Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 240041
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
441 PM PST Tue Jan 23 2018

.SYNOPSIS...A stormy pattern will keep conditions wet and
unsettled through the end of this workweek. Expect locally heavy
precipitation and gusty winds at times.

&&

.SHORT TERM...
The warm front was moving across the CWA at this time. The
heaviest precipitation has been falling on the south slopes of
the Olympic range and the Kitsap peninsula, where 12-hour rainfall
amounts, ending at 330 PM PST, were in the 1-1.75 inch range.

In the Cascades, the snow was starting to pick up. At Mount Baker
and Paradise, on Mount Rainier, the snow was falling at a rate of
1-2 inches per hour this afternoon. Paradise and Mount Baker will
likely receive the heaviest snowfall during this event.

There will be somewhat of a lull in the precipitation later
tonight or, at least, the precipitation will become light in the
wake of the warm front. The precipitation will increase somewhat
on Wednesday ahead of and with the cold or occluded front. This
front will sweep across the region during the day Wednesday. It
should be east of the Cascades by 4 PM, Wednesday.

Cool and showery conditions will prevail over the area Wednesday
night through Friday due to the presence of a chilly upper level
trough. The air mass will destabilize Wednesday night for a risk
of thunderstorms over the coastal waters. Expect any of the
heavier showers to contain small hail on the lowlands during this
time frame; although, the precipitation may fall as snow or a
rain/snow mix in the Cascade foothills at elevations above a
thousand feet. Elevations above this foot level could receive a
couple of inches of wet snow.

.LONG TERM...
The medium range solutions were in better agreement in regards to
the atmospheric river that is anticipated this weekend. The
forecast was heavily weighted towards the ECMWF solution, which
was preferred. The other solutions were coming into more agreement
with the ECMWF.

A deep fetch of moisture from the subtropics will become directed
at western WA during the early part of this period. Rising snow
levels, a wetter air mass, and strong transport winds will result
in periods of heavy rain across much of the CWA, beginning
Saturday and then continuing into Monday morning. Rainfall amounts
in the mountains during the 48-hour period ending at 4 PM PST
Monday are anticipated to be 7-12 inches on the Olympic range and
4-8 inches in the Cascades. Across the lowlands, amounts are
forecast to be in the 1-4 inch range, with lighter amounts
occurring in the rain-shadow just northeast of the Olympic range.
The rainfall amounts were bumped up above the model guidances
because they seemed too low. This amount rainfall on top of what
has already fallen could lead to flooding on many rivers as well
as urban and/or small stream flooding.

Anticipate an upper level trough to move over the Pacific
Northwest the end of the period for a return to colder weather.

&&

.AVIATION...Southerly flow aloft will prevail tonight and
Wednesday. A frontal system will shift inland tonight. An upper
level disturbance will move through the area late Wednesday
afternoon. The air mass is moist and stable. It will become
unstable Wednesday.

There will be a mix of VFR low clouds, MVFR, and even some IFR
ceilings tonight. The trend, if any, will be for lowering ceilings
tonight.

Areas of low level wind shear AOB 2000 AGL will persist for a few
more hours this afternoon until the front passes and southerly
winds at the surface develop/increase.

KSEA...Ceilings 3000-4000 feet should lower a bit tonight with
mostly MVFR into Wednesday. Low level wind shear should persist
until earl evening when south winds 8-12 knots develop. Schneider

&&

.MARINE...A frontal system will move inland tonight with easing
winds. Gales should be ending this afternoon or evening. Small
craft advisory winds after that will ease as well. 10-13 foot
swell for the Coastal Waters will linger into Wednesday.

Additional disturbances will move through the area over the next
few days for small craft advisory strength winds at times.
Stronger systems are expected Friday night and Sunday. Schneider

&&

.HYDROLOGY...
The stage at the Skokomish River should start shooting up shortly
in response to the heavy rain that has fallen today. The observed
stage is lagging a little behind the forecast, so the river may
reach flood stage slightly later than currently forecast. A Flood
Warning was issued earlier for the Skokomish River.

There is still the potential for an atmospheric river to impact
the area this weekend into early next week for the possibility of
excessive rainfall. At this time, the location, exact amount, and
timing of the heavy rain are still not certain. The screaming
message is that the combination of a milder (snow levels rising
above 5500 feet) and wetter air mass with strong transport winds
could could possibly lead to flooding on many rivers as well as
potential cause some urban and/or small stream flooding 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, and especially this weekend, will only
increase the threat of shallow landslides.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Warning in effect for the Skokomish River in Mason
 County.

  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.

PZ...Small Craft Advisory from 9 PM this evening to 3 PM PST
     Wednesday for Coastal Waters From Point Grenville To Cape
     Shoalwater 10 To 60 Nm-Coastal Waters From Point Grenville
     To Cape Shoalwater Out 10 Nm.

     Gale Warning until 9 PM PST this evening for Admiralty Inlet-
     Coastal Waters From Point Grenville To Cape Shoalwater 10
     To 60 Nm-Coastal Waters From Point Grenville To Cape
     Shoalwater Out 10 Nm-East Entrance U.S. Waters Strait Of
     Juan De Fuca.

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

     Small Craft Advisory until 3 PM 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.

     Small Craft Advisory until 6 AM PST Wednesday for Northern
     Inland Waters Including The San Juan Islands.

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

     Small Craft Advisory until 9 AM PST Wednesday for West Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 9 PM PST this evening for Central
     U.S. Waters Strait Of Juan De Fuca.

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

&&

$$

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