Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 442 PM EDT Sun Apr 25 2021 Valid 00Z Mon Apr 26 2021 - 00Z Thu Apr 29 2021 ...Sierra Nevada and Great Basin to the Rockies... Days 1-3.. An upper low drifts east into the Great Basin tonight as a reinforcing shortwave trough dives south off the West Coast. Existing troughing south of the low center continues to pump Pacific moisture over the Sierra Nevada tonight where heavy snow will continue to occur above the snow level with drops from 5000ft to 4000ft overnight and into the Great Basin. As the upper low/trough drifts east, pre-frontal convergence picks up and enhances ascent in the ranges of central to northeast NV to western WY tonight, spreading across northern UT on Monday. Several additional inches are likely in the Great Basin areas above the 5000 to 6000ft snow levels as 700 mb convergence maxima combine with the difulent jet flow to enhance ascent. Day 1 snow probabilities are moderate for 6 or more inches from south-central OR terrain south down the CA Cascades and Sierra Nevada across central and eastern NV ranges, the northern Wasatch in UT, over southeast ID ranges and northwest WY ranges. These shift east for Day 1.5 to including southern UT ranges, the Uinta of UT and the Big Horns of WY. The reinforcing trough shifts inland over southern CA Tuesday, promoting a southward shift of the upper low center to southern AZ by Tuesday night. Snow levels of 6000 to 7000ft over the southern Intermountain ranges keep Day 2 snow probabilities for 6 or more inches to the highest terrain of northern AZ, the western San Juans of CO and the Big Horns of WY. Lee side surface cyclogenesis over southeast CO on Tuesday allows moisture increases from the Plains to the CO Rockies by Tuesday night where upper level difluence promotes heavy snow in north-central CO ranges where Day 2.5 snow probabilities for 6 or more inches are moderate to high. ...Northern Great Lakes... Days 1-2... Low level convergence of moist return flow up the Plains and a surface ridge axis near the MN/Ontario border will promote precip enhancement late tonight through Monday over northern MN/WI and the UP of MI. A frontogenetic band of snow is expected to develop near Lake Superior late tonight where a cross-lake Superior fetch picks up additional moisture fluxes from the lake. Lee shore convergence in the arrowhead of MN combined with an upslope component of flow leads to a period of snow late tonight through Monday over northeast MN as well as snow over mostly interior portions of the western UP. Continued low level warm air advection quickly brings about a warm nose with a switch to sleet and freezing rain over these areas through the day Monday. Despite midday April conditions, most guidance maintains wet bulb temperatures around 30F for the North Shore/Arrowhead and interior UP meaning freezing rain could continue to accrete. Day 1 snow probabilities are moderate for 2 or more inches along the North Shore escarpment and moderate along the central WI/MI border. Day 1 ice probabilities for a tenth or more freezing rain are moderate over interior sections of the Arrowhead where light freezing rain/drizzle could continue into Monday evening. For Days 1-3, the probability of significant icing (0.25 inch or more) is less than 10 percent. Jackson