Area Forecast Discussion
Issued by NWS Spokane, WA

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000 FXUS66 KOTX 261235 AFDOTX Area Forecast Discussion National Weather Service Spokane WA 435 AM PST Sat Nov 26 2022 .SYNOPSIS... Motorists will encounter snow in the mountains this evening and a mix of rain and snow in the lowlands. Moderate to heavy snow will fall in the Cascades and Idaho Panhandle mountains late tonight into Sunday. Temperatures will be colder next week, with the potential for lowland snow during the middle of the week. && .DISCUSSION... Today: Wave is quickly exiting the region. Have decreased our chance of snow for this morning across the ID Panhandle as well as the Cascade mountains. Forecast challenge for today will be figuring out the stratus and fog situation. Patchy fog and stratus will continue through the morning hours across the region and then slowly dissipate through the afternoon as sunny skies above the stratus deck will provide an opportunity for drying and for it to dissipate. Winds will be light through the day and we will see temperatures in the 30s across the valleys which is average for this time of the year. Tonight and Sunday: The next wave will move in this evening bringing another round of winter precipitation and winds. Snow will start in the evening across the Cascades and north ID mountains. Precipitation will then spread south through the overnight hours. The caveat is there is a strong 120 kt jet that moves in overnight. Our winds will begin to increase late this evening and continue to increase through the night. The winds will be from the south or west, depending on location. The westerly winds will shadow out the Moses Lake area and only brings the potential for brief rain or snow to Wenatchee. Meanwhile the Cascade mountains will get slammed. Stevens Pass has a 95% chance of 12 inches of snow and a 65% chance of 18 inches. Have upgraded the watch to a warning for this evening through Sunday morning. HREF model is showing good probabilities of an inch an hour accumulation occurring through the overnight hours. Travel could be quite difficult if you plan to be on the roads Saturday night and early Sunday morning. Let`s go back to the winds for a moment...there is a 90% chance or higher of seeing gusts to 40 mph for Ritzville, Spokane, Couer d`Alene and Pullman. A 50% chance of 40 mph gusts for Moses Lake, and a 35% chance for Wenatchee. Gusts over 45 mph have the highest probabilities around Lincoln and Adams county as well into Spokane and the Couer d`Alene area with about a 50-60% chance. These winds will be strongest overnight and through about noon and will then decrease through the afternoon. The Cascade mountains, The Blues and parts of the central ID Panhandle mountains will see gusts in excess of 50 mph...but that is for locations generally above 4000 ft. The snow moves into the Spokane/COE area overnight and Sunday morning. Accumulations will be light with about a half an inch for Spokane and near an inch for COE. Valleys to the north, locations such as Newport, Sandpoint, Bonners Ferry and Ione will see 1 to 2 inches of snow. Snow will start in the central ID Panhandle mountains early Sunday morning. Have issued a winter storm watch for Sunday. Accumulations in the valleys will be about 1 to 2 inches with 4 to 7 in the mountains. There is a 90% chance of 6 inches and a 70% chance of 8 inches for Lookout Pass. Given the return holiday travel wanted to put out this watch so people are aware and can make plans accordingly. Amounts and rates won`t be as heavy as the Cascades, but a travel impact nevertheless. Sunday Night and Monday: The wave moves south Sunday night and Monday but we get a second wave to move through, mainly impacting the southern portion of Washington and the ID Panhandle. The Blue Mountains could see snow begin Sunday morning, but really picking up during the evening and continuing through Monday. Have issued a watch for them as well. There is an 90% chance of 8 inches and a 75% chance of 12 inches. Some of the ensembles as well as the NAM is spinning up a surface low across southern WA Monday morning. This will need to be monitored as this could produce higher snow amounts than what is currently in the forecast. Regardless of the low position, it looks like the Palouse could see snow throughout the day with a 70% chance of 2 inches of snow, and a 30% chance of 4 inches of snow. /Nisbet Tuesday...This period will be quietly enough as the remnants of the Sunday/Monday system drop south and east of the region. But that quiet will be eroded as the next upper level trough moves into the region. ...Widespread moderate to locally heavy