Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 423 PM EDT Sat Apr 10 2021 Valid 00Z Sun Apr 11 2021 - 00Z Wed Apr 14 2021 ...Northern and Central Rockies to the Northern Plains... Days 1-3... An upper level shortwave trough and its associated frontal band will move out of the northern Rockies and east across the northern Plains Sunday. This fast-moving system is expected to bring windy conditions and periods of snow to the higher elevations of central Montana. Several inches of snow are expected in windward areas tonight as the low level are near saturation in the forecast soundings. Tomorrow, as the upper trough departs, the drying aloft should cut down on snow shower coverage/intensity despite steep lapse rates. As the system moves across the northern Plains on Sunday and upper MS Valley Sunday night it is expected to assume a negative tilt. The post-front cold advection advects colder air east and changes rain to snow starting in North Dakota and then northern Minnesota. While widespread heavy accumulations are not expected, a general 2-4 inches is forecast in a band across northern North Dakota, with lighter amounts elsewhere in the northern Plains. Persistent moist conditions from the surface to 700 mb and return northeast flow sets up the likelihood of several inches of snow in the Absaroka and Wind River Ranges late Sunday night into Monday night. On Monday night-Tuesday, the northern Plains system shears and moves slowly. the deformation band north of the circulation sets up the possibility of a long duration light to moderate snow over northern North Dakota and northwest Minnesota. The uncertainty is how far south the precip band and colder air can progress across North Dakota and northern into central MN. Phasing differences lead to spread among solutions and more uncertainty. WPC PWPF shows 10-40 percent probabilities for accumulations of 4 inches centered over northern North Dakota on Day 3. In the Rockies, the jet stream axis and zone of moist confluent flow drops south form Wyoming into Colorado. Downstream from the closed 700 mb low over the Great Basin, moisture increases in CO and 700 mb ascent begins. Low low level flow has an easterly component to it, favoring the foothills and front range getting snow developing Tuesday. Several inches are possible in the CO ranges on Wed. The GFS is wetter/whiter, so while the version 16 has improved over the old version 15, it still has a bit of a high bias. Consequently, other models were weighted more in CO. The PWPF has a 50-60 probability of 4 inches in the CO front range Tue. For Days 1-3, the probability of significant icing (0.25-inch or more) is less than 10 percent. Petersen