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FXUS66 KOTX 041758 AFDOTX

Area Forecast Discussion National Weather Service Spokane WA 958 AM PST Mon Dec 4 2023

SYNOPSIS
Light preciptiation will spread into the region today. A wintry mix will be possible in Central and Northern Washington, otherwise rain is expected. Mild and wet weather will be in place Tuesday and Wednesday with a significant snow melt expected in the valleys and potential for minor flooding issues. Cooler temperatures return Thursday into next weekend with several opportunities for snow.

DISCUSSION

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

Today: The focus on today's weather will be an incoming warm front delivering the leading nose of a well advertised atmospheric river (AR). Prior to the moisture arriving, there are a few items to address. 1) Showers are ongoing over the Camas Prairie, Blue Mountains, and Central Panhandle Mountains. These showers are light and having minor impacts mainly due to surface temperatures in the upper 30s to lower 40s so rain will be the main p-type.
Look for these showers to wane by sunrise. 2) Warmer temperatures and snowmelt has created an very moist boundary layer with widespread low clouds and dense fog. With visibilities near or below a 1/4 mile from Spokane to Ritzville to Moses Lake and in some outlying areas, we have issued a dense fog advisory through 10AM though timing carries uncertainty and would be surprised if some zones are removed earlier if conditions warrant.

Going into Monday morning, attention will shift toward a band of light precipitation coming in from the west/southwest reaching the Cascades/SW Basin around 7-8AM and slowly traversing to the northeast reaching the ID/WA border around midday. Latest guidance is tracking slower than previous runs and this is good news for potential impacts from wintry precipitation. Thermodynamic profiles continue to support a mix of rain/freezing rain/snow for Central and Northern WA however the later the precip arrives, the less the impacts would be from any freezing rain with surface temperatures getting a few hours jump start on warming. We are also observing a very chaotic temperature regime tonight courtesy of the low clouds and fog that has developed. Locations like Moses Lake, Ephrata, and Wenatchee are right near the freezing mark.
Where we are void of low clouds such as the Methow Valley and Waterville Plateau, temperatures have plummeted to near 20F.
Within the northern mountain valleys, temperatures vary from 36F at Omak to 30F at Colville to 36F at Deer Park and will likely not change very much given the presence of low stratus. Needless to say, it will be a challenging forecast with multiple precipitation types before switching over to rain in the afternoon and thinking the greatest risk for light ice accumulations will be over the Waterville Plateau and Methow Valley where temperatures should be solidly below freezing as the precipitation arrives. By Monday night, snow levels will be surging near 8000 feet or higher as the AR begins to lay into the region.

Tuesday-Wednesday: The atmospheric fire hose will be aimed tight into the Northwest through this period delivering 48 hours of mild temperatures and periods of moderate to heavy precipitation.
The axis of moisture will take on a southwest to northeast orientation and wring out the heaviest precipitaiton across the Cascade Crest, rising terrain of the Northeastern WA, and North Idaho. Pend Oreille, Boundary, Bonner, and northern portions of Shoshone County carry 100% of receiving over an inch of rainfall.
Probabilities reduce near 40-70% into Stevens, Spokane, Whitman, Kootenai, Benewah, Latah, and southern Shoshone Counties. Rainfall amounts drop off considerably across the L-C Valley and Camas Prairie. Moderate to heavy precipitation is also likely to expand into the higher terrain of southern Ferry and Stevens Counties north of the Columbia River due to strong orographic ascent into these areas. Although ensembles only carry 20-40% chances for greater than inch, forecast rainfall amounts have increased considerably and would imagine these probabilities to spike in the next run or two. Moderate rains will also expand into the Basin at times but not on a consistent basis so areas that pick up over an inch will be localized and the 20% probabilities seem to represent this well.

Overall, we are looking at 2-3" of rainfall for North Idaho and Northeastern WA, 1-2" for Spokane to Wilbur and into the Central Panhandle Mountains, tapering off to less than an inch for the Palouse and lower Columbia Basin and less than half an inch for the L-C Valley, Camas Prairie, Wenatchee, Chelan, and Omak.

The Cascade Crest will be a different story and intercept a bulk of the moisture. Many areas will be in the 3-5" range near the crest with locally near 6". Slop over into the upper river valleys will be a fraction of this but still in the 1-2 inch range.

