Heavy Snow in Washington, November 19 to 21, 2024

Area Forecast Discussion
National Weather Service Spokane WA
410 AM PST Tue Nov 19 2024

.SYNOPSIS...
Scattered snow showers will pass through the region early
Tuesday. Weather will remain unsettled through the week with the
next winter storm Tuesday night into Wednesday bringing another
round of widespread snow. This will result in moderate to major
impacts across central Washington and minor impacts for eastern
Washington and north Idaho. Additional systems passing through
Thursday through next weekend will bring additional periods of
lowland rain and mountain snow.

&&

.DISCUSSION...

Today: A shortwave is crossing into Central WA this morning and will
slowly track to the east and northeast through the day. Cloud
tops are cooling and light radar echoes are beginning to blossom
indicative of widely scattered snow showers that were anticipated
to develop. The latest short-range models have shifted the highest
concentration of showers further west into Central WA from
roughly Wenatchee to Omak/Republic. These areas carry around a 50%
chance for an inch of snow as this activity passes through.
Lighter activity is expected on the outer perimeters of this
activity with less than a 10% for an inch of snow. Winds will
start to swing around to the east and southeast and increase in
speeds late in the day becoming breezy this evening with speeds of
10-20 mph from southeastern WA into the upper Columbia Basin and
over the ridge lines of the Cascades.

...HEAVY SNOW FOR CENTRAL WASHINGTON.  BLIZZARD CONDITIONS POSSIBLE
ON THE CASCADE PASSES...

Tuesday night through Wednesday: The main message for the upcoming storm:

- Heavy snow is expected in Central Washington
- Light snow for the remainder of Eastern WA and North Idaho
- Travel across the Cascade Passes will be extremely challenging
  with strong winds and heavy snow
- Strong southeast winds around the Blue Mountains
- Most residents of the Inland NW will be dealing with winter
  travel conditions for the Wednesday morning commute

Quite the impressive meteorological event expected to take form over
the next 12 hours with low pressure expected to undergo bomb
cyclogenesis (drop in central pressure by 24mb/24 hours) reaching
a central pressure around 950mb around sunset. While this low will
remain nearly 350 miles off the WA Coast, it will influence the
weather over the Inland Northwest drawing gusty easterly winds
across the landscape. In the upper-levels, a 100+ kt jet from the
southwest will steer a plume of subtropical moisture inland
resulting in widespread precipitation. A cool trough passing
through the region today is providing the necessary cooling for
p-type to start as snow for most areas, but through the day
Wednesday, the battle will be on for the warmer air to push
northward.

Overall changes for the storm system have been minor in regards to
the snow forecast but with some caveats. First, a winter weather
advisory has been expanded into the Moses Lake zone with emphasis
mainly on Ephrata, Quincy, and George for up to 2 inches and
little to no snow from Moses Lake to Quincy. Idaho Central
Panhandle was also added to the advisory with light snow arriving
during the morning commute before retreating to the mountains
Wednesday afternoon-night.

Further examination of the increasing hi-res models and ensemble
solutions enables us to see more spread in the some of the snow
forecasts for areas. Wenatchee is one of these areas. HREF suggest
near a 80% chance for dewpoints to rise to 33F around 1AM and
100% by 5AM. This can be used a proxy when there may be a shift
from snow to rain...or could possibly just fall as very wet snow.
The UW 4KM WRF members indicate the range of outcomes from 2" to
7". We see a similar signal for locations below 1000 feet along
the Columbia River toward Omak. Consequently, these lowest
elevations would be the areas that come with some uncertainty.
Above 1000 feet, confidence is higher for snowfall amounts falling
well into the ranges indicated in the winter storm warnings. On
the contrary, easterly flow is a good recipe for damming the cold
air in place in the lee of the Cascades so thinking once the
heaviest precipitation arrives, it will likely remain consistent
at that p-type until late in the event when rates decrease. Would
not rule out a rain/snow mix at the onset for locations like Moses
Lake, Ephrata, and Wenatchee if afternoon temps reach the upper
30s but confidence is high for wetbulb cooling to quick drive the
temps near freezing. Another ingredient leaning toward more snow
than rain...current temperatures in the 20s/30s in the valleys
and teens in the mountains with dewpoints in the teens and
20s. This increases confidence some for a cooler/snowier scenario
and forecast is leaning in this direction, especially with
moderate to heavy precip rates arriving overnight.

