Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 425 PM EST Fri Nov 06 2020 Valid 00Z Sat Nov 07 2020 - 00Z Tue Nov 10 2020 ...A major winter storm is expected for much of Montana this weekend with heavy snow and strong winds... ...Overview... A cold core mid-level low currently stretched meridionally over the CA/OR border will continue to shift south to central CA into Saturday before opening and ejecting northeast across the Great Basin through Saturday night. This feature closes off again over eastern MT Sunday and slowly tracks northeast into the Canadian Prairies into Monday. Meanwhile, a reinforcing shortwave currently over southwest Yukon and riding a powerful jet streak draped across southern AK will amplify as it digs over BC through tonight and closing off as it shifts down the West Coast Saturday afternoon through Sunday. Interaction with the low shifting northeast from eastern MT Sunday night makes for a broad, deep and positively tilted trough stretching from the northern Rockies and across the Great Basin Sunday night. This feature then shifts east onto the Great Plains Monday. ...Western U.S.... Days 1-3... Elevated, tropically sourced moisture spreading into The West ahead of the low moving into CA will allow brief heavy snow to shift south down the Sierra Nevada (snow level around 4000ft) tonight and southern CA ranges (including the San Gabriels and San Bernadinos with snow level around 5000ft) Saturday. Snow levels dip to around 5000ft as the system shift northeast across the Southwest through the UT ranges including the Wasatch, the San Juans and western ranges of CO, into WY through Saturday night. WPC probabilities for snowfall greater than 4" on Day 1 and 1.5 are moderate to high across these areas as well as western NV. The reinforcing low dropping from WA down the West Coast Saturday night/Sunday producing the larger mid-level low will bring more widespread and longer duration heavy snow to the Sierra Nevada, Southern CA ranges, Mogollon Rim, and Wasatch and San Juans. Some lighter snows may also accumulate down into the valleys as snow levels fall up to 2000ft lower than the system tonight into Saturday. Day 2 WPC snow probabilities for 6 inches or more are across are high for the Sierra Nevada and higher ranges across NV to southern UT before being moderate on Day 3 for southern UT, the Mogollon Rim and White Mtns of AZ as well as high for the San Juans in CO. ...Northern Rockies and Montana... Days 1-3... The combination of sharp height falls and intense upper divergence will drive pressure falls, and surface cyclogenesis Saturday in WY with only a slow shift northeast across the northern High Plains through Sunday night. Strong advection ahead of this system will transport anomalous moisture (PWs of +2 standard deviations) up the Great Plains as Pacific and Gulf moisture lift northward. The establish stationary front near the MT/WY border will maintain cold air over MT with advective precip forming on the High Plains late Saturday. Initially, the column outside of the terrain will be too warm for snow, but intense forcing through synoptic and mesoscale ascent, including a developing and intense TROWAL and isallobaric acceleration into the deepening low will dynamically cool the column to allow for a changeover from rain to snow. Periods of freezing rain are also possible in eastern MT into northwest ND Sunday with a snow/mix/rain line forming over eastern MT. Day 2.5 ice probabilities for a tenth inch of ice are 10 to 20 percent over far northwest ND to far northeast MT. However, the majority of the precipitation from the Northern Rockies and points northeast will be snow, and this should be extremely heavy at times. Strong ascent coupled with anomalous moisture should produce snowfall rates that will exceed 2"/hr as the TROWAL pivots overhead, and WPC probabilities have increased further today. There are now high probabilities for 12 or more inches across the eastern slopes of the MT Rockies and across north-central MT on Day 2 and 2.5. Locally 20" is possible in a few locations, and this event could challenge November record snowfall across parts of Montana. Additionally, strong winds will accompany this snowfall to produce significant blowing and drifting. Jackson