Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 230553
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
953 PM PST Mon Jan 22 2018

.SYNOPSIS...A weak upper level ridge of high pressure will move
through the area tonight. The next front to reach the area will
spread precipitation into western Washington beginning Tuesday
morning with the precipitation continuing until the front moves
through the area Wednesday morning. Heavy snow is possible in the
mountains. A cool upper level trough will set up offshore later
Wednesday and remain through Thursday.

&&

.SHORT TERM...Satellite imagery shows flat upper level ridge over
western Washington this evening. Next front well offshore with the
parent 993 mb low near 47N/136W and the associated front
trailing back to the southwest. Temperatures at 9 pm/05z were in
the mid 30s to mid 40s.

Weak upper level ridge exiting quickly tonight with precipitation
out ahead of the incoming front spreading over the area Tuesday
morning. Parent low associated with the front moving north on
Tuesday. This will slow down the eastward movement of the front to
a crawl with the front still offshore at 00z Wednesday. Snow
levels will be around 2500 to 3000 feet. Will upgrade the watch
for the Cascades to a winter storm warning and issue a winter
weather advisory for the Olympics. Easterly gradients increasing
Tuesday morning especially over the northern portion of the area.
A wind advisory is in effect for the San Juans and the Admiralty
Inlet area.

Front slowly moving to the coast Tuesday night into Wednesday
morning with the steady precipitation continuing. Snow levels
slowly rising in the Cascades Tuesday evening into Wednesday
morning but easterly gradients will keep the cool air in place in
the passes. Easterly gradients will relax enough for the wind
advisory to come to an end early Tuesday evening.

What is left of the front moving through the area on Wednesday
keeping rain at times in the forecast. Precipitation rates will
ease on Wednesday so will only have the Winter Storm Warning for
the Cascades up through the early morning hours.

A couple of upper level troughs moving through western Washington
Wednesday night into Thursday keeping precipitation in the
forecast for the remainder of the short term. Felton

.LONG TERM...Previous discussion follows. The medium range
solutions were not in very good agreement regarding the location
and timing of a potential atmospheric river event during this
period. The ECMWF was faster and showed heavy rain impacting the
CWA as early as Saturday. It also showed the brunt of the heaviest
rain impacting the Olympics and north Cascades (Snohomish county
northward). The GFS didn`t set up the atmospheric river event
until early next week, and indicated that the heaviest rainfall
will occur across the southern part of the CWA (King County
southward).

The screaming message is that, at some point, we can expect
expect the possibility of excessive rainfall sometime this weekend
or early next week. However, the location of the heaviest rain
was still uncertain.

The medium range solutions were in more or less agreement that a
cold upper level trough will move over the area during the end of
the period. Felton


&&

.AVIATION...A flat upper ridge axis currently over Western
Washington will exit the area late tonight with nothing in the way
of showers expected in the very near term and VFR conditions area-
wide. A warm front will arrive on Tuesday morning...returning the
prospect of rain to the area and as such pulling cigs down into low-
end VFR to MVFR conditions. Moderate westerly flow aloft will become
strong southwesterly Tuesday morning.

KSEA...VFR cigs expected tonight. Warm frontal rain will arrive
around 14z Tue. Cigs will start to lower thereafter, eventually
falling into the 020-030 range late Tue morning. Reduced vsbys
possible at times from 18z-24z Tue during periods of moderate rain.
Otherwise, low-level wind shear will reach criteria for inclusion in
the TAF on Tue owing to E-SE surface wind of 8-13 kt and 2000-foot
winds that will be southerly around 35-40 kt for much of the day.
Haner/SMR

&&

.MARINE...A warm front will move east through the waters
Tuesday...preceded by strong east to southeast pressure gradients.
This will allow 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 Tuesday evening following the
warm front. All that being said...inherited headlines look good and
will remain in place.

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/SMR

&&

.HYDROLOGY...The Skokomish River fell below flood stage this
morning but is receding slowly tonight. Another round of rain
Tuesday into Wednesday will push the river back up to near flood
stage on Wednesday and a flood watch has been issued.

There is a potential for an atmospheric river setting up over the
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 at the
thresholds where landslides become probable. The additional
rainfall this week will only increase the risk of shallow
landslides. Felton

&&

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

     Flood Watch from Wednesday morning through Thursday morning for
     Hood Canal Area-Lower Chehalis Valley Area-Olympics.

     Wind Advisory from 9 AM to 6 PM PST Tuesday for Admiralty Inlet
     Area-San Juan County.

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

     Gale Warning until 3 PM PST Tuesday 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 for hazardous seas until midnight PST
     tonight 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 1 AM to 6 AM PST Tuesday for East
     Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland
     Waters Including The San Juan Islands.

     Gale Warning from 6 AM to 6 PM PST Tuesday for 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.

     Gale Warning from 10 AM to 6 PM PST Tuesday for Admiralty Inlet.

&&

$$

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