Subj : Major Winter Storm Discus To : All From : Dumas Walker Date : Tue Apr 02 2024 03:19 pm FOUS11 KWBC 021924 QPFHSD Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 324 PM EDT Tue Apr 2 2024 Valid 12Z Tue Apr 02 2024 - 12Z Fri Apr 05 2024 ....Great Lakes... Days 1-2... ....Major late season winter storm this week... The stage is being set for a multi-day major April winter storm that will bring about a myriad of precipitation types (heavy snow, heavy rain, sleet, freezing rain), which combined with strong winds from the Upper Midwest to New England will result in numerous impacts tonight and through the end of the week. The origins of this winter storm begins with a vigorous northern stream disturbance diving south and phasing with a southern stream disturbance tracking northeast into northern Illinois and southern Wisconsin. This interaction will result in the rapid deepening of a more consolidated 500mb low over the Upper Midwest and quick intensification of a surface low by 06Z tonight over Lake Michigan. The latest forecast calls for the storm to deepen by as much as 20mb over the next 18 hours. As the 500mb low rapidly deepens, intense vertical velocities on the northwest and western flanks of the 850mb low, induced by intense PVA aloft and exceptional 850-700mb FGEN gives rise to a potent deformation axis that will become primarily snow this evening from northeast Iowa to central Wisconsin. As the storm occludes overnight, the TROWAL on the backside of the of the storm will continue to be the focus for heavy snow over northern Wisconsin and the U.P. of Michigan. The U.P. pf Michigan, in particular, will sport the best chances for ripping snowfall rates of >2"/hr thanks to the lake enhanced bands off Lake Superior and along the more elevated terrain of the central U.P.. By 12Z Wednesday, NAEFS shows 500mb heights that, according to NAEFS, fall below the observed CFSR database (1979-2009) over Illinois, illustrating the highly unusual nature of a cyclone that intense over Illinois for early April. It is on the northern flank of the low Wednesday AM where snowfall rates will be most significant. Due to the upper low being cut off from the mean flow to the west and the upper level omega block over eastern Canada and the northwest Atlantic, the storm system will be slow to move east on Wednesday, prolonging the period of heavy snow in northern Wisconsin and the Michigan U.P.. Latest WPC probabilities show high probabilities (>70%) for snowfall totals >8" from northern Wisconsin to the Michigan U.P.. Farther south, there are moderate-to-high chances (50-70%) from just north of the I-94 corridor in southern Wisconsin up to areas just west of Green Bay. The WSSI sports Major to even locally Extreme Impacts for areas neighboring Green Bay and in the central Michigan U.P. The localized Extreme Impacts are depicted along the Huron Mountains through Wednesday afternoon. The Hurons currently have high chances (>70%) for >24" of storm total snowfall. Snow Amount is the primary driver in the WSSI algorithm, but the WSSI is also showing in some parts of eastern Wisconsin and even near the tip of Michigan's Mitt, that some Moderate Impacts as a result of Snow Load and Blowing Snow are expected. ....Northeast... Days 2-3... While the upper low in the Great Lakes occludes Wednesday afternoon, farther east, an impressive IVT over the Southeast will see some of its associated moisture stream north into the Northeast Wednesday morning and run into an air-mass just sufficiently cold enough to support a wintry mix of sleet and freezing rain from the Catskills and Adirondacks to the Berkshires. This wintry mix will translate farther north and east through the Worcester Hills, the Green Mountains, and White Mountains by Wednesday afternoon. This air-mass supporting the onslaught of wintry precipitation is not expected to leave any time soon due to the upper level omega block mentioned in the Great Lakes section that is locking in a dome of cold Canadian high pressure over Quebec. In actuality, what this omega block will do in part is to help keep the storm track farther south. As the occluded front works north through the Mid-Atlantic Wednesday afternoon, intense PVA and robust 850mb FGEN from southern PA to the southern New England coast will rapidly intensify an area of low pressure tracking from the Delaware Valley Wednesday evening to along the Long Island coast by early Thursday morning. The key to this forecast lies with when the surface low takes over. Most of the interior Northeast will be dealing with a wintry mix due to the >0C warm nose in the 800-750mb layer. However, once the 850mb low forms, winds will shift more out of the E-NE, reducing the warm nose aloft and leading to a sudden changeover to snow. Latest guidance shows this happening somewhere in the 03-09Z Thursday timeframe, which given this coincides with snow falling overnight, will maximize the opportunity for rapid accumulations on all surfaces. By 12Z Thursday, just about everyone from Upstate New York and interior New England to even the coast of New Hampshire and Maine can expect to be all snow.Snow fall rates late Wednesday night and through Thursday morning are likely to be between 1-2"/hr with wind gusts topping 40mph in many cases, especially along the New England coast and in the higher elevations. The storm looks to occlude off the eastern Massachusetts coast with the cold conveyor belt (CCB) of snow to the north of the low lasting over the northern Appalachians and much of Maine through Thursday evening. Due to the upper level