Gale Warning 100, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
1008 PM PST Thu Dec 28 2017

.SYNOPSIS...A pair of Pacific weather systems will produce
rain through Friday evening with a wintry mix for the Cascades and
passes. The system on Friday will bring the heaviest precipitation,
falling as snow, sleet or freezing rain in most passes. Breezy or
windy conditions are becoming more possible late Friday afternoon
and evening behind this system's surface low as it tracks inland. A
few rivers could also reach minor flood levels. Lingering showers
should end on Saturday as higher pressure builds over the region
with dry weather for the start of the new year.

&&

.SHORT TERM...Doppler weather radar this evening showed a decreasing
trend in precipitation as the first system was moving onshore.
Associated warmer air flooded into Western Washington today with
high temperatures today about 5 to 15 degrees warmer than Wednesday.

Easterly pressure gradients across the Cascades have maintained
snowy conditions with local freezing rain in the passes for messy
conditions. With the first system moving onshore this evening, winds
in the passes should turn westerly or ease at the least, and permit
brief warming there as precipitation eases. As the next system
approaches early Friday, anticipate pass winds to return to a cooler
easterly direction.

The next system has more moisture than the first for another surge
of a wintry mix in the passes Friday. The combination of a return to
cool easterly flow through the passes and fluctuating snow levels
will likely create a mix of snow, freezing rain and even a little
sleet in Snoqualmie and Steven passes, creating hazardous driving
conditions. Have updated the winter weather statement.

More on this second system. An upper low was near 49N/145W this
evening with a trough extending southward near 145-150W. An upper
ridge was along the coast with a 140 kt westerly jet across southern
British Columbia. Further to the southwest was a 100 kt SW jet max
establishing a jet couplet situation that synoptically tends to
create surface low intensification. The 12z progs moved in that
direction. The 00z NAM initialized well when compared with satellite
imagery and buoy 46006, and intensifies a surface low as it tracks
onshore late Friday. The 00z GFS was not as well initialized but
still tracks the low across the Olympic Peninsula. Given this track
east across the Olympic Peninsula, windy conditions to the south of
low along the coast and the Puget Sound area southward should
develop late Friday. Have issued a Special Weather Statement
regarding this growing wind potential.

Rain with this system's warm front should increase after 12z Friday
and continue into Friday evening as the associated cold front moves
onshore. The associated upper trough is forecast to move onshore
Friday night and Saturday with showers tapering off during the day.
Ridging aloft is expected to build just offshore for dry weather
early next week. Buehner

.LONG TERM FROM THE PREV DISCUSSION...
High pressure will gradually weaken Tuesday through Thursday of
next week. Weak systems may approach the area but any
precipitation looks to be light. At this point, no significant
weather is shown by models next week. Mercer

&&

.AVIATION...
Strong zonal or westerly flow aloft will prevail overnight. The
low level flow will be easterly or offshore. EXpect wdsprd MVFR
CIGs and areas of IFR CIGs/MVFR VSBYs.

KSEA...Expect MVFR CIGs (1-3K ft) to prevail thru Friday evening;
although, there will be occasional CIGs of 500-1000 ft thru at
least daytime Friday. Visibilities will generally be in the 3-5SM
range. Light easterly winds will become strong southerly Friday
evening.

&&

.MARINE...
A 1007 mb low well off the Oregon coast will continue moving
northeast overnight. This low will deepen to near a 1000 mb as it
moves across Cape Flattery late Friday. The cold front extending
from the low will move across the coastal waters late Friday and
the interior waters Friday evening. Expect strong onshore flow
with and behind the front. The flow will weaken on Saturday.

&&

.HYDROLOGY...
There will be heavy rain at times in the mountains tonight and
Friday. The area of heaviest rain will drift south tonight, then
move back north Friday. The snow level will be around 5000 feet
south and 3000 feet north on Friday. Then a cold front will move
through the region Friday night, lowering the snow level below
2000 feet and ending any heavy rain.

A few rivers could have minor flooding from this event. The most
likely flooding will be on flood prone rivers in the southern half
of the CWA, such as the Skokomish River in Mason County, the
Snoqualmie River near Carnation and the White River at R Street
bridge in Auburn, both in King County, and the Newaukum River in
Lewis County. A flood watch is in effect for the southern two-thirds
of Western Washington. It highlights the possibility of minor
flooding on a few rivers. Burke

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Watch from Friday afternoon through Saturday afternoon for
     Admiralty Inlet Area-Bellevue and Vicinity-Cascades of
     Pierce and Lewis Counties-Cascades of Snohomish and King
     Counties-Central Coast-East Puget Sound Lowlands-Eastern
     Strait of Juan de Fuca-Everett and Vicinity-Hood Canal Area-
     Lower Chehalis Valley Area-North Coast-Olympics-Seattle and
     Vicinity-Southwest Interior-Tacoma Area-Western Strait of
     Juan De Fuca.

     Winter Weather Advisory until 6 AM PST Friday for Cascades of
     Snohomish and King Counties-Cascades of Whatcom and Skagit
     Counties.

     Winter Storm Warning from 6 AM to 10 PM PST Friday for Cascades
     of Snohomish and King Counties-Cascades of Whatcom and
     Skagit Counties.

     Winter Weather Advisory until midnight PST tonight for Cascades
     of Pierce and Lewis Counties.

PZ...Gale Watch from Friday afternoon through Friday evening 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 Watch from Friday afternoon through Friday evening 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.

     Gale Watch from Friday evening through late Friday night for
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Gale Watch from Friday evening through late Friday night for
     Puget Sound and Hood Canal.

&&

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