Flooding of low lying areas and/or urban areas with poor drainage are a major concern. It seems very reasonable to state that nearly all streams,creeks and rivers will experience some rises but river models do not forecast flooding on any main stem rivers.
The recent snow-pack is holding anywhere from 0.30-0.80" of additional moisture that will contribute to runoff due to temperatures warming above freezing and remaining there through Wednesday night. Current forecast has a few record warm overnight lows for Wednesday such as Omak at 38F and Spokane at 42F. There will also be an increase risk for rock slides in steep terrain.

A cold front will sweep through Wednesday night bringing an end to the precipitation and ushering much cooler temperatures however, delayed runoff and any ongoing flooding issues will likely linger well into Thursday. /sb

Thursday through Monday: There is high confidence of wintry precipitation returning to the forecast beginning Thursday. There is high confidence of an offshore PNW trough amongst the ensemble clusters. Snow levels would crash from 8-10 kft Tuesday to 2.5-4 kft Thursday. The lowlands would see a mix of rain and snow with this system, so accumulations are minor. The best chances for accumulating snow are currently in the mountain passes. Here are the probabilities for 4, 8, and 12 inches of snow for the passes from 4 AM Thu - 4 AM Fri:

Stevens: 80%, 45%, 25% Lookout: 65%, 25%, 5%

Fear not, there is a greater chance for valley snow to return Saturday. A blocking high over CA will amplify an anticyclonic jet streak and a surface low forms in the exit region (Gulf of Alaska)
and draws moisture from subtropical latitudes into the region. Snow levels would favor snow mostly everywhere except the Basin.
Additional chances for winter precipitation come again early next week. Keep those shovels accessible and stay tuned. Here are the probabilities for 8, 12, and 18 inches of snow for the passes from 4 AM Sat - 4 AM Sun:

Stevens: 60%, 45%, 20% Lookout: 25%, 5%, 0%

/db



AVIATION
18Z TAFs: Areas of dense fog are continuing to impact portions of the region including the Spokane corridor. It is expected to continue through 21Z. Light precip will accompany a passing front. Precip type in the lee of the Cascades and across the northern mountains from Winthrop to Omak to Colville will be a mix of rain/snow/freezing rain. Temperatures across the Inland Northwest are generally above freezing. Threat of frza is minimal.
A few pockets are possible in the Northern Valleys where surface temps are still sub-freezing. Precipitation chances will wane from SW to NE through the day, but confidence is high the low clouds and IFR/MVFR conditions will persist and potentially lower again before additional precip arrives tonight. IFR/MVFR conditions are expected to last through Tuesday morning.

FORECAST CONFIDENCE AND ALTERNATE SCENARIOS: TAF site temperatures are above freezing bringing moderate confidence of no fzra at these areas. Northern valley locations are still subfreezing keeping moderate confidence of frza at these location. Confidence of improving conditions from LIFR are low as depending on timing of front. Warmer air overrunning snow is a favorable setup for persistent low clouds and even as rain is falling, it is possible for ceilings and vis to continue to lower, especially if rainfall intensities are light. Continued moist boundary layer keeps high confidence of return to LIFR conditions tonight.

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 42 37 48 43 46 35 / 80 100 100 100 90 80 Coeur d'Alene 43 38 46 41 46 35 / 80 100 100 90 90 80 Pullman 44 41 49 42 49 36 / 70 80 80 80 80 90 Lewiston 49 46 55 42 54 40 / 50 40 50 60 70 80 Colville 36 33 43 39 42 30 / 80 100 100 100 90 80 Sandpoint 38 36 45 40 44 34 / 80 100 100 100 100 90 Kellogg 43 40 46 40 47 35 / 80 100 90 90 90 90 Moses Lake 40 37 48 45 49 34 / 70 90 90 70 70 40 Wenatchee 37 36 41 38 43 33 / 70 90 80 60 60 40 Omak 38 35 43 37 42 31 / 80 100 80 60 60 50

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

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

Winter Weather Advisory until 1 PM PST this afternoon for Central Chelan County-Moses Lake Area-Waterville Plateau- Wenatchee Area.

Winter Weather Advisory until 4 PM PST this afternoon for Northeast Mountains-Okanogan Highlands-Okanogan Valley- Western Okanogan County.

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

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