Tuesday, December 6, 2016

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Multiple Atmospheric Rivers to Impact Sierra Over the Next Week

While November brought decent snowfall and a nice start to the ski season, storms were all small to moderate with limited moisture for Sierra standards. December will start off much different. A deep plume of moisture originating in the tropics can be seen on satellite that will begin to reach the West coast starting tomorrow afternoon. Wave one will occur Wednesday night into Thursday night, and a second wave will impact the region Friday night into Saturday.

Currently we are transitioning from a cold and mostly dry continental airmass to a much warmer and moist Pacific air mass. The map below shows temperatures at 700 hPa (~10,000 feet) with cold air (purples, -10 to -12 C) over Tahoe and really cold air (whites, -15 to -20 C) over the Pacific Northwest and Northern Rockies.

By tomorrow afternoon temperatures begin to rise a bit, but will still be cold enough to snow down to lake level and probably Reno, too. You can see the yellow colors (warm) out in the Pacific being advected (transported) towards CA. 

By Thursday morning temperatures continue to rise, and then things get interesting. The cold air in place from the day before may be difficult to mix out and snow levels could stay low longer (we've seen this in the past). It is also possible that snow will transition to rain, or even freezing rain, Thursday morning at lake level. Snow level could rise above pass level by Thursday evening. 

Snow level will remain relatively high with second wave Friday night, but the snowpack should build up pretty nicely above ~7,000-7,500 ft. The second AR is likely to impact the region early next week, but we won't get into those details yet. The bottom line is several storms will bring abundant precipitation to the region through the next week. The graph below shows the accumulated precipitation (bottom) and snowfall (top) over Donner Summit based on many different forecast ensemble members. The thin lines are individual forecast members and the thick line is the average of all forecasts. Snowfall will heavily depend on temperatures and snow levels. 

Wave one will bring 0.5-1 inches of liquid to the Sierra Crest Wednesday afternoon Thursday afternoon. Wave two could bring about the same of maybe more Friday night into Saturday.

Lake level could see a slushy accumulation that will likely melt by Thursday afternoon, mid elevation (7,000-8,000 feet) could see 6-12 inches by Saturday morning, and upper elevations may see 1-2 feet of base building pineapple pow by Saturday morning! More later on next weeks storms. 


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