Small Craft Advisory 15, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
944 AM PST Tue Jan 23 2018

.SYNOPSIS...A series of Pacific storms will impact the region this
week for wet and unsettled conditions. It will also be locally
windy at times.

&&

.SHORT TERM...
Precipitation ahead of a warm front offshore overspread the CWA
this morning. This front is forecast to dissipate as it moves
across the WA coastal waters this evening. Expect strengthening
southerly flow in the lower to mid levels of the atmosphere to
cause a rain shadow to develop downwind of the Olympic range. This
will impact places such as Sequim, Port Townsend, and Oak Harbor.

Meanwhile, the Pacific storm that has begun impacting the CWA will
bring heavy snow at times to the Cascades this afternoon through
tonight. The heaviest snow is expected to occur on the south or
southwest facing slopes, where snow could fall at a rate of 1-2
inches per hour; therefore, a Winter Storm Warning is in effect.
The Olympic range, where a Winter Weather Advisory is in effect,
is expected to receive less snow or about 6-12 inches.

Locally heavy rain will also fall on the lowlands, mainly from the
Puget Sound Region southward and the central coast. Total rainfall
amounts during the 24-hour period ending at 4 a.m. PST Wednesday
are forecast to be about 1-2 inches. This amount of rain will
probably result in minor flooding on the Skokomish River later
tonight or Wednesday. Elsewhere, rainfall amounts will generally
be in the quarter to half-inch range.

In addition to the locally heavy precipitation, expect gusty winds
today over parts of the northwest interior, where a Wind Advisory
is in effect.

Expect the cold or occluded front to sweep across the area during
the day Wednesday. The steady precipitation will become showery
behind the front and snow levels will fall into the 2000-3500
foot range behind the front.

A chilly upper level trough will remain off the coast Wednesday
night through Thursday for cool and unsettled conditions. The air
mass may be unstable enough Wednesday night and Thursday for
isolated thunderstorms, especially over the coastal waters. It
will also be locally windy.

.LONG TERM FROM THE PREV DISCUSSION...
A pretty cold upper trough will move through the region Friday.
Overnight low temperatures will be mostly in the 30s, so higher
hills could see a dusting of snow in heavier showers Thursday
night and Friday morning. Showers will continue Friday.

A very wet weather system seems likely to impact the region at some
point Saturday through Monday. Timing and location of the heaviest
rain remains uncertain, but flooding on rivers beyond the most flood
prone ones is possible. Burke

&&

.AVIATION...A warm front will pass east-northeast across Western
Washington today. East-southeast pressure gradients and strong
southwest flow aloft will set the stage for areas of low-level
wind shear through this afternoon, especially where the surface
wind keeps a strong east component. A diffuse cold front will pass
Wednesday as an upper trough moves to about 135W. The air mass
will be moist and stable through tonight, becoming modestly
unstable on Wed afternoon. Cigs will continue to lower through
mid-day, then generally remain below 030 through Wed. Strong SW
flow aloft today will become moderate on Wed.

KSEA...Cigs continue to lower with the approach of a warm front,
falling into the 015-025 range by around 21z and staying there
through Wed. Heaviest rain will be in the 22z-04z time frame,
leading to reduced vsbys as low as 4-5SM. Showery weather will
continue thereafter. Until about 03z, the combination of a gentle
easterly surface wind component and 2000-foot southerly winds
around 30-40 kt will meet criteria for including low-level wind
shear in the TAF. Once surface wind becomes southerly early this
evening, the shear will lessen below criteria.  Haner

&&

.MARINE...A warm front will move east through the waters
today with strong east to southeast pressure gradients. This will
support southeast gales over the coastal waters, and over the
inland waters from Admiralty Inlet north. Strongest winds over the
inland waters will be focused from Admiralty Inlet up to Haro
Strait. Southeast gradients will relax this evening following the
warm front.

From Tuesday night through Saturday, a low pressure trough will
pass about every 12 to 18 hours. Forecast winds could be
conservative during any single forecast period due to the effect
of averaging out so many small, compact, fast-moving features.
The strongest trough may come around Friday night. Haner

&&

.HYDROLOGY...
More heavy rain could cause the Skokomish River to climb above
flood stage late tonight or Wednesday; therefore, a Flood Watch
is in effect for this river. Any flooding that occurs is
anticipated to be minor.

There is a potential for an atmospheric river to impact he area
this weekend or early next week for a period of significant
rainfall. At this time, the location of where the heaviest rain
will occur was still uncertain due to disagreement in the medium
range models. The main message is that the combination of rising
snow levels and heavy rainfall could lead to flooding on some
rivers (besides the Skokomish River) late this weekend or early
next week.

The USGS landslide threshold indices were near or slightly above
the thresholds where landslides become probable. The additional
rainfall this week will only increase the threat of shallow
landslides.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Storm Warning until 6 AM PST Wednesday for Cascades of
     Pierce and Lewis Counties-Cascades of Snohomish and King
     Counties-Cascades of Whatcom and Skagit Counties.

	 Flood Watch in effect for the Skokomish River in Mason County.

     Wind Advisory until 6 PM PST this evening for Admiralty Inlet
     Area-San Juan County.

     Winter Weather Advisory until 6 AM PST Wednesday for Olympics.

PZ...Small Craft Advisory for rough bar until 10 AM PST Wednesday for
     Grays Harbor Bar.

     Gale Warning until 3 PM PST this afternoon 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 6 PM PST this evening for Admiralty Inlet-
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance
     U.S. Waters Strait Of Juan De Fuca-Northern Inland Waters
     Including The San Juan Islands.

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

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