Small Craft Advisory 20, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
830 PM PST Sat Feb 4 2017

.SYNOPSIS...A cold upper level low offshore will maintain cool moist
southwest flow aloft over Western Washington through Sunday, with
periods of mainly rain. The low will weaken and move across the area
Monday morning with a little light snow likely. Another low will keep
the air mass cold with scattered rain or snow showers on Tuesday. A
warm front will bring rain and warmer conditions later in the week.

&&

.SHORT TERM...A deep upper low offshore will spin bands of rain into
Washington over the next 36 hours. Temperatures and dew points are
all in the 40s so the precipitation will be rain. The one exception
is northern Whatcom County where the temperature is near freezing. A
wintry mix of precipitation is likely there tonight and a winter
weather advisory is in effect.

Southwest flow aloft will continue to bring snow to the Cascades,
where a winter storm warning is in effect tonight. The Cascades of
Pierce and Lewis counties will continue to have heavier snow Sunday
and Sunday night, so have extended the warning there through Monday
morning.

The upper level low will begin moving slowly east tonight and will be
near the northern tip of Vancouver Island early Sunday morning. The
low will weaken into an upper level trough Sunday night, then move
east across the area Monday morning. Snow levels will drop to near
sea level as increasing Fraser river valley outflow combines with the
cold air aloft in the trough. Fortunately, the trough doesn't look
like a big precipitation producer, but it still looks like there
could be enough precipitation for a few inches of snow late Sunday
night through Monday. GEFS plume plots for snow support this amount.

The Fraser river valley outflow Sunday night and Monday may be
strong enough for winds to reach high wind warning criteria. The 18Z
GFS forecasts the BLI-YWL gradient getting up to -22.3 MB at 09Z
Monday. The 00Z NAM is at -18.4 MB and the ECMWF around 16 MB. A high
wind watch for western Whatcom and San Juan counties addresses this.

The upper level trough moves east of the area Monday afternoon, but
another weaker upper level low will form just off the central B.C.
coast. This feature will maintain cold moist westerly flow aloft
over W WA until it moves E of the area on Wednesday. The air aloft
with the trough will remain cold, but not quite as cold as on
Monday. Likewise, Fraser River outflow will continue but it will not
be as strong. The air mass over W WA will be cold enough for snow
showers to continue but the snow level will probably be a few
hundred feet above sea level. Finally, the showers moving across the
area will be scattered with little snow accumulation. Burke/Kam

.LONG TERM...Previous discussion...The GFS and ECMWF are in fairly
good agreement through next Saturday. Once the upper level tough
moves E on Wednesday a warm moist frontal system will take aim on
the PacNW Wednesday night. The leading edge of the warm front rain
should reach the area after midnight Wednesday night, followed by
the warm front itself late Thursday morning. The cold front will be
right behind, crossing the area Thursday evening. Rainfall amounts
could be healthy with 2-3 inches over the mountains and an inch plus
over the interior lowlands. Thursdays highs should reach the upper
40s. The associated surface low will move NE across Vancouver island
early Friday morning. The GFS operational run is showing significant
pressure gradients, but is an outlier compared to the ensemble
solutions.

An upper level ridge may build over the region on Saturday, but
don't say dry day on the way, yet. Kam

&&

.AVIATION...Strong southwest flow aloft over Western Washington
this evening will continue tonight and Sunday, ahead of a deep
upper low west of Vancouver Island near 48N/131W. A cold front
over Western Washington, just east of the Puget Sound region at
8 pm, will move slowly across the Cascades tonight. Moist onshore
flow will follow the front, and another frontal wave will lift
more widespread precipitation into the forecast area again Sunday
afternoon.

Ceilings are mainly 020-040 this evening, with widespread rain
along the front and scattered showers to its west. There are
locally lower IFR conditions in rain. Conditions will probably
improve somewhat tonight as precipitation decrease, but moist
onshore flow will continue and the guidance on ceilings tonight is
decidedly mixed. We should see deterioration again Sunday
afternoon as the next frontal wave arrives. Precipitation could
start falling as snow over the lowlands Sunday evening.

KSEA...South wind 10-16 kt gusting 24 kt, easing to 4-10 kt Sunday
and shifting to northeast late in the day. In the short term,
ceilings should improve at KSEA due to less precipitation. It's
very difficult to say whether this will continue through the night
though, as some of the guidance shows IFR ceilings developing
overnight in moist onshore flow. A transition from rain to snow is
likely later Sunday evening, with accumulating snow probable by
Monday morning. McDonnal

&&

.MARINE...A cold front over the Western Washington interior this
evening will move slowly across the Cascades tonight, with
southwesterly onshore flow following the front. Small craft
advisory winds are likely for most of the forecast area tonight,
with winds easing and becoming south to southeast on Sunday.

A low center will track southeastward through southwest
Washington Sunday night and Monday, while high pressure remains
over British Columbia. As a result northeast offshore flow will
develop across Western Washington, with strong Fraser River
outflow winds in the northern inland waters, through the Strait of
Juan de Fuca, and across the northern coastal waters. Northeast
gales are likely over the northern inland waters. Weaker Fraser
outflow will continue from Monday afternoon into Tuesday.

&&

.HYDROLOGY...No river flooding expected for the next 7 days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Storm Warning until 4 AM PST Monday for Cascades of
     Pierce and Lewis Counties.

     Winter Storm Warning until 4 AM PST Sunday for Cascades of
     Snohomish and King Counties-Cascades of Whatcom and Skagit
     Counties.

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

     High Wind Watch from Sunday afternoon through Monday morning for
     San Juan County-Western Whatcom County.

PZ...Small Craft Advisory until 9 AM PST Sunday 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 until 3 AM PST Sunday for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca-Northern Inland Waters Including The
     San Juan Islands-Puget Sound and Hood Canal.

     Gale Watch from Sunday afternoon through Monday afternoon for
     Northern Inland Waters Including The San Juan Islands.

&&

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www.weather.gov/seattle/gafd/latest_webafd.html