011
FXUS66 KOTX 011200 CCA
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
500 AM PDT Sat Jun 1 2024

.SYNOPSIS...
A chance for light showers will return for the northern mountains
today, but most locations will mainly only see thickening clouds
and possible some sprinkles. A dynamic system pushes across the
region Sunday into Monday, bringing widespread rain, followed by
gusty winds, and the potential for thunderstorms on Monday. Next
week will see a warming and drying trend. Confidence is increasing
for temperatures warming into the 80s and 90s.

&&

.DISCUSSION...

...DYNAMIC SPRING STORM TO BRING WIDESPREAD RAIN SUNDAY INTO MONDAY
FOLLOWED BY A CHANCE FOR THUNDERSTORMS AND WINDY CONDITIONS...

Today and Tonight: A weak shortwave trough of lower pressure will
push across the Inland Northwest today. Clouds will increase and
thicken over much of the region. Moisture will be lacking with this
precursor shortwave though ahead of the "big show" spring storm
system over the following 48 hours. Best chances for showers (at 30-
50%) will be across the northern mountains. Otherwise, most valley
locations and the basin will largely only see the potential for
sprinkles. There is a 20-25% for thunderstorms near the Canadian
border today with this weak system. A westerly pressure gradient will
tighten across the Cascades late this afternoon into this evening.
Winds will increase into the Wenatchee Area out of the Wenatchee
River Valley, on the higher benches of Wenatchee and Chelan, on the
Waterville Plateau, and into the western portion of the basin; these
areas will see the potential for wind gusts of 25-35 mph into the
evening.

Sunday through Monday night: A dynamic spring storm system remains
on track to bring the potential for moderate impacts to portions of
the region. There are three variables of note that will be monitored
as this storm system approaches: (1) rain, (2) wind, (3)
thunderstorms. A strong 150+ kt jet streak will be directed into
the Northwest. The upper level trough will take on a slight
negative tilt and the timing of this feature during the day on
Monday could play a big role in organizing convection across the
Inland Northwest. It will also bring with it a moderate strong
Atmospheric River (AR 2 to 3 level event) that will be focused
more so across the state of Oregon, but a good portion of the
moisture will penetrate across eastern Washington and into the
Idaho Panhandle. Ensembles have sped up the arrival of rainfall
with the AR moving in during the day on Sunday. The earlier timing
makes sense as the strong westerly jet should act to bring in the
moisture plume fairly quickly. Widespread stratiform rain will be
upon the region by Sunday afternoon. Strong westerly flow across
the Cascades will keep rainfall amounts down in the lee of the
Cascades with the focus of heavier rainfall along the Cascade
crest and in the favorable upslope areas of the Idaho Panhandle
(see rainfall section for amounts and probabilistic guidance).

The next potential for widespread impacts will be from the winds
that pick up as the rain shuts off, or becomes convective late in
the morning into the afternoon and evening. The convective element
with the cold front passage increases the potential for moderate
impacts with winds. About 1/4 of the ECMWF ensemble members
indicate the potential for 50 mph wind gusts on the Spokane West
Plains and on the Palouse. This is largely due to a deep mixing
layer right as the mid level cold front clears the area and the
surface front moves through. Deterministic model guidance shows up
to 300 J/kg of surface based CAPE as early as 10AM-12PM late
Monday morning. Potential is increasing for convection to form
along the cold front with 25-45 kts of unidirectional shear in the
0-6 km layer. A CAPE of near 300 J/kg with this shear profile
would be sufficient for convection to organize along the cold
front. There may not be a lot of lightning due to relative low
Equilibrium Levels (EL) up to around 20 kft, but strong dynamics
with the advancing vorticity maximum at the base of the upper
level trough should be sufficient to make up for the lack of
instability. The NAM is also showing around 500 J/kg of downdraft
CAPE. Putting it all together and there is good potential for
enhanced winds with convection that forms by late morning into the
afternoon.

