Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

000
FXUS66 KSEW 061540
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
740 AM PST Mon Dec 6 2021

.UPDATE...Light precipitation continues across the region at this
time, with rain being generally being reported south of Skagit
County at this hour. The latest 15Z 925 and 850mb meso- analysis
shows warm air advection underway across the region. Expect snow-
levels to quickly rise from southwest to northeast this morning,
ending any chance for mixed precipitation and/or light snow by
late morning.

WSDOT traffic cams revealed a snowy scene across portions of Whatcom
County this morning, with some areas seeing between 1 and 2 inches
of snow accumulation. Snow has decreased in intensity in the last
hour, with only light snow currently falling. A mid-level shortwave
trough (visible on Water Vapor imagery) will enhance
precipitation across Whatcom County in the next 1-2 hours. Expect
an additional shot of snowfall as a result before snow changes to
rain by late morning. Additional accumulations of 0.5 inch or less
can be expected in Whatcom County before precipitation changeover
occurs later this morning.

Precipitation will gradually taper this afternoon, with light
showers likely lingering in the Cascades and along the outer coast
through tonight.

Pullin

&&

.PREV DISCUSSION... /issued 324 AM PST Mon Dec 6 2021/

SYNOPSIS...Colder conditions will continue this week with
multiple systems making their way through the area. These will
bring mostly lowland rain, mountain snow, and periods of breezy
conditions. Wintry precipitation is possible in the lowlands at
times this week, with minor accumulations possible. Confidence is
increasing in active weather continuing into next weekend.

SHORT TERM /TODAY THROUGH WEDNESDAY/...Radar imagery early this
morning shows activity across western Washington. Most of the
precipitation has been light rain so far, but there have also been
reports of light snow falling mixed in with rain in many
locations. This will generally be the trend through the morning
hours as the snow levels remain low, generally between 500-1000
feet. The best chance for very light accumulations between 0.5-1
inch this morning is in Whatcom and Skagit counties, with the
rest of the area likely to see that rain/snow mix. From Snohomish
County south, a light dusting of snow is possible on various
surfaces this morning but should not be impactful. Additionally, Stevens
and Snoqualmie Pass will likely only see between 4-6 inches. Snow
levels will rapidly rise from south to north by mid to late
morning today to between 4000-5000 feet this afternoon. Rainfall
will likely taper off this evening.

Tuesday looks to be mostly dry with some moisture lingering in the
Cascades and Olympics as a weak ridge builds in. Cloudy skies will
remain and isolated areas of fog are possible by Tuesday morning.
Temperatures will be slightly warmer on Tuesday, generally in the
upper 40s to low 50s.

A stronger system will approach the area by Tuesday night into
Wednesday, bringing additional lowland rain, mountain snow and
breezy winds. Gusty southwest winds are expected Wednesday morning
between 25-35 mph, and should ease by the evening. Snow levels
will lower Wednesday night to between 1000-1500 feet. As such,
more impactful snow will occur in the Cascades with the Passes
potentially receiving 8-12 inches of additional snow. Those with
travel plans across the Passes should continue to check the
forecast and monitor the road conditions.

LONG TERM /THURSDAY THROUGH SUNDAY/...A broad trough lingers over
the greater northwest on Thursday with additional mountain snow
and lowland rain expected. Snow levels will remain lower, still
between 1000-1500 feet. With northwesterly flow aloft expected to
continue, despite a shortwave ridge providing very temporary
relief on Friday, activity will continue.

The next system looks to be onshore by Saturday morning and has
the indications of being another prolonged rain event...We
know... Model ensemble analysis at this stage is already
highlighting a notable increase in QPF and increased winds for the
coming weekend. There is a lot to learn about this system in the
coming days and much can change including, but not limited to
event timing, rain amounts, expected snow level, winds, and storm
track. Overall, stay tuned to the forecast for updates as much
more will come into focus over the coming days.

Kristell

AVIATION...Increasing mid and high clouds early this morning with
widespread rain/snow showers across the area. Expect ceilings to
trend toward lower MVFR to IFR with reduced visibilities in these
heavier showers. Rain/snow mix or light snow at most terminal this
morning, with minor accumulations possible at KBLI. Otherwise,
transition to all rain expected from south to north through the
morning, with mostly rain by 18-21z. Residual moisture likely
maintains lower MVFR ceilings through much of the remainder of the
period, even as showers decrease. Predominantly southerly winds 5-7
kt through the period.

KSEA...VFR ceilings early this morning, though expect lowering to
MVFR toward 13-14z as steadier showers arrive. Expect rain/snow mix
through the morning push, with transition to all rain in the 18-20z
window.  Winds mostly southeast this morning and through the day at 5
to 7 kt. Cullen

MARINE...Increasing southerly winds over the waters this morning
ahead of the next frontal system. Current small craft advisory for
the coastal waters and the west entrance of the Strait of Juan de
Fuca remains in effect. Elsewhere, winds 15 to 20 kt will be common,
with a few local gusts to 25 kt possible around the east entrance of
the Strait. However, these don`t appear widespread or frequent
enough to warrant an advisory for these areas. The next front
arrives late Tuesday into Wednesday, with likely small craft
conditions over most waters and possible gales over the coastal
waters, followed by a strong west push through the Strait of Juan de
Fuca. Yet another, perhaps even stronger, system arrives late Friday
into Saturday. Meanwhile, seas will build to around 8 ft today with
this system. Expect seas to ease Tuesday in between fronts, but to
reach 10 to 13 feet behind Wednesday`s front and likely remain
elevated through the second half of the week, particularly over the
weekend. Cullen

HYDROLOGY...No river flooding is expected in the next 7 days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory until 1 PM PST this afternoon for
     Western Whatcom County.

PZ...Small Craft Advisory 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-West Entrance U.S. Waters
     Strait Of Juan De Fuca.

&&

$$

www.weather.gov/seattle


USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.