000
FXUS66 KOTX 040023
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
423 PM PST Sun Dec 3 2023

.SYNOPSIS...
Two weather systems, today and again Monday, will bring a threat
for a light wintry mix to portions of the Inland Northwest. Much
warmer and wetter weather will be in place Tuesday and Wednesday
with a significant snow melt expected in the valleys and potential
for minor flooding issues.

&&

.DISCUSSION...

Sunday night through Monday: Precipitation has ended across much of
the region behind the passage of a cold front this afternoon.
Widespread low level clouds will stick around for most of the
region, keeping most areas a bit warmer tonight. The exception will
be for the lee of the Cascades where downsloping winds, courtesy
of the strong zonal flow aloft, brings clearing to the Cascade
valleys. This will allow temperatures to cool more than what the
NBM is suggesting.

Another warm front will lift north across the region early Monday
morning for another period of mixed precipitation. As the warm air
surges north, chances for freezing rain will increase across the
Cascade valleys, the Waterville Plateau Monday morning, with the
threat increasing into the northeast Washington valleys by the
later morning and the afternoon. Cold temperatures in these
valleys will be hard to scrub out at the onset of precipitation
with a couple hours of freezing rain before transitioning to rain
by the late afternoon and evening. Confidence for freezing rain
lowers across the Moses Lake area, where thick cloud cover tonight
may inhibit tonight`s cooling for areas around Moses Lake and
south. We have decided to issue Winter Weather Advisories for
these areas as any ice accumulations on untreated roads and
sidewalks will result in slick and hazardous conditions. Rain
will be the dominant precipitation type for the Spokane/CdA
areas, the Palouse, L-C Valley, and lower Idaho Panhandle.

...MILD AND WET WITH PERIODS OF MODERATE TO HEAVY RAINFALL MONDAY
THROUGH WEDNESDAY...

Monday night through Wednesday night: A moderate atmospheric river
(AR) event will bring the potential for flooding impacts to the
Inland Northwest. A strong warm front will bring warmer, mild
temperatures and an extended period of moderate to heavy rain to
portions of the region. The AR will remain directed over the
Cascades and into the Northeast Mountains and Northern Panhandle for
a 48 hour period resulting in an extended period of wet and
increasingly mild conditions for Monday night through Wednesday
night. The cold front is delayed a bit by 6-12 hours from what
models were showing this time yesterday. The delay with keeping the
AR more so over the northeast portion of the region results in a
little bit less rainfall for places like the Palouse region into the
southern and central Idaho Panhandle.

*Precipitation Accumulations: The National Blend of Models (NBM)
indicates between 4 to 5 inches at the Cascade crest over this
period, and between about 1.0-2.0 inches in the Central Panhandle
Mountains to between 1.5 to close to 3.0 inches over the Northeast
Mountains to the Northern Panhandle. The 90th percentile (or "worst
case scenario") suggests near 6 to 6.5 inches at the Cascade crest
and up 3 to 4 inches for the Idaho Panhandle. The potential for 4
inches is more likely for the Northern Panhandle. These will be the
areas of focus for the heaviest precipitation, and much of this will
fall as rain (even at the highest elevations). Precipitation will
see a transition of snow to rain in the Cascades and northern
mountains on Monday with snow levels starting out around 3,000 to
3,500 feet and increasing above pass levels Monday night. Then we`ll
see snow levels dropping again with cold front passage on Thursday,
but the bulk of this period will be all rain for the most part. The
Spokane Area and Palouse will see the potential for 1-2 inches.
Areas in the lee of the Cascades won`t be completely shadowed out,
but will see significantly less in the range of 0.25-0.75 inches for
the AR event.

*Temperatures: The other factor with this AR event will be the mild
temperatures with highs on Monday warming into the mid 30s to mid
40s, and warming further across the easter two-thirds of the
forecast area into the upper 40s and upper 40s to low 50s by
Tuesday. Low temperatures Monday night look to stay above freezing
except for the east slopes of the Cascades where temperatures look
to hover closer to the freezing level. Temperatures look to remain
above freezing Tuesday night into Wednesday away from the Cascades
as well before the colder air behind the cold front drops overnight
temperatures back down near to or below freezing for Wednesday
night. This will result in two solid days of mild temperatures and
melting snow for at least the low elevations and mid slopes. Extreme
eastern Washington and the Idaho Panhandle in particular will see
mild temperatures that will enhance melting snow at lower to mid
elevations.