* Rain Amounts: This is the part of the forecast where confidence is
  highest. Overall, precipitation accumulations have increased a bit
  since 24 hours prior. In general, the Cascade crest is looking at
  storm total rainfall of between 1.75-3.0 inches south of Lake
  Chelan and between 1.0-1.75 inches north of Lake Chelan. Places
  such as Omak, Wenatchee, and Moses Lake will see downsloping off
  the Cascades with much less in the way of total accumulations in
  the range of 0.05-0.20 inches. Rainfall amounts will then
  precipitously increase over extreme eastern Washington and in the
  Idaho Panhandle with around 0.5-1.0 inches expected for Colville,
  Spokane, Pullman, and Lewiston. Then between 1.0-2.0 inches
  expected in the Idaho Panhandle with the westerly slopes of the
  higher terrain likely wringing out the most moisture and may
  eclipse the 2 inches mark. A near certainty that the Cascade crest
  will see at least an inch and around an 80% chance for at least
  1.5 inches. The Idaho Panhandle has a 30-60% chance for at least
  an inch and around a 10% chance for more than 1.5 inches.

* Winds: Increased wind speeds a bit for the afternoon. Synoptically
  driven winds in the absent of convection are expected to be
  sustained 25-30 mph with gusts of up to 40-50 mph across the
  exposed areas of the basin. This will includes in the lee of the
  Cascades across the Wenatchee Area, into the Moses Lake Area,
  Spokane Area, Palouse, and lower Asotin and Garfield County. Winds
  look strong enough that a wind advisory may be needed if models
  continue their trend upwards over the weekend.

* Thunderstorms: As previously mentioned, the potential for
  thunderstorms will be greatest with the surface front from late
  morning into the mid afternoon hours. This will be the period of
  the best combination of moisture, instability, and forcing -- the
  three ingredients need for convection. Instability will be a
  limiting factor, but the combination of dynamics along the cold
  front and aloft should be enough to overcome this deficiency.
  Model trends have increased the potential for organization of
  convection with cold front passage.

* Impacts: Best potential for moderate impacts will be with any
  thunderstorms that develop because they will be capable of
  becoming organized and would enhance the potential for wind
  gusts at the surface. It`s possible that in this scenario that
  we would see 50+ mph wind gusts with confidence increase a bit
  for this scenario. It will be interesting to see how Convective
  Allowing M models (CAMs) depict this potential as we get that
  data for Monday on Saturday. There is the potential for moderate
  impacts with just the synoptically driven winds with the passage
  of the cold front as well. Difficult travel is expected for high
  profile vehicles. Winds will also have the potential for light
  weight objects to become airborne. See the "Hydrology Section"
  below for hydro impacts. /SVH

Tuesday...Ensemble model guidance is in fair agreement that the
Inland NW will be under zonal flow with a 120kt westerly jet poised
over southern BC. The jet is also expected to push a weak system
over the Cascades in the morning and into north Idaho by afternoon.
This system will be much weaker than the Sunday/Monday one. It will
also feature a sizable rain shadow given persistent moderate
westerly flow in the mid-atmosphere. This will me especially
apparent in the afternoon as moderate cold air advection develops at
700 mbs in the lee of the Cascades. Rainfall amounts will generally
be a tenth of an inch or less. The instability wont be sufficiently
deep for thunderstorms, or at least it looks like that at this
point. It will also remain breezy on Tuesday, however nothing like
the speeds forecast for Monday. We should see wind gusts of 30-35
mph in the afternoon across portions of central Washington.

