Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 231653
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
853 AM PST Fri Feb 23 2018

.SYNOPSIS...Cold and unsettled weather will continue through next
week. A weather system will move through the area later today and
tonight with lowland snow transitioning to rain. Another system
will cross the area Saturday night and Sunday with localized
lowland snow possible again. There will be a bit of a break on
Monday before more systems arrive in the middle of the week.

&&

.SHORT TERM...It looks like a messy and tricky forecast for the
next few days.

A weather system will arrive from the northwest later today. The
air in place is cold enough for snow down to sea level. At the
same time, southerly flow near the surface will increase. This
usually means rain, not snow near sea level. The upshot is
precipitation should start as snow this afternoon over the
interior but then change to rain at the lowest levels as snow
levels rise to at least a few hundred feet. The coast will warm a
little quicker with mostly rain expected. Advisory amounts this
afternoon and evening seem most likely over the northwest
interior and near the Cascade foothills where cold air will be
more entrenched. The central sound area, including Seattle will be
a bit shadowed and snow amounts are expected to be less. Further
south should warm a little quicker, likely limiting accumulations.

A second upper short wave will drop down from the northwest late
tonight with another shot of precipitation. Snow levels should be
high enough, at least initially, for rain most areas but there
could be problem areas like Whatcom county near the Canadian border,
near the Cascade foothills, and in a Puget Sound Convergence
Zone. Elsewhere, rain could also change to snow for most of the
lowlands Saturday morning as snow levels lower again.
Precipitation will be ending around this time so accumulations for
most area are not expected at this time.

The mountains will also pick up a decent amount of snow through
Sunday morning. High end advisory amounts are expected for now
with the heavier shot coming early Saturday morning.

There will be a bit of a break later Saturday in between weather
systems. Yet another cold upper short wave will drop down from the
northwest Saturday night and Sunday morning. There is enough
uncertainty with the details that another shot of lowland snow
cannot be ruled out but, at least for now, snow levels are
expected to be high enough to preclude lowland snow. The mountains
look to get another good shot of snow.

Monday will be a relative lull but cool north to northwest flow
aloft will continue with weak embedded short waves. Some rain or
snow showers cannot be ruled out but significant accumulations are
not expected. Schneider

.LONG TERM...Previous discussion...Little change in the pattern
through next week with much cooler than normal temperatures and
above normal precipitation continuing. Next system embedded in the
northwesterly flow aloft arriving on Tuesday with another one
possible on Wednesday. Upper level trough digging toward the area
the latter half of the week will bring the low snow level scenario
back into the forecast after Wednesday. Felton

&&

.AVIATION...Moderate northwest flow aloft will continue through
tonight. Moisture will increase this morning, with conditions
deteriorating as a front brings snow and rain into Western
Washington beginning midday. Precipitation will become more
showery and change to rain over most of the lowlands this evening,
with an upper trough moving over the area late tonight. At the
surface, southerly flow will increase.

VFR conditions prevail across Western Washington at 8 am with
ceilings mainly 12,000 ft or higher, but deterioration to
widespread MVFR and local IFR is likely this afternoon or early
evening and continuing tonight.

KSEA...Southeast wind 6-12 kt becoming south 10-16 kt gusting
25 kt by early afternoon. Conditions should gradually deteriorate
to low-end MVFR by late afternoon or early evening, with around a
half inch of snow between 2 pm and 8 pm. McDonnal

&&

.MARINE...Southerly flow will increase today as a frontal system
approaches the area and then moves inland. Strong onshore flow
will follow the front tonight into Saturday. Small craft
advisories are in effect for all marine zones. Meanwhile, seas
will build to 10 to 13 feet on Saturday.

A somewhat stronger frontal system will move southeast across the
forecast area Saturday night and Sunday, and gales are possible. A
weaker front will arrive on Tuesday. McDonnal

&&

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

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory from 4 PM this afternoon to noon PST
     Saturday for Cascades of Pierce and Lewis Counties-Cascades
     of Snohomish and King Counties.

     Winter Weather Advisory from noon today to noon PST Saturday for
     Cascades of Whatcom and Skagit Counties.

     Winter Weather Advisory from noon today to 8 PM PST this evening
     for San Juan County-Western Skagit County-Western Whatcom
     County.

     Winter Weather Advisory from 4 PM this afternoon to 4 AM PST
     Saturday for East Puget Sound Lowlands.

PZ...Small Craft Advisory until 4 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-West Entrance U.S. Waters Strait Of
     Juan De Fuca.

     Small Craft Advisory until 6 PM PST Saturday 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.

     Small Craft Advisory from 1 PM this afternoon to 10 PM PST this
     evening for Admiralty Inlet.

     Small Craft Advisory from 1 PM this afternoon to 4 AM PST
     Saturday for Puget Sound and Hood Canal.

&&

$$

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