Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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870
FXUS66 KSEW 210012 AAA
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
330 PM PST Tue Feb 20 2018

.SYNOPSIS...Light snow across southwest Washington this evening will
shift south into Oregon by midnight. Partial clearing later tonight
will allow temperatures to fall with Wednesday morning lows in
the low to mid 20s, except upper teens possible in colder valleys.
Another relatively weak system will drop south just off the
coast Wednesday night and may bring some light snow, mainly to the
coast. A break between systems later Thursday

&&

.SHORT TERM...Radar imagery was interesting to watch today as bands
of precipitation expanded over the southwest interior and north
over parts of central/south Puget Sound. Steadier snowfall and
accumulations mainly stayed within the advisory area covering
Thurston, Mason, Lewis, and Grays Harbor county. A spotter in
Thurston county reported 3.5 inches at 300 pm but also noted
snowfall was decreasing significantly. So far, little or no snow has
accumulated in the main Tacoma to Seattle/Bellevue I-5 corridor as
anticipated. pockets of light snow or flurries are still falling
around the Sound, but this should shift south early this evening and
end. skies may partially clear later tonight and temperatures should
drop back to the low or mid 20s again. Locations that have fresh
snow on the ground, mainly the southwest interior, could dip to the
upper teens if clouds clear quick enough.

Wednesday will be dry most of the day, but another weak low pressure
system arriving from the north will begin to spread light snow into
the northern coastal areas late in the day. This system is weaker
than the current low, but is much closer in proximity to the coast.
Almost all models keep most precipitation near the coast and imply
an inch or two could fall from Forks to Hoquiam/Aberdeen Wednesday
night, with just flurries or very light snow over the interior and
not likely to amount to more than a dusting if that.

Another break between systems is expect later Thursday and Thursday
night. The flow aloft will remain northerly with the long wave ridge
still centered out near 140-150W. This should maintain the northerly
flow pattern and keep temperatures from warming much until the next
stronger system arrives Friday.

.LONG TERM...Global and longer range meso models agree on driving a
strong front south-southeast through the area Friday. It should
produce some breezy or locally windy conditions and a period of
onshore flow. Models disagree considerably on the trajectory of the
approaching frontal wave and how much temperatures moderate before
precipitation develops. The ECMWF has been the most consistent in
driving cold air aloft over the area with the frontal precipitation,
limiting thickness rises. Unfortunately the 12z ECWMF was not
available today, and will be interesting to see the the 00z
solution tonight. The GFS moderates temperatures the most with 1000-
850 mb thickness values rising to near 1300M and fluctuating around
that value into the weekend. That is often the value referred to for
determining if snow levels reach near sea-level. This late in the
winter season, thickness values likely need to be lower than 1300M
to see snow down to sea-level. There may also be the issue of
shadowing off the Olympics depending on track of the approaching
front and flow aloft. Much of Puget Sound could be end up with much
lighter rain/snow amounts than the rest of Wrn WA. The pattern looks
great for orographic enhanced snowfall along the Cascades with snow
levels below passes. Ski resorts could pick up over a foot of snow
Friday/Friday night and possibly more over the weekend.

A cold showery trough will reside over the region over the weekend.
Some models bring a system in from the west and introduce a threat
of lowland snow on Sunday. Other models like the GFS keep a closed
low over the region with rain/snow showers but would not be a steady
significant precipitation producer. However, this would maintain the
ridge off to the west and could allow additional cold northern
stream systems to arrive early next week. Mercer


&&

.AVIATION...Northerly flow aloft will continue tonight, then become
more westerly Wednesday as a weak upper trough approaches the area.
The air mass is stable with areas of high and mid level moisture
over the north, and low level moisture over the south. South of a
KHQM-KSHN-KTCM line, precipitation will continue as light snow
tonight. North of that line there are light snow flurries and even
some rain mixed in. The northwest interior has been dry all day.
Northerly winds tonight will gradually push all of the precipitation
and most of the clouds out of the region. A weak system Wednesday
will increase cloudiness in the afternoon, but any precipitation
will probably be confined to the coast.

Areas of MVFR or even IFR ceilings with light snow will persist in
the southwest interior this evening. VFR ceilings 4-6k ft in the
north gradually improve to no ceilings in the far north. Conditions
will improve tonight, with gradual clearing from the north.

KSEA...VFR ceilings will continue this evening, with light rain or
snow showers possible in the vicinity of the terminal. No snow
accumulation is expected. Ceilings will rise this evening with just
scattered or broken high clouds into Wednesday morning.
Northerly winds will increase to 8-12 kt this evening. CHB

&&

.MARINE...Winds will become northerly and offshore tonight as a weak
low center moves south off the Oregon coast. Northeast winds will
increase to small craft advisory strength in the Northern Inland
Waters and through the Strait of Juan de Fuca tonight. Have added a
small craft advisory for northerly winds in Puget Sound later
tonight.

A weaker low will move south through the area Wednesday night. At
this time, winds are forecast to remain below 20 kt.

A stronger system will arrive Friday and will remain around the area
through the weekend. This system is likely to bring at least small
craft advisory strength winds to all waters, and could bring gale
southerlies or westerlies to some waters. CHB

&&

.HYDROLOGY...
River flooding is not expected the next 7 days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory until 6 PM PST this afternoon for
     the Central Coast.

     Winter Weather Advisory until 10 PM PST this evening for Lower
     Chehalis Valley Area-Southwest Interior.

PZ...Small Craft Advisory until 3 AM PST Wednesday for Central U.S.
     Waters Strait Of Juan De Fuca-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-East Entrance U.S. Waters Strait Of Juan De Fuca-
     Northern Inland Waters Including The San Juan Islands-West
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 7 PM this evening to 4 AM PST
     Wednesday for Puget Sound and Hood Canal.

&&

$$

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