Heavy Snow in Washington, November 22 to 24, 2018

Area Forecast Discussion
National Weather Service Seattle WA
818 PM PST Wed Nov 21 2018

.SYNOPSIS...Broad troughing across the eastern Pacific will bring
several systems into the area over the next several days. A stronger
system will affect the area Thursday and Thursday night, bringing
accumulating snowfall to the mountains. Another system moves thru
Friday with another round of lowland rain and mountain snow
expected. Weak ridging looks to keep the weekend fairly dry before
another unsettled period begins next week.

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.SHORT TERM /NOW THROUGH SATURDAY/...Radars show areas of rain over
mainly southwest Washington this evening that will spread through
the area overnight, and then a stronger shortwave will arrive Thu.
No updates this evening.

This next wave will have more impressive dynamics than today's
system as well as considerably colder air aloft. This is likely to
result in higher precipitation rates with the initial band of
precipitation followed by rain and snow showers through Friday. Weak
instability (100-200 J/kg CAPE) could result in some locally
intense rain or mountain snow showers Thursday night through Friday.
Snow levels will hover between 3000-4000 feet through Friday before
dropping to around 2500 feet by Friday evening. Due to the showery
nature of the post-frontal showers, confidence in exact rain/snow
totals is moderate at best. However, with the combination of locally
intense snowfall, holiday travel, and the relative lack of snow thus
far this season, we opted to issue a Winter Storm Watch for the
Cascades & Olympics for late Thursday morning through Friday
evening. Travelers should be prepared for winter travel and
recreation conditions. This means considering alternate travel
plans, having an emergency supply kit, and monitoring the forecast
and conditions for the latest information. Call 5-1-1 for the latest
road conditions.

A brief break in the action is expected Friday Night/Saturday before
the next, relatively weak, wave moves through Saturday Night and
Sunday. This will be a quick-hitting system with snow levels
generally between 2500-3500 feet at the mountain passes. -Wolcott-

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...
Saturday night's frontal system starts to lose steam quickly as
it moves inland...due in part by another shortwave ridge building
overhead. Aside from some scattered light rain showers...Sunday
may not turn out to be all that bad.

An interesting pattern could be setting up for next week as model
guidance is showing some decent cyclone activity across the
eastern Pacific. Deep moisture looks to push into the region
sometime on Monday into early Tuesday. Rain and mountain snow seem
probable at this point. Discrepancy exists thereafter...with the
GFS creating a very strong cyclone off the BC and Washington coast
whereas the ECMWF keeps a more open wave/progressive trough
pattern in place. Although neither solution is dry have preferred
to follow the ECMWF with this forecast package...as its pattern
seems to be fairly consistent with the current. As just mentioned
tho...the weather will remain unsettled and the more classic PacNW
weather looks to be here to stay finally.

Kovacik

&&

.AVIATION...A batch of rain over southwest Washington will shift
through the areas overnight and then another front will arrive
Thursday afternoon and evening. The air is moist and will
become more unstable behind the front Thursday afternoon as the air
aloft cools. The mountains will be obscured Thursday.

KSEA...There are just some areas of showers on the radar this
evening but steady rain will likely start up overnight ad the area
of precip moving into southwest Washington shifts north. There will
probably be a break in the precip midday, and then another batch of
rain will arrive by evening. Light winds this evening will
give way to stronger southerly breezes Thursday.

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.MARINE...A front will move through the waters Thursday. Gales are
expected over the coastal waters and small craft advisory conditions
are expected elsewhere on Thursday as the front moves through the
area. A series of fronts will give active weather to the waters From
later Saturday into the middle of next week.

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.HYDROLOGY...No flooding is indicated at this time but several
opportunities will need to be monitored. Wet pattern returns today
and continues through Friday as several weather systems move across
the area. Still looks like generally between 2-4 inches across the
higher terrain of both the Cascades and Olympics...with locally
higher amounts possible. These amounts could push the Skokomish
River close to or above flood stage Thursday and Friday. Lowering
snow levels during the period could reduce storm runoff into the
streams as well. Will see a brief break in rainfall this weekend.

Models continue to advertise a fairly wet and unsettled pattern next
week. This would cause additional river rises with increasing
hydro/river concerns. Will continue to monitor.

CEO

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.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Storm Watch from Thursday morning through Friday evening
     for Cascades of Pierce and Lewis Counties-Cascades of
     Snohomish and King Counties-Cascades of Whatcom and Skagit
     Counties-Olympics.

PZ...Gale Warning from 4 AM to 4 PM 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-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 for rough bar until 2 AM PST Friday for
     Grays Harbor Bar.

     Small Craft Advisory from 6 AM Thursday to midnight PST Thursday
     night 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-West Entrance U.S.
     Waters Strait Of Juan De Fuca.

     Small Craft Advisory until midnight PST Thursday night for
     Admiralty Inlet.

     Small Craft Advisory from 3 PM Thursday to midnight PST Thursday
     night for Puget Sound and Hood Canal.

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