omega block still in place, the upper low over the Northwest will continue to keep periods of snow in the forecast in the Green and White Mountains, but now with the air- mass modifying and gradually diminishing upper level support, a mix of rain and snow showers will be possible through Friday afternoon. Last but not least, upslope enhancement in the central Appalachians of eastern West Virginia will also ensue Thursday evening and into the day on Friday with elevations >3,000ft most likely to see heavy snow through the end of the work-week. In terms of impacts, this will be an exceptional one for the Northeast given not only the heavy, wet snow that is expected, but the prolonged round of strong winds combined with highly saturated soils in the Northeast. The WSSI shows Major Impact potential (considerable disruptions to daily life, widespread closures and disruptions) in parts of the Adirondacks, the White Mountains, and as far east as the Kennebec Valley of central Maine. Moderate Impacts (hazardous driving conditions, closures and disruptions possible) are higher confidence in the areas expecting Major Impacts, but are also possible in parts of the Catskills, Green Mountains, Berkshires, Worcester Hills, and in parts of the central Maine Highlands and along the central Maine Coast. Snow Amount is the primary driver in the WSSI algorithm, but a combination of Snow Load and Blowing Snow is also included with Moderate Impacts possible. With the expected impacts from Blowing Snow correlated to strong winds and the Snow Load component present as well, the exceptionally saturated soils throughout the region is leading to increased concerns for extensive tree damage and power outage potential. WPC PWPF sports high chances (>70%) for snowfall totals >12" in the Adirondacks, as well as the Green and White Mountains, and into portions of central Maine. There are even some moderate- to-high chance probabilities (50-70%) for >24" in parts of the White Mountains. Please see our Key Messages below for the ....Western U.S.... Days 2-3... An upper level trough originating in the Gulf of Alaska will plunge south and be located off the Pacific Northwest coast by Wednesday afternoon. Pacific moisture streaming out ahead of the upper trough will result in some mountains snow along the Cascade Range and over the Olympics during the day on Wednesday. By Wednesday evening, a mid-upper level frontal band will focus a heavier swath of precipitation from northern California to the Northern Rockies where snow will fall from the Trinity/Shasta and the Blue Mountains of eastern Oregon to the Boise/Sawtooth and Bitterroots overnight. As the upper trough continues to dived south off the West Coast, the current of Pacific moisture will work its way south along the spine of the Sierra Nevada where upslope enhancement will prompt heavier snowfall rates over the central and southern Sierra Nevada throughout the day. The enhanced snowfall rates are also a byproduct of a compact and robust 500mb low at the base of the upper trough moving into central California. This leads to falling snow levels that support heavier snowfall totals occurring as low as 5,000ft. By Thursday night, 500mb and 700mb heights over central California are forecast by NAEFS to be below the 0.5 climatological percentile and will even allow for some locally heavy snowfall amounts in the Transverse Ranges through Friday. Snowfall rates will back off some across most of the Pacific Mountains and Great Basin on Friday, but modest lift and steep lapse rates will still keep mountain snow in the forecast from as far south as the Peninsular Range to as far north as the Bitterroots. Latest WPC PWPF shows high chances (>70%) for snowfall accumulations >12" in portions of the Blue Mountains and both the central and southern Sierra Nevada above 6,000ft, while similar high chance probabilities for >8" of snow are present in the Trinity/Shasta, the Oregon Cascades, and into parts of the Boise/Sawtooth mountains. The WSSI-P sports moderate-to-high chances (50-70%) for Moderate Impacts in the central and southern Sierra Nevada, the Blue Mountains, and the central Great Basin in central Nevada through Friday afternoon. Mullinax ***Key Messages for Early April Nor'easter*** ---Western Great Lakes heavy snow Tue night Heavy snow will develop over Wisconsin and expand into Upper Michigan overnight. Heavy snow will continue over much of the region through Wednesday, with additional heavy snow across portions of Upper Michigan, before ending on Thursday. As much as 1-2 feet of snow is expected in parts of northern Wisconsin and the U.P. of Michigan. ---Western Great Lakes heavy snow Tue night Heavy, wet snow and some sleet will envelope the Northeast Wednesday afternoon through Friday. Significant snow accumulations over 12 inches are likely over northern New York and central New England. ---Significant impacts from heavy snow and wind The combination of heavy snow rates and gusty winds will result in hazardous travel due to whiteout conditions and snow-covered roads. The wet snow and high snow load combined with strong wind gusts may also cause tree damage and power outages. ---Moderate coastal flooding in the Northeast Prolonged onshore flow late Wednesday and continuing through Thursday will result in moderate coastal flooding for portions of the Northeast coast. Impacts include widespread roadway flooding, coastal and bayside flooding, impassable roads, and some damage to vulnerable structures. $$ --- SBBSecho 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (21:1/175) .