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FXUS66 KOTX 030603 AFDOTX

Area Forecast Discussion National Weather Service Spokane WA 1003 PM PST Sat Dec 2 2023

SYNOPSIS
The next weather system will move into the Inland Northwest early Sunday morning. Much warmer air will push in from the south, gradually turning all the snow to rain, with only high elevation snow expected by early next week. Much warmer but wet weather will continue through the week, with a significant snow melt expected in the valleys.

UPDATE...Update this evening was to remove parts of the Cascades from the winter storm warning as snow was pretty much done for the night. Light snow may continue at times at Stevens Pass, and left that out through 4am. Another round of snow moves into the Cascades after 4 am and then into eastern WA and north ID after 9am. Temps across portions of the Methow and Okanogan Valleys have lowered into the upper teens given the decreased sky cover and light winds. Have also lowered max temperatures for these areas tomorrow. Updated precip type and amounts for Sunday. Looks like for northern WA and ID precip type will be snow through the morning. Also kept precip type as snow for Spokane until 10am and then bring a mixture of rain and snow to the area. Another concern is the potential for freezing rain starting about 7 am and continuing through 1 pm. The areas of concern are the Cascades and adjacent valleys, into Wenatchee, Waterville Plateau stretching across the upper Columbia Basin, northeast valleys towards Priest Lake. There is about a 10% chance of freezing rain for these areas, and ice accumulation would just be a glaze. As far as snow amounts Sunday in the valleys goes...most valleys will see less than an inch. The exception to that will be in the Cascade valleys like Leavenworth and the Methow Valley where they could see 1 to 3 inches of snow. /Nisbet

DISCUSSION
Saturday night through Sunday: The cold front that brought widespread heavy, wet snow to the region this morning has exited to the east, with rising pressure in its wake and a brief period of beautiful, sunny skies for most areas. A resultant +11 to +13 mb pressure gradient from Yakima to Lethbridge, AB is supporting breezy southwest winds across the Inland Northwest with sustained winds 15 to 20 mph and gusts up to 35 mph. Winds peaked around 10AM-12PM this morning at KGEG as the front moved through and will continue to decrease through the early evening. A persistent Puget Sound Convergence Zone (PSCZ) behind the front continues to bring moderate to heavy snow to Stevens Pass which will see an additional 2 to 4 inches of snow through the early evening. Additionally, upslope snow showers will continue across the Idaho Panhandle mountains into the early evening, with winter driving conditions over Lookout Pass.

The next system begins to move into the area late tonight into early Sunday morning as a warm front lifts from the south. Snow levels will rapidly rise through the morning on Sunday as the front lifts, bringing a mixed bag of precipitation types.
Precipitation looks to begin as rain for areas south of I-90 in the Columbia Basin and as a rain/snow mix or all snow north of I-90. As the front lifts, colder air will be harder to scrub out initially in the Cascade and northern valleys, with a brief transition to freezing rain before completely transitioning to rain in the late morning. Use caution when venturing out Sunday morning, as roads will be slippery.

Winds will pick up again Sunday afternoon as the cold front pushes across the region, with the breeziest winds across the Cascades and in the lee of the Blue mountains. Winds on Sunday will be notable weaker than Saturday with areas across the eastern Columbia Basin into the Palouse and up into the Spokane and Coeur d'Alene metros will experience gusts around 20 to 30 mph.

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

Monday through Wednesday: A moderate atmospheric river (AR) event will bring the potential for flooding impacts to the Inland Northwest. Much, if not all, of the low elevation snow that has and accumulate over this weekend will melt as temperatures warm up and snow levels rise. The time extent of the AR is more impressive with this incoming moisture plume more so than the magnitude itself.
Model consensus has the plume pushing in Monday afternoon with a strong warm front, and then the region will be under moist isentropic ascent through Monday night into Tuesday night. Cold front passage will then occur on Wednesday and will shift the atmospheric river to the southeast at that time.

*Precipitation Accumulations: The National Blend of Models (NBM)
indicates between 4 to 5 inches at the Cascade crest over this period, and between about 1.5-2.5 inches in the Idaho Panhandle.
The 90th percentile (or "worst case scenario") suggests near 6 to 6.5 inches at the Cascade crest and up to around 3 inches for the Idaho 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 Monday morning with snow levels starting out around 3,000 to 3,500 feet.
Then we'll see snow levels dropping again with cold front passage, but the bulk of this period will be all rain for the most part. The Northeast Mountains, Spokane Area, and Palouse will see the potential for 1-2 inches with the higher accumulations over this area across the Northeast mountains.
Areas in the lee of the Cascades won't be completely shadowed out, but will see significant less in the range of 0.25-0.75 inches or so.

