Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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740
FXUS66 KSEW 030446
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
845 PM PST Fri Dec 2 2016

.SYNOPSIS...Scattered showers will taper tonight. A front will
arrive Saturday night, followed by a cold upper trough Sunday
through Tuesday. Snow levels will fall to around 500 feet with snow
possible down to sea level at times Monday and Monday night. Cold
and dry weather will develop Tuesday. A system arriving Wednesday
night and Thursday could bring some low elevation snow before warm
air brings rising snow levels.

&&

.SHORT TERM...Showers have mostly become scattered and will mostly
continue to taper tonight. There is a Puget Sound convergence zone
in Snohomish County and this will probably persist through the
night. It is forecast to stay more or less north of Seattle. Snow
levels jumped up today to around 5000 feet south and 4000 feet north
but should fall behind the front tonight to around 3500 feet. A few
inches of snow could fall in the higher passes, and especially along
higher west facing slopes.

Rain will increase Saturday afternoon and evening as another impulse
moves through westerly flow. Snow levels will rise a bit to 3500 to
4000 feet, then fall to 2500 feet late Saturday night and Sunday.
Washington will be on the cold north side of the low/front. The
WRFgfs and QPF on other models suggest 1 to 2 feet of snow is a good
possibility in the mountains so a winter storm watch is in effect
for late Saturday afternoon through Sunday afternoon. It will rain
in the lowlands but colder air aloft will start to filter in Sunday
night behind the front with lowering snow levels.

Models agree the coldest air of the season so far will move into
western Washington Sunday night. Temperatures will be near or below
freezing. Some precipitation is likely, and it is likely to be snow
over most areas. The best chance for some accumulation will be with
the convergence zone from far north King county to Snohomish county
and locations with a few hundred feet of elevation. The WRF4km
suggests 1 or 2 inches is possible, more with the convergence zone.
The GEFS ensembles show about 2 inches plus or minus 1 inch in
Seattle for the period Sunday night through Monday. Burke/Mercer

.LONG TERM...The coldest temperatures of the year are likely by
Tuesday, between systems. With some clearing and very cold air mass
aloft, lows could dip into the 20s for most areas. Highs may struggle
to reach 40 Tuesday.

The next system will approach on Wednesday. Models differ
significantly on how much cold air will be in place and the timing
of incoming precipitation. It is possible there could be several
hours of overrunning snowfall Wednesday night and Thursday. Most
likely any snow or frozen precipitation would switch over to rain
pretty quickly but some lowland accumulation is not out of the
question. By Thursday, the air mass will moderate and precipitation
will be all rain in the lowlands. Rain tapers to showers toward the
weekend. Mercer

&&

.AVIATION...Westerly flow aloft over Western Washington this
evening will continue tonight and Saturday. The air mass is
moist, mainly below 7000 ft, and slightly unstable. Moisture aloft
will increase late Saturday as the leading edge of the next
frontal system arrives.

Conditions are mostly MVFR across the area this evening with
ceilings 015-025, but there is also isolated IFR conditions and
areas of just scattered clouds. Scattered light showers continue,
mainly in the Puget Sound convergence zone and along the west
slopes of the Cascades. Cloud cover between 015-025 will probably
become more widespread tonight, but on the whole there will not be
much change through Saturday.

KSEA...South winds 8 to 16 kt tonight and Saturday with occasional
gusts as high as 24 kt. Expect mainly VFR conditions. McDonnal

&&

.MARINE...Onshore flow will continue tonight. The gale warning
that was issued for the central and eastern Strait of Juan de Fuca
will be downgraded to a small craft advisory with the evening
forecast issuance. Westerly swell will build to 21 to 22 ft over
the outer coastal waters and 19 ft over the inner coastal waters
late tonight and Saturday morning. Small craft advisories for
hazardous seas will continue through Saturday over the coastal
waters and at the west entrance to the Strait of Juan de Fuca.

Another frontal system will move east across the waters late
Saturday night and Sunday morning. This front is expected to bring
small craft advisory conditions to most of the waters. The flow
will turn northerly offshore on Monday as a cold dome of high
pressure moves south through British Columbia. Cold Fraser River
outflow winds of 20 to 30 kt will probably develop across the
northern inland waters and Strait of Juan de Fuca Monday night or
Tuesday, and gales are possible. McDonnal

&&

.HYDROLOGY...No river flooding is expected over the next week.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Storm Watch from Saturday afternoon through Sunday
     afternoon for Cascades of Pierce and Lewis Counties-
     Cascades of Snohomish and King Counties-Cascades of Whatcom
     and Skagit Counties-Olympics.

PZ...Small Craft Advisory until 10 PM PST this evening 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 hazardous seas until 6 PM PST Saturday
     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 6 PM PST Saturday for
     Grays Harbor Bar.

     Small Craft Advisory until 11 PM PST this evening for Central
     U.S. Waters Strait Of Juan De Fuca-East Entrance U.S.
     Waters Strait Of Juan De Fuca.

     Small Craft Advisory for hazardous seas until 6 PM PST Saturday
     for West Entrance U.S. Waters Strait Of Juan De Fuca.

&&

$$

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