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FXUS66 KSEW 230333
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
733 PM PST Tue Jan 22 2019

.UPDATE /TONIGHT/...As of 5PM PST, snowfall rates at Paradise
Ranger Station had increased to 2 inches per hour with 11 inches
fallen today. A combination of short range models including the
HRRR, SREF, RAP, and NAM show intense precipitation continuing
until the frontal system now moving through the coastal waters
pushes inland. Therefore, the Winter Weather Advisory for the
Cascades of Pierce and Lewis Counties was upgraded to a warning
for up to another foot of snow possible by midnight tonight.
Despite snow levels rising, easterly flow through the passes is
keeping precipitation at Snoqualmie and Stevens passes in the form
of snow this evening, so will let the warning continue there and
points northward as well.

Heavy rainfall is falling in the Skokomish River basin in Mason
County and the river is beginning to respond. It remains possible
that the river will briefly reach flood stage early Wednesday
morning, so will maintain the flood watch and will issue a warning
overnight if flooding appears imminent.

The remainder of the forecast appears to be in good shape this
evening, so updates to the Aviation and Marine sections follow,
and the previous forecast discussion follows those segments. Albrecht

&&

.AVIATION...A warm front is lifting northeastward through the
central portions of the the area this evening and a trailing cold
front is over the coastal waters at 7 PM PST. The cold front will
sweep inland overnight. An associated weakening surface low near
50N 130W will slide ESE through the northern interior of Western
Washington Wednesday morning. Westerly flow aloft will become more
northwesterly on Wednesday. At the surface, brief post-frontal
onshore flow will rapidly ease on Wednesday as weak high pressure
moves over the area. The air mass is moist and stable.

The passing warm front and incoming cold front is producing
widespread MVFR and areas of IFR conditions across the area.
Steady rainfall will change to showers with conditions improving
slightly behind the cold front late tonight or early Wednesday.
But rather widespread IFR conditions are expected to redevelop
later Wednesday morning as weak high pressure moves into the area
and the air mass remains moist and stable. Albrecht

KSEA...Low MVFR to IFR conditions tonight may improve for a few
hours late tonight into early Wed, but IFR conditions in low cigs
are expected to dominate late Wed morning through Wed night. S
wind 15G25 kt this evening will ease overnight then becoming light
and variable Wed morning through Wed night. Albrecht

&&

.MARINE...A warm front is lifting northward through the inland
waters this evening. A cold front over the coastal waters will
sweep inland after midnight tonight. A weakening low near 50N 130W
will fill as it moves ESE through the northern inland waters Wed
morning. Gale conditions continue across the southern third of the
coastal waters while small craft advisory conditions are occurring
elsewhere. Post frontal onshore flow will keep small craft
advisory conditions are expected to rapidly decrease Wed morning.

High pressure will build into the waters on Wed with winds all
areas easing. However, small craft advisory conditions for
hazardous seas over 10 ft will continue through Wednesday evening
and possibly a bit longer. Hence the small craft advisories for
the coast and the west entrance to the strait were stretched out
into Wed night.

High pressure is expected Thu through Sat across the area. Then
offshore flow may increase on Sun as high pressure builds into the
interior of British Columbia.

Coastal flood advisories remain in effect for the shorelines
along the inland waters from Point Roberts to the south end of
Puget Sound and Hood Canal around the Wednesday morning high tide.
Tides are near the highest values for the year and will be
accompanied by relatively low pressure and lingering longer-period
anomalies. Any salt-water flooding that occurs is expected to be
brief and minor. Albrecht

&&

.PREV DISCUSSION... /issued 312 PM PST Tue Jan 22 2019/

SYNOPSIS...A frontal system precipitation will drive rain,
mountain snow, and breezy conditions over parts of the area this
evening into Wednesday morning. Drier, benign weather will return
Thursday and continue into the weekend. A couple of weak weather
systems may brush the region Thursday night and again Sunday for a
chance of showers mainly over the north.

SHORT TERM /TONIGHT THROUGH FRIDAY/...Precipitation has spread
through the are this afternoon, with snow falling above 3000 to
4000 feet in the mountains. Expect precipitation to pick up this
evening as the cold front moves inland. This will also boost snowfall
the Cascades through the evening hours, where a Winter Storm
Warning is in effect for the Cascades from Whatcom County
southward through King County through, and Winter Weather
Advisory is in effect for the Cascades in Pierce and Lewis
Counties. In addition, expect windy conditions along the coast
into this evening and breezy to locally windy over the interior as
the front works onshore. Winds are expected to remain sub-
advisory with little or no impacts anticipated at this time.
Precipitation will transition to showers Wednesday, with snow levels
holding around 4000 to 4500 feet.

