Heavy Snow in Washington, December 9 to 10, 2023

Area Forecast Discussion
National Weather Service Spokane WA
1049 AM PST Sat Dec 9 2023

.SYNOPSIS...
Widespread snow arrives today with potential for moderate snow
amounts going into Sunday. Travel across the mountain passes could
be challenging. Next week looks seasonal and drier than normal as
a ridge of high pressure settles over the region.

&&

.DISCUSSION...

...WIDESPREAD SNOW SATURDAY AFTERNOON THROUGH SUNDAY. HEAVY SNOW
OVER THE CASCADE PASSES...

Today through Sunday night: In the next 24 hours, a plume of subtropical
moisture associated with a weak atmospheric river will over top a
dome of cold air currently enveloping the Inland Northwest. This
will bring our next bout of winter weather and smiles to those who
are tired of the rain and ready for some snow. Unfortunately,
this bout of winter weather does not come with pure snow and a
transition from snow to rain is a likely scenario at some point
during the event. This is largely due to the subtropical origins
of this moisture plume and the nature of the air mass to warm in
efforts to carry the volume of water vapor that is set to arrive.
How quickly this transition occurs is the major challenge of this
forecast, leads to wide range in snowfall amounts, and presents a
decent bust potential for snow amounts at any given location.
Another element that is being considered and adding to uncertainty
is the later timing for snow to start across the eastern third of
WA and North Idaho. If surface temperatures warm above freezing
which carries a 50-80% chance, snow will be falling initially with
temperatures of 33-36F. The air mass is dry enough such that
wetbulb temperatures support snow as the main p-type but until
temps can wetbulb down to 32F, accumulations will be minimal. This
also suggest that moisture will readily melt on roadways at the
start of the event and then could become quite icy overnight as
snow intensities pick up. Avoiding travel late Saturday night into
early Sunday morning could be to your advantage. On the other end
of the spectrum, it has gotten quite cold tonight with many
locations in the twenties. If the high clouds are thick enough
through the day, models may be overdoing the warming for today and
the cold air could be more stubborn to scrub out. This could lead
to snowfall totals on the higher end of the spectrum.

* Onset of snow/snow intensities: Snow will arrive in the
  Cascades and portions Central WA between 10AM-1PM. Snow will
  continue to expand eastward through the day reaching the eastern
  third of WA roughly 5-7PM and North Idaho 6-8PM. By about 1PM,
  snow intensities will ramp up over the Cascades with 80-100%
  chance for snowfall rates of 1"/hour and 30-40% chance for rates
  near 2"/hour. Look for these rates to continue into Saturday
  evening making travel across the passes very challenging. Snow
  fall rates for the lowlands in Central WA will pick up around
  2-3PM and continue into the early evening. Snowfall rates will
  be closer to 0.5"/hour. Similar rates will expand into eastern
  WA with time reaching Spokane 6-8 PM and North Idaho 8-10PM. For
  the lowlands, expect these snowfall rates to last roughly 4
  hours. Rates at Lookout Pass will also approach 1"/hr (70%
  chance) during this time and continue into early Sunday morning.

* Transition to rain: We continue to see a wide range in
  possibilities how and when this will play out. As mentioned in
  previous discussions, deterministic models are hedging toward a
  slower transition while the ensemble means are hours quicker.
  The HREF for example has the transition at Moses Lake around
  6PM, Spokane around midnight, and Coeur D Alene and Sandpoint by
  2AM. Deterministic models delay this nearly 3 hours.
  Consequently, this 3 hour difference is having huge
  ramifications on potential snow amounts for this event. Some of
  the deterministic models suggest snow will remain the dominant
  p-type throughout the event for locations along and north of Hwy
  2. GFS is one of these models. When comparing it to nearly 60
  other ensemble member solutions, one could argue that this is an
  outlier.

