FXUS66 KSEW 111731

Area Forecast Discussion
National Weather Service Seattle WA
931 AM PST Tue Dec 11 2018

.SYNOPSIS...A series of vigorous fronts will move through the
area over the next week. The first one will move onshore today
with locally windy conditions and heavy mountain snow. There will
be a lull on Wednesday followed by the next system Wednesday night
and Thursday. This system will hang up over the area before
pushing inland later Friday. Another system will arrive on Sunday.


.SHORT TERM...A strong frontal system is currently moving through
the area. Wind advisories are verifying at the coast and
Northwest Interior. There has been some off and on high wind at
one observation site on Lopez Island but no other reports of high
wind. Winds will ease a but later today as the front moves inland
but it will remain breezy to windy in places. The winter storm
warning for the Cascades also looks on track. Snow has begun to
accumulate in the Cascades and this will continue today and
tonight before tapering. Snow levels will rise to 4000-4500 feet
later today which could turn snow to rain in the passes but then
snow levels will come down again tonight. 1-3 feet of new snow
still looks like a good forecast through tonight with the highest
amounts above 4000 feet.

Wednesday will be a relative lull in between frontal systems. The
next in a series of frontal systems will bring more rain, wind,
and mountain snow Wednesday night and Thursday. Advisory strength
winds are likely again coast and Northwest Interior. Headlines
will also likely be needed for the Cascades where another 1-2
feet of snow is possible.

This next system differs a bit from recent fronts in that it will
hang up across the area on Thursday, weaken, and then lift north
as a warm front Thursday night. This is all in response to another
system strengthening offshore. This frontal wave will move
through the area as a cold front on Friday. An associated deep
surface low will track offshore and move into Vancouver Island.
Models are starting to come into better agreement with this idea
but there are still differences with timing, location, and
strength of the low. Regardless, it will likely be another breezy
to windy day in spots Thursday night and Friday with additional
wind related headlines possible. Schneider

.LONG TERM...Previous discussion...So far models continue to show
a break in the action on Saturday with an upper level ridge. But,
this break is short- lived at we return to wet weather Saturday
night and Sunday. Beyond then, looking into early next week, it
looks like the pattern will remain active but the timing, strength
and tracking of the systems is off in the models. But in general,
more rain and mountain snow is in our future. 33


.AVIATION...Vigorous upper level trough west of BC and associated
frontal system will affect western Washington thru early Wednesday.
Thru the afternoon today a warm front will lift N/NE across the
local area, with a cold front expected to trail behind it this
evening into tonight. Strong gradients are suggestive of continued
breezy conditions with some decent gusts behind the warm frontal
boundary. Wind shear is also a concern as recent VAD wind profile
data from KATX show wind speeds increasing to around 45kts at
2000ft. In addition, ceilings are expected to remain low-MVFR to IFR
into the evening hours. As the cold front passes tonight, ceilings
have a better chance of improving somewhat, altho winds will likely
still remain breezy. A break in between systems is expected for the
first half of Wednesday before another strong system moves in late
Wednesday and leads to another increase in winds and ceiling issues.

KSEA...Should begin to see wind speed/gusts increase anytime now at
SeaTac, as guidance suggests strong gradients and VAD wind profiles
show strong winds through much of the atmosphere. This is in
association with a warm front which will affect the area thru the
afternoon. Cig will remain in the MVFR/IFR category thru this
timeframe. Cold front will sweep thru this evening and help improve
cigs somewhat but winds will continue to remain breezy. Steady rain
is likely for much of today before tapering to showers later this
evening and overnight.



.MARINE...A strong upper level shortwave trough currently offshore
the BC Coast will amplify thru the day and move southeast. An
associated warm front will lift N/NE from the Washington Coast thru
the Cascades, followed by a trailing cold front. Based on current
observations and the above synoptic analysis, will leave headlines
as is, with gale warnings for the coastal waters/West Strait and
Northern Inland waters/East Strait/Admiralty Inlet with small craft
for remaining waters. The Gale Warning for the previously mentioned
inland waters may need to be extended thru 00z with the afternoon
package should projected trends continue ahead of the front. Winds
will relax somewhat after frontal passage tonight but the next
system will move into the region fairly quickly by late Wednesday
where winds will pick back up. A series of systems will continue to
affect western Washington thru at least early next week where marine
headlines are expected to continue.



.HYDROLOGY...The current storm over western Washington today has put
down 1.5 to almost 2.5 inches of precipitation over the west side of
the Olympics. Rain shadowing is occurring over the central puget
sound region with mostly a tenth of an inch or less. The Cascades
have received 0.5 to almost 1.4 inches so far. The forecast is for
another 1 to 2 inches of rain today before tapering off. The snow
level over the Olympics has jumped up to 7000 feet this morning but
will come back down tonight - after the heavy precip is over. This
is allowing plenty of storm runoff from the rain. This will swell
rivers over the region, but as of now only the Skokomish River is
expected to exceed flood stage.

After a break on Wednesday the next storm in a series impacts the
area starting Wednesday night and may hang on into Friday. This will
renew flooding on the Skokomish and continue pushing other area
rivers higher. Over the weekend into next week another potent storm
will impact the area and early indications are it could produce some
widespread flooding.


WA...Winter Storm Warning until 4 AM PST Wednesday for Cascades of
     Pierce and Lewis Counties-Cascades of Snohomish and King
     Counties-Cascades of Whatcom and Skagit Counties.

     Wind Advisory until 1 PM PST this afternoon for Admiralty Inlet
     Area-San Juan County-Western Skagit County-Western Whatcom

     Wind Advisory until 10 AM PST this morning for Central Coast-
     North Coast.

PZ...Small Craft Advisory for rough bar until 4 AM PST Wednesday for
     Grays Harbor Bar.

     Gale Warning until 4 AM PST Wednesday 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 1 PM PST this afternoon for Admiralty Inlet-
     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 Wednesday for Central U.S.
     Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 10 PM PST this evening for Puget
     Sound and Hood Canal.




NWS SEW Office Area Forecast Discussion

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