Wednesday-Friday...There is very good ensemble agreement that a
strong ridge will develop by midweek over the entire Western US in
response to a deep trough forming between 140-150w. The result will
be a significant warming and drying trend. 850 mb temperatures will
climb from the 8-12C range on Tuesday to the 20-23C range on Friday.
This translates to highs in the warming from the mid 60s to mid 70s
on Tuesday to the 80s and potentially the lower 90s on Friday.
However by Friday there are some significant model differences. Most
of the GFS ensemble solutions move the aforementioned offshore
trough nearly to 130w on Friday. This still results in similar
temperatures, however it also introduces a small chance of showers
and thunderstorms, mainly for the SE corner of WA into NC ID.
Cluster analysis suggests this scenario is supported by 75% of the
GFS members and around 30% of the Canadian members. Meanwhile a dry
solution is supported by all of the ENS solutions.

Saturday and Sunday...Model differences continue between the
unsettled GFS solutions (nearly 60% of its runs), and the much drier
and warmer ENS solutions. If the latter models verify it would
remain dry through and very warm through the weekend. By Sunday
there the models are not in good agreement. The ENS solutions have a
578 dm ridge centered over the Cascades, Canadian solutions push the
ridge into western MT and the GFS runs have a much weaker ridge over
central Montana. So needless to say, confidence is not high, however
the majority of the ensembles support it being dry next weekend. As
for temperatures there is a wide variation in solutions. The
Canadian runs have 850 mb temperatures in the lower to middle 20s
which would support highs in the 90s across most valley locations on
Saturday with a slight cooling for Sunday. Meanwhile the GFS runs
are nearly 10F cooler with an increasing chance of precipitation
across the eastern third of WA and the ID Panhandle on Saturday
expanding across the northern mountains and Cascades on Sunday. So
stay tuned and hopefully we will see some better model agreement as
we get closer in time, but right now odds favor a very warm and dry
scenario. fx

&&

.HYDROLOGY...
There is a Hydrologic Statement in effect focusing on rises to
stream, creeks, and some mainstem rivers. Base flows remain low
across the region, and a good portion of the Inland Northwest is
within a moderate drought (including the Cascades) and even a severe
drought in the central portion of the Panhandle. Due to the ongoing
drought conditions, the rain that is expected will largely be
beneficial. With that said, the amount of water with a little bit of
snow left to be melted in the highest elevations of the Cascades
will result in a steep rise on the Stehekin River. Flooding is not
anticipated at this time but will need to be monitored. Another
potential impact will be for rock slides on steeper slopes in the
Cascades and across the Idaho Panhandle. /SVH

&&

.AVIATION...
12Z TAFS: A shortwave trough of lower pressure will pass across the
Inland Northwest today. Clouds will lower and thicken this morning;
however, ceilings will largely remain above 6 kft with VFR
conditions. Best chances of showers today will be across the Cascades
and over the northern mountains. A chance of sprinkles is possible
at KPUW-KGEG-KSFF-KCOE. There will also be a 20% chance for
thunderstorms near the Canadian border this afternoon. Thunderstorms
are expected to remain north of airports from the Methow Valley,
Omak, Colville, and Sandpoint. Winds will pick up out of the west
into KEAT Saturday afternoon and evening with sustained winds up to
15 knots and gusts up to 25 knots.

FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: High confidence in
VFR conditions and thunderstorms remaining north of TAF sites. /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        72  49  64  52  63  43 /  20  10  90 100  90  20
Coeur d`Alene  69  48  63  51  59  43 /  20  10  80 100 100  30
Pullman        69  48  64  51  59  41 /  10  10  80 100 100  20
Lewiston       77  55  72  57  69  49 /  10  10  70 100 100  20
Colville       69  41  64  45  63  36 /  30  20  90 100  90  30
Sandpoint      66  47  61  50  57  41 /  20  20  90 100 100  60
Kellogg        68  51  61  52  56  43 /  10  20  80 100 100  60
Moses Lake     78  49  67  53  70  46 /  10   0  80  80  50  10
Wenatchee      75  54  65  54  65  48 /  10   0  90  70  50  20
Omak           75  48  67  51  69  44 /  20   0  90  80  60  10

&&

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

$$