Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 100620 CCA
Area Forecast Discussion...CORRECTED
National Weather Service Seattle WA
1020 PM PST Fri Dec 9 2016
Corrected the W/W/A hazard headlines.
.SYNOPSIS...A Pacific front will move across the area overnight
bringing gradual warming and rising snow levels. Showers will
continue Saturday through Sunday with snow levels remaining low
over the north interior. High pressure building over British
Columbia will spread cold air southward across the area on Monday.
Some snow showers are possible as the cold air mass spreads south.
Dry northerly flow aloft next week will maintain a cold dry
weather pattern for much of next week.
.SHORT TERM...Precipitation type has been a problem since this
afternoon as low level easterly flow ahead of an incoming warm
front keeps cold air in place east of about Interstate 5. Areas to
the east continue to see snow mixing in with the rain at times,
and some freezing rain has been reported in western Snohomish
County. Showers moving into the Cascades have produced impressive
snowfall amounts in the East Puget Sound Lowlands -places like
North Bend, Maple Valley, Snoqualmie Falls, and Gold Bar. With the
easterly flow continuing ahead of the incoming front, and
impressive precipitation rates with the front seen on observations
and radar, opted to extend the Winter Storm Warning for the East
Puget Sound Lowlands -specifically for areas near the Cascades-
until 3 AM PST Saturday.
Warm air aloft will spread into all but the extreme east part of
the Fraser Valley east of Bellingham on Saturday, but moisture
will be limited after tonight. Hence the Winter Weather Advisories
for Western Whatcom County and the mountains are expected to come
to an end during the early morning hours on Saturday as well.
A warmer -though not yet warm- and showery pattern will prevail
Saturday and Saturday night across the area with temperatures
below, but nearer, to average for mid December. Moist westerly
flow aloft will give more showers on Sunday.
The weather pattern will become interesting again late Sunday or
Sunday night through early Tuesday. The extremely cold air that
has been over Alaska will begin to relocate toward warmer climes.
The models are in agreement that some of this modified arctic air
will push south over the B.C. interior this weekend eventually
piling up over the southern B.C. interior on Sunday. This will set
up another Fraser River valley outflow event, most likely on
Monday. At this point it looks there could be a short period of
snow over the interior of W WA sometime Monday as the modified
arctic front moves southward across the interior. At the same time
the arctic air also pushes westward out the Strait of Juan de
Fuca, usually bringing upslope snow to the land areas along the
western strait. The timing of when the modified arctic front moves
across W WA is still highly uncertain and probably won`t be
resolved until later this weekend. Other details like snow amounts
and distribution are still uncertain. Albrecht/Kam
.LONG TERM...From the previous long term discussion: After the
modified arctic air spreads over W WA, the GFS, ECMWF, and
Canadian models generally maintain dry northerly flow aloft over
the area. Skies should clear from the north Monday night through
Tuesday. Confidence is still not that high though since the models
have differences in the location of an offshore trough and how
close it may get to W WA. For now though the forecast is for
mostly sunny, dry and cold conditions. Kam
.AVIATION...A weak upper level trough of low pressure will move
across the area late tonight, and will be followed by a weak upper
level ridge. Strong wly flow aloft will weaken. The low level flow
will become sly. Anticipate widespread MVFR CIGs (areas of MVFR
VSBYS) overnight. There will be pockets of -sn or -fzra at the
lower elevations of Whatcom County and the Cascade valleys/
KSEA...MVFR CIGs are expected through Saturday. CIGs may briefly
lift into VFR category overnight or Saturday morning. Ely winds
will become sly. Anticipate the rain to be mixed with snow at
times with any of the heavier showers overnight.
A surface trough will move across the area overnight, followed by
a stronger one on Saturday. Expect increased onshore or westerly
flow in the wake of the trough on Saturday. Onshore flow will
weaken on Sunday and then become northerly. Northerly flow will
strengthen on Monday.
.HYDROLOGY...No flooding is expected over the next week.
WA...Winter Storm Warning until 3 AM PST Saturday for East Puget
Winter Weather Advisories are in effect for the mountains
and the western Whatcom lowlands.
PZ...Gale Warning in effect from 4 AM to 9 PM PST Saturday for
coastal waters from Cape Flattery to Cape Shoalwater from
10 out to 60 NM.
Small Craft Advisories are in effect for the remaining
An illustrated version of this forecast discussion can be seen at