Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 432 AM EDT Fri Mar 27 2020 Valid 12Z Fri Mar 27 2020 - 12Z Mon Mar 30 2020 ...Southwestern U.S. to the Western Great Lakes... Days 1-2... Deep positively tilted trough will begin to tilt neutrally, and eventually close off as it lifts northeast from the Four Corners into the Central Plains. This mid-level low will deepen quickly Saturday as it moves towards the Great Lakes, aided by increasing upper level ventilation within the LFQ of a 150+kt subtropical jet streak. Beneath this features, surface low is expected to develop in the lee of the Rockies and then lift northeast towards the U.P. of Michigan while also deepening. Before the surface low develops, SW mid-level flow will interact with a low-to-mid level front dropping southeast across the Central Rockies. This will support periods of heavy organized mountain snows in the San Juans, eastern Wasatch, and CO Rockies, with easterly upslope flow developing in response to dropping surface pressure driving heavy snow into the High Plains. WPC probabilities for 8 inches are high on D1 in the Front Range of CO. Ascent will become intense along and north of the path of this low. In addition to the height falls and jet level diffluence, WAA will become strong, spreading precipitation northward into the Northern Plains, and a pronounced theta-e ridge is forecast to wrap cyclonically around the low as a robust TROWAL develops into Minnesota. Collocated with this TROWAL, a potent deformation band is likely to develop and pivot across MN/WI/U.P. of MI. While the low-level thermal structure of the atmosphere will be marginal for snow, the strongly forced ascent should dynamically cool the column to cause p-type transition during periods of heaviest precipitation from rain to snow. While there exists still some uncertainty into where this band may setup, the non-NAM consensus is for a swath of heavy snow from S-Central MN into northern WI and the Arrowhead of MN. The heaviest snow may reach 1"/hr at times within this band, and WPC probabilities have increased for 4 inches to 50%, highest in the Arrowhead where isolated amounts over 6 inches are possible. ...Northeast... Day 3... A warm front extending from the significant low pressure near the Great Lakes will lift northeastward through the Mid-Atlantic and into New England. This warm front will briefly stall as a wedge of high pressure extends down from Maine, before progressing further northeast during Sunday. As this occurs, strong WAA will spread precipitation northward, with isentropic ascent further enhancing the coverage and intensity of the precipitation. At the same time, a wave of low pressure is likely to develop along the boundary near Cape Cod, and strengthen as it moves northeastward late in the forecast period. As precipitation expands into New England, it will initially be snow across Central and Northern New England, with some sleet also potentially mixed in due to wet bulb cooling affects from antecedent dry air. As the WAA persists, p-type will gradually transition to rain everywhere except the terrain of NH and most of ME away from the coast where ageostrophic flow between the wedge and the developing low should keep the column cold enough for snow. It is in this area where accumulations are expected to become moderate to heavy, and WPC probabilities for 4 inches are high across the northern half of Maine. ...Pacific Northwest to the Northern Rockies... Days 2-3... A deepening upper trough will extend the length of the Pacific Coast and shift onshore Sunday. Two distinct shortwaves embedded within this trough, combined with persistent confluent mid-level flow will provide ascent and moisture into much of the terrain of the West Saturday and Sunday. This will spread periods of snow across much of the terrain of the West, with snow levels varying from around 4000 ft in the Washington Cascades, to as high as 7000 ft in UT/CO. The heaviest snowfall during the period is expected in the Olympics and Cascades where persistent moisture and forcing leads to high WPC probabilities for 12 inches on each day, with 4 ft of snow possible in the highest terrain. Elsewhere, WPC probabilities for 6 inches are moderate on D2 in the OR Cascades, Blue mountains, and Sierra, spreading south and east into the Northern Rockies, Wasatch, and CO Rockies on D3. For Days 1-3, the probability of significant icing (0.25-inch or greater) is less than 10 percent. Weiss