392
FXUS66 KOTX 271133
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
433 AM PDT Sat Apr 27 2024

.SYNOPSIS...
The weekend and early next week will feature showery weather over
the mountains with seasonal like temperatures. Sunday and Monday
have the potential to be windy with the passage of a frontal
system. This system will also bring snow to the Cascade mountain
passes Sunday night and Monday morning. Temperatures begin to warm
mid next week onward, but showers remain in the forecast as well.

&&

.DISCUSSION...

Today through Monday: The next few days will feature breezy to
locally windy conditions as several shortwaves swing across the
PacNW. The first shortwave is evident on water vapor near 130W
this morning and will arrive today. This wave is encountering
split flow which will split its energy into Oregon and along the
Canadian Border. A second shortwave arrives Sunday but will only
offer a glancing blow with majority of its energy once again
deflected along the Canadian border as midlevel flow in buckling
to the southwest in response to the third most compact shortwave
diving toward the region. This third, most aggressive system, will
arrive Sunday night and Monday. Each of the waves will force
pressure falls on the lee side of the Canadian Rockies while
cooler, maritime air piles up west of the Cascades creating
surface high pressure. This will setup a tight pressure gradient
across the forecast area delivering the multi-day winds. On
average, the pressure gradient from say Portland, OR to Kalispell,
MT will average around 7-8mb for Saturday and Sunday with winds
generally blowing out of the south and southwest. On Monday, the
pressure gradient increases toward 12 mb. In general, wind speeds
will be as follows: Saturday- South/Southwest 10-15 gusts to
25-30 mph; Sunday - South/southwest 15-20 mph G 30-35 mph; Monday-
Southwest becoming West/Northwest 15-25 mph G 30-40 mph. There
will be a few areas with locally stronger speeds including the
Waterville Plateau, exposed ridgelines of the Cascades, and our
wind prone areas in the foothills of the Blue Mountains.

We cannot rule out the threat for patchy blowing dust but this comes
with very confidence and is very hard to predict, especially this
time of year. Thinking Monday will carry the main risk in the
Western Columbia Basin around Moses Lake and Othello due to areas
most active with planting.

Precipitation will come in the form of showers with a few weak
thunderstorms.  Showers on Saturday and Sunday will mainly focus
over the Cascade Crest and into the rising terrain of Northeastern
WA and North Idaho. Showers will expand at times into the
northern Basin for Saturday and Sunday. On Monday, most areas will
be vulnerable to precipitation given the stronger forcing. Not
overly excited about the lightning threat but tough to call it
none with 100-300 J/kg of CAPE each afternoon. Greatest risk on
Saturday will be over the northern mountains. T-storms have been
removed on Sunday then including for much of the Idaho Panhandle
and eastern third of WA for Monday.

Precipitation type will mainly be rain in the lowlands and mix of
rain/snow in the mountains through Sunday. Sunday night and
Monday, snow levels come crashing down closer to 3000 feet or
slightly lower. Would not rule out some wet snow mixing down to
2500 feet at times. The main impacts will be snow returning to the
mountain passes Sunday afternoon through Monday. My main concern
for any impacts will be after sunset Sunday into Monday morning
when 2-3 inches will be possible for Stevens Pass. This comes with
a 30% chance of occurring. Washington Pass is expected to receive
1-2 inches with similar amounts for Lookout Pass. Motorists
planning overnight trips across the passes should be cautious of
winter travel conditions.

Temperatures will be a few degrees cooler today vs Friday with
little changes into Sunday. Readings will be close to 30-year
averages or in the upper 50s to mid 60s. Temperatures will drop a
handful of degrees on Monday as the air mass cools roughly 4
Celsius topping out in the 50s for all locations. If clouds clear
and winds decrease Monday night, there will be a good chance for
freezing temperatures Tuesday morning. /sb

Tuesday through Friday...The extended forecast remains uncertain
with a division in ensemble predictions beginning Wednesday night.
For Tuesday through daytime Wednesday, the atmosphere remains
unsettled with afternoon showers and the potential for isolated
lightning strikes over mountainous terrain and far Eastern WA.
Temperatures Tuesday night will drop into the mid 30s, leaving the
area with the potential for frost. Those with sensitive plants may
want to keep an eye on Tuesday morning lows near freezing within the
mountain valleys and across the upper Basin. Showers will return
Wednesday afternoon, but the coverage will remain over the
mountainous terrain. Winds will increase each afternoon, with gusts
of 15 to 20 mph, before decreasing each evening.

Beginning Wednesday night, ensembles begin to differ. Some ensembles
continue with a troughing pattern and the arrival of a shortwave
ridge, while others slide a ridge into the region. If the ridge
remains off the coast, temperatures will be cooler than the current
forecast with the region remaining in the trough. But confidence is
increasing that ridge will shift onshore, warming temperatures more
than the current forecast by the end of the week. Afternoon will
also contain enough instability that could allow for spring showers
to pop up. Details remain uncertain on specifics this far out, so
stay tuned. /KM

&&

.AVIATION...
12Z TAFS: One shortwave is traversing the region this morning
producing bkn-ovc skies across the northern mountains and Idaho
Panhandle and spotty light showers. A second shortwave arrives 17-
04Z and combined with afternoon heating will result in expanding
ceilings between 5-8k ft AGL region-wide along with widely
scattered light showers. A few cells in the afternoon and early
evening will be capable of a slightly deeper depth producing
heavier downpours and lcl ceilings as low as 3000 ft AGL. This
also comes with a 10-20% chance for lightning strikes with the
focus north of a line from Coeur D Alene to Omak. Another area
that will need to be monitored for lightning strikes is south of a
line from Lewiston to Mullan. Winds will increase this afternoon
with speeds of 10-15 mph across the Columbia Basin and Okanogan
Valley. Gusts will be in the vicinity of 25 mph. Showers decrease
after 03-04z for most sites but a completely dry forecast is not
expected given the unsettled nature of the upper-level trough
over the region.  /sb


FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS:
There is a small threat for scattered to broken MVFR stratus
developing this morning 15-18z between Pullman and Spokane. This
comes with low confidence to impact the terminals but HREF carries
a 50% prob of MVFR cigs for areas in between. Confidence is low
for lightning strikes. SREF has a 5-10% chance for general
thunder; HREF keeps the best chance along the Canadian Border but
produces enough afternoon CAPE to keep from removing in the
general forecast. Given the low probabilities, did not mention in
the TAF forecasts. As mentioned above, will need to keep an eye
south of Lewiston for cells developing around the Camas Prairie
capable of enough growth for lightning strikes.  /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        59  40  58  40  53  33 /  40  30  10  10  60  10
Coeur d`Alene  57  41  55  38  50  32 /  40  40  40  20  80  30
Pullman        56  39  56  38  48  33 /  20  10  20  30  80  30
Lewiston       63  46  64  44  56  38 /  20  20  10  20  60  30
Colville       57  34  58  38  52  30 /  70  60  30  30  70  20
Sandpoint      56  41  52  39  48  32 /  70  70  70  50  90  40
Kellogg        54  43  52  36  45  32 /  70  40  70  50  90  50
Moses Lake     64  39  64  42  58  36 /  20  10   0   0  20   0
Wenatchee      60  42  61  39  55  37 /  10  10   0  10  10   0
Omak           62  39  63  38  58  33 /  40  10   0  10  30  10

&&

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

$$