NWS Seattle, WA wrote its last area forecast discussion before the first event at Jan 4, 8:26 PM PST, 5 hours 34 minutes earlier.
Area forecast discussion, Jan 4, 8:26 PM PST, 5 hours 34 minutes before
Area Forecast Discussion
National Weather Service Seattle WA
826 PM PST Tue Jan 4 2022
.UPDATE...Only made minor tweaks to the forecast this evening,
mostly to the marine forecast but to also reflect the snow in
Whatcom County.
Water vapor imagery this evening continues to depict a sheared
lobe of vorticity along the southern edge of a weakening shortwave
trough located around the southern BC Coast, while near the sfc
the leading edge of a cold airmass lies around the US/Canada
border, with a warm front near the southern OR Coast extending
westward offshore.
Scattered showers continue across the area this evening, per MRMS
radar imagery that now includes data from KATX! These showers are
supported largely by shortwave energy over BC in tandem with an
associated narrow 100kt jet streak near 500mb, along with perhaps
some increasing WAA at 700mb well ahead of the frontal boundary.
Provided some of the cooler Fraser outflow across W Whatcom Co,
precip is falling as snow, with general accumulations in the
0.5-1.5 inch range. Elsewhere, mainly rain is being reported, but
there could be a rain/snow mix in spots as temps generally remain
in the mid to upper 30s. Precip should push east into the Cascades
over the next few hours, with not much in the way of additional
accumulation expected up in Whatcom Co (maybe 0.5-0.75 inches).
Winter Weather Advisory for the Cascades will continue through
6AM.
The lull in precip activity will be rather short on Wednesday as a
strong warm front begins to lift north towards the area. Still
agree with the ongoing forecast regarding precip type. Very strong
WAA should help promote synoptic lift across the area, while also
helping aid in some dynamic cooling of the atmospheric column. The
question will remain how much Fraser outflow makes it into the
area and how far south it'll get, but generally think the dynamic
cooling will play a big role. A quick glance thru forecast
soundings from the RAP, NAM, and GFS suggest saturation of most of
the entire column from 18z onward, including thru the DGZ. This
should promote heavy precip/snow development aloft that will have
difficulty melting even upon contact with the ground. The result
should be a period of heavy wet snow or "chunky rain" for many
lowland locations before WAA and frontal passage then yields
better warming of the column from south to north. Most guidance
is supportive of more of an isothermal layer vs warm nose during
the transition period, making the transition more of a snow to
rain vs freezing precip for the most part. Precip should be all
rain by Thursday with temperatures much warmer than we have seen
recently.
Coastal flooding remains a possibility for shorelines along the
interior waters again on Wednesday morning around high tide. An
advisory will remain in place for this with no changes to it
planned. Previous discussion follows with an updated marine and
aviation section.
Kovacik
&&
.PREV DISCUSSION... /issued 309 PM PST Tue Jan 4 2022/
.SYNOPSIS...Moist west flow continues tonight with snow levels
remaining low. A warm front moves up from the southwest Wednesday
night with snow levels increasing rapidly except in the Northwest
Interior and Cascade foothills. Warm front stalling over Western
Washington Thursday. Trailing cold front arrives Friday. Possible
drying trend over the weekend with upper level ridge trying to
build over area.
&&
.SHORT TERM /TONIGHT THROUGH FRIDAY/...Moist, westerly flow
continues over western WA tonight with showers mainly affecting
the coast and mountains. Snow levels remain low (around
500-1000ft) with a rain snow mix possible in the lowlands (but
little to no accumulation expected). Temperatures overnight will
be in the lower 30s with 20s in western Whatcom (due to Fraser
River outflow winds). There were some reports of icy roads and
sidewalks this morning and looks like we may see a repeat
tonight.
We'll see a brief break in the action on Wednesday, then another
threat of accumulating lowland snow Wednesday night into early
Thursday. This is a classic overrunning setup with cool N/NE flow
(Fraser Outflow) and low snow levels while a warm front spreads
moisture up from the south. Timing is crucial - how far south do
the north winds reach and when does the warmer air mass move
inland? At this point, it looks like areas from Olympia south will
see little to no accumulation. Seattle/Tacoma/Everett may see an
initial rain/snow mix but accumulations are generally under an
inch. These areas will all see a change over to rain early
Thursday morning as warmer air moves inland. Farther north, like
northern Skagit, Whatcom and the San Juans, the changeover will be
slower and therefore accumulating snows will be possible
(especially from 11pm-5am timeframe). This area may also see a
brief period of freezing rain before changing over to all rain
Thursday morning (depending on how cold temps get and when they
get scoured out). Accumulating snow is also possible on the
Eastside for areas close to the Cascades like North Bend and Black
Diamond. All areas should be rain by Thursday afternoon with
temperatures mostly in the 40s with lower 50s possible in the
south sound.
This pattern also brings more heavy snow to the Cascades with
freezing rain possible Thursday morning at Snoqualmie Pass (east
winds keeping pass temps below freezing). Winter Storm Warnings
are in effect with another 1-2 feet of snow expected. Check
Northwest Avalanche Center for mountain conditions.
Meanwhile, heavy rain will fall SW WA through the period which
brings the potential for river flooding on the Chehalis,
Skookumchuck, Newaukum and White Rivers (the Skokomish too). See
hydrology discussion below.
