Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 403 AM EST Tue Nov 26 2019 Valid 12Z Tue Nov 26 2019 - 12Z Fri Nov 29 2019 Day 1... ...Oregon/California into the Great Basin... Models continue to show a low of unprecedented track and strength taking aim at the southern Oregon coast on Tuesday. A rapidly deepening, compact cyclone is forecast to drop southeast toward the Oregon/California border on Tuesday. Most of the guidance show 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 winds will support widespread moderate to heavy precipitation across southern Oregon and northern California. With snow levels dropping to as low as 500 ft, 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. Meanwhile, lighter accumulations will extend farther east into eastern Oregon and northern Nevada. ...Central Plains to the Upper Great Lakes... An upper trough moving east 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 and assume a negative-tilt, 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. This is supported by several HREF members which indicate precipitation rates of 0.15-0.25 in/hr developing over south-central to eastern Nebraska during the late afternoon and evening hours. As the system tracks northeast, the HREF members suggest precipitation rates will decrease some over northwestern Iowa followed by some reintensification over southern Minnesota during the overnight hours. For the 24-hr period ending 12Z Wednesday, WPC probabilities for snowfall accumulations of 8-inches or more are 50 percent or greater from from northeastern Colorado and far northwestern Kansas to southern Minnesota. Day 2... ...Western U.S.... A powerful low moving into southern Oregon Tuesday evening is expected to 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 northern California by early Thursday, it will not be before snowfall totals likely well-exceed three feet across portions of the southern Cascades, the northwestern California ranges and the Sierra. On Wednesday into early Thursday - significant snowfall accumulations are also 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. WPC probabilities indicate that snowfall accumulations of a foot or more are likely across some of the higher peaks of the southern Nevada and Utah ranges to the Wasatch and Uintas by Thursday morning. 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 tracks 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, followed by a period lake-enhanced snow showers as the system departs to the east in the afternoon and evening, will likely result in heavy snowfall accumulations across the U.P. of Michigan and northern Wisconsin. ...Northeast... As the upper low moves east of the upper Great Lakes, models show the surface low over eastern Ontario quickly weakening as a new low forms over the Gulf of Maine Wednesday night. This will bring a period of mixed precipitation resulting in some light snow and minor ice accumulations across northern New England, with the heaviest accumulations centering across far northern New Hampshire and northern Maine. Day 3... ...Southern California/Southwest/Central and Southern Rockies... A series of shortwaves diving south along the West Coast will continue to amplify the upper trough over the West, carving out 500mb heights 3.5-4 standard deviations below normal across Southern California Thursday into early Friday. A well-defined shortwave digging into the base of the trough is expected to support additional heavy snows along the South California coastal ranges during the afternoon into the evening hours, with heavy snows developing farther east once again across the southern Nevada mountains into Utah during the evening and overnight hours. By Friday morning, heavy snows are expected to extend southeast along the Mogollon Rim into the White Mountains of Arizona, while developing farther northeast across the San Juan Mountains in southwestern Colorado. For Days 1-3, the probability of significant icing (0.25-inch) is less than 10 percent. Pereira