NWS Seattle, WA wrote its last area forecast discussion before the first event at Dec 21, 3:00 PM PST, 0 minutes earlier.
Area forecast discussion, Dec 21, 3:00 PM PST, 0 minutes before
Area Forecast Discussion
National Weather Service Seattle WA
300 PM PST Tue Dec 21 2021
.SYNOPSIS...An incoming weather system will drop down along the
British Columbia coast, bringing another round of lowland rain and
mountain snow to the region tonight. Wet and unsettled conditions
will then continue through much of the week. A switch to a much
colder weather pattern is likely over the weekend, bringing with it
the increased potential for lowland snow chances.
&&
.SHORT TERM /TONIGHT THROUGH FRIDAY/...Daytime heating proved to be
a bit on the disappointing side thanks to lingering clouds. Obs at
the time of this writing /around 2 PM PST/ show that the bulk of W
WA failed to get out of the 30s with the exception of locations
along the coast that managed to creep into the lower 40s. Speaking
of the coast, already starting to see some precip there, albeit
generally light. That will gradually make its way inland this
evening and make for a soggy night tonight. This will make for three
points of discussion. First, short term hi-res models still
suggesting a very brief window for the prospect of freezing rain for
portions of W Whatcom county...mainly from Bellingham north. It is a
pretty narrow window, between 00Z and 03Z /4-7 PM PST/, and given
that Bellingham is still sitting around 31 degrees...well, it is
entirely possible. Travelers in the area would be well advised to
use caution. Second point of discussion is the fact that while
forecast amounts have been trimmed slightly, snowfall amounts for
the 24 hour period starting 12Z Wed morning still look to be on
track to meet Winter Storm Warning criteria. As such, will upgrade
Watch with the afternoon forecast package. Last point will be
thunder threat along the coast and over the coastal waters. With
activity over the waters tending to be overforecast to begin with,
current model solutions showing a little CAPE and some instability
look to warrant some slight chance wording over the waters, as there
appears to be a non-zero chance there. Getting inland however, it
just does not look like the support is there, even with this being a
warm front. As such, limited any activity to the immediate coastline.
The front will linger over the area for much of Wednesday, allowing
the cold front to move into the area as early as Wednesday evening,
providing minimal, if any, break in the precip. The northern and
southern portions of the Cascades seem to be favored, while the
Central Cascades and Olympics may see lesser amounts. Not
significantly lesser amounts, mind, thus instead of trying to get
fancy with the forecast and splitting things up with warnings for
the heavier spots and advisories for the lower, opted to just go
with a blanket Warning. A series of shortwaves Thursday will keep
precip activity going, and as such have opted to extend inherited
timing of the now Winter Storm Warning to cover these features as
well. Will leave it for future shifts to extend or trim as the
situation merits. Late Thursday night and into Friday looks to bring
in yet another system that looks to pair with some Fraser outflow
over the area. While the Fraser outflow will not have an immediate
impact, the incoming system will ensure that the weather looks to
remain wet for the duration of the short term.
Warmest temps of the week expected tomorrow with the warm front as
most lowland locations will see high temps flirt with 50 degrees.
After that, Thursday and Friday look to return to cooler temps with
highs generally in the lower 40s.
Conversely, overnight lows will start a cooling trend, starting
relatively warm tonight right around 40 then gradually cooling
Wednesday and Thursday night into the mid to upper 30s. 18
.LONG TERM /SATURDAY THROUGH TUESDAY/...The aforementioned Fraser
outflow will start to impact W WA weather starting Saturday while
the upper level low sits and churns off the WA coast allowing for
moisture to continue to cycle into the area. Yes, these elements
might just align for the prospect of snow but consultation with
both ensemble data and deterministic models shows that amounts will
not look to be terribly impressive. Even with the upper low and
colder temps lingering throughout the weekend, total snowfall
Saturday through Monday only looks to be 1-3 inches for this entire
period with not a single 6-hour block of snowfall exceeding a
quarter of an inch. Temps initially will have a hard time supporting
snow as highs Saturday will still warm into the upper 30s to around
40 in the lowlands, but as the weekend progresses, the Fraser impact
to temps will be more widespread, with highs Sunday in the lower to
mid 30s and Monday even colder still with highs in the lower 30s and
most lowland locations struggling to get above freezing. Once the
upper level low moves east of the area Monday, activity should taper
off. Deterministic models are split on Tuesday, with the GFS
bringing in a shortwave that would allow for some showers while the
ECMWF actually continues dry conditions. Ensembles lean a bit drier,
but not completely dry. Temps Tuesday however do not see much in the
way of improvement, so if anything falls that day, it will likely be
white.
As always when trying to forecast snow in W WA, stay tuned to the
forecast as this is always a difficult phenomena to try and pin
down. 18
&&
.AVIATION...A ridge of high pressure aloft will continue to
weaken and slide east tonight as a stronger upper level low pressure
system moves into the area. This will increase southerly flow across
the area. Low cigs and fog were stubborn today and it looks like IFR-
MVFR conditions will just persist into the overnight. Showers will
increase at terminals after 03Z tonight with reductions in both vsby
and cigs at all terminals overnight. Southerly winds will begin to
increase especially towards 12Z Wed and will become gusty. Southerly
winds of 12-18kts and gusts of 25 kts will then be possible through
the end of the TAF period on Wed.
KSEA...IFR conditions to persist this evening and into the
overnight. Vsby will diminish as well in rain showers after 03Z
tonight. These low cigs to persist through 18Z Wed. Light
southeasterly surface winds of 4-6 kts this afternoon will increase
after 03z as well and become gusty on Wednesday. Johnson
&&
.MARINE...Southerly flow increasing tonight and Wednesday with a
front moving through the area - Small Craft Advisories are in
effect. A period of onshore is expected Thursday and Friday. Strong
Fraser River outflow winds are likely over the weekend. Johnson/33
&&
.HYDROLOGY...A frontal system will bring approximately 2-3 inches
of rainfall to the south slopes of the Olympic mountains tonight
through Wednesday night. This could lead to minor flooding on the
Skokomish River in Mason County. Snow levels will start around 5500
feet this evening and lower to 3000-4000 feet on Wednesday- but with
the river already running at an elevated level, this amount of
additional rainfall could be enough to push the river above flood
stage around Wednesday night. A flood watch has been issued.
Elsewhere, river flooding is unlikely for the next 7 days. 16/14
&&
.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Storm Warning from 4 AM Wednesday to 4 PM PST Thursday
for Cascades of Pierce and Lewis Counties-Cascades of
Snohomish and King Counties-Cascades of Whatcom and Skagit
Counties-Olympics.
Flood Watch from Wednesday afternoon through Thursday afternoon
for Hood Canal Area-Lower Chehalis Valley Area-Olympics.
PZ...Small Craft Advisory until 4 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 10 PM this evening to 4 PM PST
Wednesday for Admiralty Inlet-East Entrance U.S. Waters
Strait Of Juan De Fuca-Northern Inland Waters Including The
San Juan Islands-Puget Sound and Hood Canal.
Small Craft Advisory from 10 PM this evening to 10 AM PST
Wednesday for West Entrance U.S. Waters Strait Of Juan De
Fuca.
&&
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