Small Craft Advisory 256, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
1021 PM PST Sun Dec 9 2018

.SYNOPSIS...The weather pattern will be more active over the next
week as a series of fronts move through the area. The next one,
stronger than the front Sunday evening, will arrive on Tuesday.
An upper trough and onshore flow will keep the weather cool and
showery Tuesday night into Wednesday. Additional vigorous frontal
systems will affect the region through the rest of the week into
next weekend.

&&

.SHORT TERM...Satellite imagery shows front moving into the
Cascades this evening. Doppler radar has a little break behind the
front with more shower activity moving inland along the coast.
Northwest Avalanche Center mountain observation sites reporting 3
to 4 inches of new snow so far this evening in all locations
including Snoqualmie pass. Temperatures in the lowlands at 9
pm/05z were in the lower to mid 40s.

Showers will continue through Monday morning as the trailing upper
level trough moves into the area. Orographics not that strong
with model 850 mb winds overnight southwest 10 to 20 knots but
good enough to keep the light precipitation going in the Cascades.
Snow level remaining just below 3000 feet so all of the passes
will get a few more inches of snow overnight. Will keep the Winter
Weather Advisory going until 4 am Monday. Upper level trough axis
moving east of the area Monday afternoon with weak high pressure
building in at the surface. This will give the lowlands a little
bit of a break with the showers mostly confined to the mountains
by mid afternoon.

Next system still timed into the area on Tuesday with rain most of
the day. This system will be stronger than the one that just moved
through Western Washington with the jet aimed right at the area.
Snow levels rising with the warm advection out ahead of the front
on Tuesday then lowering back down to near pass levels Tuesday
night behind the front. Heavy snow is possible in the Cascades
Tuesday night with good post frontal orographics.

Upper level trough behind the front moving through quickly later
Tuesday night into early Wednesday morning with another mini
break in the precipitation for most of the lowlands. 00z model
runs a little faster with the frontal system Wednesday night into
Thursday with rain spreading over most of the area Wednesday
evening. Will wait for at least another model run with the faster
timing before changing the forecast. Quick update to transition
the precipitation type to showers in the forecast this evening for
the lowlands. No other changes to the forecast. Felton

.LONG TERM...Previous discussion follows. Frontal systems affect
the region each day but the timing and details vary quite a bit
between the models. Periods of rain with snow only in the
mountains is the best forecast for now-- and some blustery winds
are likely. The ECMWF and GFS-FV3 maintain a tendency to split the
system coming in on Friday while the 12z GFS has much less of a
split at 120hrs:
At 12z Friday the GFS has a 992mb low at Destruction Island and
another 1000mb low off the southern Oregon coast--but the southern
low doesn't amount to much and by midday there is a low in the
990s somewhere between Seattle and Bellingham. The UW WRFGFS takes
a 991mb center through the Chehalis Gap at daybreak and then
generates some snow for the Puget Sound area--that seems unlikely,
but something to see if the Euro and FV3 become less splitty with
time. Speaking of splitty--the 12z FV3 has a 986mb low going into
Vancouver Island with surprisingly little gradient over Western
Washington by Friday afternoon because it shifts more of the
dynamics into Oregon and Northern California by that time.

&&

.AVIATION...An upper trough will settle over the area tonight
behind the front then move inland on Monday with a short wave
upper ridge moving over Western Washington. Flow aloft westerly
with flat surface gradients. The air mass is moist and stable.

With plenty of low level moisture and little flow in the lower
layers of the air mass into Monday morning. Widespread MVFR
ceilings across the area with isolated ceilings below 500 feet
mainly over the Kitsap peninsula. Slow improving trend late
Monday morning with ceilings rising to near 5000 feet by mid
afternoon Monday.

KSEA...Ceilings in the 1000-1500 foot range improving to around
2000 feet Monday morning and to near 5000 feet Monday afternoon.
South southeast wind 4 to 8 knots through Monday. Felton

&&

.MARINE...A series of fronts will bring southerly gales as times
to the Coastal Waters, entrances to the Strait, and Northern
Inland waters.

Monday will be a relative lull in the active weather with the
headline being for some 10-12 foot west swell over the Coastal
Waters.

Next front to reach the area looks to bring another round of
gales Monday night and Tuesday morning. There will also be a
westerly push behind this system with small craft advisory
strength west winds in the Strait and perhaps gales Tuesday night.

After a break on Wednesday the next front will reach the area on
Thursday with another round of gales likely in the usual spots.
This system is expected to hang up across the area into Friday
with a surface low eventually moving onshore. Models are in poor
agreement with details like where the low will move onshore and
how strong it will be. This means there is a great deal of
uncertainty with the wind and sea forecast by Friday. Felton/Schneider

&&

.HYDROLOGY...A series of fronts will cross the area over the next
week, bringing a period of rain roughly every other day. The
cumulative effect could be enough to push the Skokomish river in
Mason county to near flood stage by late this week. The best
chance for flooding on the Skokomish is probably Thursday or
Friday when a frontal system could hang up over the area. The
hanging up over the frontal system late in the week also brings
up the possibility of flooding on other rivers flowing out of the
Olympics.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory until 4 AM PST Monday for Cascades of
     Pierce and Lewis Counties-Cascades of Snohomish and King
     Counties-Cascades of Whatcom and Skagit Counties.

PZ...Gale Watch from Monday evening through Tuesday 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.

     Small Craft Advisory until 10 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 for rough bar from 4 AM to 10 PM PST Monday
     for Grays Harbor Bar.

     Small Craft Advisory until 10 PM PST this evening for East
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 4 AM PST Monday for Northern Inland
     Waters Including The San Juan Islands.

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