Forecast Discussion for SEW NWS Office
000
FXUS66 KSEW 221013
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
313 AM PDT Thu Oct 22 2020

.SYNOPSIS...Quiet weather expected today. A strong weather system
will move through the area on Friday into early Saturday bringing
lowland rain, mountain snow and cooler temperatures. In the wake
of the system, chilly temperatures expected with the potential for
the first widespread freeze of the season. Dry weather expected
for the remainder of the weekend into next week.

&&

.SHORT TERM /TODAY THROUGH SATURDAY/...Dry weather across the area
today as northwest flow aloft remains and high pressure shifts
east ahead of next storm system. Patchy fog has developed around
the South and Chehalis River Valley with chilly temperatures in
the 30s to low 40s and likely frost in colder spots. Clouds ahead
of next system will push into the area throughout the day into
tonight so overnight lows tonight should be several degrees
warmer.

Attention remains on stronger weather system that will drop
southeast on Friday. At upper levels, elongated negatively-titled
trough will swing south from Canada with attendant surface low
moving southeast from off the coast of Vancouver Island. Model and
ensemble guidance still not in full agreement on strength of
upper level trough and more so with track of surface low as it
moves onshore into Western Washington. The trend over the past
several runs has been slightly further northward but spread of
ensemble member lows lies anywhere from north coast to the
southwest interior. This has major implications on tangible
weather, especially with snow levels and subsequent amounts and
lends itself to pretty high uncertainty in that regards. Current
forecast reflects northward trend but remains in the middle of the
envelope. Details are as follows.

1. Precipitation: System arrives Friday morning with precipitation
shield spreading into the area from the northwest throughout the
day. Widespread rain likely at the coast and into the lowlands
with totals on the order of 0.1 to 0.5". At higher elevations,
liquid totals will range from 0.5 to 1.5", heaviest in the
Cascades. May see a convergence zone develop as system exits
Friday evening into early Saturday but will wait to see how hi-res
guidance evolves.

2. Snow Levels and Amounts: This remains the high uncertainty
factor of the forecast and hinges largely on the track of the low
and just how much warm air is pulled up into the Cascades in
southerly flow as system moves onshore. Latest forecast is
substantially warmer than previous but still not quite as warm as
some of the guidance suggests. As it stands currently, snow levels
may start out around 2000 feet at onset of precipitation and then
rise steadily throughout the day, likely much above Snoqualmie
Pass and possibly above Stevens Pass. South of I-90, snow levels
could rise as high as 6500-8000 feet. This cuts snow forecast
substantially, especially for Snoqualmie Pass which may only see a
few inches before switching over to rain. 4-8 inches is still in
forecast for Stevens Pass but may be adjusted downward if
northward/warmer trend continues. Those with travel plans across the
passes Friday should keep on eye on the forecast as details will
likely be adjusted in the next 24-36 hours and could go either way.
The northward trend also lowers the probability of a bit of snow
mixing in with rain across foothill communities. Wrinkle in the
ointment would be if a convergence zone or convection banding
develops - which could briefly lower localized snow levels on the
order of 1000 feet or so. Still not expecting accumulating snow on
roadways in those locations as road temperatures remain warmer.

3. Winds: Breezy southerly winds begin to develop across much of the
region Friday as the low approaches. Gusts will likely increase to
20-30 mph through the afternoon, with the strongest gusts generally
north of Everett and near the water - including through the San Juan
Islands, northern portions of Whidbey Island, and near the coastal
portions of Whatcom and Skagit counties. As the low continues to
advance to the east and south, expect the winds to flip to be from
the north/northeast. Stronger Fraser outflow may develop to bring
gusts in the 30-40 mph range from western Whatcom county to the
southwest across the San Juan Islands and into the Strait of Juan de
Fuca. The strength of the winds, as well as the timing of a change
in direction, will remain very dependent on the track of the low, so
expect this portion of the forecast to be refined as the low
continues to approach the area.

4. Temperatures: Friday morning lows should be several degrees
warmer than this morning and likely above freezing with an
increase in cloud cover and southerly flow ahead of the system.
It`ll cool a few degrees for Saturday morning but likely still
above freezing in most locations. Big cooldown arrives by Saturday
night into Sunday morning - details in long term section below. Afternoon
highs on Friday will drop into the 40s for most locations with
the exception of the coast which will likely still be in the 50s.

Models will likely fluctuate with the details until system arrives
Friday so expect further adjustments to the forecast over the next
day. Stay tuned here and to our social media channels for the
latest.

CEC/Cullen

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...Much quieter next week as
ridge of high pressure amplifies over the eastern Pacific and then
gradually shifts inland through mid-week. This leads to a dry
forecast and gradual warming trend in temperatures. Sunday morning
will be the coldest of the period and likely of the season so far
with lows dropping into the upper 20s to low 30s. This will likely
lead to the first widespread freeze and end to the growing season
for most locations. Monday morning will also be chilly but maybe a
degree or two warmer. Tuesday and Wednesday mornings will be back
above freezing again. Afternoon highs will climb from the 40s on
Sunday back into the low 50s by Tuesday and mid to upper 50s by
Wednesday.

CEC

&&

.AVIATION...Light northwesterly flow aloft continues today. Mix of
fog and stratus around the region early this morning leading to a
mix of MVFR to LIFR flight conditions. The most widespread fog will
remain around the south Sound and also extending into the foothills.
Improvement expected through the day, with more widespread VFR
conditions under mostly clear skies this afternoon. Generally
northerly surface winds through the day today, becoming southerly at
the end of the period. Increasing mid-level clouds late this evening
from the northwest ahead of the next frontal system.

KSEA...MVFR ceilings around 2500 ft early this morning may briefly
lower closer to IFR thresholds for a few hours around 14z.
Otherwise, scattering trend with VFR conditions expected from around
19z into the evening. Light winds this morning, with return of
northerly surface winds 6 to 9 kt this afternoon.    Cullen

&&

.MARINE...Light winds across the waters today in between systems.
The next front approaches late tonight, with a return of stronger
southerly winds across the coastal waters by early Friday. A low
then enters the waters, with a sharply increasing surface pressure
gradient. Flow will turn offshore on the north side of the low by
late Friday afternoon (through the northern inland waters and Strait
of Juan de Fuca, as well as the northern coastal waters), but
northwest over the coastal waters south of the low.

Strengthening Fraser outflow late Friday evening through early
Saturday may lead to strong gales - as a result, a Gale Watch has
been issued. Small Craft Advisories will likely be needed for the
remainder of coastal waters at times Friday through Saturday. It is
possible that gales could develop over portions of the coastal
waters and near the west entrance of the Strait of Juan de Fuca into
Saturday, but confidence is not as high at this point.

Meanwhile, seas over the coastal waters remain somewhat steep but
continue to gradually subside through the day today, before again
building Friday and Friday night as the low approaches. Expect seas
over the coastal waters reaching 9 to 12 feet Friday night, before
easing into early next week. Cullen

&&

.HYDROLOGY...No river flooding expected for the next 7 days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Frost Advisory until 8 AM PDT this morning for Cascades of
     Pierce and Lewis Counties-East Puget Sound Lowlands-Lower
     Chehalis Valley Area-Southwest Interior.

     Winter Storm Watch from Friday morning through late Friday night
     for Cascades of Snohomish and King Counties-Cascades of
     Whatcom and Skagit Counties.

PZ...Gale Watch from Friday evening through Saturday morning for East
     Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland
     Waters Including The San Juan Islands.

&&

$$

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NWS SEW Office Area Forecast Discussion