Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 242157
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
257 PM PDT Sun Mar 24 2024

.SYNOPSIS...

Monday and Tuesday will feature seasonal temperatures, with periods
of rain and snow showers. A wetter system arrives Wednesday and
Thursday, delivering widespread precipitation before warmer and
drier weather returns next weekend.

&&

.DISCUSSION...
Tonight through Tuesday night. The area of low pressure responsible
for rain and snow over Idaho and southeastern Washington this
afternoon will continue to drift east with precipitation tapering
off from northwest to southeast. While Central WA received a brunt
of the moisture on Saturday, it was a whole different story this
afternoon with dry, continental air flooding in from north. This
has resulted in quite the temperature gradient this afternoon with
locations like Omak and Moses Lake in the upper 50s while
portions of North Idaho such as the Silver Valley remains stuck in
the mid 30s with wet snow.

As the low drifts southeast this evening and overnight, drier air
will get pulled toward the departing system allowing the thick
cloud cover over Eastern WA and North Idaho to eventually break
and for some areas, clear out completely. This will promote good
radiational cooling and chilly overnight temperatures. More
importantly, this may give the Inland Northwest a chance to view
the Northern Lights. Earlier today, The Space Weather Center
issued a geomagnetic storm alert; this can be found at
swpc.noaa.gov. If these elevated space weather conditions were to
continue into the overnight hours, would not rule out a viewing
for areas absent of cloud cover. My lowest confidence for clearing
is south of a line from Kellogg to La Crosse. This includes
locations like Lewiston, Winchester, and Pullman-Moscow. The
further north and west you go, the better the odds for clearing
skies. The clearing will be greatest between 8PM and 3AM then
probabilities increase for cloud cover with many areas outside the
lower Columbia Basin carrying a 50-70% chance for some clouds by
5AM. This will come in the form of fog in the sheltered northern
valleys, low stratus between Ritzville and the West Plains, and
increasing midlevel clouds as the next system approaches. Good
luck to those eager to view the rainbow in the night sky and be
sure to check the aforementioned space weather page for updates
throughout the night.

The Inland Northwest will be positioned within a mean upper-level
trough into Tuesday afternoon. This will keep 500 mb temperatures
cool (-26 to -29C), lapse rates modest, and fuel on and off
showers through the period. Showers on Monday will be a bit more
synoptically forced as midlevel waves drops in from the northwest.
The highest PoPs will be found in the rising terrain of the Idaho
Panhandle, Northeastern Mountains, and Cascade Crest but not
ruling out showers in the lee of the Cascades and Basin. It is
likely this activity will be light in nature as they contend with
the antecedent dry air that arrived today (SUN). There is less
midlevel forcing for Tuesday however 500mb temperatures will start
off cooler (closer to -30C). A combination of light orographic
flow and afternoon heating will initiate convection delivering the
second day of isolated to scattered showers. There will also be a
small threat for lightning from the Camas Prairie northward into
the Central Panhandle Mountains. This comes with low confidence
and roughly a 10-15% probability via the NBM and 10-20% chance on
the SREF. The air mass will begin warming aloft late Tuesday first
in Cascades then toward the ID/MT border Tuesday night as the
trough departs and shortwave ridging translates inland ahead of
the next wet system.

Temperatures over the period will not stray far from 30-year
averages with highs in the 40s to 50s and lows in the 20s and 30s.
/sb

Wednesday and Thursday: An organized frontal wave moves into the
region between Wednesday and Thursday. Temperatures are forecast
to be slightly below normal Wednesday and near normal Thursday.
The leading warm front pushes up to the Cascades Wednesday
morning, with the occluding cold front coming on its heels toward
midday. That front then pushes across eastern WA and ID through
the afternoon and evening. Then overnight into Thursday an
unstable upper trough comes in. The primary frontal wave comes
with an increased moisture tap, with PWATs 140-180% of normal,
before starting to decrease some into Thursday. Altogether this
will bring expanding rain and mainly mountain snow to the region
Wednesday, starting near the Cascades and western basin in the
morning and expanding through the region toward afternoon and
evening. Some snow or a rain/snow mix is possible Wednesday
morning around the Cascade valleys and across the north and
eastern rim of the Columbia Basin and northeast valleys, including
the Spokane area and Palouse. Yet the main snow impacts are
expected in the mountains. Overall PoPs peak Wednesday evening.
PoPs start to wane in the lee of the Cascades overnight, but the
potential for precipitation elsewhere remains high through
Thursday with that upper trough on the heels of the main front.

* Rain amounts: The 24-hour probability of >=0.10 inches of rain
  (i.e. a wetting rain) is around 80-100% over a good portion of
  the our CWA, with 60-70% around the deeper Columbia Basin and
  near the Okanogan County/BC border. The 24-hour probability of
  >=0.25 inch of rain is around 60-90% over the Cascade crest,
  northeast WA and the ID Panhandle and about 20-50% around the
  deeper Columbia Basin and lee of the Cascades. So this looks
  like a wet system. Hydrograph forecasts show some rises with
  this system, but at this point nothing is projected to reach
  action level much less flood stage. However we still will have
  to watch for at least ponding of water in poor drainage areas
  and typical urban areas and some minor rock/mudslides may be
  found. Confidence in any of that is low.

