Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 354 PM EST Sat Feb 27 2021 Valid 00Z Sun Feb 28 2021 - 00Z Wed Mar 03 2021 ...Northern Plains to the Upper Peninsula of Michigan... Day 1... A trough over the northern Rockies becomes increasingly positively tilted as a reinforcing trough allows the northern stream to separate and eject east across the northern Plains tonight and the northern Great Lakes Sunday. Lee side surface cyclogenesis currently over southwest KS lifts northeast this evening tracking across central IA tonight and central WI Sunday morning. This low will deepen in the left exit region of a jet streak currently intensifying over the southern Plains. This deep layer synoptic ascent will be aided by locally intense mesoscale forcing as strong low to mid-level fgen develops NW of this surface low. Continued evidence that some CSI banding with possible upright convection (and possible thundersnow) is possible especially from near KFSD northeast into southwest MN, then again for northern WI across the U.P. of Michigan due to negative theta-e lapse rates and -EPV intersecting the saturated DGZ. 12Z HREF snowfall probabilities for 1"/hr increased again, and there is likely to be a narrow band of 1-2"/hr snowfall rates this evening into the overnight over southeast SD and southwest MN then again Sunday morning through midday from northern WI to the western U.P. Day 1 WPC probabilities for 4 or more inches are moderately high under the mid-level trough over southwest SD and the Black Hills, near the Buffalo Ridge in southwest MN, and then over northern WI to the central U.P. where there are 30 to 40 percent for 6 or more inches. From southeast SD to northwest WI, including the MSP metro, are continued Day 1 WPC probabilities or at least 30 percent. ...Northeast... Days 1-3... The shortwave trough lifting northeast over the northern Great Lakes Sunday afternoon will direct warm air advection across the Northeast Sunday/Sunday night. Locally trapped cold air should allow for some light icing over north-central PA Sunday morning and the northern Appalachian Valleys over interior New England Sunday night. Day 1/2 ice probabilities are generally 20 to 30 percent for a tenth inch or more of ice in north-central PA, in the Adirondacks, and the White Mtns of NH. A potent trough with associated Arctic cold front will dig across the Great Lakes Sunday night and the Northeast/New England through Monday. This will reinforce broad cyclonic flow across the eastern CONUS and be accompanied by rapid and potent height falls along with PVA to drive an area of snow across the eastern Great Lakes and New England Monday afternoon through Monday evening with lake effect snow continuing through at least Monday night. While snowfall with this quick moving feature are expected to generally be modest, WPC probabilities for 4 inches are as high as 30% in the favored upslope terrain of the Adirondacks and Greens on Day 2.5 with local snow belt areas also likely to see a few to several inches of snow where the bands set up. Additionally, while accumulation outside of the terrain should be light, there continues to be potential for snow squalls as low-level CAPE climbs to 50-150 J/kg coincident with negative 0-2km theta-e lapse rates. These squalls could race across New England and Upstate New York/Pennsylvania producing briefly intense snowfall rates and lowered visibility late Monday and Monday night. ...Southern High Plains... Day 2... As the trough over the northern Rockies becomes more positively tilted and amplified tonight a closed low spins off the end and tracks east from AZ over NM Sunday into Monday and across the southern Plains Monday night/Tuesday. Gulf-sourced moisture currently spread over the eastern half of TX is drawn west to the Panhandle Sunday night which is is lifted topographically and from the approaching low, resulting in locally moderate snow over eastern NM into the TX Panhandle late Sunday night/Monday morning. Day 2 snow probabilities are 10 to 20 percent for 2 or more inches along the central NM/TX Panhandle border. ...Pacific Northwest... Days 1 & 3... A weak shortwave trough overtopping the Northeast Pacific ridge crosses BC tonight and provides some lift in continued onshore NWly flow into western WA and makes for mainly moderate northern Cascades and Olympic snows into Sunday morning as snow levels rise from 2000 to 3000ft overnight. Day 1 snow probabilities are 20 to 30 percent for 6 or more inches for the higher Cascades. The northern portion of an amplifying long wave trough approaching the West Coast Monday weakens as it reaching the Pacific Northwest coast Monday night as a low closes off CA. Some additional moderate precip can be expected Monday night with Day 3 snow probabilities generally 20 to 30 percent with snow levels around 2000ft. For Days 1-3, the probability of significant icing (0.25-inch or greater) is less than 10 percent. Jackson