Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 434 PM EDT Mon Apr 11 2022 Valid 00Z Tue Apr 12 2022 - 00Z Fri Apr 15 2022 ...Pacific Northwest... Days 1-3... Two distinct closed mid-level lows embedded within confluent and moist mid-level flow will spread heavy precipitation, including snow, across the Pacific Northwest from the Olympic Range south through the Sierra and east as far as the Northern/Central Rockies this week. The first low is moving onshore this evening and will advect into the Northern High Plains by Wednesday morning. Closely following this lead low, a second impulse of similar strength will dig out of the Gulf of Alaska and slide onshore WA State while weakening late Wednesday. Even as this secondary feature weakens, continued lobes of vorticity around it will maintain ascent within the moist column. These closed lows will gradually amplify an otherwise broad trough across the west, driving the best Pacific jet energy southward, and the best moisture is likely to slowly shunt to the south with time. Modest surface lows along the coast combined with the deep layer ascent and lowering snow levels beneath the cold cores aloft will create heavy snowfall all 3 days of the period, with even light lowland snow possible, especially D1 into D2 when snow levels fall to 500 ft or less. Heavy snow on D1 is most likely in the WA and OR Cascades where more than 12 inches of snow is possible in the higher terrain, with significant pass level snowfall also likely. Heavy snow exceeding 8 inches D1 is also likely to spread through the Blue Mountains and into the Northern and Central Rockies including the Wasatch and CO Rockies. By D2, the heaviest snow is likely to shift south such that the heaviest snow, likely exceeding 8 inches once again, is focused in the OR Cascades and into the Trinity/Shasta region. By D3 as the moisture plumes shunts further southward, heavy snow will become focused in the northern CA ranges and into the Sierra. 3-day snowfall may locally exceed 4 feet in the OR Cascades. ...Northern Rockies through the Northern Plains... Days 1-3... ...Significant, potentially historic, blizzard becoming more likely for portions of the Northern Plains... A closed low emerging out of the Pacific Northwest will deepen as it shifts into the Northern Plains Wednesday morning, reaching to as low as -3 standard deviations from the climo mean with respect to 500mb heights according to NAEFS ensemble tables. As this low shifts eastward, it will combine with increasingly coupled upper jet streaks to drive pressure falls in the lee of the Rockies, and a surface low is expected to develop and then shift northeast towards the Upper Midwest Tuesday night into Wednesday. At this time, the closed low aloft is progged to capture the surface low driving an occlusion, and potential retrograde of this feature, before it finally ejects into southern Canada on Thursday. Downstream of this low, warm and moist advection will become impressive noted by the 850mb LLJ surging northward from the Gulf of Mexico at 40-50 kts, and lifting the impressive theta-e ridge northward. This is then progged to loft cyclonically into an impressive TROWAL around the strengthening low pressure, while isentropic ascent at this time at the 290-295K surfaces becomes intense and moist, with mixing ratios reaching 5g/kg to fuel copious moisture across the region. While heavy snow is likely in the terrain from the Northern Rockies through the WY ranges, the heaviest snowfall is likely to occur from eastern MT through much of western, central and northern ND. It is in this area where two distinct bands of heavy snowfall, one within a laterally translating WAA band Tuesday into Wednesday, and then a a secondary pivoting deformation band, could produce record-setting snow amounts. Snowfall rates will likely reach 2"/hr as impressive ascent intersects the deepening DGZ, and despite modest SLRs due to heavy snow and fracturing from strong winds, locations across eastern MT through much of ND will likely receive well over 1 foot of snowfall during the event. Guidance has trended a bit NW today with the 12z suite, and the GFS/FV3 continue to hold their serve southeast of consensus, so there remains uncertainty into the exact placement of the low and its heaviest snowfall. However, the current WPC probabilities are highest across far eastern MT through north-central ND where both impressive bands may occur. In these areas, it is possible that more than 3 feet of snow will accumulate as shown by NBM and HREF probabilities. With strong winds expected as well, this could become an historic, life-threatening blizzard for parts of MT and ND before the event begins to wind down on Thursday. For Days 1-3, the probability of 0.10 inch or more of ice is less than 10 percent. Weiss ~~~Key Messages for April 11-14 Blizzard~~~ --A long duration, significant winter storm will bring heavy snow to portions of the Intermountain West through tonight with a blizzard over portions of the Northern Plains tonight into or through Thursday. --A combination of heavy snow and strong wind gusts will produce blizzard conditions with significant blowing and drifting of snow from the eastern slopes of the Northern Rockies across the Northern Plains. --Travel will be very difficult to impossible and there is potential for power outages and tree damage. Significant impacts to young livestock are also possible. --Snowfall may reach record levels in parts of eastern Montana and western/central North Dakota, with maximum storm total snowfall in excess of two feet likely.