001 FXUS66 KOTX 072242 AFDOTX Area Forecast Discussion National Weather Service Spokane WA 242 PM PST Sun Nov 7 2021 .SYNOPSIS... An occluded front will bring mountain snow and a mix of rain and snow to the valleys through this evening into tonight. Cascade passes will receive another several inches of snow by Monday morning. A stronger storm will impact the Northwest on Tuesday with moderate snow accumulations in the mountains and wet snow possible for valleys in the morning. By the end of the week, there is a decent chance we could be looking at warming temperatures with a flat ridge building in, but also remaining active with moisture pushing in over the top of the ridge over the region. && .DISCUSSION... ...MANY NORTHERN VALLEYS EXPECTED TO SEE THE FIRST SNOW OF THE SEASON THROUGH TUESDAY... Tonight through Monday: A shortwave trough of lower pressure is pushing into western Washington this afternoon. This disturbance is expected to shift across the Cascades in the evening with increasing chances for precipitation primarily over the northern zones, but could also see some light precipitation into the Columbia Basin as well. Precipitation type will be a bit more tricky with this next disturbance. Thermal profiles suggest snow for the mountain valleys, but precipitation intensity is a bit uncertain and lighter intensities may make it difficult for some valley locations sitting in the lower to mid 40s this afternoon to sufficiently cool down enough for all snow. Even if these northern valleys do see snow, it may be light enough that it has a hard time accumulating -- especially on roadways. I have a dusting of snow for places like Republic, Colville, Deer Park, Priest River, Sandpoint and Bonners Ferry as people wake up early Monday. Some places may see up to between 0.5 to 1.0 inches. Sherman Pass is expected to receive between 1 to 2 inches. More moderate snow accumulations will fall near the Cascade crest with an additional 3 to 5 inches expected for Stevens Pass and Washington Pass. Monday night through Tuesday: This portion of the forecast feature a moist occluded front. The timing of the front is coming into better focus, but there are still some timing differences. Namely the 18Z NAM solution is considerably faster by roughly 6 hours, and is also the the wetter and stronger solution. The NAM is the outlier, but presents a much snowier solution that the official forecast will show. As far operational guidance is concerned, the forecast most closely follows the 12Z GFS; although, the 12Z ECMWF has a similar solution to the GFS. The GFS/ECMWF is slower in timing bringing in the front Tuesday morning. Precipitation looks to really pick up across the east slopes of the northern Cascades through the morning hours where easterly to southeasterly flow right out ahead of the front will generate favorable orographics. Precipitation type will be tricky for the Wenatchee Area up into the Okanogan Valley primarily down right at the Columbia and Okanogan River. These lowest elevations are expected to see either rain/snow mix or non-accumulating snow. The higher benches above 2,000 feet will more likely see wet snow, and this includes the Waterville Plateau. Further up into the Cascade valleys, we should see thermal profiles supportive of snow where valley accumulations will be in the 1 to 3 inch range and locally up to 4 inches. The mountains in the Cascades will see around 3 to 8 inches with the crest more likely to be in the range of 5 to 10 inches. These areas will most likely need an advisory for snow through Tuesday. Uncertainty for snowfall just increases across the rest of the eastern Washington and into the Northern Panhandle. Big contributing factors for the uncertainty is timing of the front and precipitation intensity. Dynamics along the front will be strongest over the Cascades and see some weakening is it pushes into the Panhandle. The GEFS and ECMWF ENS looks to bring the front into these areas later in the morning into the early afternoon hours as temperatures are warming up. The northern valleys will undoubtedly start out as snow. It will likely be wet and may have a hard time effectively sticking to the roads. Grassy surfaces will more likely see accumulations. General accumulations for Republic to Colville to Deer Park to Sandpoint and Bonners Ferry will be in the 0.5 to 1.5 inch range. The Highway 2 corridor from the Waterville Plateau to Spokane and into Coeur d'Alene are expected to see accumulations up to around a half of an inch