Area Forecast Discussion Issued by NWS Spokane, WA
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000
FXUS66 KOTX 231730
AFDOTX
Area Forecast Discussion
National Weather Service Spokane WA
1030 AM PDT Sat Mar 23 2024
.SYNOPSIS...
The mild spring weather will come to a close. Expect cooler and
wetter weather this weekend. As snow levels gradually fall this
weekend minor impacts from winter weather are forecast in the
mountains, as well as north Idaho Sunday morning. An active
weather pattern continues through next week with a series of
storms bringing mountain snow, and mainly rain in the valleys
except some snow at times overnight for the northern valleys.
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.DISCUSSION...
Today through Sunday night: A deep upper level low pressure system
is spinning off of the Oregon coast with the ridge axis centered
over the Idaho Panhandle. Southerly flow across eastern Washington
will pump moisture up into the region. An occluded front is
beginning to push up into central Washington and this will supply
the lift along with upslope flow into the east slopes of the
Cascades. The heaviest precipitation this morning will occur closer
to the east slopes of the Cascades where on the order of 0.25-0.50
inches is expected around Wenatchee, Waterville, Chelan,
Leavenworth, and the Methow Valley. Some areas in the Cascades will
see 20-30% chance of between 0.50 and 1.00 inch. Snow levels will be
right around 4,500 feet and Stevens Pass may see some slushy snow,
but not expecting snow to accumulate on the road itself.
The frontal band will spread across eastern Washington and into the
Idaho Panhandle through the morning hours. Dynamics aloft will spin
up a weak surface low over the northern portion of the Columbia
Basin. This will result in a TRough Of Warm Air aLoft or TROWAL like
feature that extends across the northern portion of the forecast
area by this afternoon into this evening. The surface low will then
migrate essentially across the Highway 2 corridor into the Spokane-
Coeur d`Alene area late tonight into Sunday morning. The forming
deformation band (or TROWAL) will wrap in around this low and begin
to shift east with precipitation being focused into the Idaho
Panhandle by Sunday afternoon.
The most challenging part of this forecast has been snow levels and
snow accumulations in the lowlands. Model ensembles continue to
waver on the potential for snow in the Northern Panhandle. The 00Z
ECMWF Ensemble run shows 2 inches of snow as the 75th percentile at
Coeur d`alene and around 3.5 inches at Sandpoint using a 10:1 snow to
liquid ratio (SLR). A 10:1 SLR probably is a bit high, but there is
a higher than average bust potential here with the most likely
scenario painting the potential for up to an inch of snow an less
for the lowlands across the Purcell Trench. Road temperatures figure
to be too warm for much snow to accumulate, but that could be
overcome if snow falls heavier than anticipated under the
deformation band. Grassy surfaces will have the best potential for
seeing upward of 2 inches to as much as around 4 inches or so.
Sherman Pass and Lookout Pass stand to see the best potential for
snow and winter travel conditions. Snow accumulation over these
passes look to be in the range of 3-6 inches. Confidence is only
moderate in these accumulations right now and holding off on issuing
a Winter Weather Advisory due to this being a marginal event and
lack of confidence at this point. Below are updated probabilistic
guidance on snow accumulations for areas of concern off of the
National Blend of Models (NBM):
*Chance of at least 1" of snow: Sandpoint = 60%, Colville = 15%,
Spokane = 8%, Coeur d`Alene = 30%
*Chance of at least 2" of snow: Sandpoint = 30%, Colville = 5%,
Coeur d`Alene = 10%
*Chance of at least 4" of snow: Sandpoint = 5%, Stevens Pass = 10%,
Lookout Pass = 60%, Sherman Pass = 35%
*Chance of at least 8" of snow: Lookout Pass = 10%
Temperatures will cool below normal for this weekend with highs in
the 40s and 50s. Cooler temperatures will be felt more so across
northeast Washington and into the Idaho Panhandle where more steady
precipitation is expected. Low temperatures over extreme eastern
Washington and into the Idaho Panhandle will be dipping near to and
below the freezing mark, especially as the low pulls away Sunday
night.
There will be breezy north to northeast winds down the Okanogan
Valley and Purcell Trench over the weekend. Expect sustained winds
of 10-20 mph with gusts up to between 25-30 mph. /SVH
Monday and Tuesday...The cool and relatively strong upper level low
which will impact the area this weekend will migrate south and
east of the Inland NW resulting in drier weather, at least for a
good portion of the area. The precipitable water values plummet as
upper level west-northwest flow moves into the area. Despite the
drying, we will be left with relatively unstable atmosphere and
given some daytime heating we will see some light showers develop
Monday afternoon. The best chances will occur over the SE quarter
of WA and into the ID Panhandle with a rain shadow over central
WA. The depth of the instability isn’t sufficient to generate
charge separation and thus no thunderstorms are expected, however
graupel showers certainly are possible given the cool cloud base
temperatures. Although the precipitation chances should taper off
over the evening hours, a weak band of moisture is expected to
traverse eastward across the region overnight and into Tuesday
associated with a weak upper level trough. The trough will lose
steam as it heads to the ID/MT border and it looks like Tuesday
will be the wetter of the two days. Precipitation amounts on
Monday will generally be less than a tenth of an inch. On Tuesday
precipitation amounts will generally range from 0.05-0.15” with up
to a quarter inch possible over the central ID Panhandle. As for
precipitation type most will fall as snow on Monday at elevations
above 3000’ with a mix of rain/snow/graupel below. On Tuesday the
snow levels will rise to 3500-4000’.
