Small Craft Advisory 246, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
936 AM PST Wed Nov 21 2018

.UPDATE...With the combination of expected modest snowfall
totals, a holiday travel period, and a relative lack of snowfall
at the passes thus far this season we've opted to issue a Winter
Storm Watch for the Cascades and Olympics above 3000 feet for late
Thursday through Friday. We will be digging in a refining
additional details regarding this system with the afternoon
package. -Wolcott-
&&

.SYNOPSIS...Broad troughing across the eastern Pacific will bring
several systems into the area today with rain chances
increasing. 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.

&&

.SHORT TERM...
Morning satellite data reveals an amplified mid/upper level trough
across the far eastern Pacific /roughly located along 130W/. Water
vapor imagery can confirm numerous perturbations rotating about
the trough...with IR imagery showing an extensive cloud shield out
ahead of the trough. Sfc analysis around 09z this morning courtesy
of WPC shows a cold front located out ahead of the large
trough...with a few weak sfc waves made out along the front.

Above analysis is representative of a more active pattern across
the region. Local radar imagery from KLGX is showing light shower
activity finally reaching the western Washington coastline and
pushing inland. The first in a series of shortwave troughs
/embedded within broad scale trough/ will eject NE into SW BC
later this morning. A weak jet streak with right entrance region
over W Washington will supply enough lift for shower activity to
hold as it pushes inland. By this afternoon another subtle
shortwave will push into western Washington and bring another
round of showers to the area right on the heels of this morning's
activity. QPF amounts today are light enough that no hydro
concerns are expected. In addition, snow levels will also remain
above 5000ft so snow accumulations are not expected to become a
concern.

No appreciable break is expected as we go into the Thanksgiving
holiday on Thursday. A stronger impulse will be following quickly
behind Wednesday's late day shortwave...carrying with it much
stronger dynamics in its left exit region along with the wave
itself taking on a slight negative tilt. This system is expected
to bring another round of widespread /and heavier/ rainfall with
it as it pushes across the area thru the day. As the colder core
of the system moves into the area later into the day...H5 temps
will drop to -25C or greater...helping to establish a more
unstable atmosphere as snow levels begin to drop noticeably.
MUCAPE values are popping up in the 100-200 J/Kg range as mid
level lapse rates approach 7C/km or greater. A greater concern for
impactful snow accumulation is developing across portions of the
Cascades beginning late Thursday, especially if decent instability
comes into fruition. Will likely need a winter weather advisory
for snow amounts for portions of the Cascades beginning Thursday
afternoon and continuing thru Friday. Please stay tuned to the
latest forecast information if you plan to be traveling across
this area towards the end of the week.

Although Thursday's system will be relatively fast
moving...another strong shortwave will drop down on its heels from
the NW by early Friday. Snow levels will drop between 2500-3000ft
with rain and snow continuing. Additional accumulations in the
higher terrain will be likely. At this time...for this
event...lowland snow is not a concern. As the wave passes to the
SE late in the day Friday, western Washington will finally see a
break in the constant activity. Weak ridging will quickly slide by
on Saturday. Some light showers could linger into the first part
of the day but overall expect most locations to be dry. A frontal
system will be approaching coastal locations by the evening
hours...with rain showers possible.

Kovacik

.LONG TERM...
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 frontal system moving through the coastal waters
will push inland this afternoon and east of the Cascades this
evening. The air mass is moist and stable and will become slightly
more unstable behind the front this evening. The air mass will
become more unstable behind Thursday's front.

A weak and splitting frontal system is moving through the coastal
waters this morning. Despite clouds and light rain ahead of the
front, conditions in the interior are generally VFR with more
MVFR conditions along the coast. MVFR ceilings are expected to
spread into the interior late this afternoon with the front then
conditions will likely improve to lower end VFR this evening as we
get into a lull between frontal systems.

The next frontal system upstream will spread steady rainfall and
widespread MVFR conditions to the area after midnight tonight and
continuing through 18Z Thursday. Albrecht

KSEA...VFR conditions will lower to BKN030 with the front after
about 22Z this afternoon then improve slightly behind the front
early this evening. CIGS will lower to OVC020-025 after midnight
tonight ahead of the next frontal system. Southeast wind 4 to 8 kt
becoming south 8 to 12 kt behind the front, then backing to SE
10-14 kt late tonight. Albrecht

&&

.MARINE...A cold front moving through the coastal waters this
morning will move onshore during the afternoon hours and through
the inland waters by early this evening. Gale force winds are
occurring over the coastal waters ahead of the front and at the
eastern entrance to the Strait of Juan de Fuca. Winds will
diminish behind the front from the west this afternoon through
tonight.

Another front now seen near 138W will push eastward across the
area on Thursday. Gales are possible over the coastal waters, and
from the eastern entrance to the Strait northward through the
Northern Inland Waters with this second system. Gale Watches are
in effect for this second frontal system on Thursday and will
likely be upgraded to warnings this afternoon.

A series of frontal systems will affect the waters Saturday
through the middle of next week. Albrecht

&&

.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

&&

.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 Watch from Thursday morning through Thursday afternoon 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-East Entrance U.S.
     Waters Strait Of Juan De Fuca-Northern Inland Waters
     Including The San Juan Islands.

     Gale Warning until 4 PM PST this afternoon 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-East Entrance U.S. Waters Strait Of
     Juan De Fuca.

     Small Craft Advisory for rough bar from 1 PM to 5 PM PST this
     afternoon for Grays Harbor Bar.

     Small Craft Advisory until 2 AM PST Thursday for Northern Inland
     Waters Including The San Juan Islands.

     Small Craft Advisory until 4 PM PST this afternoon for Central
     U.S. Waters Strait Of Juan De Fuca-West Entrance U.S.
     Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 10 PM PST this evening for Admiralty
     Inlet.

&&

www.weather.gov/seattle