Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 418 AM EDT Mon Mar 27 2023 Valid 12Z Mon Mar 27 2023 - 12Z Thu Mar 30 2023 ...Northeast... Day 1... Mid-level vort max will stream eastward out of the Midwest and along the NY/PA border into Southern New England today/tonight as a weak surface low pressure zips across the Mid-Atlantic. Brief WAA and 700mb FGEN will support light to moderate QPF in a very marginal thermal environment during the day. After sunset, as the wave moves eastward, some cooling aloft and down to the surface will allow a changeover to snow in especially higher elevations across central NY into the southern Adirondacks, Catskills, then into the Green Mountains and Berkshires where a couple inches are likely. WPC probabilities for at least 4 inches of snow are low -- generally no higher than 15%. ...Colorado and High Plains... Day 1... Shortwave along the UT/CO border early this morning will move eastward today into KS. An inverted trough at the surface will lie beneath an area of 700mb FGEN which will help enhance snowfall early D1 over the northeast CO plains into parts of NE/KS. Several inches are possible in the first 12 hours of D1 with perhaps up to 6" per the HREF mean. WPC probabilities for at least 4 inches are >50% over northeastern CO near and southeast of I-76. ...Great Lakes and Northeast... Days 2-3 Surface cold front beneath a shortwave trough on the south side of the deep Hudson bay upper low will push through the Upper Great Lakes and across Lower Michigan late Tue into Wed from northwest to southeast. Limited moisture with the front itself will only result in an inch or less of snow, but NW flow behind the front should promote a few inches of Lake Effect snow across the U.P. overnight Tuesday. Shortwave will start to deepen as it moves through Ontario, which should act to help wring out more snowfall over the Northeast, especially over the Adirondacks into the northern Green and White Mountains into Wednesday. WPC probabilities of at least 4 inches are generally under 50% over the region. ...Oregon/California and Great Basin... Days 1-3... Vigorous upper low west of Washington this morning will continue southward just west of 130W, wrapping in another system from the southwest today and continuing to deepen through Tuesday. Snow will overspread the OR Cascades and especially the NorCal ranges tonight into early Tuesday as a precipitable water plume (0.50-0.75" or +1 to +2 sigma) surges into the area in advance of the cold front and back around the wrapped occlusion. Snow levels will rise to around 4000-5000ft when the heavier snowfall arrives before falling back 1000-2000ft lower post-FROPA as the snow starts to ease back. The system, though anomalous with 500mb heights near -3 sigma, will remain progressive and continue to sink southward then southeastward paralleling the northern/central CA coast Wed into early Thursday as it then opens up into a strong trough by the end of D3 (12Z Thu) as the upper jet stretches northeastward across the Great Basin. This will focus most of the heaviest snowfall over the NorCal ranges and through the Sierra where 3-day totals will likely exceed 1-2ft (with >4ft at the highest elevations). Snowfall rates will be intense at times with 1-3"/hr rates likely per the HREF probs and WPC snowband tool. Snowfall will also progress southward into the San Bernadinos and San Gabriels in Southern California Wed-Thu as snow levels drop to near 4000ft. 4 inch snowfall probabilities there are moderate (>40%) and more than 6 inches is possible at the peaks. To the northeast and east, height falls and sufficient moisture on southwest flow will spread light to moderate snow across interior Oregon into central Idaho and across Nevada into Utah, where snow will fall over most locations except the lowest valleys. WPC probabilities for at least 8 inches D2-3 in the Great Basin are highest (>70%) over the Blue Mountains into Idaho. For Days 1-3, the probability of significant icing greater than 0.25" is less than 10%. Fracasso