Snow Amounts: Snow will increase over the Cascades and central
Washington Tuesday evening and ramp up overnight. Snowfall rates
of 1-2 inch/hour will be possible at times. 6-10 inches is
likely. The rising terrain of Chelan and Okanogan County will
carry a 60% chance for a foot or more. With the easterly winds,
amounts are actually lower right at Stevens Pass (6-10") but
higher around Coles Corner to Leavenworth likely due to orographic
effects and some shadowing in this unique flow pattern. Also
seeing some of hi-res models showing something in Winthrop which
is near 7" but 24" on the ridgeline just east.

Snow will spread northeast into far eastern Washington and the Idaho
Panhandle very late Tuesday night and closer to the Wednesday
morning commute. Timing right at the Spokane and Coeur d’Alene
areas is close to 5AM. Amounts look to be much less compared to
central Washington, but models are suggesting a 3 to 5 hour period
of moderate snow rates early Wednesday morning before switching
over to rain. Amounts are on the order of 1-4 inches with some
valleys of NE WA and N ID holding snow longer into the period and
closer to 6 inches. Snow advisories have also been issued for the
Blue Mountains with 5-8 inches and 1-3 below 4000 feet.

There will be a transition toward light rain with the quickest
transition occurring in southeastern WA and working its way into
Northeastern WA and North Idaho. While the valleys of Central WA
show a wintry mix or transition to rain, this comes with more
uncertainty due to models tendency to warm things too quick under
easterly flow. Yet, the air mass will become increasing wetter
with time and eventually dewpoints and surface temperatures in the
lower elevations will budge and climb above freezing. The Cascade
river valleys including the Methow Valley are areas that will
most difficult to scrub out and could remain snow throughout the
event but confidence is high that any additional QPF will be light
after the morning.

As for winds, we did issue a wind advisory for the foothills of the
Blue Mountains. Wind gusts up to 50 mph are a concern. Models
have been consistent that stronger southeast winds will descend in
the foothills of the Blue Mountains further south near Walla
Walla and points south where high wind warnings are in effect from
the Pendleton office. Strong winds will also be possible
channeling northward through the Snake River spilling out near
Clarkston. Generally gusty winds will be found across the Columbia
Basin with gusts around 30 mph. In the Cascades, winds will be
quite strong on the ridgetops and squeezing out through the gaps.
A wind ninja run made for for Highway 2 indicate 30-50 mph around
the Nason Creek Area (Coles Corner to Stevens Pass) and wind gusts
in excess of 50 mph on the higher peaks. This supports the idea
of blizzard conditions for the warnings in effect. /sb

Thursday through Tuesday: Models show a deepening surface low
south of the parent low off the CA coast that will skirt northward
along the Pacific coastline during the day Thursday. This will
bring a warm front through the region Thursday night into Friday.
For most locations, the dominant precipitation type overnight into
Friday will be rain as snow levels rise to around 3000-5500 feet.
However, the Methow Valley and far northern valleys appear to be
cool enough to support snow Thursday night into Friday morning.
During the day Friday, snow levels rise to around 4500-7000 feet.
With PWATs running around 160-250% of normal, Thursday through
Friday look very wet. Rain amounts will range from 0.5 to 2.0” in
the lowlands with the greatest totals in north Idaho. The
increased snow levels, rain, and low to mid elevation snow melt
will contribute to increased water into area rivers and streams.
No flooding is forecast at this time but this will continue to be
monitored. Snow levels drop below 2500 feet into Sunday as the
upper level low moves inland. The Cascades, extreme eastern
Washington, and north Idaho have additional chances for light
snowfall into next week with the upper level low overhead. /DB&&

