104
FXUS66 KOTX 121211
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
511 AM PDT Wed Mar 12 2025

.SYNOPSIS...
A low pressure system will deliver mainly rain in the valleys and
snow to the mountains through Thursday morning. 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. The weekend will be breezy to windy
with a cold front passage. Temperatures remain near to or below
normal through the week.

&&

.DISCUSSION...

KEY POINTS
* Moderate to heavy snow accumulations over mountain passes in the
  Cascades to the Okanogan Highlands tonight through Thursday
  morning.
* Active weather pattern will continue into the weekend with
  additional rounds of mountain snow. Lowland valley snow
  accumulations are expected Saturday morning.
* Winds will be breezy Thursday afternoon, and breezy to windy over
  the weekend.

Today through Thursday: An elongated trough of lower pressure will
slowly push into the Northwest over the next 48 hours. A shortwave
disturbance moving through this morning will focus light
precipitation over the eastern half of the forecast area with slop
over precipitation over the Cascade crest. The main impact with
precipitation this morning will be for light snow accumulating over
Stevens Pass.

There will be a stronger shortwave disturbance that pivots around
the base of the upper level trough into the region Wednesday night
into Thursday morning. Diffluence aloft and some weak isentropic
ascent over the region this afternoon will keep light showers going
at times. Warmer temperatures will nose in on the front side of the
trough with snow levels climbing. Snow levels are expected to climb
up to around 5,000 feet over the Central Panhandle Mountains, and up
to 4,000 feet over the Cascades. Higher snow levels with a higher
sun angle and lighter intensity precipitation should result in snow
that falls over the higher mountain passes to melt on roads --
primarily referring to Stevens Pass and Sherman Pass. The warmer
temperatures with the cold pool aloft nosing in with the upper level
trough will introduce very weak low level instability over the
Columbia Basin. Better instability with 100+ J/kg of surface based
CAPE is expected more so over the lower portions of the Columbia
Basin. I removed the mention of thunderstorms from the forecast for
this afternoon north of Moses Lake, to Ritzville, to La Crosse and
kept in a 15 percent chance south of this line.

The shortwave disturbance for Wednesday night into Thursday will
bring a more robust vorticity maximum with stronger dynamics aloft.
Deterministic model guidance is in good agreement with a 994 mb
surface low spinning up over north-central Oregon this evening and
deepening to a 990 mb low over Moses Lake to the Waterville Plateau
overnight before ejecting across the Okanogan Highlands/Northeast
Mountains by Thursday morning. The track of this low will bring low
level easterly flow into the Cascades resulting in favorable upslope
flow. Models have been consistent with higher precipitation totals
from the east slopes of the Cascades over to the Okanogan Highlands.
Snow levels will be lowering overnight. Colder air wrapping into the
low from the west will result in snow levels lowering to below 3,000
feet in the Cascades by midnight, and from around 4,200 feet to
below 3,000 feet over the Okanogan Highlands form this afternoon to
early Thursday morning. It looks like a good set up for periods of
heavy precipitation to develop over north-central Washington. The
higher intensity precipitation could result in snow accumulations of
1 inch per hour over Stevens Pass and Sherman Pass for the overnight
hours into Thursday morning. A winter weather advisory is in effect
for Stevens Pass that covers light snow of 1 to 3 inches through
this morning, and then the moderate snow accumulations of 5 to 8
inches Wednesday night into Thursday morning. The Winter Storm Watch
above 4,000 feet over the Okanogan Highlands has been upgraded to a
Winter Storm Warning for 5 to 11 inches in the mountains. Both the
advisory and warning are out through noon Thursday with the warning
going into effect at 5 pm this evening.

Thursday in the post frontal air mass will be breezy with winds of
20-25 mph and gusts up to around 35 mph across the exposed areas of
the basin. Convective snow and graupel showers are also expected in
the afternoon into the evening.

Thursday night into Friday: A continuation of afternoon graupel
showers is expected on Friday. Temperatures up at 500 mbs will cool
down to around 32 degrees below zero over northeast Washington and
into the Idaho Panhandle. Mid level lapse rates will be fairly steep
early into the afternoon, but will become less unstable through the
day with a shortwave ridge of higher pressure nudging in from the
west. Temperatures will be cooler Thursday and Friday with highs in
the 40s. /SVH