An upper level ridge that has been building over the Eastern Pacific
will begin to shift slowly onshore Wednesday evening. This shift in
the large scale pattern will set up a dry northwesterly to north
flow over Washington Thursday and Friday. Models have been pretty
consistently showing a weak system riding up and over the ridge and
into western Washington Thursday night into Friday morning. This
continues to look very weak and will likely result in little more
than some light showers for parts of the area. In general it looks
like the biggest weather impact to the area will be patchy morning
fog in the favored areas.

LONG TERM /SATURDAY THROUGH TUESDAY/...The offshore upper ridge
will will build further and fully shift onshore Saturday. It
continues to trend towards a pleasant start to the weekend with some
morning fog followed by sunshine and above average temperatures. The
upper ridge shifts offshore late in the weekend, which may allow a
weak system to clip the area from the northwest Sunday afternoon or
evening. Models have been trending weaker with this, so the dry
weather may in fact extend fully through the weekend. With the upper
level ridge firmly anchored over the Eastern Pacific into early next
week, it continues to look generally dry and benign into the early
part of next week.

AVIATION...A warm front will move inland this evening. A
trailing cold front will move onshore tonight. An associated
weakening surface low moving east across the northern interior
tonight. Westerly flow aloft will become northwesterly on Wednesday.
At the surface, onshore flow will rapidly ease on Wednesday as weak
high pressure moves over the area. The air mass is moist and stable.

Widespread low clouds and occasional reduced visibilities in rain
will prevail tonight as a cold front pushes inland. The rain will
change to showers Wednesday morning but the low levels will remain
moist for continued low clouds.

KSEA...It looks quite soupy with lots of low clouds and reduced
visibilities tonight and Wednesday. South wind 5-10 knots will
increase to 12-18 knots tonight and become gusty then ease again on
Wednesday. Schneider

MARINE...A warm front will move inland this evening. A trailing
cold front and an associated filling low will move inland across the
northern part of the area tonight. Most areas will have small craft
advisory strength winds tonight with marginal gales possible over
the southern Coastal Waters this evening.

Onshore flow will rapidly ease on Wednesday. Weak high pressure will
move over the area on Thursday and linger in the vicinity through
Saturday for a period of relatively light winds. Offshore flow will
develop on Sunday as high pressure builds into British Columbia.

A coastal flood advisory is in effect for the Central Coast for
minor tidal overflow around the time of high tides early this
afternoon. A second round of minor tidal overflow is possible for
the inland waters Wednesday morning due to high astronomical tides
and tidal anomalies due to relatively low atmospheric pressure.
Schneider

HYDROLOGY...The Skokomish River, and rivers flowing off the north
Cascades will see sharp rises tonight due to a period of heavy rain.
One half to one inch of rain has already fallen over the Olympics.
This system will push through relatively quickly, but with snow
levels above 5000 feet, there may still be up to 3-4 inches storm
total rainfall over the Olympics. This will drive the Skokomish
River up to near or above flood stage. Rivers will recede on
Wednesday and this should be the last flood threat for the next 7
days. No other river flooding is expected for western Washington
during this time.
jbb

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Coastal Flood Advisory from 5 AM to 10 AM PST Wednesday for
Admiralty Inlet Area-Bremerton and Vicinity-Everett and
Vicinity-Hood Canal Area-San Juan County-Seattle and
Vicinity-Southwest Interior-Tacoma Area-Western Skagit
County-Western Whatcom County.

Winter Storm Warning until midnight PST tonight for Cascades of
Pierce and Lewis Counties-Cascades of Snohomish and King
Counties-Cascades of Whatcom and Skagit Counties.

Flood Watch from midnight PST tonight through Wednesday
afternoon for Hood Canal Area-Lower Chehalis Valley Area-
Olympics.

PZ...Small Craft Advisory for rough bar until 9 PM PST Wednesday for
Grays Harbor Bar.

Small Craft Advisory from midnight tonight to 3 AM PST Thursday
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 midnight PST tonight for Coastal Waters From
Point Grenville To Cape Shoalwater 10 To 60 Nm-Coastal
Waters From Point Grenville To Cape Shoalwater Out 10 Nm.

Small Craft Advisory until 3 AM PST Thursday 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 9 AM PST Wednesday for East Entrance
U.S. Waters Strait Of Juan De Fuca.

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

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

Small Craft Advisory from 2 AM to 9 AM PST Wednesday for Central
U.S. Waters Strait Of Juan De Fuca.

Small Craft Advisory until midnight PST tonight for Admiralty
Inlet.

Small Craft Advisory until noon PST Wednesday for Puget Sound
and Hood Canal.

&&

$$

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