* Snow amounts: snow amounts in the mountains are bit higher than
  the guidance was showing this time yesterday. The snow that has
  the potential for impacts will be around the Cascade crest and
  over the north-central WA passes. The official forecast for
  Stevens Pass right now has about 3-6 inches of snow Wednesday
  and Wednesday night and about 1-3 Thursday. Similar amounts are
  in the forecast around Sherman Pass. Meanwhile the Idaho Passes
  have about 1-3 inches over both days. Some individual 48-hour
  snowfall probabilities from the National Blend of Models,
  between 4 AM Wednesday to 4 AM Friday, are found below:

                       >=1"  >=2"  >=4"  >=6"
        Stevens Pass    95%   89%   72%   50%
        Blewett Pass    73%   56%   29%    8%
        Sherman Pass    83%   73%   46%   24%
        Loup Loup Pass  73%   56%   20%    8%
        Lookout Pass    75%   58%   24%    7%

* Winds and thunderstorm threat: Also in the Wednesday to Thursday
  time frame breezy conditions are expected, especially over the
  Basin, Spokane Area and Palouse with gusts near 20-30 mph
  possible. Nothing highlight-worthy at this time, but worth
  knowing about. The only other thing to monitor for is possible
  embedded thunderstorms Thursday afternoon with some conditional
  instability, with CAPE values around 100-200 J/kg over north and
  eastern WA and ID. Overall at the moments models paint about a
  10-15% probability of thunderstorms Thursday afternoon.

Thursday night to Sunday: the overall potential for precipitation
starts to wane to a greater extent going into the end of the week,
as the broader system moves out and comparatively weaker impulses
pass through a drier atmosphere (with PWATs back to around
80-110% of normal). PoPs do not end entirely, except in the lee of
the Cascades, but the higher potential retreats to the mountain
zones Friday and even more so on Saturday. Also by this time frame
some snow may mix down toward the valley floors for the overnight
and morning hours, but with little to no impact expected.
Otherwise it still looks like mainly lowland rain and mountain
snow potential. By Sunday the main potential for precipitation
backs up to the Cascade crest and along the ID/MT border. It
still looks a little breezy going into next weekend, with gusts
near 15-20 mph. Temperatures are expected to push slightly above
seasonal normals by this weekend, with highs ranging from the
mid-50s to mid-60s by Sunday. The warmer of that range is forecast
around the deeper Columbia Basin and L-C Valley. /Solveig

&&

.AVIATION...
18Z TAFS: Steady precipitation falling in the form of rain and
snow will continue to bring IFR to MVFR conditions for North Idaho
and much of southeastern WA including Bonners Ferry (K65S),
Sandpoint (KSZT), Coeur d`Alene (KCOE), Spokane Intl (KGEG),
Pullman (PUW), and Lewiston (LWS). Between 19-22Z, the system will
slowly exit to the southeast and drier air will seep in from the
north. Ceilings will lift with a 80-90% chance for VFR skies
between Spokane, Sandpoint, Coeur D Alene, and Pullman between
02-05Z. There is lower confidence around Lewiston with the
moisture lingering longer into the night and favorable upslope
flow. There are little to no aviation concerns for Central WA this
afternoon and evening where the dry air has already settled in.
Mainly breezy northerly winds will be the impact gusting to 20 mph
around Omak and Ephrata. After 08z...Patchy fog and/or low
stratus is expected to develop within our northern mountain
valleys and northeastern Columbia Basin. This comes with a 50-70%
chance via the HREF.

FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: There is high
confidence for MVFR to IFR conditions through 20Z given recent
observations. There is moderate confidence for the ceilings to
lift as indicated in the impacted TAFS given the March sun angle
and drier air arriving from the north. Confidence is low for
fog/stratus coverage overnight and early Monday 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        30  49  35  52  33  50 /  10  20  20  40  10  60
Coeur d`Alene  28  46  33  47  31  47 /  10  30  30  60  10  50
Pullman        31  47  35  48  31  49 /  20  50  50  70  10  50
Lewiston       36  54  39  55  36  56 /  40  40  50  50  10  40
Colville       31  49  31  52  29  47 /   0  10  20  40  10  60
Sandpoint      26  44  33  44  32  45 /  10  20  30  80  30  60
Kellogg        26  44  36  43  32  47 /  30  50  50  80  20  50
Moses Lake     34  57  35  59  35  52 /   0  20  10   0  10  70
Wenatchee      38  54  37  56  36  47 /   0  10  10   0  10  70
Omak           33  57  36  59  35  51 /   0  10  10  10   0  70

&&

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

$$


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