Weather Service Report


976 
FXUS66 KSEW 091756
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
956 AM PST Fri Dec 9 2016

.SYNOPSIS...An approaching upper level trough with moist westerly
flow aloft will maintain showers over Western Washington through
this weekend. Residual spotty sub-freezing air over the area this
morning will mix out this afternoon. 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 the week. 

&&

.SHORT TERM...Warm air spreading northward behind a warm front has
been enough to push temperatures to just above freezing for most of
W WA this morning. As a result the showers moving across the area
have been falling as primarily rain. Some residual cold pockets
where temperatures are still hanging just below freezing may still
get some brief light snow or light freezing rain. Most notable would
be the east Puget Sound like Maple Valley, Fall City, Duvall, east
of Snohomish. The freezing temperatures will likely mix out before
midday. The other main spot would be western Whatcom county where
sub0-freezing cold air is still spilling out of the Fraser River
valley. The winter weather advisory continues until noon today and
could be extended if warming is slower to take hold.

For this weekend, the stacked upper level and surface low near
50N/135W this morning will slowly move E toward W WA, moving
overhead Saturday night. The approaching low will maintain moist
westerly flow aloft over the area with lots of showers. After the
low has dissipated early Sunday morning, a broad upper level trough
will remain over the region, allowing the moist westerly flow aloft
and showers to continue. Temperatures will be pretty close to normal
with highs in the lower to mid 40s and lows in the 30s. Kam

.LONG TERM...The weather pattern will become interesting again late
Sunday through 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, mostly likely on Monday. At this point
it looks there could be a short period of snow over the interior of
W WA sometime on 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. 

After the modified arctic air spreads over W WA. Dry northerly flow
aloft will persist over the area through most of next week, for
colder than normal temperatures. Kam

&&

.AVIATION...An upper level low will remain over the Pacific
Northwest and British Columbia through Sunday. Moderate southwest
flow aloft. Moist at all levels and weakly unstable near the coast. 

Warmer air aloft has changed precipitation to mostly rain except
snow continues along the Strait and over Western Whatcom county at
the CLM and BLI terminals. A period of Freezing rain is possible at
the BLI terminal later today and even possibly into this evening
with any ice accumulation there less than a quarter inch. Another
warm front tonight should switch over any residual freezing
precipitation to rain by midnight. Cigs mostly MVFR today.

KSEA...Precipitation has transitioned to rain which will continue at
times through tonight as another weak warm front passes the terminal
around midnight. Mostly MVFR cigs. Some break in rain Saturday with
some VFR SCT-BKN low or mid clouds possible. More rain moves in
Saturday evening with lowering cigs. dtm

&&

.MARINE...A warm front will cross the waters late this afternoon and
evening and pass through the interior waters by midnight. A trailing
998 mb surface low will track across the north coastal waters and
north interior Saturday afternoon and night. Gales are likely
Saturday and Saturday evening over the coastal waters. Small craft
winds most other waters through tonight except Puget Sound, although
southwesterly Small craft winds could occur behind the warm front
later tonight. dtm


&&

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


&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory until 3 AM PST Saturday for Cascades of 
     Pierce and Lewis Counties-Cascades of Snohomish and King 
     Counties-Cascades of Whatcom and Skagit Counties-Olympics.

     Winter weather advisory for East Puget Sound lowlands, western
     Skagit county until 4 PM. 

     Winter Weather Advisory until midnight PST tonight for Western
      Whatcom County.  

PZ...Small Craft Advisories are in effect for all waters except 
      the Puget Sound/Hood Canal.

  Gale Watches are in effect for the coastal waters from Cape
 Flattery to Cape Shoalwater from 10 NM out to 60 NM and
 for the east entrance to the Strait of Juan de Fuca for
  Saturday.

&&

$$

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Western Regional Climate Center, wrcc@dri.edu