Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 191749
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
949 AM PST Fri Jan 19 2018

.SYNOPSIS...An upper trough over the region will maintain cool
and showery pattern today. Snow showers above 2,000 feet could
produce a few inches of snow in the passes. A stronger and wetter
front will bring more rain, mountain snow and windy conditions
late Saturday night and Sunday. Additional fronts will will keep
the weather active through next week with temperatures trending
below normal.

&&

.SHORT TERM...In the big picture, a longwave trough will persist
over the eastern Pacific in some form or fashion for at least the
next 5 days. This will maintain active west-southwest flow aloft
over Western Washington with ejecting systems crossing the area
with regularity. Will be hard to find even a 12-hour period of dry
weather. We will generally remain under or poleward of the jet
stream, keeping snow levels seasonably low, i.e. at or below 3000
feet.

Now for the details. The much-diminished low center that brought
recent high surf to the coast will be moving north up Queen
Charlotte Sound later today. A negatively tilted occluded front
will be crossing the forecast area this afternoon, enhancing
precip. A larger mass of rain is already spreading onto the coast,
and it will spread inland this afternoon. Southerly gradients will
continue to increase in advance of the front today, so breezy or
windy conditions will be common. The coast will get a wind shift
as the front passes later this afternoon from southerly to
westerly.

Next up will be a sharp and vigorous occluded front that will
cross the coast early Sunday morning. East-southeast gradients
really pack up and strengthen in advance of this feature, peaking
in strength around 12z (4 am) Sunday. UIL-BLI will peak near -6
mb, PDX-BLI near +7 mb, AST-UIL near +5 mb, and SEA-YKM near -8
mb. Southerly 925 mb wind speeds will peak near 60 kt along the
coast and 55 kt over the north interior. GFS-based marine MOS
guidance peaks wind speed at Destruction Island near 50 kt and
Smith Island at 45 kt. GFS MOS for Whidbey NAS peaks at 31 kt,
while the usually more conservative NAM MOS peaks Whidbey at 33
kt. Will be heavily considering going with a High Wind Watch later
today for the coast and the standard 4 north interior zones.
Besides wind, this front will come with an enhancement of precip.
With snow levels topping out only near 3000 feet, moderate
snowfall amounts are expected in the mountains. Advisories or
possibly warnings may eventually be needed for the mountains.

An unstable air mass will follow on Sunday afternoon, with
abundant showers everywhere and thunder possible along the coast.
Haner

.LONG TERM...From Previous Discussion: The weather will remain
unsettled through next week as a series of fronts track through
the Pac NW. There`s little break in the action. Late in the week,
models show a cool upper level trough overhead with heights in the
530s. This keeps snow levels low, near 500-1000 ft. It`s possible
we could see rain/snow showers on the higher hills. However, with
southerly/onshore flow snow amounts look spotty and light. 33

&&

.AVIATION...A weak upper level trof and associated surface trof
offshore will move across the area later today. The flow aloft
will be westerly. Contd low level onshore flow. Areas of MVFR CIGs
will become more widespread late today. Expect MVFR VSBYs with the
heavier showers. The mtns will be obscd.

KSEA...Expect MVFR CIGs (2-3k ft) to develop late today and then
persist through at least Saturday morning. Southerly winds will
increase to 10-15 knots and become gusty this afternoon. Expect
the winds to diminish overnight.

&&

.MARINE...
A trough extending from a 990 mb low off the northwest coast of
Vancouver Island will move across the area today. Expect another
trough to move across the area on Saturday. A stronger frontal
system will impact the Washington waters on Sunday for the
potential for widespread gale force winds.

&&

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

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...High Surf Advisory until noon PST today for Central Coast-North
     Coast.

PZ...Gale Watch from Saturday afternoon through Sunday morning 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.

     Gale Warning until 6 PM PST this evening for Coastal Waters From
     Cape Flattery To James Island 10 To 60 Nm-Coastal Waters
     From James Island To Point Grenville 10 To 60 Nm-Coastal
     Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm.

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

     Small Craft Advisory until 6 AM PST Saturday for Coastal Waters
     From Cape Flattery To James Island Out 10 Nm-Coastal Waters
     From James Island To Point Grenville Out 10 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-Puget Sound
     and Hood Canal-West Entrance U.S. Waters Strait Of Juan De
     Fuca.

     Small Craft Advisory until midnight PST tonight for Admiralty
     Inlet.

&&

$$

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