On Wednesday there is good ensemble agreement that we will see a
good influx of moisture moving into the area ahead of another
trough digging off the coast. Precipitable water values will rise
up to nearly 150% of normal with strong low-level warm air
advection resulting in widespread precipitation during the day.
The offshore trough will near the coast on Wednesday night into
Thursday morning which will end the precipitation in the lee of
the Cascades overnight. However the precipitation will be slower
to end over the remainder of eastern WA and the ID Panhandle as
upper level cooling will lead to a rapid destabilization of the
atmosphere and numerous showers. There is some question whether
the atmosphere will be unstable enough to support thunderstorms.
Yesterday we saw a handful of lightning strikes with an atmosphere
that was slightly more unstable that we suspect Thursday will be.
As usual the Canadian ensemble members are by far the most
unstable and are dragging the NBM numbers up with it. So for now
we will leave it out of the forecast. Between Wednesday and
Thursday we are expecting a significant amount of precipitation.
Totals will likely range from 0.10-0.20” over central WA with
amounts ranging from 0.20-0.40” over eastern WA and N ID. The warm
air advection on Wednesday will push snow levels up to 4000-5000’
before dropping to 3000-3500’ on Thursday. So even though this is
a wetter system, most of the snow will only impact the higher
mountains. Stevens Pass could see up to a half foot of snow with
slightly lower amounts at Lookout Pass. However I suspect that
whatever falls during the daytime hours will have a hard time
accumulating on the roads.
For Friday into Saturday, yet another upper level trough moves
south to southeast toward the NorCal coast with unstable
conditions remaining over the Inland NW. That should return us to
seeing afternoon and evening showers but nothing significant from
a precipitation standpoint.
For temperatures, we are looking at readings temperatures
remaining slightly cooler than normal for this time of year. Highs
will be in the mid 40s to mid 50s from Monday through Thursday
with a subtle warming by the end of the week. Lows will generally
be in the 30s. fx
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.AVIATION...
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18Z TAFS: Low pressure moving into the region will bring rain and
lowering ceilings down to 2,000-4,000 ft agl through this
morning. Moderate rain at times into the east slopes of the
Cascades may reduce visibility down to 4SM-6SM at times between
KEAT to KOMK. A surface low will spin up near KMWH/KEPH this
afternoon with a deformation band and stratiform precip forming
across the northern mountains of eastern Washington into North
Idaho. This portion of the region will continue to see MVFR
conditions through the afternoon into Saturday evening. The
surface low will push east into the Idaho Panhandle late Saturday
night with stratiform precip wrapping in around the backside of
the low. Snow levels will drop over northeast Washington into the
northern Idaho Panhandle Saturday night with rain transitioning to
snow across the Spokane-Coeur d`Alene corridor (KGEG-KCOE) and
airports north (KDEW-KCQV- KSZT-K65S) Sunday morning.
FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: There is high
confidence of ceilings lowering and MVFR conditions with rain
today. MVFR conditions will impact the KEAT-KMWH-KGEG-KSFF-KCOE-
KPUW terminals today. JW
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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-- End Changed Discussion --
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.PRELIMINARY POINT TEMPS/POPS...
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Spokane 49 34 44 30 49 33 / 70 60 60 10 30 30
Coeur d`Alene 47 32 41 26 46 33 / 80 60 60 10 30 30
Pullman 50 37 45 30 48 34 / 60 40 90 30 50 60
Lewiston 56 40 52 35 54 39 / 60 30 80 30 30 50
Colville 46 32 48 26 50 30 / 90 70 50 10 10 20
Sandpoint 44 30 39 25 44 31 / 90 90 60 10 30 40
Kellogg 47 34 40 28 46 36 / 90 70 80 20 40 50
Moses Lake 57 37 57 35 56 34 / 20 30 20 10 10 0
Wenatchee 52 40 56 36 54 36 / 90 50 10 10 10 10
Omak 50 37 57 35 56 35 / 90 80 20 0 10 10-- End Changed Discussion --
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.OTX WATCHES/WARNINGS/ADVISORIES...
ID...None.
WA...None.
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$$