gales are certainly possible.

Strong northeast offshore flow will develop Sunday as high
pressure builds into southern British Columbia. Gale northeast
winds are likely for the Northern Inland waters and much of the
Strait of Juan De Fuca.

Northeast offshore flow will continue Monday through Thursday.
Gale force northeast winds will probably come down to small craft
advisory strength for the Northern Inland waters Monday night.
Schneider

&&

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

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisories are in effect for much of the Puget
	 Sound region and the western Whatcom/Skagit County lowlands.

 	 Wind Advisory in effect from 6 AM until 6 PM PST Sunday for
 	 San Juan County, Western Skagit County, and Western Whatcom
	 County.

PZ...Gale Warning in effect from 6 AM Sunday to 4 AM PST Monday
	 for the central/eastern Strait Of Juan De Fuca and the
 	 Northern Inland Waters.

	 Small Craft Advisories are in effect for the remaining
 	 waters except the Puget Sound/Hood Canal.

 	 Small Craft Advisory for rough bar in effect until noon PST
 	 Sunday for the Grays Harbor Bar.

&&

www.weather.gov/seattle

An illustrated version of this forecast discussion can be seen at
www.weather.gov/seattle/gafd/latest_webafd.html