River Flood Warning 1, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
857 AM PST Thu Jan 3 2019

.SYNOPSIS...A frontal system will bring rain, mountain snow, and
wind at times through early Friday. An upper trough will maintain
somewhat unsettled weather through the weekend. Another wet
system may arrive the first part of next week.

&&

.UPDATE / TODAY /...Current forecast in pretty good shape this
morning. Did up precipitation amounts for the Olympic Peninsula
and north Cascades for next 8-12 hours as latest hi-res guidance
has trended upward. Deep moisture fetch off the eastern Pacific
is leading to steady, heavy rainfall rates around 0.5in/hour
which should continue through most of the day until the cold front
swings through later tonight. Will continue to monitor rivers
feeding off the Olympics and north Cascades for flooding
concerns. See Hydrology discussion below for the latest. Wind
gusts of 30-40 mph continue along the Pacific coastline and north
interior and will pick up a bit during the day with gusts to 50
mph possible, peaking this afternoon and evening. Elsewhere
breezy conditions with gusts in the 20-40 mph range. With
saturated soil, this may still be enough for some localized tree
damage and power outages. Previous discussion follows. CEO

.SHORT TERM / THURSDAY THROUGH SATURDAY /...Little change in the
forecast today with the warm front remaining over the northern
portion of the area. Rain will spread south covering the entire
area this morning with the exception of the favored rain shadow
area at the east end of the Strait of Juan de Fuca. Winds
increasing during the day as a cold front approaching the coast
increases the surface gradients. Wind advisories will remain in
effect for the Northwest Interior, Admiralty Inlet and coastal
zones for southerly winds 20 to 35 mph with gusts 45-50 mph. Snow
levels rising this morning to at least around 5000 feet. A small
pocket of cool air trapped in Snoqualmie pass early will mix out
this morning. Highs today will be in the upper 40s to mid 50s.

Warm front lifting north with the cold front stalling over the
area tonight as an upper level trough back out near 138w digs
south. With the front stalling have extended the wind advisory
along the coast until 10 pm this evening. As the cold front starts
to sag south cooler air will move into the North Cascades this
evening with snow levels lowering to around 3500 feet. Will
convert the winter storm watch to a warning for the North Cascades
tonight into Friday. Slower transition to the lower snow levels
over the remainder of the Cascades along with decreasing
precipitation rates. Will only go for an advisory for the central
and southern Cascades. Windy conditions tonight with the winds
easing overnight as the cold front slowly moves south. Lows
tonight will be in the lower to mid 40s.

Cold front moving south of the area Friday morning with the steady
rain changing to showers. Winds easing slowly during the day with
the model surface gradients putting the KOLM-KBLI gradient below 3
mb by 00z Saturday. Snowfall rates will decrease in the Cascades
but will still be high enough to keep the winter storm warning for
the North Cascades going throughout the day. The winter weather
advisory for the Central and Southern Cascades will come to an
end by afternoon. Snow levels will remain around 3500 feet. Highs
on Friday will be near 50.

A little bit of a break in the action Friday night as the
precipitation dries up over Western Washington. Weak upper level
low approaching the Northern California coast with the flow aloft
over Western Washington becoming southwesterly. Moisture streaming
northeast out of the low approaching Western Washington by 12z
Saturday. Lows Saturday morning will be in the mid 30s to lower
40s.

Moisture associated with the weakening upper level low continuing
to stream into the area on Saturday with most of the precipitation
remaining south and west of the area. Model solutions have been
having trouble latching on to a consistent solution for Saturday
and wit with the current solutions bringing rain close to the
area will go with a chance of rain on Saturday for most of Western
Washington. The best chance for a dry day will be in the north.
Highs on Saturday will be in the mid 40s to lower 50s. Felton

.LONG TERM...Extended models in pretty good agreement with a weak
frontal system followed by an upper level trough moving through the
area Saturday night into Sunday. Models having trouble with the
timing of the next system, a warm front moving up from the south.
The ECMWF moves this feature over the area on Monday while the GFS
is 18-24 hours slower bringing the system in on Tuesday. More
inconsistency Tuesday night and Wednesday with the ECMWF bringing
a cold front through while the cold front on the GFS gets torn
apart as it moves into the area on Wednesday. With the lack of
model consistency will go with a broad brush rain likely forecast
for most of the Monday through Wednesday time frame. Felton

