312
FXUS66 KOTX 220456
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
956 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...
Evening update: areas of mostly light rain continue to spread
over the region at this hour and it is expected to persist over
much of the eastern third of WA and ID tonight and Wednesday. The
main low will continue to drop south into western WA and
eventually near central WA/OR border tonight into Wednesday
morning and moisture wrapping into this features will wrap the
precipitation back north and west too, before that precipitation
also starts to expand back into central WA, before things start to
decrease throughout the region later tomorrow evening/early
overnight. Some enhanced bands of precipitation are expected near
the Cascade crest later tonight into the overnight, with some of
that falling as snow in the passes such as Stevens and Washington.
/Solveig


/PREVIOUS 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...
06Z TAFS: A moist system will bring MVFR/IFR conditions with rain
over the east third of WA and ID, including GEG/SFF/COE/PUW/LWS.
Some patchy fog/mist is possible toward the early morning too. The
precipitation wanes briefly around TAF sites in the morning, but
increase in the afternoon through evening at all TAF sites as the
low tracks just south of the area, before starting to pull away
later Wednesday evening. There will be a small risk for t-storms
in the afternoon, with cig/vis improving to MVFR/VFR.

FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS:
High confidence in MVFR ceilings, decreasing to IFR ceiling
overnight/early Wednesday, then moderate to high confidence in
ceiling improving to MVFR and then VFR after 17-20Z.
-----------------------

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.
&&

$$