* Potential snowfall amounts: The forecast is still taking close
 to a middle of the road approach after the latest round of edits
 and tweaking. General amounts are between 2-4 inches for the
 Upper Columbia Basin between Grand Coulee and Spokane and into
 the northern mountain valleys. Thinking the Okanogan Valley will
 receive the least amount of QPF for this event and amounts on the
 low side of the advisory or closer to 1-2". For the Cascades,
 1-2 feet along the crest and 6-10" for the upper ends of the
 river valleys. Amounts closer to the Hwy 97 corridor will be
 closer to the 2-5" range. Northeastern WA and North Idaho will
 remain snow for much of the event and have the potential for 3-6
 inches with the exception of Bonners Ferry where local
 downsloping effects will reduce amounts. Heavier amounts are
 expected in the Central Panhandle Mountains and latest forecast
 continued to support 5-10 inches. Given the higher elevations of
 the Palouse (Eastern Whitman County into Idaho), snow should hold
 longer into the event with 4-7" and this goes for Cd'A Area as
 well. Also coming up with similar amounts along the US95 corridor
 across the Camas Prairie and Blue Mountains. For the lower
 Columbia Basin and L-C Valley, amounts range from a trace to
 1.5".

* Freezing rain: There remains a moderate threat for a few hours
 of freezing rain in the lee of the Cascades around Wenatchee,
 Chelan, Waterville Plateau, and between Ephrata and Grand Coulee.
 Potential ice amounts remain below a tenth of an inch and this
 should have low impacts falling on snow vs bare grounds.

* Confidence levels: By far, my lowest confidence extends from
 Chelan to Spokane to C'dA and points south. Snowfall amounts for
 locations like Wenatchee, Chelan, Grand Coulee, Spokane, Cd'A,
 and Pullman differ by as much as 5 inches when looking at
 individual ensemble members. Ranges within different suites of
 models vary from 1 to 6". Moving further north into Northeastern
 WA and North Idaho including Republic, Chewelah, Sandpoint, and
 Kellogg, there is higher confidence for at least 2-3" then
 additional amounts on top of that also range from 1-6" range. So
 locations like Kellogg and Sandpoint could end up with 7-8" if
 the stars align.

Precipitation and the plume of subtropical moisture will exit the
region from west to east through the day Sunday with mainly
lingering mountain showers continuing Sunday night and dry
conditions for much of the Basin. /sb

Monday through Saturday:
After a very busy period of active weather, the Inland Northwest
will be mostly under the influence of an upper level ridge which
will keep temperatures above normal and precipitation chances at a
minimum.

The overall upper air pattern Monday afternoon includes a
shortwave exiting the region toward the southeast as a ridge
builds off the PacNW coast in its wake. That shortwave will
continue to dig into an impressive trough over the 4-corners area
in the southwest US. The deep trough results in an amplifying
ridge along the western coastal areas. By midweek, the ridge axis
will shift east centered over the Inland Northwest.

Given this upper pattern and abundant low level moisture, we
anticipate that fog and stratus will develop under the ridge. This
will lead to typical gloomy days Monday and Tuesday in many of the
low elevations. The challenge with fog and stratus is always
determining if and when areas break out and skies clear. For now,
we have hedged toward a cloudy forecast with most regions
experiencing some reduction in visibility due to fog.

That changes Wednesday as the ridge shifts inland, flattens, and
a weak shortwave moves inland. This will shift the surface winds
around from the east bringing drier air and eroding the fog and
stratus. The break from the clouds will be short-lived as the
shortwave spreads mid and high level clouds out ahead of it. The
chance of rain/snow is low with this system outside of the terrain
enhanced precipitation. The current forecast has a mix of rain and
snow at the low elevations and only minor accumulations of snow at
the higher elevations.

That system moves through Thursday night resulting in clearing
skies under a building ridge. The ridge once again amplifies
between two deep troughs, one over the southern plains and a
second cutoff low well off the coast of California. This pattern
looks to hold on through Friday and into the weekend with
temperatures at or above normal for this time of year. Given the
very low sun angle and limited day light, it is difficult to get
temperatures to warm substantially without sufficient mixing.

