418
FXUS66 KOTX 212315
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
315 PM PST Thu Nov 21 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...
Thursday night through Friday: The Inland Northwest will remain
in a broad southwest flow aloft into the weekend as an upper level
trough remains nearly stationary off the offshore. The surface
low that brought the significant weather to western and central
Washington continues to weaken and drift to the northwest as weak
waves bring showery weather into the region. With an upper level
ridge centered over the Aleutians, this low pressure will remain
within the confines of the broad trough . Meanwhile, the next
system expected to move through the Pacific Northwest has started
to rotate through the base of the upper level low and will
approach the coast this evening.

As the showers this afternoon near their demise, the next round of
precipitation will quickly begin its entrance into eastern
Washington and north Idaho around midnight. A wave that is currently
located about 40N, 135W will strengthen over the next couple hours
as it approaches the northern California coast. By this evening,
models show this low traveling up the Pacific Northwest coast
through Friday morning and remaining offshore Vancouver Island
through the weekend.

While this low travels up the Pacific Northwest coast overnight
tonight, it will tap into the moisture plume that has been aimed
into northern California and transport more warm, moist air into
the region. Current PWAT Values across the Inland Northwest in the
0.45 to 0.60 inch range will increase to 0.75 to 1.20 inches as
this plume surges northward. This will bring widespread moderate
to heavy precipitation tonight through Friday. Similar to the
previous low pressure system, this low will strengthen the
easterly pressure gradient across Washington and initially focus
strong upslope flow into the east slope of the Cascades until the
flow shifts back to the southwest by early Friday afternoon. By
Friday afternoon and evening, precipitation will continue into far
eastern Washington and north Idaho through much of Saturday.
Given several days of warm southwest flow into the region now,
snow levels have come up above the valley floor for most
locations. The exception will be for the Methow Valley which
typically hangs on to colder air much longer than any other
valleys in our area. With that said, the Methow Valley Airport is
currently sitting at 35F/34F right now. Model soundings suggest
precipitation to begin as a rain/snow mix and transition to a
moderate to heavy snow between 5/6AM and 10AM on Friday. Given the
temperature profile, this snow will likely be very heavy and
dense before transitioning to rain by the late morning.

Rain Amounts/Impacts: With this next swath of precipitation, the
ECMWF Extreme Forecast Index is highlighting central Washington
(including the Columbia Basin) as an anomalous event with regards to
QPF during this timeframe. Current probabilities for over a half
inch of liquid through Friday is about a 40 to 50 percent chance.
Further east, the NBM is giving far eastern Washington and north
Idaho a 80 to 100 percent chance for over a half inch. For over an
inch, far eastern Washington (Spokane, Pullman) have a 10 percent
chance and northern Idaho has 20 to 30 percent chance. Around
Sandpoint, there is a 90 percent chance for over an inch.
Following the wet period, this additional precipitation will
result in steep rises over small creeks and streams. Places near
steep terrain will be susceptible for rock and mud slides.

Saturday: Precipitation will gradually decrease from west to east as
a frontal boundary pushes across the region early Saturday, except
for over north Idaho where moist upslope flow into the Idaho
Panhandle support showers through the entire day. As the front moves
through, snow levels will fall to 3000-3500 feet and transition
precipitation to snow across the mountains. Snow will struggle to
accumulate on roads initially, but wintry driving conditions will be
possible over the higher mountain passes after sunset. /vmt

Sunday through Thursday: A broad upper-level low will spin offshore
through the weekend and into early next week, sending weak
disturbances rippling through the Inland Northwest and keeping
chances for precip in the forecast. Seasonably cool and showery
conditions will continue through the period with intermittent snow
expected in the mountains and varied precip types expected for the
valleys depending on location and timing. Snow levels look to stay
well below pass levels for the entirety of the forecast, and will
drop between 1k and 2.5k ft each morning allowing for light snow in
some lower elevation areas during the early morning hours. Models
are suggesting we`ll have a decent shot at seeing light lowland snow
mainly along and north of US-2 with up to an inch of accumulation.
Heading into the workweek, upper-level flow will shift northwesterly
as the broad offshore low moves inland. Where exactly that low will
track remains uncertain. Some models are showing it moving directly
over the Inland Northwest, while others suggest it will dive down
into California. Precip amounts we`ll see through the middle of next
week will be largely dependent on which path the low takes.
/Fewkes

&&

.AVIATION...
18Z TAFS: Eastern Washington and north Idaho continue to be in a
very moist southwest flow aloft with periods of showers expected
for the next 12 to 15 hours. By 08Z a more widespread swath of
precipitation will move into the region and bring moderate rain to
all of the TAF sites.

Currently, light rain moving across southeast Washington into
central and north Idaho this morning is resulting in widespread
IFR to MVFR conditions for many locations. Showers will be on and
off through the day with conditions bouncing between MVFR/VFR
through the next couple of hours. Conditions look to gradually
degrade this evening as the next band of preciptiation approaches
the region.

FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS:
Confidence is high for the area to remain under a stratus deck
with MVFR conditions at KGEG-KSFF-KCOE-KEAT-KMWH although there
could be intermittent periods of improvement to VFR through the
early evening.


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

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  49  38  44  33  42 /  70 100 100  60  40  30
Coeur d`Alene  37  46  37  42  32  41 /  80  90 100  90  70  50
Pullman        39  49  38  43  31  42 /  80  90 100  90  50  30
Lewiston       43  55  42  49  34  47 /  70  90 100  80  20  10
Colville       31  41  33  41  28  39 /  70  90 100  60  50  50
Sandpoint      35  41  34  41  30  37 /  90 100 100  90  80  80
Kellogg        37  46  36  40  31  39 /  80 100 100 100  80  60
Moses Lake     38  49  38  48  33  45 /  90 100  70  20  10  20
Wenatchee      36  43  33  44  31  41 /  90 100  50  20  30  30
Omak           35  42  34  44  30  42 /  70  90  80  20  20  20

&&

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

$$