.AVIATION...
12Z TAFS: A shortwave lifting through the region will bring
scattered light snow showers through this morning into the early
afternoon. This placement of the showers has shifted west focusing
more over Central WA between Wenatchee and Omak/Republic. Light
snow accumulations will be possible at Chelan, Winthrop, Omak, and
Republic btwn 12-18z. These showers may result in MVFR conditions
with a drop in ceilings and visibility. Otherwise, the focus will
be the next storm system approaching. E/SE winds will increase
through the day with ceilings lowering in Central snow increasing
in the East slopes around 00z and expanding eastward 08-15z. IFR
or lower conditions are likely for KEAT to KOMK and any terminals
within the Cascades. Significant snow accumulations are expected
for these areas. Lighter snow amounts will spread east for all
remaining terminals into Idaho 08-15z (except Lewiston). /sb

FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS:
Low clouds with ceilings around 1-2K ft AGL have developed over
the valleys of NE WA and N ID. As flow shifts to east this
morning, there is potential for these clouds to drift into
Spokane-Coeur d'Alene corridor though this comes with low
confidence. There is moderate confidence for snow showers in
Central WA this morning but low for coverage in Eastern WA and
North Idaho. Confidence is high for heavy snow in the Cascades
overnight into early Wednesday morning and bands of lighter snow
to spread into E WA and N ID early Wed morning. /sb

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

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        37  29  40  32  42  39 /  20  60  90  60  30  80
Coeur d'Alene  37  26  39  32  41  37 /  10  30  90  80  40  80
Pullman        40  29  40  34  43  41 /  10  70  80  60  60  90
Lewiston       44  34  51  38  47  43 /  10  60  70  40  50  70
Colville       34  24  35  27  39  34 /  40  40 100  70  30  80
Sandpoint      36  22  35  30  39  35 /  30  20 100  90  60  80
Kellogg        35  23  39  34  40  36 /  20  30  90  90  70  90
Moses Lake     40  33  46  34  45  40 /  40 100  60  40  50  80
Wenatchee      36  31  39  33  38  35 /  80 100  70  30  40  70
Omak           35  29  38  32  39  35 /  70  90  90  40  30  70

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...Winter Weather Advisory from midnight tonight to 10 PM PST
     Wednesday for Northern Panhandle.

     Winter Weather Advisory from 10 PM this evening to 10 AM PST
     Wednesday for Coeur d'Alene Area-Idaho Palouse.

     Winter Weather Advisory from 10 PM this evening to 7 AM PST
     Thursday for Central Panhandle Mountains.

WA...Winter Weather Advisory from midnight tonight to 10 PM PST
     Wednesday for Northeast Mountains.

     Winter Weather Advisory from 10 PM this evening to 10 AM PST
     Wednesday for Northeast Blue Mountains-Spokane Area-Upper
     Columbia Basin-Washington Palouse.

     Wind Advisory from 7 PM this evening to 10 AM PST Wednesday for
     Lower Garfield and Asotin Counties.

     Winter Weather Advisory from 10 PM this evening to 8 AM PST
     Wednesday for Moses Lake Area.

     Winter Storm Warning from 10 PM this evening to 4 PM PST
     Wednesday for Okanogan Highlands-Okanogan Valley.

     Winter Storm Warning from 4 PM this afternoon to 10 AM PST
     Wednesday for Central Chelan County-Waterville Plateau-
     Wenatchee Area.

     Blizzard Warning from 4 PM this afternoon to 10 AM PST Wednesday
     for Western Chelan County.

     Winter Storm Warning from 4 PM this afternoon to 4 PM PST
     Wednesday for Western Okanogan County.

&&