267 FXUS66 KOTX 071055 AFDOTX Area Forecast Discussion National Weather Service Spokane WA 255 AM PST Sun Nov 7 2021 .SYNOPSIS... Several weak systems will bring the chance of rain or snow to the region through the weekend and into Monday. Cascade passes may receive several inches of snow over the weekend. A potentially stronger storm could impact the area on Tuesday with snow in the mountains and a mix of rain and snow in the valleys. By the end of the week, there is a decent chance we could be looking at warming temperatures with a strong ridge forecast to build over the region. This should push the precipitation chances toward the Canadian border or northward. && .DISCUSSION... Today through Monday: A low traveling through Central BC will continue to pull moisture and cold air through the region. The trend of showers across the Inland Northwest is expected to last through early Monday morning. For today, snow levels will fluctuate with elevations of 2500-3000ft during the day and dip to around 1500-2000ft overnight. Precip during the day will be generally rain for the lower terrain and mountain snow. During the overnight periods, the precip could be snow for the lower terrain but accumulation is expected to remain a couple of tenths at most. The Cascades will receive the brunt of the system with amounts around 10 inches possible. An advisory has been issued for the Passes through early Monday morning. For Monday, most of the region can expect a brief break as shallow ridge builds into the region. The Cascades will still continue to impacted be snow showers with amounts of a few inches possible. The rest of the Inland Northwest can expect a cool, dry day. Another Low will begin to press into the region late Monday night and Tuesday. Winds during the afternoon will be breezy with gusts into the low 20 MPH range. Highs will in the 40s and low 50s. Lows will be in the 20s and low 30s. /JDC ...Significant snows still looking possible for the northern valleys of eastern Washington into the Idaho Panhandle... Late Monday night into Tuesday... This continues to look like the most interesting and impactful portion of the forecast as the ensemble means continue to push a very deep offshore trough deep offshore trough northeast through southwest BC before weakening by Tuesday night over eastern BC. Although they all show this, there are some specific details that need to be resolved, and that is what to make of a surface low that forms off the SW Oregon Coast Monday evening. The models have backed of significantly on the depth of the low. Yesterdays runs showed an impressive 980 mbs low from the GFS and 989 mbs from the NAM and moved the low progressively northeastward through Tuesday morning. Both the NAM and GFS have backed off significantly on the depth of the low which will result in a much weaker deformation band of precipitation forming north of the low. Although this feature is forecast to be weaker now, both models still are forming an west- east deformation band of precipitation over the southern half of eastern Washington late Monday night and moving it toward the Canadian border by daybreak. The NAM solution is the strongest and wettest of the short-range models with a 1002 mb low just south of Omak by sunrise. This solution produces heavy 6 hours precipitation amounts extending from the western Palouse to the Methow Valley of 0.30-0.70 inches during the early morning hours and then shifts it gradually northeast and weakens it through early afternoon. Meanwhile the GFS is much weaker with 1008 mb low near the Tri-Cities by early morning and a less intense deformation band. It also takes the band of precipitation into the northern portions of eastern Washington and north Idaho during the morning. Precipitation amounts from the GFS are more in line with the EC and the GEFS ensemble and paint amounts ranging from 0.15-0.35 inches across the northern third of eastern Washington and the north half of the Idaho Panhandle. This is the solution we will go with, however if the NAM pans out, our forecast will prove much too dry. While the track and depth of the low is uncertain, the 850 mb temperature forecast remains fairly consistent with sub-zero 850 mb wet-bulb temperatures expected to persist over the northern third of WA and the northern ID Panhandle. This will certainly lead to snow above 3000’ but what about the valleys? This is hard to answer as forecast highs are expected reach into the lower 40s by afternoon, but there is a very good chance we should see valley snow before things change over to rain. So how much snow will fall is the big question. If we believe the NAM it would be significant amount of snow and would likely necessitate a winter storm warning for some of the northern valleys, as well as the mountains. However with the lighter GFS we are likely only looking at 1 to local 2 inch amounts for valley locations such as Colville, Sandpoint, Republic, and possibly Omak. But how effectively the snow will accumulate will be tough based on surface temperatures right around freezing. For colder locations such as the Methow Valley and upper Wenatchee valley we are more likely to see 2 to 4 inch amounts. While the valley forecast looks tough, its a much easier call for the mountains above 3000 feet. We will likely see another 6-11 inches of snow at Stevens Pass and several inches of snow at Sherman Pass and Lookout. Originally all the models all weakened this deformation band by afternoon as the upper level trough offshore shifted inland. Now there is a hint that we could see a secondary band of moderate precipitation form over southeast Washington into north-central Idaho as the trough shifts inland. The NBM doesn’t show this, but have hedged the pops and precipitation amounts accordingly. This band should consist of rain, but it would bring snow to the Blue Mountains. The other thing we are now seeing is a little stronger wind fields associated with the approaching surface low. The NBM is showing the possibility of high winds just south of our forecast area (western foothills of the Blues) and some gustier winds across the Columbia Basin of 20-30 mph and we have adjusted accordingly. By Tuesday night into Wednesday there is fairly good agreement that the offshore trough will eject east of the area and drift across the prairie provinces of Canada. This should bring a drying trend in the lee of the Cascades with orographic showers continuing at the Cascade Crest and over the Panhandle mountains. For Thursday through Saturday there is still good agreement between the ensemble means that we should see a rapidly amplifying ridge pattern over the Western US. This should push the upper level jet and storm track northward and take most of the moisture with it. But how far north will it go? The trends have been farther south with time, echoing what the Euro ensembles were hinting at. So this means rather than a dry forecast, we are likely to see wet weather at least near the Cascades and the Idaho Panhandle due to the presence of a very moist air mass combined with swift westerly or zonal flow. fx && .AVIATION... 06Z TAF: Moist upslope westerly flow into the Panhandle will result in showers continuing into the overnight hours. Snow levels will drop through the night with a dusting of snow possible for Sandpoint and Bonners Ferry. A moist boundary layer is expected to result in low stratus over northeast Washington and into the Panhandle for Sunday morning with marginal VFR conditions possible. Some light flurries or sprinkles, maybe a stray snow shower, may come out of the stratus but confidence is low. Cloud bases are expected to lift to between 4-6 kft agl in the afternoon, before the next precipitation potential comes in after about 00-02Z including possible -shra/-shsn. /Cote' && .PRELIMINARY POINT TEMPS/POPS... Spokane 46 31 48 33 46 32 / 10 30 0 30 80 30 Coeur d'Alene 44 32 46 32 45 32 / 20 40 10 20 80 40 Pullman 44 28 47 33 45 32 / 0 20 0 30 80 40 Lewiston 51 35 52 39 53 38 / 0 10 0 20 70 30 Colville 43 26 44 26 43 26 / 20 70 10 30 80 30 Sandpoint 41 30 42 28 42 32 / 40 80 20 20 90 60 Kellogg 41 33 44 32 45 32 / 10 40 10 20 90 60 Moses Lake 49 31 50 37 51 33 / 10 10 0 50 50 10 Wenatchee 47 31 46 36 46 32 / 30 30 10 60 70 30 Omak 46 30 45 33 44 30 / 40 40 10 50 80 20 && .OTX WATCHES/WARNINGS/ADVISORIES... ID...None. WA...Winter Weather Advisory until 4 AM PST Monday for Western Chelan County-Western Okanogan County. && $$