Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 423 AM EST Mon Nov 25 2019 Valid 12Z Mon Nov 25 2019 - 12Z Thu Nov 28 2019 Day 1... ...Pacific Northwest the Northern Rockies... An amplifying shortwave centered along the leading edge of a strong upper jet is expected to dive southeast across California into the Great Basin and Four Corners region on Monday. Ongoing precipitation over the Northwest early Monday is forecast to continue, while shifting farther to the south. In addition to the southern shift, snow levels are forecast to drop through the across the western Oregon - increasing the threat for locally heavy accumulations across the southern Cascades on Monday. Meanwhile, additional heavy accumulations are possible across the Blue Mountains in northeastern Oregon and far southeastern Washington, as well as the Bitterroot Range in northern Idaho. ...Central Rockies to the High Plains... As the previously noted shortwave continues to dive southeast into the Great Basin, expect locally heavy snows to develop over the Uintas and Wasatch Monday morning. Then by late Monday afternoon and continuing into the evening and overnight hours, snows will develop farther east across northern Colorado, southeastern Wyoming and western Nebraska. As low pressure develops east of the Rockies, increasing upslope flow north of the low, along with low-to-mid level frontogenesis, is expected to support heavy snowfall accumulations from the Laramie and Medicine Bow Mountains and the Front Range eastward into High Plains - extending into the southwestern sections of the Nebraska Panhandle Tuesday morning. Day 2... ...Oregon/California into the Great Basin... The global guidance is in generally good agreement, showing a rapidly deepening, compact cyclone dropping southeast toward the Oregon/California border on Tuesday. Most of the guidance shows the surface low pressure decreasing by nearly 30mb in 12 hours, with a central pressure dropping below 980mb as it nears the coast Tuesday afternoon. Beginning late Tuesday and continuing into early Wednesday, powerful onshore flow will support widespread moderate to heavy precipitation across southern Oregon and northern California. With snow levels dropping to near the coast, widespread heavy snowfall accumulations are expected from the southern Cascades and the coastal ranges of southwestern Oregon and northwestern California to the Sierra. By Wednesday morning, widespread snowfall accumulations of 12-24 inches, with locally heavier amounts can be expected across these areas. Lighter accumulations will extend farther east into eastern Oregon and northern Nevada. ...Central Plains to the Upper Great Lakes... The upper trough forecast to move across the central Rockies early Tuesday is expected to amplify, with a closed low developing as it moves east across the central Plains on Tuesday. Models show the upper system continuing to deepen with a strong surface cyclone developing and tracking northeast from central Plains and lower Missouri valley late Tuesday, toward the Great Lakes Wednesday morning. In addition to the strong upper forcing, low-to-mid level frontogenesis will likely support a stripe of moderate to locally heavy snows northwest of the low center. In comparison to previous runs, the general consensus of the overnight models has shifted the axis of heavier amounts a little farther to the northwest across the central Plains into the upper Mississippi valley, with the heaviest amounts extending from northeastern Colorado to southeastern South Dakota and northeastern Nebraska on Day 2. Day 3... ...Western U.S.... A powerful low moving into southern Oregon Tuesday evening is expected to quickly weaken as it moves farther inland Wednesday morning. However, a well-defined shortwave digging south of the center will continue to amplify the flow - carving out a deep upper trough along the West Coast Wednesday into early Thursday. This will shift the axis of heaviest snows farther to the south along the central into the southern Sierra, with heavy snows developing across the higher elevations of the Southern California coastal ranges on Wednesday as well. While snows are expected to diminish across much of Oregon and California by early Thursday, it is expected that three day totals will likely well-exceed three feet across portions of the southern Cascades, northwestern California ranges and the Sierra. On Wednesday into early Thursday - lighter, but significant snowfall accumulations are expected farther east, particularly along an southwest-northeast oriented baroclinic zone extending from southern Nevada/Utah into northern Utah. This is expected to support some locally heavy totals across the higher terrain of the region. Meanwhile, southwesterly flow ahead of the low to the north will draw deeper moisture into the northern Rockies. This along with strengthening upslope flow and low level convergence will raise the potential for heavy snow along the Lewis Range in northwestern Montana late Wednesday into early Thursday. ...Upper Great Lakes... Models show low pressure continuing to deepen as it track from the Mississippi valley to the upper Great Lakes on Wednesday. Moderate to heavy synoptically driven snow as the low passes to the south on Wednesday morning and afternoon, followed by a period lake-enhanced snow showers as the system departs to the east, are expected to result in heavy snowfall accumulations across the U.P. of Michigan. For Days 1-3, the probability of significant icing (0.25-inch) is less than 10 percent. Pereira