Tornado in Washington, December 18, 2018

Area Forecast Discussion
National Weather Service Seattle WA
840 AM PST Tue Dec 18 2018

.SYNOPSIS...
An unsettled weather pattern will continue all the way through
the holidays as occasional storms across the Pacific make their
way into the region. Today's system will taper to showers by
tonight, carrying through Wednesday before another system brings
rain and wind to the area Thursday. A trend to more showers is
possible Friday and Saturday before another system moves in
Sunday. Unsettled weather possible for the holidays early next
week.

&&
.UPDATE /TODAY/...Minor updates to today's forecast this morning
mainly to lower snow levels across the Cascades to around
2500-3000 feet. Snow is falling at Snoqualmie Pass and I-90 is
closed in both directions near the pass due to snow on the roads
and multiple spinouts. Snow will likely continue at times
throughout the day so travelers should keep an eye on road
conditions and plan accordingly. Elsewhere across the lowlands,
showers will continue behind the front. Latest hi-res guidance
indicates some marginal instability later this morning - afternoon
hours, so cannot rule out a thunderstorm or two but
confidence/coverage too low for inclusion into the forecast at
this point. Will continue to monitor that today. Still waiting on
latest model runs to further refine impacts for Thursday's
system. Previous discussion follows. CEO

.SHORT TERM...
Active weather continues this morning across the CWA. Water vapor
imagery reveals two tightly wrapped centers of low pressure
located west of the BC. These lows are embedded within a broader
cyclonic flow regime across the northern Pacific, which is
helping to usher in a deep and extensive channel of moisture into
the Pacific Northwest. Finally, an analysis of the lower levels
reveals a slow moving frontal system pushing across western
Washington.

Given the synopsis presented above, it is not surprising that
radar imagery reveals an extensive area of light to moderate
rainfall across the lowlands/snow across mountains. Precipitation
will continue to move across the area thru much of the day before
tapering to showers tonight and early Wednesday. Earlier this
evening, winds were rather breezy/gusty across coastal locations
and the northern interior, where high wind warnings were valid.
Observations largely behaved according to forecast and these
headlines were allowed to expire. On the other hand, snow levels
have been tricky tonight, with observations showing 24 hour snow
totals from Snoqualmie Pass to Crystal Mountain in the 6-9 inch
range. Have therefore opted to issue a Winter Weather Advisory for
the Cascades of Lewis and Pierce counties above 3000ft thru 6pm
this evening, matching the timing of the remaining winter weather
headlines. Snow levels will not rise today, but precip should wane
later in the day, allowing headlines to likely be able to end
this evening. Heavy rainfall will continue to need to watch rivers,
but please refer to hydro section for details on that.

A weak ridge of mid/upper level high pressure will help push the
strongest jet energy to the north late tonight and Wednesday,
helping keep precip to more rain/snow showers in the
lowlands/mountains. This ridge looks rather dirty so do not expect
long periods of clearing thru Wednesday evening, and not to
mention the system responsible for the ridge will be moving
quickly towards the region. Another atmospheric river of moisture
looks progged to affect the region Wednesday night and Thursday,
bringing with it what looks to be another round of high wind/wind
advisory headlines. Snow levels ramp back up towards 5000ft so
snow concerns are not high at the moment. As the system passes
through the day, snow levels will then drop significantly to
below 2000ft but precip will wane noticeably.

Kovacik

.LONG TERM...
As the Thursday system departs, Friday has the potential to see
some showers with snow level below 2000ft. An unsettled pattern
will follow us into the weekend where guidance suggest a quick
moving system may skirt across the area on Saturday before a more
significant system moves in for Sunday. 06z GFS agrees more with
EPS/GEFS/GEMS ensembles on the 500mb pattern than does the 00z
ECMWF, which unfortunately is the wetter solution. This would
bring through an open wave trough in the mid/upper levels and
several compact sfc lows, one of which may track across the
western peninsula of Washington. This could be an event similar
to the previous few, with rain/wind/mountain snow concerns.

Remainder of the extended will likely remain unsettled. Could see
a brief break late Sunday but there seems to be rather good
consensus even in the ensemble data of even better Pacific energy
approaching the PacNW for the holiday. Exact track and trends in
weather-related impacts are yet to be determined but another round
of decent rainfall could be possible, which would essentially be
rubbing salt in the wounds at this point.

Kovacik

&&

.AVIATION...A frontal system will move east of the area this
morning. A weak upper ridge will move over the area tonight.
Westerly flow aloft will prevail today and tonight with low level
onshore flow. The air mass is moist and slightly unstable. It
will become stable tonight.

The trend will be for modest drying in the lower levels later
today with MVFR ceilings decreasing and mostly going away in the
afternoon .

KSEA...MVFR low clouds should lift and scatter today. VFR low
clouds are expected after that into tonight. Gusty south winds
10-20 knots will ease a bit tonight. Schneider

&&

.MARINE...Onshore flow will prevail today in the wake of a front
with small craft advisory strength winds most waters. Onshore flow
will ease on Wednesday. The next strong frontal system will move
through the area Wednesday night and Thursday with gales most
waters and possibly storm force winds for the Coastal Waters.
Recent model runs have begun to diverge with this system with the
GFS now about 12 hours faster than the ECMWF. That means there is
a bit of uncertainty with when the strongest winds will occur and
how strong they will be.

Models are also showing poor run to run consistency with fronts
moving into the area Friday and Saturday. Confidence is high
stormy weather will continue during this period but the details
are somewhat up in the air.

A period of high surf is likely late tonight and Wednesday
morning as 20 foot swell reach the coast. The latest wave model
has backed off slightly with these waves however so it looks
marginal at best. Coastal flooding is also still possible
Thursday but the different model solutions mean confidence is not
high. If the GFS is correct, coastal flooding probably won't
happen. If the ECMWF solution pans out, coastal flooding is more
likely. Schneider

&&

.HYDROLOGY...Flooding continues on the Skokomish river in Mason
county. The river pushed into moderate flood stage this morning
and will slowly crest this afternoon. The Skokomish River is
likely to remain above flood stage all week.

A Flood Watch remains in effect for Lewis county through this
afternoon. Heavy rainfall in southwest Washington, especially over
the Willapa Hills, will force sharp rises on the Chehalis river
system. The Chehalis River near Doty is forecast to crest just
below flood stage. However, minor flooding is possible if rainfall
rates in the basin exceed guidance.

Another round of heavy precipitation is likely on Thursday but so
far it looks like this will mainly affect the Skokomish. Otherwise,
additional river flooding is not anticipated. 33

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Watch until 4 PM PST this afternoon for Cascades of Pierce
     and Lewis Counties-Southwest Interior.

     Winter Weather Advisory until 10 PM PST this evening for
     Cascades of Pierce and Lewis Counties-Cascades of Snohomish
     and King Counties.

     Winter Storm Warning until 10 PM PST this evening for Cascades
     of Whatcom and Skagit Counties.

PZ...Small Craft Advisory until 10 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-Northern Inland Waters
     Including The San Juan Islands-West Entrance U.S. Waters
     Strait Of Juan De Fuca.

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

     Small Craft Advisory until 4 AM PST Wednesday for East Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 4 PM this afternoon to 4 AM PST
     Wednesday for Central U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 4 PM PST this afternoon for Admiralty
     Inlet.

     Small Craft Advisory until 4 PM PST Wednesday for Puget Sound
     and Hood Canal.

&&

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