Lake-effect snow, Appalachian upslope snow and a Sierra storm line up for Nov. 11-14

A three-part winter pattern takes shape across the northern tier and high terrain from Wednesday night through Saturday, according to the National Weather Service's Weather Prediction Center on Nov. 11, in a discussion running from 12Z Wednesday to 12Z Saturday. The discussion is a planning forecast built on probabilities, not warnings, and no part of the pattern involves flooding — the Weather Prediction Center put the chance of rainfall exceeding flash flood guidance below 5% nationwide for the same days. What it involves is lake-effect snow off the Great Lakes, upslope snow in the Appalachians and the Northeast's high terrain, and a wet, heavy Sierra storm arriving Friday.

The Great Lakes piece starts first. A trough digging south from Ontario sends a cold front through the region, followed by northerly flow over lakes that are still mild — Lake Superior at about 8 to 12 degrees Celsius and Lake Michigan at about 10 to 15 degrees — while air at about 850 millibars drops to between minus 5 and minus 10 degrees. That 15-to-20-degree difference is the engine for lake-effect snow. The coldest surface air settles over Michigan's Upper Peninsula, where multi-banded snow is expected from late Wednesday evening into Saturday. Through Saturday morning, the probability of at least 4 inches is over 70% over the Huron Mountains, away from the warmer lakeshore, with lighter snow over the rest of the Upper Peninsula and northern Lower Michigan.

The same northerly flow, running down the long axis of Lake Michigan, could spin up a single robust lake band aimed at northwestern Indiana on Thursday evening into Friday. Temperatures there are marginal, so the band would fall as rain and snow, but a narrow area of modest wet snow is possible, with the probability of at least 4 inches at about 20% to 60%. Lake bands are notoriously narrow — a few miles can separate pavement from a near-whiteout — so drivers crossing the Indiana lakeshore late Thursday and Friday should expect conditions to change quickly over short distances.

In the Northeast, low pressure over the Corn Belt on Wednesday arcs through the Midwest and into the central Appalachians overnight into early Thursday. The Weather Prediction Center is candid that forecast models have struggled with how this system ejects and how far north its moisture reaches, so placement carries more uncertainty than usual. The general picture for Thursday into early Friday: snow focused on the terrain, mostly rain in the valleys, with perhaps under half an inch of snow even in lower spots Thursday night into early Friday. Above 1,500 feet, the probability of at least 4 inches is over 50%, and some areas above 2,500 feet could reach at least 8 inches, at better than a 40% chance.

The central Appalachians get a long event of their own. Behind the front, cold air advection and moist northwesterly flow forced upslope along the western side of the mountains keep precipitation going from Thursday night into early Saturday. Temperatures through most of the air column drop below freezing by late Thursday night, changing rain to snow, and light to moderate snow accumulates into the first half of the weekend, especially above 3,000 feet. The probability of at least 4 inches is over 40% across the high terrain and as high as 90% in the best-aligned upslope areas, especially across the high terrain of West Virginia and along the Tennessee and North Carolina border. Valley routes will see far less — the classic Appalachian trap of a dry-looking climb that ends in snow at the ridge.

The Sierra Nevada storm is the heaviest of the three. A strong upper-level wave drives an atmospheric river into the range on Friday — strong, though weakening, with integrated vapor transport diminishing to 250 to 400 kilograms per meter per second by Friday night. Moderate to heavy upslope snow begins Friday afternoon and continues past the forecast period's end Saturday morning. Snow levels start near 8,000 feet and fall to 6,000 to 7,000 feet Saturday, and the snow will be wet and heavy, with snow-to-liquid ratios of 9 to 11 to 1 at best. The probability of at least 8 inches is over 50% above 6,500 to 7,000 feet, and of at least 12 inches over 50% above 7,000 to 7,500 feet. The probability of moderate travel impacts runs 40% to 90% on the probabilistic winter storm severity index, mainly above 7,000 to 8,000 feet, and the discussion calls travel across some passes difficult.

One piece of good news runs through the discussion: the probability of significant ice anywhere in the contiguous United States is less than 10%, so this is a snow-and-timing problem, not an icing problem. The practical steps are the standard ones, applied early. Check state 511 systems and the local National Weather Service office before rolling — this is a national outlook, and the warnings that govern any given stretch of road come from those local offices as the events get closer. Build slack into Upper Peninsula runs from Wednesday night, Appalachian high-terrain runs from Thursday night, and any Sierra crossing from Friday afternoon. Where a load can safely wait out a band or a pass closure, the forecast says these systems move on by Saturday — the expensive outcome is getting caught mid-route in the Indiana band or on a Sierra grade in wet, heavy snow.