Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 457 PM EDT Mon Apr 1 2024 Valid 00Z Tue Apr 02 2024 - 00Z Fri Apr 05 2024 ...Southwest/Central and Southern Rockies... Day 1... Positively-tilted upper trough will finally exit the region by Wednesday, split apart by an exiting vort max over northeastern NM this evening and trailing stretched vorticity across northern Mexico. Light to perhaps modest mountain snow will continue this evening/overnight as residual moisture over AZ and CO/NM subsides. Additionally, another vort max will help wring out a few more inches of snow over the San Juans and especially the Sangre de Cristos in CO/NM tonight, where several inches of snow are likely. WPC probabilities for at least an additional 6 inches of snow are highest (>50%) over the Sangre de Cristos. ...Pacific Northwest... Days 2-3... Amplified northern stream will bring in a digging trough to the PacNW starting Tuesday night/early Wednesday and continuing through Thursday as it slowly advances eastward/southeastward. High snow levels in advance of the cold front (~7000ft) will trend down to around 3000ft across the Cascades by early Wednesday and perhaps to 2500ft Wednesday night into early Thursday, progressing through Oregon and into NorCal and also into the northern Great Basin. Moisture source will be twofold... the lead source from the subtropics Wednesday and the subsequent lesser moisture flux from the northern stream Thursday as additional backside vorticity deepens the trough along and just off the West Coast. The precipitation will come in waves, tied to the moisture sources, but slowly/steadily progress down the coast through D3 (and beyond) with lowering snow levels into the many of the mountain passes. WPC probabilities for at least 8 inches of snow for days 2-3 are highest (>50%) in the northern WA Cascades and over northwestern Montana, over the OR Cascades and especially the Blue Mountains (which may see the heaviest totals) then southward into the NorCal ranges (Shasta/Siskiyous/Trinity) and the northern Sierra, generally above 6000ft or so. ... Great Lakes into the Northeast... Days 1-3... ...Significant late season winter storm this week... Models continue to indicate that a long duration late-season winter storm will develop mid-week, producing widespread gusty winds, especially on the coast, and heavy snow over portions of the Great Lakes and the interior Northeast. Upper pattern Tuesday morning over the central/eastern CONUS will feature the positively-tilted trough out of the Southwest, with a lead vort max moving northeastward from the mid-MS Valley into the Upper Great Lakes, and a digging upper low out of southern Manitoba moving into the Corn Belt, initiating a bit of a Fujiwara interaction by Wednesday and a rapidly deepening surface low over Lower Michigan. This phasing/interaction will result in a maturing, occluded, and slowing system occupying much of the Great Lakes that will become vertically stacked as southern stream energy out of the Southeast/Gulf help form another low over the Mid-Atlantic that moves northeastward toward/across Cape Cod. This new low will become the parent low to a new occluded front into the Atlantic, allowing for a slowing of that low around the Gulf of Maine late Thursday into early Friday. First part of the system will center over the western Great Lakes in response to the developing low from the southern stream. Models continue to show a track favorable for heavy snow developing across northern Lower Michigan, supported initially by low-to-mid level frontogenesis and favorable upper jet forcing centered across the region late Tuesday evening. With the incoming northern stream upper low out of Canada, models have trended farther west with the system, which will wrap around snowfall farther west into Wisconsin Tuesday evening into Wednesday. Temperatures will be fairly marginal, except for over the U.P. of Michigan, and SLRs will be on the low side as well (<10:1 on the southern side of the snow shield). Strong dynamics could support heavier rates (>1"/hr per the WPC Snowband tool) over eastern WI into northern Lower MI tomorrow evening. As the system becomes vertically stacked, snow rates will decrease over the region but snowfall will continue through Wednesday and into most of Thursday. WPC probabilities for accumulations of 4 inches or are highest (>50%) over much of central/eastern WI into much of the U.P. of Michigan and northern Lower MI. Second phase of the system will be its affect on the Northeast, starting Tuesday afternoon over the Catskills from lead WAA but encompassing much of the Northeast by Wednesday afternoon. At that point the triple-point low will start to become the primary surface feature as it tracks off the Mid-Atlantic coast toward southeast Mass. Global model guidance remains in overall good agreement with a healthy area of QPF and marginal thermal environment, tied to elevation and time of day (typical for late- season systems). This will also result in lower SLRs for many areas outside the mountains (7-10:1) with rain or mixed precipitation (perhaps a modest amount of sleet due to the strength of a warm nose aloft, advecting in oceanic air off the Atlantic/Gulf of Maine). The low will likely move into the Gulf of Maine and slow as the front yet again extends eastward, elongates, and spawns a new triple point, taking much of the QPF eastward into the Atlantic late Thursday into Friday (just beyond this forecast period). Colder air will wrap around the low before its exodus, changing rain to snow to lower elevations, but with limited accumulation. The heaviest snow will likely be along/east of the White Mountains with the best moisture flux and cold enough temperatures. There, WPC probabilities for at least 12 inches of snow for this event are high (>90%). Over the eastern Adirondacks into the Green Mountains, probabilities for at least a foot are also high (>70%), but those decrease into the Hudson and Connecticut River Valleys to <30%. There will likely be a notable gradient between mountain and valley locations due to the marginal thermal environment during the highest QPF. Elsewhere, there is a broad area of probabilities of at least 4 inches from the event, stretching from western NY eastward along the I-90/86 (Rt 17) corridor into central New England but likely west of I-95 in northeast Mass. Fracasso *** Key Messages for Early April Nor'easter *** ---Long duration winter storm A large storm system will produce gusty winds and late-season heavy snow across portions of the Great Lakes and the Northeast mid to late week. ---Western Great Lakes heavy snow Tue night Heavy snow likely develops over Wisconsin Tuesday afternoon and expands overnight into northern Lower Michigan. Light to moderate snow will continue over much of the region through Wednesday into Thursday before gradually ending. ---Northeast snow and wind Wed-Fri Secondary low pressure development along the Mid-Atlantic coast will likely bring heavy, wet snow and some sleet to the Northeast Wednesday afternoon through Friday. Significant snow accumulations are likely over parts of interior central/northern New England. ---Significant impacts from heavy snow and wind The combination of heavy snow rates and gusty winds will likely result in hazardous travel due to low visibility and snow-covered roads. Additionally, the wet snow and high snow load may cause tree damage and impact infrastructure.