Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 111703 CCA
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
900 AM PST Wed Jan 11 2017

.SYNOPSIS...Snow in Lewis County will end this morning. Drier and
more stable high pressure will build from the north through Friday.
Westerly flow aloft will develop later Friday allowing temperatures
to moderate. A weak warm front will arrive Saturday night with mainly
an increase in clouds. A series of warmer and wetter systems will
arrive early next week.

&&

.SHORT TERM...Lewis County has been on the edge of a heavy snow
event in northern Oregon. Reports this morning show mainly 2-4
inches of snow fell there overnight. In Thurston County up to an
inch fell, and north of there precipitation has been flurries at
most. The clouds give out altogether north of Seattle, with clear
skies over the north interior. The upper low over north Oregon is
forecast to slowly move southeast today and tonight. Snow should end
in Lewis County this morning, and the snow advisory there will be
allowed to expire at 10 AM.

Clearing will proceed from the north, though probably slower than
earlier thought. The Seattle area is forecast to clear this evening,
with the Olympia area clearing after midnight. A cold and dry air
mass will remain over the region through Thursday. Highs will be in
the 30s today and Thursday, with overnight lows in the teens and 20s.

On Friday, a temporary flattening of the upper ridge could allow
some mid and high clouds to spread across the area, but the only
precipitation in the forecast is a low chance for the North Coast.
High temperatures will rise a bit, into the 35-40 range. Burke

.LONG TERM...Previous discussion...Upper ridging should rebound on
Saturday, and guidance has been trending drier and drier for
Saturday, so lowered PoPs further with this forecast package.
Another disturbance will flatten the ridge a bit on Sunday, with a
bit more of a chance of precip. Around next Tuesday, the jet stream
pattern will really consolidate into broader westerly flow across
the northeast Pacific, pushing a potentially wet front into the
region. Relating to the 00z GFS model`s heavy QPF next Tuesday,
only two of the 20 ensemble members have precip as heavy as the
operational GFS next Tue, so that does not support much concern
yet for flooding.  Haner

&&

.HYDROLOGY...River flooding is not expected through at least Monday
of next week. A period of heavy rain around next Tuesday will cause
rises on area rivers. Minor flooding on a river or two cannot be
ruled out but is unlikely at this time. Note: Snow is not a
significant contributor to flood flows on western Washington rivers.
Even when there is snow on the ground, river flooding is almost
exclusively caused by runoff from heavy rain.

&&

.AVIATION...A low pressure system at the surface and aloft over
northwestern Oregon will fill today. An upper level ridge to the
northwest of the area will slowly build southeast into the area
tonight and Thursday. A 1022 mb high over southwestern British
Columbia will slowly sag southeast into eastern Washington tonight
through Thursday.

Snow and MVFR conditions associated with the filling low over
northwestern Oregon reaches northward to just north of KCLS in the
southwestern Interior of Western Washington. Areas from about
KHQM-KSEA are just seeing mid level clouds...though a few snow
flurries with no accumulation are hitting the surface at times
around KTCM-KGRF. Expect conditions to dry from the north today as
the low fills, then mainly clear conditions with good
visibilities to dominate this later this afternoon through
Thursday. Albrecht

KSEA...Mid and high level clouds will gradually dissipate by 00Z
this afternoon leaving good VFR conditions tonight and Thursday. A
few snowflakes may fall from the mid level deck at times til 20Z,
but the lower levels are too dry to allow many to reach the ground
and no accumulations are expected. Winds 03011-15KT will slowly
relax to 6-8 kt late this afternoon and tonight. Albrecht

&&

.MARINE...A 1023 mb high over southwestern British Columbia will
slowly sag into eastern Washington tonight through Thursday. In
the mean time, a 1005 mb low over northwestern Oregon will
gradually fill today. Fraser outflow will relax midday then
offshore flow this afternoon will subside tonight. Light offshore
flow is expected Thursday through Saturday.

A weak front will move through the coastal waters on Sunday.

Gale warnings for the central portions of the Strait of Juan de
Fuca and for the Northern inland Waters have been extended until
noon today then will be converted to a small craft advisory though
this afternoon. Small craft advisory conditions will remain over
the northern 2/3 of the coastal waters and the western portion of
the Strait of Juan de Fuca through this afternoon. Other
advisories will be allowed to expire at 9 AM. Albrecht

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...High Wind Warning until 9 AM PST this morning for San Juan
     County-Western Whatcom County.

     Winter Weather Advisory until 10 AM PST this morning for
     Southwest Interior.

PZ...Small Craft Advisory until 3 PM PST this afternoon 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-West
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from noon today to 3 PM PST this afternoon
     for Central U.S. Waters Strait Of Juan De Fuca-Northern
     Inland Waters Including The San Juan Islands.

     Gale Warning until noon PST today for Central U.S. Waters Strait
     Of Juan De Fuca-Northern Inland Waters Including The San
     Juan Islands.

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.weather.gov/seattle/gafd/latest_webafd.html



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