736
FXUS66 KSEW 171106
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
306 AM PST Thu Jan 17 2019

.SYNOPSIS...An extensive area of low pressure will bring rain,
mountain snow, and some wind to western Washington today and
tonight. Another fast moving system will bring another quick
period of moderate rainfall Friday night and Saturday. The pattern
will then remain fairly active Sunday thru mid next week as
several weak systems pass by. By late next week, a high amplitude
pattern develops across the country, with the Pacific Northwest
under a dry ridge influence.

&&

.SHORT TERM...A 10z water vapor imagery analysis reveals an
extensive area of low pressure off of the Washington/Oregon coast
beginning to lift northeast. Near the sfc an occluded front
extends out ahead the system`s sfc low, which is stacked
underneath of the parent mid level low. This is evidence of a
weakening system, which guidance depicts well as the low is
progged to continue northeast and eventually become an open wave
trough.

This morning MRMS radar imagery reveals an initial swath of
precip lifting north across the CWA. Offshore flow and a
noticeable easterly gradient has allowed for some breezy winds out
of the Snoqualmie Pass area into places like North Bend where
gusts around 30 mph have been noted. Gradients are beginning to
relax so do expect these winds to continue to gradually ease. The
flow will continue offshore thru the day today with scattered rain
showers likely as the mid level low remains in the vicinity of
the area. Model guidance does suggest some stronger winds just off
the sfc between 925-700mb tho feel lack of appreciable mixing
should inhibit these winds from reaching the sfc, at least
initially today. Nonetheless, by this afternoon as winds shift
onshore as the main area of low pressure lifts northward, do think
stronger winds will reach the sfc, at least across the coast and
the north interior. A Wind Advisory remains in place for these
area thru 4am Friday where gusts to at least 40 mph is expected.
In addition to the wind, as precip continues, snow levels will
linger around 3500 ft in the Cascades. Given the amount of
moisture expected with this system, snowfall amounts will likely
reach advisory levels above 3500ft. A Winter Advisory has been
issued for this afternoon thru Friday morning with accumulations
ranging from around 11 inches at Mt Baker to around 7 inches at
Stevens Pass and up to 15 inches at Paradise, Mt Rainier.

The effects of this system should begin to taper off Friday
morning into the afternoon, where a brief break in the steadier
rainfall/mountain snow is expected. Scattered showers will still
likely linger thru this timeframe and it will be short lived as a
quickly deepening shortwave trough darts across the Pacific
towards the south-central BC coast. An atmospheric river of
moisture will accompany this wave with storm track progged to hit
the Pacific Northwest. Latest model solutions still indicate a
period of decent rainfall late Friday afternoon into early
Saturday morning, although it is beginning to look like the
heaviest swath will miss us to the south in Oregon. Should this
trend continue, western Washington may be looking more into
residual scattered showers for most of the day Saturday, perhaps
even a fairly dry end to the day.

Kovacik

.LONG TERM...Active weather will continue into the first half of
the long term. The next in a series of shortwave troughs across
the Pacific will approach the region on Sunday. Consensus for now
is pointing towards a more southerly track with this system so
have essentially blended the forecast for now, as some solutions
are mainly dry, others wet. Whatever the case, it is a quick
moving system and should be exiting Sunday night. By Monday, a
ridge sets up in the mid/upper levels and western Washington may
remain rain-free. However by Monday night and Tuesday moisture
will ride atop the ridge and bring another round of rain to the
area, which may linger all the way into much of Wednesday. After
this episode of wet weather a pattern change still looks
promising, as the CONUS enters a very high amplitude long wave
pattern- one which features an eastern US trough and western
US/eastern Pacific ridge. This will allow for the return of dry
conditions late next week into the weekend.

Kovacik

&&

.AVIATION...Southerly flow aloft this morning will become
southwesterly this afternoon. Frontal system will move north of the
area early this morning with an upper level trough moving into. the
area tonight. At the surface, easterly offshore flow will continue
today with the flow becoming more southerly tonight into Friday. The
air mass is stable with mid level moisture becoming slightly
unstable tonight.

Ceilings in the 5000-7000 foot range for the most part this morning.
Just some mid level clouds this afternoon. Ceilings lowering back
down to near 5000 feet tonight.

KSEA...Ceilings 5000-7000 feet this morning. Just some mid and high
level clouds this afternoon. Ceilings lowering back down to around
5000 feet tonight. Easterly wind 6-12 knots with gusts to 20 knots
early this morning. Wind becoming south southeasterly 6 to 12 knots
around 03z tonight. Felton

&&

.MARINE...A 972mb low over the Oregon offshore waters will lift
northward to near Haida Gwaii Tonight as it slowly fills. Strong
gradients to the east of this low will give gales to all of the
waters except for Admiralty Inlet and Puget Sound and Hood Canal
through tonight.

A 998 mb low near 34N 152W is forecast to deepen to 975 to 980 mb
as it moves NE to the Queen Charlotte Sound early Saturday
morning. This low is expected to give another round of gales to
most of the waters Friday night into early Saturday.

Seas with the first system will build to 18-22 ft, highest over
the outer coastal waters. The second system may bring seas up to
30 feet to portions of the coastal waters late Friday night or
Saturday. Albrecht

&&

.HYDROLOGY...An active pattern is in store through early next as a
series of Pacific storm systems affect the local area. The first system
will impact the area today into early Friday, with the second
system following closely on its heels late Friday afternoon into
Saturday. QPF amounts during this timeframe, which have come from
a blended forecast using hi-res model data and WPC suggest the
highest amounts across the mountains, particularly the Olympics
where generally 2-4 inches can be expected which locally higher
amounts around 6 inches possible. Snow levels will hover around
3500 ft or so snow is expected across higher elevations. The
Skokomish River will need to be monitored, as it may still reach
early this weekend. No other river flooding is expected during
this timeframe. An additional inch or so of precip will be
possible Sunday through the middle of next week.

Kovacik


&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory from 4 PM this afternoon to noon PST
     Friday for Cascades of Pierce and Lewis Counties-Cascades
     of Snohomish and King Counties-Cascades of Whatcom and
     Skagit Counties.

     Wind Advisory from 4 PM this afternoon to 4 AM PST Friday for
     Admiralty Inlet Area-Central Coast-Everett and Vicinity-
     North Coast-San Juan County-Western Skagit County-Western
     Whatcom County.

PZ...Small Craft Advisory for rough bar until 4 PM PST Friday for
     Grays Harbor Bar.

     Gale Warning until 4 AM PST Friday 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 until 4 AM PST Friday for Admiralty Inlet-
     Puget Sound and Hood Canal.

&&

$$

www.weather.gov/seattle

NWS SEW Office Area Forecast Discussion

Discussion Script by Ken True at Saratoga Weather - WFO Script by SE Lincoln Weather