246
FXUS66 KOTX 262236
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
336 PM PDT Sun May 26 2024

.SYNOPSIS...
Dry and mild conditions on Monday with temperatures warming back
into 70s and 80s. Tuesday will see a return of showers and
thunderstorms to Eastern Washington and Idaho Panhandle. A few
storms could be strong with gusty outflow winds. Cool and showery
conditions continue through the end of the week with drier
conditions at the start of the upcoming weekend.


&&

.DISCUSSION...

Tonight through Monday night: The Inland NW will continue to see
relatively benign weather, but some changes will start to make
themselves known toward the end of this period. Tonight a weak
shortwave will exit through northeast WA/north ID by late evening
while a weak ridge starts amplify over the region into Monday.
Isolated shower chances near the Cascade crest and Canadian border
will wane through this evening. Skies will be partly to mostly
cloudy this afternoon and early evening before starting to thin at
least a bit into Monday. Winds will wane through sunset into
early Monday, then increase a little bit again Monday afternoon
near the Cascades/Okanogan Valley. Monday night the offshore
trough starts to pivot toward the region and moisture starts to
increase ahead of it, along with some elevated instability. This
will lead to thickening clouds, especially heading into late
Monday evening into the overnight. Shower chances will also start
to develop from the south-southwest overnight through the Upper
Columbia Basin, Blue Mountains and Palouse up toward western
Spokane county. Some guidance is more robust with this potential,
with locally heavier downpours possible with that elevated
instability and possible embedded t-storms. However confidence is
not high enough at this point to put t-storms in the official
forecast, but it will be monitored. High temperatures Monday will
be slightly warmer than normal in the 70s, with a few low 80s in
the deep basin and L-C valley.

...STRONG THUNDERSTORMS POSSIBLE TUESDAY AFTERNOON AND EVENING...

Tuesday and Tuesday night: A robust system will move into the
region this period, with showers and the potential for strong to
isolated severe t-storms. As of this afternoon the Storm
Prediction Center (SPC) is highlighting a Marginal Threat of
severe thunderstorms for Tuesday afternoon and evening over the ID
Panhandle. They have a general thunderstorm potential painted
over north-central and the east third of WA; however, that does
not entirely preclude these areas from seeing some stronger
t-storms too. The highest risk for stronger t-storms will be late
Tuesday afternoon and evening (about 1 pm to 8 pm). The largest
potential impact with the strong to severe thunderstorms that
might develop will be wind gusting near 35 to 60 mph. This could
cause localized damage. Other impacts would be dangerous
lightning, small hail and heavy downpours. With the heavier rain
we will monitor for any ponding of water and debris blows/mudslides
including near any recent burn scars or steeper terrain. While
the storms are expected generally be moving quickly, we will have
to monitor for any that may repeatedly track over the same areas
and increase hydrology concerns.

As it looks now here is the overall timing of precipitation.
Tuesday morning lingering scattered showers and isolated embedded
weak t-storms will be possible over the Columbia, Palouse the
Spokane/CdA area into the northern mountains, tracking north-
northeast. Chances will continue near the Cascade crest too.
Tuesday afternoon the next round of showers and t-storms are
forecast to develop. There may be two focus areas starting (1) a
line near the Okanogan Highlands to the northern basin after
midday with the cold front and (2) developing southwest of the
Blue Mountains. By late afternoon to evening these two respective
areas increases in coverage and strengthens as they tracks east-
northeast over more of the CWA. The primary area that has likely
PoPs or higher is over the Cascade crest, the NE WA and ID
Panhandle mountains south toward the Blue Mountains in the late
afternoon and evening. Elsewhere PoPs are in the slight to chance
category, meaning a more isolated to scattered shower/t-storm
threat. The lower lee of the Cascades has the lowest risk. Heading
into the later evening and overnight the precipitation potential
largely shifts into the mountain zones, with the highest risk
lingering at the Cascade crest and over the central ID Panhandle
to the Camas Prairie.

Outside of the t-storms winds, the general synoptic winds will
increase Tuesday afternoon into the evening and gradually decline
later overnight into Wednesday morning. Speeds of 10-20 mph with
gusts of 20-35 mph will be possible.

