Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 210330

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

.SYNOPSIS...It is clearing from the north this evening as the
system that brought some light snow to the southern and western
portions of the area moves south. Clearing will allow temperatures
to fall into the 20s tonight with some upper teens possible in
colder valleys. Another weak system will drop south just off the
coast Wednesday afternoon and Wednesday night bringing some light
snow, mainly to the coast. A break in the weather is expected between
systems on Thursday. A stronger system will give wet weather to
the area Friday into Saturday with low snow levels.


.SHORT TERM...Radars and satellite imagery show the system that
brought some snow to the coast and to the interior from about
Seattle and Bremerton southward is moving rapidly southward into
Oregon. Observations show precipitation now moving south of the
Lewis County line, and precipitation has come to an end.
Conditions have also cleared pretty rapidly on the northern
Olympic Peninsula and from about Seattle northward. With the
clearing overnight, expect low temperatures to drop into the 20s
across the area, and there may be a couple of upper teens in
normally colder valley locations - especially ones that have some
snow on the ground.

The next system can be seen on IR satellite imagery north of Haida
Gwaii. This system will race rapidly southward reaching our area
during the late afternoon and evening hours on Wednesday. This
next system is weaker than the one that hit the area today and
appears to be a bit more moisture starved. The air mass will be
cold enough that most of the precipitation will be in the form of
snow, to just above sea level. Any accumulations will be brief and
light, less than an inch, and will be most likely over the south
interior and along the coast (generally the same locations as
today`s system).

Behind Wednesday night`s system, we will see a break in the
weather as a ridge of high pressure aloft pushes southeast across
the region. Temperatures through the end of the week will be quite
a bit below normal with lows in the 20s and highs in the 30s to
lower 40s.

Will issue an evening update to clear conditions faster from the
north tonight and end the winter weather advisory in the south
interior. Albrecht

.LONG TERM...From the previous long term discussion: 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

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 tonight will become northwesterly
on Wednesday as as a weak upper level trough over northern British
Columbia pushes southward toward the area. The flow aloft will
turn back to the north behind the trough Wednesday evening. At the
surface, a weak low over northwestern Oregon will drop south of
the area and will be followed by another weak low that will move
southward along the coast Wednesday afternoon and evening. The air
mass is stable and is drying from the north this evening.
Moisture will increase from the northwest Wednesday morning ahead
of the next system.

Conditions are now VFR across the area with exception to portions
of the southwest interior this evening. Some MVFR conditions can
be expected Wednesday afternoon along the coast and Wednesday
evening in the interior from about KPWT-KSEA southward. Albrecht

KSEA...VFR conditions are expected at the terminal through
most of the day Wednesday. Mid and high level moisture will
spread into the area mid morning and ceilings will drop to VFR or
higher end MVFR during the late afternoon and evening hours. There
may be some light snow showers during the late afternoon and
evening hours on Wednesday, but accumulations, if they do occur,
are expected to be light -a quarter inch or less. Northeast winds
8-10 knots tonight will become light southeasterly Wednesday
afternoon and evening then return to northeasterly overnight
Wednesday night. Albrecht


.MARINE...A weak 1019 mb low along the northwestern Oregon coast
will continue to drop south and southeast overnight. Behind the
low, northerly flow has increased over the Northern Inland Waters.
Some short term models show an increase in northeast to east winds
overnight through the Strait of Juan de Fuca and over the coastal
waters. Small craft advisories remain in effect for the coastal
waters, the western two-thirds of the Strait of Juan de Fuca, and
the Northern Inland Waters. Winds are now expected to remain below
21 knots in Puget Sound and Hood Canal, so the small craft
advisory there will be cancelled.

A weaker low will move south through the area Wednesday afternoon
and 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. Albrecht/CHB


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


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.


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