Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 041213
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
413 AM PST Sun Dec 4 2016

.SYNOPSIS...A cold front will move through Western Washington
early this morning. Colder air will move into Western Washington
behind the front later today. Snow showers are possible over the
lowlands Sunday night and Monday. Cold and mainly dry weather for
Tuesday into Wednesday with an upper level ridge offshore. A
system arriving Wednesday night and Thursday could bring more low
elevation snow before warm air brings rising snow levels.

&&

.SHORT TERM...Satellite imagery shows cold front over the
as far south as the Central Puget sound at 11z/3am. Rain continues
with and to the south of the front with a drying trend north of
the front. Temperatures at 3 am were in the lower to mid 40s.

Cold front will continue to move south this morning at be south of
the entire cwa by 18z. Satellite imagery shows just widely
scattered shower activity behind the front. Snow accumulations
have already decreased significantly in the North Cascades and the
Olympics early this morning. This trend will continue in the
Central and Southern Cascades later this morning with very weak
orographics behind the front ( 850 mb winds northwesterly 10 knots
or less ). Good snow totals overnight in the Cascades with Stevens
pass reporting 15 inches of new snow since 6 pm...Paradise 13
inches and Mount Baker and Snoqualmie pass 8 inches. The winter
storm warning for the Central and Southern cascades will remain in
effect through noon today. The winter weather advisory for the
Olympics and North Cascades will be ended. Cold air aloft pouring
into the area behind the front later this morning into this
afternoon with model 500 mb temperatures down to -38c and 850 mb
temperatures around -6c by 00z Monday.

Upper level trough along the British Columbia coast will continue
to sag south tonight with the leading edge of the shower activity
reaching Western washington early Monday morning. At this point it
looks like most of the shower activity will be confined to west of
Puget Sound before 12z Monday. With the colder air aloft lows will
be in the lower to mid 30s.

Monday morning still looks to be the best time for snow in the
lowlands. Model cross sections continue to show southwesterly
winds in the lower levels so even with the cold air aloft this
will keep the snow level mostly off the surface. Organized band of
showers associated with the upper level trough moving through
Western Washington in the 12z-18z time frame. Model precipitation
amounts are not very impressive...a tenth of an inch or so...but
with the convective nature of the precipitation and the low snow
levels will go ahead and issue a winter weather advisory for most
of the interior zones for the potential of up to 2 inches of snow
mainly above 500 feet Monday morning. Shower activity will
decrease in the afternoon. With the cold air in place highs will
only be in the lower 40s at best.

Upper level trough axis moving through Western Washington Monday
night keeping a chance of showers in the forecast. Snow levels
will be low, below 500 feet, but moisture will be diminishing
rapidly so any accumulations will be spotty at best. Lows will be
below freezing Tuesday morning, in the mid 20s to lower 30s.

Dry northerly flow aloft over the area on Tuesday with an upper
level ridge offshore nosing into the British Columbia coast. Highs
will be cool, in the 30s for the most part.

.LONG TERM...Extended models in good agreement on wednesday with
the upper level ridge over the area. Model solutions for Thursday
remain inconsistent with the GFS bringing a front into the area
on Thursday and the leading edge of the precipitation ahead of the
front moving into the area Wednesday night. The cold air will
still be in place bring up the possibility of the precipitation
being in the form of snow later Wednesday night into Thursday
morning. Strong southwesterly winds will scour out the cold air
later Thursday but not before a few inches of snow in the
lowlands. The ECMWF is much colder on Thursday with a slower
arrival of the front, not until Thursday night with the
precipitation beginning Thursday afternoon. Current forecast more
in line with the GFS solution and will stay with that scenario in
the morning zone package.

Both the GFS and the ECMWF have a warmer air mass over Western
Washington on Friday and Saturday. There are still timing
differences with the systems moving into the area but the
precipitation type will be rain for the lowlands. Felton

&&

.AVIATION...Strong westerly flow over Western Washington early
this morning will ease somewhat and veer to northwest during the
next few hours, as the frontal system moving over the region moves
quickly east. The air mass is moist this morning, but it will dry
aloft today and become unstable. The air mass will become stable
again tonight and deeper moisture will move into the area with
another system.

Once again the lowest conditions across Western Washington are in
the Puget Sound region, where ceilings are low-end MVFR to
locally IFR. Elsewhere conditions are mainly VFR with ceilings
035-060. The radar image at 2 am shows a Puget Sound convergence
zone developing around Everett; the convergence zone will probably
move south and weaken midday to early afternoon as the flow
becomes northwest. By this afternoon, we should have low-end VFR
conditions across the forecast area with scattered showers.
Another system will move into the area tonight.

KSEA...Southwest wind 10-18 kt this morning should ease a bit and
become westerly or northwest this afternoon, then become south
6-12 kt tonight. A Puget Sound convergence zone, forming over
Snohomish County early this morning, should weaken as it moves
south through Seattle and SEA around midday. Conditions should
improve to VFR with ceilings 040-060 or higher this afternoon. The
system tonight will probably bring snow showers to the terminal,
and there is a good chance of snow accumulations of 1 to 2 inches
mainly between late tonight and Monday afternoon. McDonnal

&&

.MARINE...The Pacific frontal system that moved into the forecast
area late Saturday is moving quickly east of the area this
morning. Moderately strong onshore flow behind the front will ease
today. The central and east Strait of Juan de Fuca has been
flirting with gale force west winds for the last several hours,
and I will extend the gale warning there through 7 am. Small craft
advisories are in effect for all other marine zones except Puget
Sound and Hood Canal. Westerly swells of 13 to 15 ft will subside
very gradually but remain above 10 ft through Monday.

A weak low will move southward along the west coast of Vancouver
Island tonight. As the low continues south to the Washington
coast on Monday, a cold dome of high pressure will move south
through British Columbia. As a result, the flow across Western
Washington will turn northerly offshore by late Monday. 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 night and continue Tuesday, 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.

&&

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory from 4 AM to noon PST Monday for
     Bellevue and Vicinity-Bremerton and Vicinity-East Puget
     Sound Lowlands-Everett and Vicinity-Hood Canal Area-Seattle
     and Vicinity-Southwest Interior-Tacoma Area-Western Skagit
     County.

     Winter Storm Warning until noon PST today for Cascades of Pierce
     and Lewis Counties-Cascades of Snohomish and King Counties.

     Winter Weather Advisory until 7 AM PST this morning for Cascades
     of Whatcom and Skagit Counties-Olympics.

PZ...Small Craft Advisory for rough bar until 6 AM PST Monday for
     Grays Harbor Bar.

     Small Craft Advisory until 6 PM PST Monday 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 from 7 AM this morning to 3 PM PST this
     afternoon for Central U.S. Waters Strait Of Juan De Fuca-
     East Entrance U.S. Waters Strait Of Juan De Fuca.

     Gale Warning until 7 AM PST this morning for Central U.S. Waters
     Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of
     Juan De Fuca.

     Small Craft Advisory until 3 PM PST this afternoon for Northern
     Inland Waters Including The San Juan Islands.

     Small Craft Advisory for hazardous seas until 3 AM PST Monday
     for West Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 9 AM PST this morning for Admiralty
     Inlet.

&&

$$

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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