Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 041756
AFDSEW

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

.SYNOPSIS...Much colder air aloft will filter into Western
Washington today as a cold front over Oregon moves south. Showers
will prevail today with mountain snow tapering off by midday. The
Puget Sound convergence zone will also be active today from around
Seattle northward. A surface trough brings a mix of rain and snow
down to sea level on Monday with light snow accumulation possible,
mainly above 500 feet. By Monday evening, fraser outflow develops
with snow showers down to sea levels moving south into the greater
Puget Sound area overnight. Mostly dry and colder temperatures
prevail Tuesday and Wednesday with lows in the 20s. Precipitation
may begin as snow across the area Wednesday night or Thursday
morning as the next system arrives before switching over to rain.
Unsettled weather persists Friday into the weekend with rain and wind
at times.

&&

.SHORT TERM...The freezing level is down to near 1500 feet this
morning as cold air aloft filters south into the region. Moist
westerly flow that brought 1 to 2 feet of snow to the mountains
overnight has shifted south over Oregon and precipitation has become
showery across the area. A few inches of snow could still fall in
the Cascades today, especially where the convergence zone lingers
over King and Snohomish counties. Scattered showers in the lowlands
will be around today, possibly mixed with some wet snow this morning
but no accumulations. Sunbreaks are also likely in some areas today
with a break between systems allowing highs to warm into the 40s.

A cold upper low along the B.C. coast will slide southeast toward
the Pacific Northwest tonight bringing a near -40C cold pocket at
500 mb into the area by Monday morning. Weak surface low pressure
will close off near the north Washington coast late tonight,
bringing a surface trough into the area Monday morning. Light
southerly/southeast flow will help keep temperatures near the surface
just above freezing but the air aloft will be cold enough to bring a
mix of rain and snow showers down to sea-level. The main snow level
will be near 500 feet and any accumulations will likely be on
hilltops and away from water. Model QPF amounts suggest local
amounts of a trace to 2 inches are possible, mainly on grassy
surface during the morning. Road temperatures will be warm initially
so not much more than some slush on some roadways outside the main
urban areas is expected. The exception to this could be near hood
canal where cold air can dam up, and also over Western Snohomish
county north to Whatcom county where slightly cooler temperatures
reside and where convergence zone activity is most likely. Locally 2
inches could fall from around Everett north. A winter weather
advisory was issued for interior lowlands for Monday morning but may
need to add Whatcom county to this.

Models have trended toward a stronger surface high settling in over
southern B.C. Monday through mid week. BLI-YWL gradient reaches -12
mb by 21z on the new 12z GFS, similar to the 00z ECMWF. This should
initiate fraser outflow, with peak gradient close to -16 mb Monday
evening. This favors bringing colder low dew point air into the
north interior by late afternoon, and filtering south into greater
Puget Sound during the evening. The 4kmWRF and other meso models are
developing a convergence band along the modified arctic outflow
boundary which could reach the Seattle area during the evening and
possibly linger from north of Tacoma to Everett through early
Tuesday morning. Showers would fall as all snow down to sea-level
and some models produce locally significant accumulations around the
north Seattle to Everett corridor. If this occurs, temperatures will
probably be in the upper 20s with icing on roadways and potential
significant impacts on the Monday evening and/or Tuesday morning
commutes. Even if precipitation does not develop, black ice will
remain a hazard during the overnight and morning hours Tuesday
through Thursday. Will need to evaluate all the latest data and
consider if the chances are high enough to warrant a watch for parts
of the metro area Monday night. 4 inches of snow are needed to meet
warning criteria so a statement may have to suffice, will decide by
the afternoon forecast issuance. Outflow could also produce several
inches of lowland accumulation over the San Juans, and also the
Strait including Port Angeles and Sequim which can get upslope
accumulation in this pattern.

Colder and drier air will filter in Tuesday. Highs will likely stay
in the 30s and lows dip into the 20s WEdnesday morning. Forecasts
should be monitored closely over the next several days. Mercer



.LONG TERM...Previous discussion...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...Moderate west to northwest flow aloft. Post frontal
onshore flow today will give decreasing showers to the mountains.
Convergence zones over the central portions of the Puget Sound area
and over Skagit and Snohomish counties will gradually dissipate this
afternoon. The air mass is somewhat moist and unstable. Moisture
will increase from the west late tonight into Monday morning and
snow levels will drop to around 500 feet with the onset of
precipitation.

Conditions are mainly VFR across the region today with MVFR
conditions within showers with convergence zones. Expect conditions
to become VFR most places during the afternoon hours as showers
diminish. The air mass is unstable, so some showers will continue.

Conditions will deteriorate from the west on the coast late tonight
and in the interior Monday morning as a system moves in from the
west. Rain or mixed precipitation can be expected at the lower
elevation terminals with snow or mixed rain and snow at higher
terminals like KPAE and possibly KSEA. KPAE could remain
predominately snow Monday morning with 1-3 inch accumulations.
Slushy accumulations of an inch or two is possible at KSEA.
Albrecht

KSEA...Variable wind 5 to 15 knots today will become southeast to
south 5-8 knots tonight. The convergence zone over the terminal this
morning will slowly dissipate during the afternoon hours with MVFR
conditions improving to VFR by 23Z. Snow showers will begin around
the terminal about 12Z-15Z Monday and 1-2 inch slushy accumulations
are possible Monday morning. Albrecht

&&

.MARINE...Post frontal onshore flow will gradually decrease today.
Winds will remain in the small craft advisory category on the coast,
but are expected to drop below that threshold later this afternoon
in the northern inland waters and through the central and eastern
portions of the Strait of Juan de Fuca. Seas will remain hazardous
on the coast 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 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/Albrecht

&&

.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.

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 until 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-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.


&&

$$

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