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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.

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