and locally up to an inch possible. Sherman Pass and Lookout Pass are expected to see accumulations between 1 to 3 inches. These areas outside of the east slopes of the Cascades will have a hard time achieving advisory level accumulations -- that is unless the stronger and wetter model solutions verify. Winds will increase through the day on Tuesday as well. They look to become generally breezy across the basin, into the Palouse Area, and West Plains. Wind gusts are expected to be up around 20 to 30 mph. The combination of winds and warming temperatures in the afternoon will likely melt off all of the snow that falls in the valleys. The morning commute may be a bit messy though. /SVH Wednesday through Saturday: The upper level trough begins to eject northward while the trailing trough shoves off to the east, leaving eastern WA/northern ID in a westerly and drier flow pattern as we enter mid-week. Lingering showers sliding over the Cascades and in north-central Idaho will be likely as the jet begins to shift northward and amplifying a ridge to our west. Wednesday should be a fairly dry day after lingering morning showers. Increasing heights and a subtle warm nose pushing in from the south (seen at 850mb and 700mb) could help to provide for several hours of less cloud cover (honestly, not ideal conditions to see truly clear skies) could help radiate out some moisture as temperatures cool down again Wednesday night. Didn't add mention of fog/low stratus to the forecast with this package, but it looks like there could be chances for that to develop. The Cluster Analysis of ensemble guidance is in good agreement with one another about building a ridge in late this week. The GFS ensemble is still the outlier in not wanting as strong of a ridge as the EC and Canadian ensemble groups, and similarly the GFS suite is also one of the wetter solutions of the bunch. Thursday and Friday will be under the influence of a westerly flow aloft from some form of a ridge developing off-shore. Snow levels and temperatures at 850mb and 700mb will be slowly warming, however it looks like there could be a threat of mixed precipitation type, even freezing rain. The airmass will be modifying with more precipitation on the way and a nose of warm air aloft. The NBM conditional freezing rain would suggest there is a decent (30-50%) probability of seeing freezing rain across the Columbia Basin Thursday. Forecast soundings are almost indicative of that pattern as well. Confidence in this right now is low, but it is worth mentioning and looking at as we move forward. The period through late week overall looks cloudy and wet for the Inland Northwest, and the challenge will be accumulation amounts and precipitation types by late week. Winds should be fairly light through late week as of now. /Dewey && .AVIATION... 18Z TAF: A moist boundary layer will result in fog at the Lewiston Airport and a persistent inversion is keeping this fog in longer than models are indicating. Diurnal heating should begin to break up the fog between 19-20Z. There is low stratus and patchy fog with ceilings between 300-600 ft agl in the Methow Valley that will be slow to lift if it does at all today with easterly winds pushing moisture up that valley. Otherwise, we are looking at low stratocumulus for airports in northeast Washington and the Idaho Panhandle including Colville, Deer Park, Sandpoint, and Bonners Ferry with ceilings between 2-4 kft agl; expect widespread mountain obscurations across this portion of the region, as well as up along the Cascade crest. A shortwave disturbance will move in this evening with light snow possible in the mountain valleys of northeast Washington and the Northern Panhandle. Snow accumulations will be minor, but low stratus with marginal VFR conditions can be expected that will persist into Monday morning. /SVH && .PRELIMINARY POINT TEMPS/POPS... Spokane 32 48 33 47 31 45 / 30 10 30 90 20 10 Coeur d'Alene 34 46 32 45 32 43 / 40 20 20 90 30 20 Pullman 29 47 32 46 31 44 / 20 10 30 90 30 20 Lewiston 36 52 39 53 38 51 / 10 10 20 70 20 20 Colville 28 44 26 45 24 43 / 70 10 20 100 20 10 Sandpoint 34 42 27 40 31 40 / 80 40 10 100 60 30 Kellogg 34 44 33 44 34 39 / 40 30 20 100 60 30 Moses Lake 32 50 37 50 30 49 / 10 0 50 80 10 0 Wenatchee 31 46 36 47 33 47 / 30 10 60 90 20 10 Omak 31 45 34 46 30 47 / 40 10 40 80 10 10 && .OTX WATCHES/WARNINGS/ADVISORIES... ID...None. WA...Winter Weather Advisory until 4 AM PST Monday for Western Chelan County-Western Okanogan County. && $$