The past two winters in North America had left their mark on many communities with unprecedented snowfall. During the 2014-2015 winter, Boston was left with record breaking snowfall—108.6 inches. On January 22-23, 2016 Baltimore, Maryland had record total snow accumulation of 29.2 inches. Last winter, Bismarck, North Dakota saw snow events of 18.7 inches, 7.1 inches, 12.5 inches, and 9.6 inches all in a 40-day span between Thanksgiving and early January. Snow dump usually indicates a place where excessive snow is removed and dumped. However, excessive snow has made entire cities the dumping grounds for Mother Nature’s surplus, but why?
Formation of Heavy Snow
For heavy snow to accumulate from a system several conditions must align. A ramp for warm air to go up to form snow must exist, warm and moist air needs to be brought up the ramp, and rising motion is needed to make going up the ramp easier. That rising motion is required to cool the air and condense/freeze the moisture, which leads to snow formation if cooling and condensation can occur fast enough. In the Northeast and Midwest, the ramp is a wedge of dense cold air brought in from Canada. For the Dakotas and Plains, this wedge of cold air is augmented by the elevation increase from the moisture source (the Gulf of Mexico) to the region. In the West, the terrain itself acts as the ramp, producing prolific snow on one side of a mountain and a desert on the other side.
The air in the cold wedge must be cold enough to preserve the newly formed snow falling from above in a frozen state. Air must be just warm enough at the top of the ramp to hold lots of moisture, but cold enough to not melt the new falling snow. If the air at the top of the ramp is too warm, freezing rain or sleet will occur as the melted snow falls through the cold air wedge below and freezes in midair (sleet) or as it hits the ground (freezing rain). If the cold layer were very thin and the ground warmer than freezing, the melted snow would simply hit the ground as rain.
Additionally, a moisture source is necessary to feed the system. For the Midwest, Dakotas, and Plains, moisture flows north into the region from the Gulf of Mexico. For the Northeast, the source is the Atlantic Ocean fed by the Gulf Stream. The West has the Pacific Ocean to provide the moisture. In a region if there is an absence of mountains, a surface low pressure must be present to create some rising motion to begin snow formation by forcing air up the ramp. Motion around a jet stream creates the rising motion for low pressure to form—however, the jet stream is reliant on a surface temperature gradient to form, which is related to the low-level cold air existing.
Tracking the Snow
Heavy snow almost always falls just northwest of the low-pressure track, so the surface temperature gradient that the low-pressure track follows commonly sits to the southeast of the heaviest snow. For example, the Dakotas were commonly hit with heavy snow in the winter of 2016-17 due to low pressure areas moving from Nebraska to Minnesota. Moisture from the Gulf of Mexico was easily available as these areas of low pressure passed southeast of the Dakotas, leaving repeated messes to clean up. The cold air wedge can easily be seen—during and after the 18.7 inches of snow that fell in Bismarck, North Dakota on November 27-30, 2016, highs in the 50s just before the snow gave way to a wedge of cold air that led to highs around 30 and lows in the teens. A follow up snow of 7.1 inches a week later brought a new wedge of cold air that had lows in the -20s at its worst.
Things are a bit different in the West, as rising motion comes from air being pushed up mountains as it circulates around areas of low and high pressure—for example, Reno, Nevada had their heaviest snow of the 2016-17 winter on a day where an area of low pressure was off the coast of Oregon and an area of high pressure sat over New Mexico. On the other side of a mountain range, Fallon, Nevada (55 miles away) had far less snow on the same day.
On top of that, lake effect snow is its own unique category. A typically benign flow of cold air across the Great Lakes can produce exceptional snowfall if it passes over the same part of the Great Lakes for days on end. This doesn’t require any area of low pressure nearby or ramp—cold air brought over the warm lake becomes warm and moist enough to rise on its own. Eventually, this air is slowed down by moving from the smooth lake to rough land, helping to produce snow just off the lake. As the patterns that lead to such cold air moving across the Great Lakes can persist for many days far easier than any other heavy snow setup, frequent snow squalls and impressive snow totals are routine every winter in parts of Michigan, Ohio, Pennsylvania, and New York.
A region gets dumped on repeatedly when the upper level pattern and jet stream get stuck in one place frequently and this happens repeatedly all winter. El Niño/La Niña and/or some other oceanic temperature features (such as the former ‘warm blob’ in the northeast Pacific that ruled winters in the early 2010s) can reinforce this repetition. In these events, cold air gets repeatedly pushed down the same path while the jet stream is passing through the same areas to form low pressure in mostly the same areas. If this is locked in badly enough, the only way to dislodge the pattern is for spring to start and its higher sun angle to change surface temperature patterns and break down the upper level pattern. The same can happen in the West, with the wind patterns stuck in place bringing moisture up the same mountains, as happened last winter.
2017-2018 Winter Season
Weather is unpredictable. However, for this upcoming winter the Dakotas appear to have a better than normal chance to return to a more typical winter—cold and relatively dry. Bismarck, North Dakota sees 53 inches of snow on average each year. While this winter looks like it’ll be closer to that normal than last year’s 71.6 inches, it only takes one heavy snow event to cause plenty of financial havoc—something that can easily happen in an otherwise benign winter, especially in the Dakotas. The winter of 2012-2013 is an excellent example, as the total snow for Bismarck was 57.3 inches. However, 17.3 inches of that fell on April 14, 2013.
Frequent cold air intrusions should extend further east than last year, setting up the temperature gradient more into the Midwest and Northeast, leading to better chances of frequent heavy snow in the Northeast and in lake effect prone areas of the Great Lakes. This also makes this winter look like it’ll be much colder than the last two. The West overall also looks drier, but parts of the Pacific Northwest look to rival last year’s snow—not likely California levels of wet from last winter, but still seeing snow that would produce undue stress on some municipal budgets and try the patients of commuters alike. Snow dumping is not a new phenomenon, but the last couple of years have made winter a historic event for some locations. We can only wait and see what Mother Nature has in store for this winter season.