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Area forecast discussion
National Weather Service Seattle Washington
938 PM PST Tue Dec 10 2019

Synopsis...a weak front is pushing across the Cascades this
evening, with lingering light rain and mountain snow showers. A
brief break is expected Wednesday morning before the next, much
stronger system moves into the area late Wednesday through early
Friday. This system will bring heavy snow to the Cascades,
particularly above 3500 to 4000 feet. Active weather will continue
into next week.

&&

Short term /tonight through Friday morning/...not much change
needed to the forecast for tonight as today's front continues to
advance eastward across the Cascades, with some lingering showers
in the Post-frontal air mass. Snow levels have been slightly lower
than expected, with snow observed around Snoqualmie Pass, so
trended the forecast to reflect this. Otherwise, showers will
linger in the higher terrain overnight, but generally come to an
end elsewhere outside of perhaps some light drizzle out of the low
stratus deck. Remainder of the previous short term discussion
remains valid and follows. Cullen

Attention then turns to the next system that will impact the
region Wednesday into early Friday. This system will bring much
more precipitation to the region, along with breezy conditions.
The headlines are as follows:

* lowland rainfall: most lowland areas will see 0.50-1.25 inches
of rainfall, with higher amounts of 1.5-2.00 inches along the
coast.

* Mountain precipitation: the mountains will generally see 1.5-2.5
inches of liquid equivalent precipitation, with locally higher
amounts around 3.0-3.5 inches.

* Snow levels: snow levels will generally begin between 4000-5000
feet, then lower to 2500-3500 feet late Thursday night into
Friday morning. This means that Stevens Pass will likely
experience significantly more snowfall than Snoqualmie Pass.

* Snow amounts: accumulations generally 3-6 inches below 3500 feet
(again, mainly early friday), 1-2 feet between 3500-4500 feet,
and 2-4 feet above 4500 feet. (A Winter Storm Watch was issued
for the cascades)

* gusty winds: breezy southerly winds are expected with this
system with gusts generally 20-30 miles per hour. Stronger winds with gusts
35-45 miles per hour will be possible across and near the coastal waters,
through the Strait of Juan de Fuca, and in the north interior
(waters near and north of whidbey island).

The heaviest and most widespread precipitation is likely to occur
Wednesday evening/night, with precipitation becoming more
showering in nature Thursday. In fact, there will exist a slight
chance of thunderstorms along the coast and southwest Washington Thursday
afternoon and potentially again Friday as somewhat cooler air
moves in aloft. -Wolcott-

Long term /Friday afternoon through Tuesday/...as we move into
Friday afternoon through Saturday, precipitation chances will
begin to diminish. That said, the pattern is messy and the door
will be open for a weak system to brush the area over the weekend,
so I wouldn't bet on a completely dry forecast.

Looking out further, there is good ensemble agreement in the
continuation of an active pattern with the development of a mean
trough just off the pnw coast. This will not only keep the door
open for more energy to move through the area, but also sets up a
more favorable pattern for wetter and warmer than average mid-
December conditions. This matches well with the latest CPC 6-10
day and 8-14 day guidance. -Wolcott-

&&

Aviation...today's front is currently advancing through the
Cascades, with some showers lingering in the moist, Post-frontal
air mass. Expect fairly widespread low stratus persisting across
much of the region overnight, with some improvement early
Wednesday. However, any improvement will be short-lived, as
conditions deteriorate again with steady rain and lower ceilings
in conjunction with the front Wednesday afternoon and into the
evening.

Ksea...low stratus will likely maintain mostly IFR/LIFR
conditions overnight. An improving trend toward MVFR possible
through the morning, but this improvement looks short-lived as the
next front arrives by late afternoon for lowering ceilings and
reduced visibility in steadier rain. Cullen

&&

Marine...a stronger frontal system approaches the coastal waters
late tonight and early Wednesday, before crossing the region
Wednesday afternoon and evening. A Gale Warning is up for the
coastal waters and the East Entrance to the Strait of Juan de Fuca
and northern inland waters have a good chance for gales Wednesday
afternoon or evening as well. Surface pressure gradient increases
a bit on Thursday as a trough crosses the waters. The next
disturbance moves onshore well south of the region over the
weekend. Meanwhile, a large west swell will arrive on Thursday,
building seas into the 17-20 ft range over the coastal waters.

&&

Hydrology...a wet storm system will impact the region Wednesday
afternoon through Friday. Given precipitation amounts and snow
levels in the 4000-5000 foot range - most rivers are not expected
to approach flood stage. There is some concern that the Skokomish
River in Mason County could approach flood stage. We will continue
to monitor this situation. -Wolcott/bower-

&&

Sew watches/warnings/advisories...
Washington...Winter Storm Watch from Wednesday afternoon through Friday
morning for Cascades of Pierce and Lewis counties-Cascades
of Snohomish and King counties-Cascades of Whatcom and
Skagit counties.

Pz...Small Craft Advisory until 3 am PST Wednesday 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.

Gale Warning from 3 am to 7 PM PST Wednesday 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.

Gale watch from Wednesday afternoon through late Wednesday night
for East Entrance U.S. Waters Strait of Juan de Fuca-
northern inland waters including the San Juan Islands.

Small Craft Advisory until 4 PM PST Wednesday for West Entrance
U.S. Waters Strait of Juan de Fuca.

Small Craft Advisory from 4 am Wednesday to 4 PM PST Thursday
for Admiralty Inlet.

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