Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 356 PM EST Sat Nov 07 2020 Valid 00Z Sun Nov 08 2020 - 00Z Wed Nov 11 2020 ...Northern Rockies and Montana... Days 1-3... ...Blizzard winter storm to impact Montana through Sunday night... An opening low will eject northeast out of the Southwest and cross the Northern Rockies late tonight into Sunday, closing off again over east-central Montana which will cause it to slow before lifting into Manitoba Sunday night. Height falls in advance of this feature combined with intense mid/upper-level divergence and upper diffluence within the left exit region of a jet streak will enhance lee-side surface cyclogenesis over northern WY before tracking across western ND Sunday and Manitoba Sunday night. Gulf of Mexico and Pacific moisture being drawn northward in the warm conveyor belt ahead of this system is anomalous with PWs of +2 standard deviations above the climo mean and mean mixing ratios along the 300K isentropic surface as high as 6g/kg. This moisture will drawn into a rapidly developing TROWAL tonight through Sunday over central MT which is further focused by a 1030mb surface high shifting south along the Canadian Rockies. Deep layer ascent through the aforementioned synoptics will combine with intense mesoscale forcing through upslope flow, isentropic upglide, and strong deformation to cause heavy precipitation to develop across the Northern Rockies and then gradually spread northeastward. Initial precipitation is snow in the northern Rockies and will be rain on the High Plains of central MT this evening in a warm column. However, rapid dynamic cooling of the column, especially as the low strengthens and isallobaric acceleration occurs from the north, will cause an abrupt transition to snow, with a swath of mixed rain/sleet/freezing rain in a stripe northeast of the surface low over northeast MT and northwest ND through Sunday. Freezing rain should only feature minimal accretion with Day 1 WPC probabilities for 0.1" less than 10%. Snowfall rates are likely to be intense as strong omega intersects a saturated DGZ. HREF probabilities continue to depict high probabilities for 1"/hr over a couple swaths of west-central MT through tonight and north-central MT Sunday afternoon with low probabilities for 3"/hr rates, so it is likely that 2"/hr rates develop tonight and Sunday. It is likely that most of the precipitation will fall as snow, and with heavy rates continuing within the slowly advecting system, major snow accumulations are forecast. Day 1/1.5 WPC probabilities are greater than 60% for 12 inches for much of west-central and north-central MT, including Great Falls and Glasgow with isolated probabilities for 18" or more over 40%. Accumulations in excess of 20" are possible in the favored upslope terrain as well, or where any banding lingers for a long duration, and this event could challenge November record snowfall across parts of Montana. In addition to this heavy snow, strong winds are expected to produce blizzard conditions at times. ...Western U.S.... Days 1-3... A reinforcing trough over the Pacific NW will track south just inland of the West Coast through Sunday when by then will have formed an anomalous and large upper gyre that spans the Great Basin and northern Rockies. This feature opens and shifts east onto the Great Plains Monday with a northern Plains/Upper Midwest low expected to develop Tuesday. The next shortwave reaches the Pacific Northwest Monday night. This looks to be just the next disturbance in an active pattern for the Northwest and central CONUS that lasts through at least the next week. Both of these upper lows will be accompanied by Pacific moisture within associated jet streaks, through which the combination of moisture and ascent will produce widespread moderate to heavy snow across much of the western terrain through Sunday night. The anomalously low heights will produce falling snow levels all the way to the Mexican border, so heavy snow accumulations are expected for the San Gabriels and San Bernadinos, as well as the Mogollon Rim, and most higher ranges of the Great Basin. Particularly heavy snow is expected in the San Juans tonight where orthogonal mid-level flow, and instability will enhance precipitation in many ways. There is a likelihood of 0.5"/hr precipitation over the San Juans tonight which should lead to particularly heavy convective snow. Day 1 WPC probabilities are high for 18" there and the next round Sunday night into Monday brings the risk of storm total snow of 48". Other ranges that will likely receive significant snowfall include the Sierra Nevada and San Gabriels/San Bernadinos of CA, the Uintas and southern Wasatch in Utah, and parts of the Mogollon Rim. Here, Day 1/2 WPC probabilities are moderate high for 8 or more inches, with local amounts greater than 12 inches likely above 4000-5000 ft. On Monday the complex upper low begins to fill and shift east with ridging shifting into the West, cutting off most of the forcing and leaving lingering light to moderate snow across the Four Corners. However, quickly behind this exiting system the next shortwave digs into the Pac NW begin to dig into the PacNW with an accompanying Pacific jet streak to provide moisture. Snowfall Monday night/Tuesday will increase across the Olympics and Washington Cascades as well as the Blue Mtns of OR and northern ID Rockies, where Day 3 WPC probabilities are moderate for 8 or more inches. ...Central Plains to Upper Midwest... Day 3... The reinforcing low currently over the Pacific Northwest swings northeast from the Four Corners Monday night, amplifying over the southern High Plains and allows rapid surface cyclogenesis over the Upper Midwest Tuesday. A swath of snow develops in the comma head of this system late Monday night over western KS/NE and shifts northeast to MN Tuesday. Differences in track are leading to lower probabilities at this time (generally 20 to 30% for 2" on Day 3), but given ample Gulf of Mexico moisture ahead of this system and rapid cyclogenesis, mesoscale banding is likely to produce locally heavy snow and should be monitored closely. Jackson