With the details outlined here is a look at the ingredients for
Tuesdays system and some caveats that make precise coverage still
tricky. At the start of the day the areas will be in southwest
flow with the parent trough off the coast and the cold front
pushing into the Cascades. Moisture will deepened ahead of it to
provide fuel for the convection, with PWATs around 0.75 to 1.0
inches (or 140-160% of normal). Surface dew points increase to the
upper 50s to mid-50s, highest over the eastern third of WA and
ID. Between 18Z Tuesday-03Z Wednesday, SBCAPE values between
100-1500 J/kg expand over portions of eastern WA and ID. The
higher end of that range is found over the WA/ID border eastward
the MT border. MUCAPE values between 1500-2500 J/kg are found
around the central Panhandle southward. Additionally 0-6km bulk
shear values average around 50kts, which helps develop more
organized t-storms. With the cold front/upper trough moving toward
the eastern third of WA and ID toward the late afternoon and
evening, all these things converge to bring this stronger t-storm
potential. A potential inhibiting factor is cloud cover and
temperatures. Highs are forecast to be in the 70s to low 80s
(similar to Monday). However the cloud cover increasing of the
system may inhibit temperatures from reaching their full potential
and also may limit the overall convective potential. With that
said, the upper trough/cold front look like decent kickers as are
other ingredients. Shower/t-storms are likely in the region, but
we will have to continue to fine-tune precise coverage and timing.
/Solveig

Wednesday through Friday: A trough will be established over the
region on Wednesday delivering cooler than normal temperatures
and continued threat for showers and thunderstorms. Initially,
these will be more of the hit or miss variety forming with the
assistance of afternoon heating however by the late afternoon and
evening, several deterministic models are hinting of an embedded
shortwave rounding the base of the trough. This could create more
organized bands of showers and embedded thunderstorms if it comes
to fruition. These would be more of the weak variety storms with
lightning, brief downpours of small hail/rain, and wind gusts up
to 30 mph as the main threats. It will be cold enough aloft to
support snow down to 4000 feet so those venturing in the
backcountry should dress accordingly. Our heaviest cells will be
capable of a quick 0.25" of rainfall but this should be close to
the higher limits given the lack of shear and PWATS decreasing
near 0.50"

The trough begins to drift eastward on Thursday with shortwave
ridging moving toward the WA Coast near 130W yet there will be
enough cool air aloft for one more afternoon of convective showers
and weak thunderstorms. By Friday, consensus of the ensembles is
for the ridge to continue to shift inland with the ridge axis
nearing the Cascades Friday evening. Can`t rule out one more day
of afternoon showers in the Idaho Panhandle but this comes with
moderate uncertainty and National Blend of Models are trending
toward drier conditions.

Blustery west to southwest winds will continue on Wednesday.
Sustained winds across the Columbia Basin and through the Cascade
Gaps will remain in the 15-20 mph range with gusts up to 30 mph.
Outside these windier locations, winds will be more in the 5-15
mph range with gusts 20-25 mph. Winds decrease further on Thursday
but still come with some breezes up to 25 mph in the lee of the
Cascades and lower Columbia Basin. Winds continue to wane Friday.

Temperatures start off in the 60s Wednesday then 60-70s for Thursday
and Friday with readings increasing a few degrees each
consecutive day. The combination the cool trough and drier air
will bring another concern for frost to our sheltered northern
valleys. Thursday morning looks to be the coldest morning. Here
are probabilities for Thursday morning lows of 34F or colder at
communities most vulnerable:

Winthrop - 15%
Republic  - 40%
Colville - 40%
Priest Lake area - 50%

Next weekend: Forecast leans toward a warming and drying pattern
under the influence of a ridge of high pressure. Closer
examination of the 100 member ensemble suggest there are
differences with the amplification of the ridge and duration over
the Inland Northwest. There is high confidence for mild
temperatures but uncertainty if readings will remain in the
70s-80s or just 80s. The placement of the ridge will have
ramifications for the extent of cloud cover and potential for
increasing rain chances by Sunday into Monday. The ensembles are
nearly split 50-50 with the differing solutions lending low
confidence for which scenario will play out. /sb

&&

.AVIATION...
00Z TAFS: One disturbance exits and high pressure builds in
overnight into Monday. Isolated showers near the Cascade crest
and northeast mountains wane after 02-06Z. Otherwise look for
BKN-OVC middle to high clouds this evening, starting to thin
overnight then remaining variable into Monday. VFR conditions
expected.

FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS:
There is high confidence for VFR conditions to continue across
the TAF Sites.

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

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        45  74  51  76  45  63 /   0   0  10  20  20  20
Coeur d`Alene  45  73  51  75  47  61 /   0   0   0  20  30  30
Pullman        43  73  50  73  45  60 /   0   0  10  20  50  10
Lewiston       49  81  57  81  53  69 /   0   0  10  20  50  10
Colville       38  72  44  73  38  62 /   0   0  10  40  40  50
Sandpoint      43  71  48  73  45  58 /   0   0   0  40  60  50
Kellogg        46  71  53  75  48  57 /   0   0   0  30  70  40
Moses Lake     45  79  52  78  45  69 /   0   0  10  20   0  10
Wenatchee      48  77  55  73  47  64 /   0   0   0  10   0  10
Omak           43  76  50  76  43  68 /   0   0  10  20  20  20

&&

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

$$