&&

.AVIATION...Moist westerly flow will continue today with rain
across Western WA. The air mass is stable. Ceilings are mostly in
MVFR to IFR range this morning with low-end VFR cigs possible this
afternoon (ceilings near 3000 feet). In addition, expect gusty S
wind to 20-30 kt at most terminals through early this evening.
The cold front will push SE through the region tonight with
showers tapering off after midnight. Onshore flow will keep the
low level air mass moist for MVFR conditions. 33

KSEA...Rain with MVFR to IFR cigs this morning. May see brief
improvement this afternoon as ceilings rise to around 3000 feet.
S winds at the surface gusting to 20-30 kt. Winds at 2000 feet
near 40 kt. Expect showers with the cold front this evening,
tapering off after midnight tonight. MVFR cigs likely through
Friday morning. 33

&&

.MARINE...Gales are likely over most waters today as a strong
Pacific frontal system moves in. Winds will ease tonight behind a
cold front, but will likely still remain within Small Craft
Advisory range. Onshore flow will prevail on Friday with post-
frontal westerly wind.

A swell train will reach the WA coast Friday morning and
afternoon with seas building to 20-23 feet - a High Surf Advisory
is in effect. Seas will subside under 20 feet Friday night.

The flow will turn offshore on Saturday as the next system moves
into northern California. Onshore flow will return on Sunday as
this system moves inland. A strong Pacific system may impact the
region early next week. 33

&&

.HYDROLOGY...QPF forecasts look to be a little more and a little
faster than previous runs...with rivers of concern...most notably
the Skokomish and the Bogachiel...reacting a bit faster than
previously forecast. However...models for the Bogachiel look to be
seriously overdone as the model calls for levels to rise to
slightly over major flood stage while internal models/guidance
shows significantly lower but still shows the river entering into
minor flood stage this evening. All that being said...will likely
issue flood warnings attached to these two rivers before the
afternoon forecast package and discussion.

As for rivers in the Cascades...will continue to monitor and revisit
for afternoon discussion.

The flood watch remains effect for rivers flowing off the Olympics
and North Cascades through Saturday.

The system for the first part of next week also has the potential to
cause minor flooding problems for the rivers flowing off the
Olympics. SMR/Felton

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory from 10 PM this evening to noon PST
     Friday for Cascades of Pierce and Lewis Counties-Cascades
     of Snohomish and King Counties.

     Flood Watch through Saturday morning for Admiralty Inlet Area-
     Bellevue and Vicinity-Cascades of Snohomish and King
     Counties-Cascades of Whatcom and Skagit Counties-Central
     Coast-East Puget Sound Lowlands-Eastern Strait of Juan de
     Fuca-Everett and Vicinity-Hood Canal Area-Lower Chehalis
     Valley Area-North Coast-Olympics-Western Skagit County-
     Western Strait of Juan De Fuca-Western Whatcom County.

     Winter Storm Warning from 10 PM this evening to 6 PM PST Friday
     for Cascades of Whatcom and Skagit Counties.

     Wind Advisory until 7 PM PST this evening for Admiralty Inlet
     Area-San Juan County-Western Skagit County-Western Whatcom
     County.

     Wind Advisory until 10 PM PST this evening for Central Coast-
     North Coast.

     High Surf Advisory from 7 AM to 4 PM PST Friday for Central
     Coast-North Coast.

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

     Small Craft Advisory from 7 PM this evening to 4 PM PST Friday
     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-West Entrance U.S.
     Waters Strait Of Juan De Fuca.

     Gale Warning until 7 PM PST this evening 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-West Entrance U.S. Waters Strait Of
     Juan De Fuca.

     Small Craft Advisory from 10 PM this evening to 4 PM PST Friday
     for Admiralty Inlet-East Entrance U.S. Waters Strait Of
     Juan De Fuca-Northern Inland Waters Including The San Juan
     Islands.

     Gale Warning until 10 PM PST this evening for Admiralty Inlet-
     East Entrance U.S. Waters Strait Of Juan De Fuca-Northern
     Inland Waters Including The San Juan Islands.

     Small Craft Advisory until 4 AM PST Friday for Central U.S.
     Waters Strait Of Juan De Fuca.

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

&&

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