*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 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 for mid to upper slopes and in the Methow Valley. 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. The Central Panhandle Mountains in particular looks to lose a lot of its mountain snow pack.

*Impacts: The potential for flooding will be a concern with rain combined with snow melt. 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. Lower elevations in the Cascades will more likely lose a good portion of the snow that has fallen and that will certainly have some contribution to rises for small stream and creeks with the addition of rain. There is higher confidence for this portion of the forecast area for rock slides in steep terrain. Flood potential will be more of a concern across northeast Washington into the Idaho Panhandle. These areas will be at risk for at least minor flooding impacts such as basement flooding and field flooding.
Ponding of water over roads will be possible, especially for those areas that see poor drainage. We will have to monitor Paradise Creek closely early next week with a forecast of 1.5 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.

Wednesday night through Saturday: There is more uncertainty with the potential for precipitation as models diverge with where to track a shortwave disturbance tracking in behind the incoming AR event for Thursday and another on Friday. The weather remains active and snow levels will be trending down. The potential for snow will mainly be in the mountains for Thursday, but we could see the return of more widespread valley snow (or a rain/snow mix) with colder temperatures Thursday night into Friday. Temperatures will be around normal for early December by Friday and Saturday coming out of the mild weather pattern early in the week. /SVH



AVIATION
06Z TAFs: Drier air has temporarily overtaken much of the region however the last vestiges of that dry air seem to be waning according to satellite data. It appears a low deck of clouds has formed just south and east of KGEG-KCOE and it may be a brief time until those clouds spread over these sites. This agrees with the latest HRRR ceiling guidance and infers we could see MVFR/IFR cigs shortly. I put 08z in the TAFS but it could be quicker than that.
Also not sure how low these clouds are, so we will get a better idea if/when these clouds move in. High clouds cover the remainder of the other sites, but based on observations we don't think there are low clouds around so we generally expect VFR conditions of the time being. The forecast becomes much more difficult overnight as the next round of precipitation spreads over the Cascades at 12z and over the WA/ID border around 16z. The precipitation type at EAT and MWH looks tough and could consist of snow and possibly some brief -FZRA. If the -FZRA does materialize it won't last for more than an hour or two, but nonetheless light ice accumulations would be possible. Along the WA/ID border we have higher confidence the precipitation will consist of snow at the TAF sites with a ra/sn mix at LWS. NBM and HRRR guidance is too warm and suggests all rain, but BUFR soundings and Hi-Res guidance says otherwise. We will hedge toward the snowier forecast but amounts should be a half inch or less. The threat of precipitation will wane during the afternoon, but suspect the lower atmosphere will retain enough moisture with the addition of snow melt for widespread low clouds and MVFR/IFR cigs.

FORECAST CONFIDENCE AND ALTERNATE SCENARIOS: High confidence for VFR conditions heading into the overnight hours for EAT LWS PUW and MWH with moderate confidence of MVFR stratus reforming overnight (MVFR/IFR cigs) for KGEG-KSFF-KCOE. Later in the forecast Low confidence for precip type for MWH and EAT. There is low to moderate confidence of seeing widespread stratus and IFR conditions across much of the region behind the precipitation from late tomorrow afternoon into Sunday evening. fx

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 24 41 32 44 38 47 / 0 100 30 90 100 90 Coeur d'Alene 23 41 33 44 39 46 / 0 100 50 90 100 90 Pullman 26 43 35 44 40 46 / 10 100 60 80 90 80 Lewiston 31 46 40 49 45 52 / 10 90 50 60 70 60 Colville 14 33 23 37 30 43 / 0 80 20 90 100 100 Sandpoint 22 38 29 39 36 44 / 0 100 70 90 100 100 Kellogg 29 43 37 44 43 46 / 0 100 70 90 100 90 Moses Lake 27 39 28 41 36 49 / 0 90 10 80 90 80 Wenatchee 31 38 32 40 34 41 / 0 90 20 90 90 90 Omak 21 35 29 39 36 45 / 0 70 10 80 100 90

OTX WATCHES/WARNINGS/ADVISORIES
ID...Winter Storm Warning until 4 PM PST Sunday for Central Panhandle Mountains.

WA...Winter Storm Warning until 10 AM PST Sunday for Northeast Blue Mountains.

Winter Storm Warning until 4 AM PST Sunday for Western Chelan County.

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