510
FXUS66 KOTX 212150
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
250 PM PDT Tue May 21 2024

.SYNOPSIS...
A wet low pressure system will bring steady rain across the region
into Wednesday. Snow levels will drop down to pass level Tuesday
night with wet snow accumulations for travel over the Cascade
crest. Another weather system will move in on Friday and Saturday
bringing more unsettled weather into the Memorial Day weekend.
Monday will be the warmest day of the holiday weekend.

&&

.DISCUSSION...

Tonight through Wednesday night: A warm front with a modest plume of
moisture is making its march across the Northwest this afternoon.
Radar mosaic is filling in across the region as stratiform
precipitation develops behind the front. The leading edge is
beginning to move into the Washington Palouse, Spokane Area, and
Northeast Mountains at the 2pm hour. Accumulations will be light but
steady, and soaking rains are expected across much of the region
through this evening. The trailing cold front will move into the
Cascades around midnight tonight. This will shift the moisture plume
eastward with drying occurring in the lee of the Cascades. Snow
levels will crash down to around 3,000 feet in the Cascades at this
time. Slop over moisture across the Cascade crest will result in a
transition of rain to snow for Stevens Pass and Washington Pass. Lee
side cyclogenesis will occur in central Washington with moisture
wrapping around the center of the low across the northern mountains
into the northern portions of the Cascades. This will be the axis
where stratiform precipitation will continue into at least early
Wednesday. A dry slot punching across southeast Washington will turn
off the faucet for the basin into the Palouse and Spokane Area where
a break in precipitation is anticipated. The upper level low with a
minus 27 celsius cold pool will be squarely over the Inland
Northwest by early Wednesday afternoon. Cold temperatures aloft will
promote convection into the afternoon, but the coverage and
intensity of convection will be limited by the cooler temperatures
at the surface. It will also be limited by drier air being entrained
into the region across the southeast portion of the forecast area.
We will then see showers diminishing through the evening on
Wednesday from northwest to southeast. Light northwest flow into the
southern and central Idaho Panhandle will keep showers persisting
there into the overnight hours on Wednesday.

* Rainfall: accumulations have been trending drier with models
  indicating a good punch of dry air moving in behind the cold
  front. We will still see a decent amount of rainfall with much of
  the eastern half of the forecast area expected to pick up at least
  between 0.15 to 0.30 inches. The Okanogan Highlands, Northeast
  Mountains and Idaho Panhandle looks to pick up closer to a half of
  an inch, and some areas potentially picking up near 0.75 inches
  when all said and done. This will all be beneficial rainfall with
  base flows for our streams and rivers running low at this time.

* Snowfall: Main impacts from snow will be for travel over
  Washington Pass and Stevens Pass once the cold front moves through
  after midnight. Total snow accumulations looks to be between 2 to
  4 inches. Road temperatures will start out warm, but snow
  intensity does look to be enough to overcome the warmer road
  temperatures with slushy accumulations possible over these passes
  by early Wednesday morning. Sherman Pass may also see light snow,
  but the coldest air will track south of this pass and may just see
  non-accumulating snowfall.

* Thunderstorm potential: Doesn`t look great for Wednesday afternoon
  due to the cool surface temperatures. It will take breaks in the
  cloud cover for sufficient warming to spark off convection. Best
  potential for a 15-20% for thunderstorms will be over the Pasayten
  Wilderness to the Okanogan Highlands and over the southeast
  portion of the forecast area (Northeast Blue Mtns into the
  Palouse).

* Temperatures: Wednesday will be the coldest day this week. The
  precipitation, cold air advection, and cloud cover will dampen
  our warming with highs generally in the 50s. /SVH

Thursday through Saturday: The Inland Northwest will be under the
influence of upper-level troughing over this multi-day span with
several shortwaves dropping through from Canada. This will not
only keep temperatures in check with readings near or cooler than
average but also keep a threat for showers and thunderstorms just
about every day. The highest precipitation chances will focus over
the Idaho Panhandle and northern mountains but the Basin will
carry at least a 20-30% chance at times. As different shortwaves
drop into the mean trough, they will cool the upper-levels with
500mb temperatures near -21C at times supporting afternoon CAPES
between 200-600 J/kg. This will be sufficient for a handful of
thunderstorms with the main hazards being heavy downpours of rain
and small hail along with lightning strikes. At this time, models
show some of the heaviest rain cores with showers Thursday through
Saturday where pockets of a quarter of an inch or more will be
possible. Although Sunday will keep a mention of showers and
storms, instability parameters will be on the decline as 500mb
temperatures warm suggesting less rainfall with any cells.

85% of the ensemble solutions support a shift in the weather pattern
late Sunday into early next week with a low pressure becoming
reestablished over the Gulf of AK and heights building northward
from the Great Basin into the Inland NW. The exact placement and
amplification of these features carries moderate uncertainty.
Forecast is leaning toward dry conditions for Monday which is
supported by 70% of the ensembles. This decreases to 50% by
Tuesday and further later in the week. There is high confidence
that these upcoming precipitation chances will come with a
moisture trajectory from the southwest indicative of a warmer
environment. This raises some concern that instability parameters
could be on the rise with more thunderstorm activity. Stay tuned
as we continue to dial this period in. /sb

&&

.AVIATION...
18Z TAFS: A moist frontal system will push east of the Cascades
today result in lower ceilings and light rain. Ceilings are
expected to lower down to between 3-6 kft agl. in the lee of the
Cascades for KEAT-KOMK-KEPH-KMWH. Rain moving into extreme eastern
Washington and the Idaho Panhandle later today (after 22Z) will
bring the potential for MVFR conditions tonight with ceilings
down to between 1-4 kft agl and visibility down to 4SM. Low
clouds will continue into Wednesday morning with numerous showers
continuing and a low potential for isolated thunderstorms where
sun breaks occur.

FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: Confidence remains
high for ceilings lowering to MVFR after 02Z Wednesday over
extreme eastern Washington and the Idaho Panhandle. HREF shows at
least a 70% chance of CIGS lowering to MVFR for KGEG-KSFF-KCOE-
KPUW, and there is a 30% chance for IFR conditions developing at
times. For KEAT to KMWH, there is a 80-90% chance of conditions
remaining VFR through the TAF period. /SVH

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

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  56  41  64  44  65 / 100  70  20  20  20  20
Coeur d`Alene  45  54  41  59  44  62 / 100  90  30  30  30  30
Pullman        43  51  39  59  40  62 / 100  80  30  10  10  10
Lewiston       51  61  46  66  47  70 / 100  70  40  20   0  10
Colville       40  56  36  64  39  63 / 100 100  30  70  40  50
Sandpoint      43  52  39  57  44  59 / 100  90  50  60  60  60
Kellogg        45  49  42  56  45  60 / 100  90  70  50  30  40
Moses Lake     42  60  43  72  45  73 /  70  50  30   0   0   0
Wenatchee      45  59  46  71  48  70 /  50  50  20   0   0  10
Omak           43  62  42  73  45  70 /  70  90  20  20  10  20

&&

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

$$