Friday night through Tuesday: Models continue to be in excellent
agreement with an unsettled weather pattern this weekend and into
early next week. The first system looks to arrive late as a surface
low nears the northwest coast.  As the low moves up the northwest
coast, weak isentropic ascent will support light precipitation
overnight. Following the push of cooler air Thursday night, 850mb
temperatures Friday night and early Saturday morning are forecasted
to be between -5C to -3C across the Inland Northwest. Snow levels
will be low enough initially to support light lowland snow for the
northern valleys, Spokane area, Palouse, and the Idaho Panhandle.
Currently, the NBM is giving the West Plains of Spokane a 30 to 40
percent chance for a half inch of snow through early Saturday. Snow
levels around 1800-1900 feet in the Spokane area make this a tricky
forecast for the lower elevations where a rain/snow mix or non-
accumulating snow is favored. The NBM has a 10% chance of a half
inch of snow. Heavier snow is expected for the mountain passes with
Stevens Pass having a 80% chance for greater than 6 inches and a 30
percent chance for greater than 12 inches.

The next system arrives between Saturday afternoon and Saturday
night as a broader trough from the Gulf of Alaska shifts
southeastward. Although there still remains uncertainty in the exact
track and strength of this system, models are in good agreement on
the presence of it to bring more precipitation to the Inland
Northwest. As it shifts southeastward, this would reorient the upper
level jet into a southwesterly position with an upper level ridge
amplifying across the western US. Models also show a deepening
surface low developing along the southern end of this trough and
bringing a push of subtropical moisture into the region. There
remains differences between the overall strength of that surface low
and trailing upper level trough. Examining model ensembles reveal
differences in the exact axis of strongest moisture and resultant
precipitation amounts for Saturday and Sunday. This second system
will also bring another round of heavier mountain snow with a 70
percent chance of 12 inches of snow at Stevens Pass and a 20 percent
chance for Lookout Pass between 5 PM Saturday and 5 PM Sunday. Rises
on smaller creeks and streams will also be a concern Sunday night
into early Monday with Paradise Creek forecasted to rise above
bankfull conditions. Given the lack of lowland snow pack, widespread
flooding is currently not anticipated.

We will also need to monitor the wind potential for Saturday and
Sunday with Sunday bringing the potential for stronger gusts with
the passage of a cold front. Currently the NBM is giving a 60 to 50
percent chance for wind gusts greater than 40 mph for the Columbia
Basin through the Spokane area and the Palouse.

Expect post-frontal showers Monday and early Tuesday as the cold
upper level trough shifts into far eastern Washington and north
Idaho. A transient ridge then shifts into the Northwest Tuesday
afternoon and evening, although there are significant differences on
how strong it will be. 35% of global ensembles are suggesting more
precipitation following quickly behind. The other 65% have varying
degrees of a stronger ridge with precipitation delayed a bit. /vmt

&&

.AVIATION...
12Z TAFS: Light rain will continue through the day across the
Inland Northwest as moisture streams across ahead of an upper
level trough of lower pressure. The boundary layer will moisten up
with low ceilings and MVFR conditions regionwide. Smaller airports
in the northern mountains from Winthrop (KS52) to Omak (KOMK) to
Colville (KCQV) to Sandpoint (KSZT)/Bonners Ferry (K65S) may IFR
conditions with lower ceilings and reduced visibility at times.
These are expected to continue through this afternoon into
tonight, and high resolution models, such as the HREF, indicates
that conditions will degrade even further this afternoon as rain
adds moisture to the boundary layer. Drier southeast winds over
the Palouse into the Spokane/Coeur d`Alene corridor looks improve
ceilings for a period overnight Wednesday; however, steady rain
into KOMK, KEAT, KEPH, and KMWH will keep conditions MVFR through
the night.

FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS:
High confidence for lowering ceilings through the morning into the
afternoon with MVFR conditions today. /SVH

-----------------------
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        50  37  47  28  46  33 /  70  80  80  20  20  60
Coeur d`Alene  48  36  46  26  44  30 /  80  70  80  30  20  70
Pullman        50  38  44  28  44  33 /  80  70  70  20  20  70
Lewiston       57  42  52  33  51  36 /  70  50  50  20  20  70
Colville       46  36  46  23  43  29 /  80  90 100  30  30  60
Sandpoint      44  37  43  27  41  29 /  80  80  90  60  40  60
Kellogg        45  38  41  29  40  32 /  90  80  90  50  50  70
Moses Lake     54  39  51  28  49  32 /  60  80  50   0  10  40
Wenatchee      48  35  47  28  44  32 /  50  80  40   0  10  50
Omak           47  36  50  25  44  31 /  70  90  80  10  20  40

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...None.
WA...Winter Storm Warning from 5 PM this afternoon to noon PDT
     Thursday for Okanogan Highlands.

     Winter Weather Advisory until noon PDT Thursday for Western
     Chelan County.

&&

$$