The air mass turns cool and showery on Friday with a passing
trough. However, temperatures will remain above freezing for rain
in the lowlands. Snow levels will lower back down to around
2000-3000 ft with more snow in the Cascades. 33
.LONG TERM /SATURDAY THROUGH TUESDAY/...Dry weather will prevail
over the weekend and into early next week with high pressure over
the area. Low temps are near freezing with highs in the lower to
mid 40s. The next incoming fronts splits as it moves inland Monday
night. Other weak system approach the area through midweek with
little impacts.
&&
.AVIATION...West-Northwest flow into Wednesday across the area.
Mainly MVFR/IFR cigs this evening. Areas of showers will taper off
tonight into Wednesday morning. Light snow across KBLI area will
taper off tonight. MVFR/IFR cigs into Wednesday morning. A system
will spread across the area on Wednesday with a rain/snow mix or
snow Wednesday afternoon into Wednesday night. The best chance for
accumulating snow will be Skagit County northwards, with light
accumulations possible for most areas. Southerly winds will
transition Wednesday morning to more light, north for the area.
KSEA...MVFR cigs into tonight. Showers will taper off tonight with
mainly dry weather Wednesday morning. IFR cigs possible late tonight
into Wednesday morning. A system will bring the chance of rain/snow
mix Wednesday afternoon with snow possible Wednesday evening into
Wednesday night. Mainly light snow accumulations possible (less than
1 inch). MVFR cigs for most of Wednesday. Southerly winds tonight
with winds on Wednesday more north and light for this area. JD
&&
.MARINE...Fraser outflow is likely to strengthen after midnight
tonight, resulting in SCA level winds over the Northern Inland
Waters. Hi-res model guidance is suggestive of these NE winds to
push down the Strait overnight tonight and through Wednesday
morning, which may also result in SCA winds. Given this potential,
have issued a SCA for the entire Strait beginning around 4am
Wednesday. It will still be worth monitoring Admiralty Inlet and
Puget Sound, but right now think the shift to N/NW after midnight
will remain between 15-20kts. As the east winds propagate down
the Strait Wednesday morning, they will eventually reach the
northern coastal waters. A SCA will remain over all coastal water
zones for much of Wednesday to account for increasing east winds
through the day, but to also capture gradually subsiding seas
which may linger around 10ft through Wed morning.
A significant frontal system is still set to push across the area
Wednesday afternoon through Thursday, which will raise the threat
for wet weather, including winter weather over the waters.
Conditions will likely then remain unstable at the end of the week
with breezy southerly winds likely. Seas are also expected to push
back up towards 10-12ft.
Refer to the public section above for information regarding the
coastal flood potential.
Kovacik
&&
.HYDROLOGY...Transition to warmer, wetter pattern late in the
week could result in high rivers flows, especially for the southern
Olympic Peninsula and southwestern Washington. The moderate to heavy
rain, combined with melting of the lowland snow could produce
flooding within the lower elevation river basins and other small
streams. The melting lowland snow will also increase soil saturation
not accounted for the landslide index, therefore there will be an
increase in the threat of landslides during the rainy period
Wednesday through Friday. The rivers most likely to have a chance
of flooding late in the week are the Skokomish, Chehalis,
Skookumchuck, Newaukum, and White Rivers and perhaps as far north
as Issaquah Creek. How much lowlevel snow remains by the time the
heavier rain begins Wednesday night will play a significant role
into how high the rivers crest and any flooding that might occur.
JBB
&&
.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Watch from late Wednesday night through Saturday afternoon
for Bellevue and Vicinity-Cascades of Pierce and Lewis
Counties-Cascades of Snohomish and King Counties-Central
Coast-East Puget Sound Lowlands-Hood Canal Area-Lower
Chehalis Valley Area-Olympics-Seattle and Vicinity-
Southwest Interior-Tacoma Area.
Coastal Flood Advisory from 6 AM to 10 AM PST Wednesday for
Admiralty Inlet Area-Bremerton and Vicinity-Everett and
Vicinity-Hood Canal Area-San Juan County-Seattle and
Vicinity-Southwest Interior-Tacoma Area-Western Skagit
County.
Winter Weather Advisory until 6 AM PST Wednesday for Cascades of
Pierce and Lewis Counties-Cascades of Snohomish and King
Counties-Cascades of Whatcom and Skagit Counties.
Winter Storm Warning from 1 PM Wednesday to noon PST Thursday
for Cascades of Pierce and Lewis Counties-Cascades of
Snohomish and King Counties-Cascades of Whatcom and Skagit
Counties.
Winter Storm Watch from Wednesday evening through late Wednesday
night for East Puget Sound Lowlands-San Juan County-Western
Whatcom County.
PZ...Small Craft Advisory until 7 AM PST Wednesday for Grays Harbor
Bar.
Small Craft Advisory until 6 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.
Small Craft Advisory from 4 AM to 1 PM PST Wednesday for East
Entrance U.S. Waters Strait Of Juan De Fuca.
Small Craft Advisory from midnight tonight to 6 PM PST Wednesday
for Northern Inland Waters Including The San Juan Islands.
Small Craft Advisory from 4 AM to 6 PM PST Wednesday for Central
U.S. Waters Strait Of Juan De Fuca-West Entrance U.S.
Waters Strait Of Juan De Fuca.
&&
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