*Impacts: The potential for flooding will be a concern as rain
combined with snow melt will result in enhanced runoff. The east
slopes of the Cascades may be okay since a lot of the snow at the
crest will likely act as a sponge to soak up the rain, but even
there will see some potential. There will be a risk for at least
minor flooding impacts such as basement flooding and field flooding.
Ponding of water over roads will be possible, especially for areas
with poor drainage. We will have to monitor Paradise Creek closely
early next week with a forecast of 1.0 to 2.0 inches over a 48 hour
period with snow melting off of Moscow Mountain expected to result
in sharp rises on this creek. A flood watch has been issued for
Chelan County and for extreme eastern Washington into the Idaho
Panhandle. The Flood Watch begins overnight Monday with concerns
decreasing in Chelan county Wednesday night and east into the Idaho
Panhandle Thursday night as temperatures cool again and snow melt
slows.

Thursday through Sunday: A colder shortwave disturbance moves
through Thursday into Thursday night. Snow levels will be down
around 2,500 to 3,500 feet for Thursday afternoon and lower down
into valley floors overnight Thursday as moisture decreases with the
exiting weather system. Then a short break between weather systems
Friday into Friday evening before we see the return of valley snow
late Friday night into Saturday. /SVH


&&

.AVIATION...
00Z TAFs: The main focus for tonight will revolve around an
expansive blanket of low clouds and fog shrouding most of eastern
WA and north ID. This blanket of clouds is expected to persist
through all of tonight so our confidence is high that all the
sites will see MVFR/IFR conditions with ceilings generally below
020 feet. The low clouds will also produce areas of drizzle and
freezing drizzle through the evening. The freezing drizzle will
avoid the TAF sites, but could impact the northern valleys from
Colville to Sandpoint through the evening. Chances for more
significant precipitation will increase by early tomorrow morning
near Wenatchee and Moses Lake and then head into the remaining TAF
sites later in the morning. The hard part of this is determining
what the precipitation types will be. For KEAT and KMWH it seems
there is a moderate chance of freezing rain while the other sites
should see rain. Other pockets of freezing rain are expected in
the lee of the Cascades and in the northern valleys of eastern WA.
All locations should see only light amounts of ice (less than
0.10"). The precipitation chances will wane from SW to NE through
the day, but confidence is high the low clouds and IFR/MVFR
conditions will persist.


FORECAST CONFIDENCE AND ALTERNATE SCENARIOS:
We have high confidence the low clouds and fog and IFR/MVFR
conditions will persist for most of the TAF sites with the
exception of KLWS. The precipitation type forecast confidence is
quite low for KMWH an KEAT. There is also a small chance that when
the precipitation arrives at KGEG tomorrow AM it may consist of
light snow.

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        29  42  35  48  40  44 /  20  80 100 100  90  90
Coeur d`Alene  29  43  37  46  40  44 /  50  80 100 100  90  90
Pullman        34  44  39  47  40  45 /  30  70  80  80  80  90
Lewiston       37  49  44  54  44  50 /  30  50  50  60  70  80
Colville       20  36  28  43  33  41 /  20  80 100 100  90  80
Sandpoint      27  38  34  45  38  42 /  80  80 100 100 100  90
Kellogg        30  43  41  46  41  43 /  70  80 100 100  90  90
Moses Lake     27  40  34  48  39  45 /   0  70  90  90  70  60
Wenatchee      30  36  34  41  33  42 /   0  80 100  90  70  60
Omak           26  36  34  44  37  43 /  10  80 100  90  60  60

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...Flood Watch from Monday evening through Thursday afternoon for
     Central Panhandle Mountains-Coeur d`Alene Area-Idaho
     Palouse-Northern Panhandle.

WA...Flood Watch from Monday evening through Thursday afternoon for
     Northeast Mountains-Spokane Area-Washington Palouse.

     Winter Weather Advisory from 6 AM to 1 PM PST Monday for Central
     Chelan County-Moses Lake Area-Waterville Plateau-Wenatchee
     Area.

     Winter Weather Advisory from 8 AM to 4 PM PST Monday for
     Northeast Mountains-Okanogan Highlands-Okanogan Valley-
     Western Okanogan County.

     Flood Watch from Monday evening through Wednesday evening for
     Central Chelan County-Wenatchee Area-Western Chelan County.

&&

$$