River Flood Warning 6, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
906 PM PST Sat Feb 3 2018

.SYNOPSIS...A frontal system will bring heavy rain to the Cascades
later tonight and Sunday. Flooding is likely for several rivers
running out of the Cascades Sunday and Monday. Weaker weather
systems will brush the area next week for a chance of rain at times.

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.SHORT TERM...Moderate to heavy rain will fall tonight and Sunday
with good orographic enhancement in the Cascades. Rainfall with this
system should be generally 3-5 inches in the Cascades with local
amounts of 4-6 inches possible. The lowlands, especially
north of Everett will also be with with two inches of rain
likely. The freezing level will be too high for snow except on
the highest peaks near the crest and the high elevations of the
volcanic peaks. This is probably a good pattern for building up
the snowpack on the glaciers, but a lousy pattern for the ski
areas. Around Puget Sound, an inch or less of rain is likely with
some rain shadowing off of the Olympics.

Precipitation will gradually decrease and snow levels will
gradually fall back to around 3500 feet late Sunday night through
Monday. Temperatures will be quite mild through Sunday, then fall
off closer to normal Monday. Showers Monday night and Tuesday will
mainly be in the mountains and will decrease as the flow aloft turns
more northwesterly and the upper ridge amplifies somewhat offshore.

.LONG TERM...The long wave pattern remains one of a ridge 135W-
140W and a through over the Midwestern to eastern US midweek
through the end of next week. A short wave trough moving through
the ridge will give increasing rain chances to the area later
Wednesday into early Thursday. The ECMWF is stronger than the
GFS solutions. Another short wave moves SE through the ridge and
into the NW sometime between Friday and next weekend with model
solutions differing on its strength and timing. There may be some
dry periods in the extended forecast, but with the uncertainty of
short wave timing and strength, the forecasts use a model blend
approach today that results in mostly cloudy conditions and a
chance of rain at times. Temperatures will likely remain a few
degrees above normal through next weekend. Albrecht

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.AVIATION...Strong northwest flow aloft tonight will continue
through Sunday over Western Washington. In the lower levels
onshore flow will continue through Sunday as well. A frontal
system will move through Western Washington on Sunday. Rain out
ahead of the front tonight into Sunday will keep ceilings in low
end of MVFR or IFR into Sunday afternoon with slight improvement
later Sunday afternoon. Visibilities will be as low as 1-3sm with
the light rain through Sunday morning. The exception will be near
Olympia where the downslope winds off the Olympics could improve
the ceilings at time to VFR.

KSEA...IFR or low end MVFR conditions continuing into Sunday
afternoon. Southerly winds 8-14 knots, rising to 12-22 knots
Sunday morning. Felton

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.MARINE...Surface high pressure over the coastal waters will
maintain onshore flow tonight. A weak cold front will move across
the area Sunday, followed by somewhat stronger onshore flow. Gale
force westerlies are forecast over parts of the Strait of Juan de
Fuca Sunday afternoon and night following the front. Surface high
pressure centered well southwest of the area will maintain weaker
onshore flow Monday through Wednesday. Another frontal system
should reach the area Thursday. Felton

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.HYDROLOGY...Heavy precipitation is expected with an incoming
frontal system tonight into Sunday evening along the west slope of
the Cascades. This system has entrained a tropical tap of moisture
originating from southwest of Hawaii but is approaching the area
from a more WNW direction (unusual for flood producing systems in
this area). Strong orographic forcing and some dynamic support
from a strong jet just to the south appears to be the primary
mechanism for the heavy precipitation producing. At this time it
appears that 3 to 5 inches of rain can be expected along the west
slopes of the Cascades with local 4-6 inch totals. Forecast
hydrographs based on a blend of the various mesoscale models has
prompted us to extend the flood watch southward to include
Thurston and Lewis counties.

At this time, it appears that most flooding that does occur will
be minor. A couple of rivers could reach moderate flood stage, and
a couple of forecast points -such as the Snoqualmie River near
Carnation and the Tolt River near Carnation could get close to
major flood stage.

Rivers will recede on Monday. Precipitation amounts after Monday
are expected to be light and snow levels will fall back to more
normal February levels.

There is a threat of shallow landslides, especially during the
period of heavy rainfall Saturday night into Sunday. A Special
Weather Statement is being issued this afternoon to address the
enhanced landslide threat during the that period. Albrecht

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.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Watch from midnight PST tonight through late Monday night
     for Bellevue and Vicinity-Cascades of Pierce and Lewis
     Counties-Cascades of Snohomish and King Counties-Cascades
     of Whatcom and Skagit Counties-East Puget Sound Lowlands-
     Everett and Vicinity-Seattle and Vicinity-Southwest
     Interior-Tacoma Area-Western Skagit County-Western Whatcom
     County.

PZ...Small Craft Advisory from 6 AM Sunday to 3 AM PST Monday for
     Coastal Waters From Cape Flattery To James Island 10 To 60
     Nm-Coastal Waters From James Island To Point Grenville 10
     To 60 Nm-Coastal Waters From Point Grenville To Cape
     Shoalwater 10 To 60 Nm.

     Gale Warning from noon Sunday to midnight PST Sunday night for
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 1 PM Sunday to 1 AM PST Monday for
     Northern Inland Waters Including The San Juan Islands-Puget
     Sound and Hood Canal.

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