snowfall looks possible late Tuesday through early Thursday... Late Tuesday night through Thursday morning...The models are in good agreement that a significant upper level trough will move toward the Washington coast during this period. The big question deals with timing and impacts. The Euro ensembles and the EC deterministic model show a deep surface low moving to the northern tip of Vancouver Island late Tuesday afternoon and then across southern BC through Wednesday. This in turn places a slow moving east-west band of snow across the northern half of our forecast area due to persistent isentropic ascent. This notion is shared more or less by the Canadian ensembles. Meanwhile the GEFS, shows a much slower solution with the upper level trough remaining well off the coast through much of Wednesday and not really bringing significant snow into the picture until Wednesday afternoon and into the overnight hours. If we simply bypass the ensemble means and look at this through a cluster analysis of all the ensemble runs about 75% of the run support this earlier Canadian/ENS solutions which suggests we could be looking at widespread snow across much of the region, including the Spokane/CdA area. Right now we are forecasting amounts ranging from 4-7 inches in the metro area, with amounts ranging from 4-9 inches from Nespelem to Chewelah to Sandpoint. If we look strictly at the chances for 6” of snow or more for Wednesday into early Thursday morning, the Spokane/CdA area has a 50-55% chance of exceeding that mark, with between a 60-70% chance for Sandpoint, Colville, Chewelah, and Priest Lake. Needless to say the percentages are even higher for Stevens and Lookout Passes. These numbers are from the NBM 4.1 which is a little snowier than the 4.0...but the trend over the last few runs is getting snowier. It could be quite messy to say the least for any travel plans. As for precipitation type there is no doubt this will fall as snow, over much of the area, with the small exception of some rain/snow over the LC Valley and extreme southern portion of the Columbia Basin. For Thursday afternoon into Friday...provided the EC/Canadian solutions prove correct...we will see the upper level trough axis push inland and weaken on Thursday, while the GEFS is stronger and slower. In any case although it wouldn’t dry out the forecast it would transition to more of a hit or miss snow shower event with steep lapse raters through the dendritic layer. This would likely favor the Idaho Panhandle with more showers at the Cascade Crest. By Friday into Saturday another trough builds off the coast, but this one is expected to dive much farther to the south. Thus if we see any snow it will likely be quite light with much heavier amounts expected over Oregon. Based on all the uncertainties with the depth and timing of the upper level trough, there are big differences in the temperature forecast. For instance, the faster Canadian/ENS solutions bring a warm sector into the region on Wednesday with highs into the upper 20s to mid 30s, whereas the GEFS doesn’t produce much of a warm sector at all and as a result has highs in the upper teens to mid 20s. And this trend continues through much of the week. fx && .AVIATION...
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12Z TAFS: IFR/LIFR fog and stratus will continue through the morning hours at terminals. The exception is LWS and EAT where VFR conditions are expected. Probabilities are high IFR/LIFR will continue through the morning and then begin to see improving conditions through the afternoon. The precip previously in TAFs for aft 00Z has been delayed and not expected till towards 12Z or after. Winds will increase aft 06-09Z which will help improve vis and cigs further.
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&& .PRELIMINARY POINT TEMPS/POPS... Spokane 36 31 35 18 25 9 / 0 40 70 20 20 10 Coeur d`Alene 36 31 35 19 25 10 / 0 60 90 30 20 10 Pullman 38 29 37 22 28 12 / 0 10 80 60 50 10 Lewiston 44 36 41 30 33 22 / 0 10 70 60 60 20 Colville 34 24 35 7 23 1 / 0 60 50 10 10 10 Sandpoint 34 31 34 18 24 10 / 10 80 90 20 10 10 Kellogg 34 32 32 21 24 10 / 10 60 100 40 30 10 Moses Lake 37 28 43 21 30 11 / 0 10 10 20 20 10 Wenatchee 39 32 39 24 30 16 / 0 30 20 40 20 10 Omak 35 26 34 16 22 8 / 0 50 30 20 10 10 && .OTX WATCHES/WARNINGS/ADVISORIES... ID...Winter Storm Watch from late tonight through Sunday evening for Central Panhandle Mountains. WA...Winter Storm Watch from Sunday morning through Monday evening for Northeast Blue Mountains. Winter Storm Warning from 7 PM this evening to 10 AM PST Monday for Western Chelan County. && $$

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