Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 040454
Area Forecast Discussion
National Weather Service Seattle WA
900 PM PST Sat Dec 3 2016
.SYNOPSIS...A cold front will bring heavy snow to the mountains
tonight. Colder air will move into Western Washington Sunday, and
snow showers are possible over the lowlands Sunday night and Monday.
Cold and mainly dry weather is likely Tuesday into Wednesday. A
system arriving Wednesday night and Thursday could bring more low
elevation snow before warm air brings rising snow levels.
.SHORT TERM...A moist wave moving through westerly flow aloft will
bring rain tonight, along with heavy snow in the mountains. A winter
storm warning covers the central and south Cascades with advisories
for the north Cascades and Olympics. The snow level is around 3000
feet so all the passes will be affected.
Sunday will be mostly dry with just scattered snow showers in the
mountains. However a colder air mass will be moving over the region.
Highs will reach the mid 40s early in the afternoon, but after that
the temperature will fall. Overnight lows will be in the low to mid
30s. More interestingly, dew points will fall below freezing Sunday
night, and remain below freezing for much of the next week. This
will introduce the possibility of snowfall in the lowlands.
The precipitation pattern Sunday night and Monday is showery. At
least some of this will be in the form of snow. The latest GFS
ensembles show one inch, plus or minus one inch, at KSEA through
Monday. This probably means most places will get some snow, with
more on the hills and less near sea level. Though accumulation on
most roads is likely to be light, the Monday morning commute may
still get messed up to some extent.
A weak impulse moves through the flow Monday which will give a
little rain or snow. Temperatures will fall further Monday night as
cold northerly flow begins. Monday night will be below freezing and
Tuesday will have highs in the mid 30s. Any precipitation will end
Monday night. Tuesday and most of Wednesday will be dry. Burke
.LONG TERM...Previous discussion...The next system to watch will
bring precipitation back into the area Wednesday night and Thursday.
With cold air in place to start, precipitation could fall as snow
initially in the lowlands. Models suggest a few inches of snow could
fall before changing over to rain Thursday morning or midday. The
flow is strong enough to scour out cold air by afternoon but there
could be several inches of snow before this occurs.
Snow will fall again in the mountains, adding to the snowpack toward
the end of the week. Precipitation will be rain in the lowlands
later Thursday and Friday. Some extended model runs show a stronger
low tracking north of our area Friday or Saturday, possibly bringing
wind and higher snow levels. There is not much agreement toward the
weekend except that the active weather pattern will continue. Mercer
.AVIATION...Strong westerly flow over Western Washington this
evening will continue tonight as a frontal system moves quickly
through the area. Lighter northwest flow will prevail behind the
front on Sunday. The air mass will remain moist, and it will
become somewhat unstable Sunday.
There is a mix of MVFR to low-end VFR conditions across the
forecast area at 8 pm, with widespread rain. Conditions should
lower somewhat tonight as rain continues, with widespread MVFR
ceilings and local MVFR visibilities. A Puget Sound convergence
zone will form late tonight, but it will probably move south and
dissipate midday to early afternoon. Conditions will improve
Sunday, probably to low-end VFR, in showery post-frontal weather.
KSEA...South winds 10 to 15 kt with gusts to 22 kt tonight. MVFR
conditions will continue until frontal passage around 09Z. A
Puget Sound convergence zone will likely develop immediately behind
the front, probably moving south and/or dissipating around midday.
Condition should improve to VFR with ceilings 040-060 Sunday
.MARINE...A Pacific frontal system will move quickly eastward
across the forecast area tonight. Small craft advisories are in
effect, except for the central and eastern Strait of Juan de Fuca
where a gale warning has been posted. Westerly swells of 14 to 17
ft this evening will continue to slowly subside tonight and
A weak low will move southward through the waters on Monday with
south to southeast flow gradually shifting to northerly. As the
low tracks south on Monday, a cold dome of high pressure will move
south through British Columbia and the flow across Western
Washington will turn northerly offshore. Cold Fraser River
outflow winds of 20 to 30 kt will likely develop across the
northern inland waters and Strait of Juan de Fuca Monday evening,
and gales are possible.
A strong Pacific front will approach the area Wednesday night and
move through the waters on Thursday. Gales are likely for most
waters except for Puget Sound and Hood Canal. McDonnal
.HYDROLOGY...No river flooding is expected over the next week.
WA...Winter Storm Warning until noon PST Sunday for Cascades of
Pierce and Lewis Counties-Cascades of Snohomish and King
Winter Weather Advisory until 7 AM PST Sunday for Cascades of
Whatcom and Skagit Counties-Olympics.
PZ...Small Craft Advisory for rough bar until 6 PM PST Sunday for
Grays Harbor Bar.
Small Craft Advisory until 6 PM 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 3 AM PST Sunday for Admiralty Inlet-
Northern Inland Waters Including The San Juan Islands.
Gale Warning 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.
Small Craft Advisory until 2 AM PST Sunday for Puget Sound and
You can see an illustrated version of this discussion at