075
FXUS66 KOTX 111152
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
452 AM PDT Tue Mar 11 2025

.SYNOPSIS...
A low pressure system will deliver mainly rain in the valleys and
snow to the mountains Tuesday night through Thursday. Winter
travel conditions will be possible over the mountain passes,
especially Wednesday night into Thursday morning. Breezy winds are
expected on Thursday with the potential for snow and graupel
showers. The weekend will remain active with more snow in the
mountains and rain and snow in the valleys. Sunday will be breezy
to windy with a cold front passage as well. Temperatures remain
near to or below normal through the week.

&&

.DISCUSSION...

KEY POINTS
* Mountain snow Tuesday night through Thursday, and moderate to
  heavy accumulations over mountain passes Wednesday night into
  Thursday morning possible.
* Active weather pattern will continue into the weekend with
  additional rounds of mountain snowfall. Valley snow this weekend
  carries greater uncertainty.
* Winds will be breezy Thursday afternoon, and there is a 20%
  chance for wind gusts greater than 40 mph on Sunday.

Tuesday through Thursday evening: The low pressure system off the
coast of Southern California will weaken and move eastward through
the day today. A low pressure system off the coast of British
Columbia will allow a shortwave trough to move through the area,
bringing a return of precipitation in the form of mountain snow and
lowland rain through Wednesday afternoon. This first wave of
precipitation will be relatively light, with much of the eastern CWA
(Northeast Mountains, Idaho Panhandle, and the Spokane/Coeur d`Alene
area) seeing a 45 percent chance and above at seeing a quarter of
an inch of rain. Snow will remain light as well and be limited to
mountain areas, except for the Methow Valley, which has lowered
snow levels. Mountain passes will see up to 8 inches of snow, with
Sherman Pass, Stevens Pass, and Washington Pass all having a 50
percent chance or higher of seeing 4 inches or more.

The trough will dig down even further as a surface low strengthens
Wednesday night through Thursday evening and ushers in a cold front,
allowing for additional rain to the lowlands and higher
accumulations of mountain snow. All mountain passes will see snow,
with Sherman, Snoqualmie, Stevens, and Washington Passes having a
greater than 70 percent chance of seeing three inches of snow in
roughly 24 hours. Lookout and Blewett Passes will also see snow, but
have a lower chance, 15 to 25 percent, of seeing 3 or more inches of
snow. This is because the overall flow will change, becoming almost
due south, which will favor higher snow amounts for the Cascades,
Okanogan Highlands, Northeast Mountains, and the Northern Panhandle.
In particular, Sherman Pass has a chance to see 7 to 9 inches of
snow in 24 hours, with a 65 percent chance or more of seeing at
least 5 inches. As such, a Winter Storm Watch above 4000 feet has
been issued for the Okanogan Highlands. Additional winter headlines
may be required for the Northern Panhandle Mountains, Northeast
Mountains, and Cascades due to these snow accumulation chances, so
stay tuned.

This second, stronger precipitation band associated with the cold
front will bring rain to the lowlands at first before lowering
temperatures drop the snow levels down. The Spokane, Coeur d`Alene,
Deer Park, and Pullman areas have a 50 percent chance or higher of
seeing an additional quarter inch of rain. However, there is a
chance these areas will see a wintry mix through Thursday evening as
snow levels lower. The lowland areas mentioned above all have a 40
percent chance or higher of seeing a trace of snow (0.10 inches).
/AS

Thursday night into Friday: A continuation of afternoon graupel
showers is expected on Friday. Temperatures up at 500 mbs will range
from between 30 to 35 below zero over northeast Washington and into
the Idaho Panhandle. Temperatures aloft will be warming through the
afternoon. Mid level lapse rates will be steeper earlier in the day
with the atmosphere becoming less unstable by late afternoon. Not as
unstable as what is expected for Thursday in the post frontal air
mass, but still unstable enough that we should see at least isolated
convective showers that will contain graupel.

Friday night through Sunday night: Model ensembles are beginning to
consolidate around the likely scenario for a pair of weather systems
for this weekend. Snow levels will start out relatively low at the
beginning of the weekend under a cold upper level trough of lower
pressure. A shortwave disturbance looks to push across the region
Friday night into Saturday. Dynamics along the front looks rather
week. Moist isentropic ascent will be the primary forcing mechanism,
but this too will be fairly weak. Mid level lapse rates will be
rather steep with 7.5+ C/km across much of the region. The 00Z
deterministic GFS also indicates 100+ J/kg of skinny CAPE across a
good portion of the region by the afternoon on Saturday. The morning
looks to start out with light snow for the mountain valleys into the
northern portions of the basin and Palouse region transitioning to
rain further south into Moses Lake and Ritzville. Snow accumulations
will be light on the order of 1-3 inches for the northern mountain
valleys and possibly up to a half inch on the higher benches around
Spokane and on the Palouse. Then we see a transition to convective
showers in the afternoon with a 10-20% chance for thunderstorms with
better chances being over southeast Washington into the Idaho
Panhandle.

