Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 429 PM EDT Tue Oct 12 2021 Valid 00Z Wed Oct 13 2021 - 00Z Sat Oct 16 2021 ...First potent winter storm spreads heavy snow from the north-central Rockies onto the northern High Plains through Wednesday... ...WY Rockies to the far western Dakotas... Day 1... A closed mid-level low lifts northeast over CO tonight before crossing the northern Plains as an occluded system through Wednesday. This cyclone will be accompanied by intense moist advection as rich theta-e air surges up the Plains from the Gulf of Mexico in conjunction with Pacific air rotating up from the southwest within the upper jet streak. Impressive upper divergence within the left exit of the jet streak around the low as well as briefly the right entrance region of another jet over the MT/ND border this evening will help drive low-level frontogenesis collocated with the best axis of deformation. At the same time, rich theta-e air will be lifted cyclonically within a TROWAL north and west of the 700mb low, providing additional ascent, with low-level easterly upslope flow also driving strong vertical motion. The area where these features overlap has shifted east a bit in 12Z guidance, over southeast MT and northeast WY to the Dakota border. Here, a band of heavy snowfall is expected (despite the marginal thermal environment), particularly from about 09Z to 15Z Wednesday and heavy amounts were adjusted farther east to now be just into the far western Dakotas (more than just the Black Hills). Strong ascent within the DGZ atop an isothermal layer supports intense snow rates, with the 12Z HREF probabilities now 30 to 70% for snowfall rates of over an inch per hour. Day 1 WPC probabilities for 8 or more inches of snow are 60-80% over east-central through northeastern WY, far southeast MT, the Black Hills and north along the western Dakotas border. After 18Z Wednesday, the low center crosses eastern ND with only lighter snow bands continuing for the Plains of far eastern MT and western ND. ...Pacific Northwest... Day 1... A shortwave trough crosses western WA this evening producing a swath of light to locally moderate precipitation ahead of a weakening cold front and associated area of low pressure over the WA/OR Cascades tonight. Snow levels will generally be around 4000ft for this case with WPC Day 1 snow probabilities for 4 or more inches 20 to 60% for the higher Cascades. Pass level snowfall in WA looks to be 1 to 2". ...Colorado... Days 2/3... A reinforcing shortwave trough shifting southeast out of the Pacific Northwest Wednesday night turns east as it crosses CO Thursday afternoon/evening. Westerly flow will capitalize on upslope enhancement over central Colorado ranges where Day 2.5 WPC probabilities of at least 4 inches of snow are 30 to 60 percent. Furthermore, lee-side cyclogenesis will allow some easterly flow which looks to enhance snow potential over the Palmer Divide where Day 2.5 snow probabilities for 4 or more inches are around 20%. The probability for significant icing (0.25 inches or more) is less than 10 percent Days 1-3. Jackson