In short, no hazardous weather or significant impacts are
expected Monday through next weekend outside of areas of fog
Monday and Tuesday followed by light mountain snow Thursday.
/AB

&&

.AVIATION...
12Z TAFS: Precipitation will spread east of the Washington
Cascades this afternoon and evening. Snow will be the primary
precipitation type for Sandpoint, Bonners Ferry, Deer Park,
Colville, Omak, and the Methow Valley. Accumulating snow with a
transition to rain is expected for Spokane, Coeur d'Alene, Moses
Lake, and Pullman. For Wenatchee, Leavenworth, and Chelan, model
soundings suggest snow changing to freezing rain late this
evening. Of the HREF members, 10 percent of them give Wenatchee a
tenth of an inch of ice while the others limit icing to a light
glaze. Snow accumulations generated by the 12z HREF are close to
the NWS official forecast with 2 to 4 inches for Wenatchee,
Spokane, Coeur d'Alene, and Colville...and 3 to 6 inches for
Sandpoint, Deer Park, Kellogg, and the Methow Valley. Low ceilings
will develop a couple hours after precipitation develops with GFS
MOS, HREF and National Blend of Models guidance advertising 500
to 1500ft ceilings by mid to late evening. Without much low level
wind, low ceilings and IFR visibilities will likely linger into
Sunday morning and possibly into the afternoon even after
precipitation ends across most of central/eastern Washington and
north Idaho. /GKoch

FORECAST CONFIDENCE AND ALTERNATE SCENARIOS:
The timing of the change from snow to rain is one of the most
challenging aspects of the forecast for places like Pullman,
Spokane, and Coeur d'Alene. While our forecast is leaning on the
High Resolution Ensemble Forecast (HREF), the operational GFS is
colder (keeping Spokane/CDA snow). This alternate scenario
suggests totals for GEG of 4 to 5 inches and 5 to 7 for COE. The
GFS Ensemble mean is lower than the operational GFS forecast and
closer to the HREF forecast, so these higher amounts have a low
probability of occurrence. /GKoch

------------------

Confidence descriptors:
Low - Less than a 30 percent chance
Moderate - 30 to 70 percent chance
High - Greater than a 70 percent chance

For additional probabilistic information for NWS Spokane airports,
please refer to the Aviation Dashboard on our webpage:
https://www.weather.gov/otx/avndashboard

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        35  30  37  33  39  31 /  20 100  70  30  20   0
Coeur d'Alene  35  30  36  33  39  31 /  10 100  80  50  30   0
Pullman        35  31  38  34  38  32 /  10 100  90  60  50  10
Lewiston       40  37  44  39  44  36 /   0 100  90  50  40   0
Colville       32  28  33  26  38  23 /  50 100  40  20  20   0
Sandpoint      32  29  34  32  35  27 /  10 100  80  50  40  10
Kellogg        33  30  36  36  40  34 /   0 100 100  80  60  20
Moses Lake     37  31  39  31  40  29 /  50 100  30  10  10   0
Wenatchee      33  31  38  33  40  32 /  90 100  40  10  10   0
Omak           34  28  35  32  40  29 /  80 100  20  10   0   0

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...Winter Weather Advisory from 5 PM this afternoon to 1 PM PST
     Sunday for Northern Panhandle.

     Winter Weather Advisory from 5 PM this afternoon to 10 AM PST
     Sunday for Coeur d'Alene Area-Idaho Palouse-Lewis and
     Southern Nez Perce Counties.

     Winter Storm Warning from 4 PM this afternoon to 4 AM PST Monday
     for Central Panhandle Mountains.

WA...Winter Weather Advisory from 5 PM this afternoon to 1 PM PST
     Sunday for Northeast Mountains.

     Winter Weather Advisory from 5 PM this afternoon to 10 AM PST
     Sunday for Spokane Area-Washington Palouse.

     Winter Weather Advisory from noon today to 8 PM PST this evening
     for Moses Lake Area.

     Winter Weather Advisory from noon today to 10 AM PST Sunday for
     Okanogan Highlands-Okanogan Valley-Upper Columbia Basin-
     Waterville Plateau-Wenatchee Area-Western Okanogan County.

     Winter Storm Warning from 10 AM this morning to 10 AM PST Sunday
     for Central Chelan County-Western Chelan County.

&&