Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 451 AM EDT Fri Apr 22 2022 Valid 12Z Fri Apr 22 2022 - 12Z Mon Apr 25 2022 ...Another Blizzard takes aim at the northern Plains... ...Intermountain West through the Northern Plains... Days 1-3... A sharp and digging upper trough currently pushing inland over CA will cross the Intermountain West today, taking on a negative tilt tonight and closing over WY/CO as the downstream strengthens in the now zonal flow and upper divergence increases. Rapid lee-side surface cyclogenesis occurs in eastern CO with the surface low track now fairly confident from western Neb to eastern ND through Saturday before shifting to Ontario Sunday. Day 1 snow probabilities for over 8" are moderately high over the Absarokas, Tetons, Bighorns, Wind River, Wasatch, Uinta, and northern CO ranges with snow levels generally 7000 ft as precip shifts ahead of the upper trough. The system crossing the Intermountain West will slow and deepen as the upper jet takes on an S-shape curve (with kissing jets maximizing ageostrophic lift over eastern MT/western ND tonight through Saturday) before splitting as the mid-level system again closes off quite vigorously by the time it reaches SD early Saturday. This will promote strong surface cyclogenesis over the northern Plains with surface low pressure quickly deepening into the low 980s mb as it transfers out of the central Rockies then lifts across 100W over SD. Strong WAA will bring PW values of over 1 inch (+2 to +3 sigma) to around 100W Friday night from the southeast, just east of the developing rain/snow line and about as high as moisture can get to still support snow. Temperatures will be marginal to start but the rapid intensification will favor strong winds (blizzard conditions) developing to the north and northwest of the low as the column cools enough to support heavy bands of snow on the Plains starting late tonight over northeast WY and southwest MT. Blizzards are notoriously hard to measure snow depth in, but snowfall rates of 1-2" per hour are likely in these bands despite SLR expected to be generally less than 10:1 into Saturday for areas outside terrain. Guidance remains well-clustered overall with the main wintry QPF axis anticipated from northeast WY, across eastern MT, and into western ND. Day 1.5 snow probabilities for over 8" depict the surge onto the Plains that begins tonight with high probabilities for northeast WY/southeast MT, expanding across western ND and far western SD (including the Black Hills which will see upslope enhancement) for Day 2. There are moderate probabilities for over 18" on Day 2 through the core of this area. Of particular note is as the low reaches peak intensity late Saturday over the eastern Dakotas a reinforcing impulse causes the system to slow, allowing longer residence of time of the TROWAL bands near the MT/ND border. There will be a p-type transition zone with potential for mainly light icing tonight into Saturday over far northeastern MT and northern ND as the low levels cool quicker than the levels just above as the system organizes and the WCB lifts into a potent TROWAL. WPC probabilities of at least 0.10" ice remain low (10-30%) over north-central ND. A trailing impulse crosses CO Sunday night. It is able to tap into elevated moisture and brings moderate Day 3 snow probabilities for over 8" to central CO Rockies. For days 1-3, the probability of significant icing greater than 0.25 inches is less than 10 percent. ...Key Messages for Northern Rockies to Northern Plains Blizzard later today into Sunday... --A significant winter storm will produce heavy wet snow and strong winds from the Rockies into portions of the northern Plains. --A combination of heavy snow and strong winds will produce blizzard conditions across portions of the northern Plains. --Travel may become very difficult to impossible. Power outages and tree damage are expected. Significant impacts to livestock are also possible. --Prepare now for impactful winter weather and delayed travel. Jackson