A stronger and wetter storm system arrives Saturday night into
Sunday bringing a weak Atmospheric River (AR). The ECMWF ensemble
indicates a surface low strengthening to around 970 mb to 975 mb
that pushes across Vancouver Island late Saturday night or early
Sunday morning. A robust warm front will push across the Inland
Northwest during this time with the sub-tropical moisture within the
warm sector delivering a good shot of precipitation. Snow levels
will be tricky. They look to start out at around 2500 feet and then
rise to around 3000-3500 feet by Sunday morning. Northern mountain
valleys could see heavy wet snow for colder pockets. The likeliest
scenario will be mostly non-accumulating snow on roads or just on
grassy surfaces. Tough to say and is the most challenging portion
of this part of the forecast. It will depend on precipitation
intensity. Confidence is high that mountain passes will see the
potential for moderate to heavy snow, particularly in the Cascades
and over Sherman Pass. Below are the probabilities for at least 1
inch of snow off of the National Blend of Models (NBM) for the
weekend:

Probability of at least 1" of snow over 48 hours next weekend:
Spokane = 20%, Colville = 20%, Sandpoint = 30%, Pullman = 20%,
Republic = 100%

The probability for our mountain passes receiving at least 6, 12,
and 18 inches of snow are as follows:

For at least 6 inches-Lookout Pass = 90%, Sherman Pass = 90%VFR conditions for most TAF sites. HREF
guidance suggests MVFR ceilings possible near PUW/COE/LWS
overnight/early Tuesday. Some low clouds may push north toward the
GEG/SFF area too, but confidence is low., Stevens Pass = 100%,
Blewett Pass = 60%, Loup Loup Pass = 40%

For at least 12 inches-Lookout Pass = 35%, Sherman Pass = 35%,
Stevens Pass = 97%, Blewett Pass = 5%

For at least 18 inches-Stevens Pass = 77%

Cold front passage looks to be early Sunday with the surface low
pushing across southern British Columbia. A tightening pressure
gradient is expected to result in windy conditions on Sunday in the
post frontal air mass. There is a lot of variability in the model
ensembles with how strong the pressure gradient will be. The 00Z
guidance suggests a 20% chance for seeing widespread wind gusts of
greater than 40 mph across the exposed areas of the basin. Winds and
mountain snow will be the main impacts to monitor as the weekend
approaches through the week. We will also need to monitor river
levels over the central portion of the Idaho Panhandle. Paradise
Creek looks to see the steepest rise and may approach its action
stage of 8 feet on Sunday. Flooding is not anticipated, but will
need to be monitored.

Monday through Tuesday: A trough of lower pressure looks to move in
over the region by early next week. This will shunt the moisture
plume associated with the AR off to our southeast. The atmosphere
will be conditionally unstable but drier overall. Expect showers at
times, especially in the afternoon as lower levels of the atmosphere
destabilize with the heating of the day.

Temperatures will generally be below normal through the extended
portion of the forecast period. /SVH

&&

.AVIATION...
12Z TAFS: MVFR/VFR status look to prevail as high stratus moves
into the area. With this will come rain to the eastern portion of
the CWA first, with KGEG-KSFF-KCOE-KPUW seeing rain beginning at
18Z. Rain for KLWS-KMWH-KEAT will start later, after 00Z. All TAF
sites will stay within MVFR/VFR status through the period.

FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS:
High confidence in MVFR/VFR conditions through the forecast
period. Possibility for low clouds near KCOE-KPUW-KLWS that may
lead to longer stints of MVFR status this morning.

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

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        48  37  49  37  48  28 /  10  80  90 100  70  20
Coeur d`Alene  47  35  47  35  46  26 /  20  80 100 100  90  40
Pullman        48  38  50  36  44  28 /  20  60  90  90  80  30
Lewiston       55  41  57  40  52  33 /  10  40  80  90  60  20
Colville       46  32  45  34  46  23 /  10  90  90 100  80  40
Sandpoint      44  35  44  35  44  27 /  20  90 100 100  90  60
Kellogg        42  36  44  34  41  29 /  30  80 100 100 100  50
Moses Lake     50  38  53  37  52  28 /  10  50  60  80  20   0
Wenatchee      47  35  47  32  48  28 /  10  60  60  70  20   0
Omak           48  33  47  33  49  25 /  10  70  80  90  40  10

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...None.
WA...Winter Storm Watch from Wednesday afternoon through Thursday
     afternoon for Okanogan Highlands.

&&

$$