937
FXUS66 KOTX 221131
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
331 AM PST Fri Nov 22 2024

.SYNOPSIS...
Friday will be quite wet with the arrival of a moist warm front
causing snow levels to rise above most mountain passes by Friday
afternoon. Small creeks and streams will see rapid rises and
there will be a threat for rock and mud slides near steep terrain.
The weekend into early next week will feature seasonably cool
temperatures and chances for valley rain and snow and periods of
mountain snow.


&&

.DISCUSSION...
Today through Saturday: A surface low off the Oregon coast is
sending a warm frontal band of moderate precip through the INW
this morning. Snow levels will be high enough for rain in most
locations. Lowland locations can expect a small few hour break in
rain this afternoon before cold front forcing increases along and
east of Moses Lake. The 9Z RAP guidance suggests the cold front
forward progress will stall in extreme eastern Washington and
north Idaho this evening and moderate rain will continue through
the night Saturday. The lowlands can expect to see around 0.5 to
1.5 of rain through Saturday morning with the greatest amounts in
extreme eastern Washington and north Idaho. Potential impacts
include: Ponding of water in urban/low lying areas and areas with
poor drainage, mud and rock slides near steep terrain, and rises
on small streams and creeks. Paradise Creek at Moscow is forecast
below action stage at the moment but the recent crest at 7.35 ft
on Sunday was accomplished with lower precipitation amounts than
what is forecasted. This will need to be watched closely.

Two tricky areas for potential snow this morning are the Methow
Valley and far north Idaho around Bonners Ferry. The Purcell
trench is hanging on to cooler surface air around 31-34 degrees
but higher elevation observations in northeast Washington and
north Idaho suggest the presence of a warm tongue aloft which will
limit the chance for snowfall accumulations. In the Methow valley,
surface conditions are similar but higher elevation observations
are a bit cooler. Winthrop is currently reporting 33 degrees and
rain but model soundings indicate the DGZ is not fully saturated
yet. The switch to all snow is expected at Winthrop around 4 AM
or so and the expectation is a switch back to light rain around
10AM when the DGZ dries out.

The cold front eventually moves into western Montana Saturday
morning dropping snow levels back to around 3500 feet. Snow will
struggle to accumulate on roads initially, but wintry driving
conditions will be possible over the higher mountain passes.
Lookout pass is expected to change to snow sometime around 4-6 AM
supported by the HREF and the experimental wetbulb 0.5C level from
the Utah Snow Ensemble. There is a 20-50% chance of 5+ of snow
from Saturday morning to Sunday morning at Lookout (higher for the
MT side of Lookout) and a 20% chance of 5+ at Stevens. The metro
model shows road temps above freezing much of Saturday at Lookout
pass, but heavier bursts of snow could overwhelm the mild pavement
temperatures quickly. Expect periods of wintry driving conditions
at the passes during this time, especially during heavier bursts
of snow. /DB

Sunday through Tuesday: An upper-low will loiter off the WA Coast
Sunday and Monday and finally eject inland on Tuesday. Models are
in decent agreement of at least two weak waves slung inland and
passing through the region bringing periods of light precipitation.
The air mass will be cooler with more typical snow levels for
this time of year. As such, intermittent snow is expected in the
mountains and mountain passes with a mix of rain and snow for the
valleys. Snow levels will waver between 1k and 3k ft each
morning/afternoon. Best chance for lowland snow will be across the
northern valleys and possibly into the Upper Columbia Basin near
US-2. Details remain uncertain at this time. NBM probabilities
indicate a 10-30% chance for 1" of lowland snow on Monday from
Waterville to St Maries and points northward.  Take this with
a grain of salt given the moist boundary layer and likelihood of
fog keeping temperatures warmer than expected limiting valley
accumulations.

Wednesday through Saturday: There is a shift in the jet stream
with high pressure building off the WA Coast. This places the
Inland NW under northwest flow aloft. This will support cooler,
more seasonal temperatures with highs in the 30s and lows in the
teens to 20s. The uncertainty in this portion of the forecast
comes with precipitation chances. Odds will favor the Idaho
Panhandle, Palouse, and Blue Mountains given the flow pattern
however, models are struggling with the placement of the ridge
axis and where potential shortwaves dropping in from the northwest
will land. More than 50% of the guidance deflect these east of
the region with light showers impacting the aforementioned areas;
20% drop low pressure systems right through the PacNW and 30% are
somewhere in between. This is reflected in the current forecast
which maintains a 20-30% chance for precipitation for the
Cascades, Idaho Panhandle, and eastern third of WA. This forecast
has potential to drastically shift one direction or another as the
large scale pattern becomes better resolved but at this time,
there is low probability for any significant travel impacts. Given
the upcoming holiday and increased travel, we will be keeping a
close eye on this evolving pattern. /sb

&&

.AVIATION...
12Z TAFS: A warm front lifting into the region is bringing the next
round of widespread precipitation. Rain has reached central
Washington including Wenatchee and Moses Lake and begin in
earnest between 12-14z for Pullman, Spokane, and Coeur d`Alene.
Increasing easterly winds is starting to break up the low fog at
KGEG but exact timing of improved visibility carries low
confidence due to this wind pattern advecting low stratus from
Spokane to the Airway Heights. Light to moderate rain will in the
morning will likely produce visibility reductions region-wide
given the significant influx of moisture. Shortly after 00z,
pressure gradients relax following the passage of the warm front
and redevelopment of fog will likely occur at airports favored by
light south winds including Spokane, Wenatchee, and possibly
Moses Lake. The trailing cold front returns shortly after with
light to moderate rain redeveloping over Idaho and the eastern
third of WA potentially as far west as Moses Lake to Colville.
This will likely inhibit fog but continue to produce IFR
conditions from rain and low stratus.

Low-level wind shear was introduced to the Spokane-CD`A area
as well we Pullman and Lewiston. A 50kt low-level jet at 850mb
will develop this morning from 180-210 degrees with surface winds
maintaining a direction from the east/southeast.

FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS:
There moderate to high confidence for widespread restrictions
today with moderate rains. The greatest uncertainty will be exact
visibility by hour and how fast conditions improve/deteriorate
behind the warm front and then resurgence of rain with the cold
front. It is certain that the boundary layer will remain saturated
and it will not take much to produce restrictions given the lack
of dry air advection. /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        48  38  44  33  43  29 /  90  90  60  20  40  30
Coeur d`Alene  44  37  42  33  40  29 /  90  90  90  30  50  40
Pullman        50  38  43  32  42  30 /  90  90  90  30  30  30
Lewiston       55  42  49  35  48  32 /  80  90  90  40  10  10
Colville       40  33  41  29  39  27 /  90  90  70  30  50  50
Sandpoint      41  35  41  31  37  27 /  90  90  90  50  80  60
Kellogg        45  36  40  31  40  27 /  90  90  90  60  60  50
Moses Lake     49  38  48  33  45  32 / 100  40   0  10  20  30
Wenatchee      43  34  44  31  40  32 / 100  10   0  10  30  40
Omak           41  34  44  31  41  31 /  90  40   0  10  30  30

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...None.
WA...None.
&&

$$