Beginning at the start of December, meteorological winter marks a three-month period through February where temperatures in the Northern Hemisphere are at their coldest. Not surprisingly, winter across a large swath of the United States is associated with much colder temperatures and wintry precipitation in the form of snow, sleet or freezing rain. Across the Great Lakes, the invasion of cold air in late fall and early winter can spell trouble for neighboring communities, as lake effect snow can significantly increase the rate at which snow falls, and as a result, causes a higher annual snowfall compared to other parts of the country aside from the Rocky Mountains and the Cascades. Since 30 September 2021, some communities downwind of the lakeshores have received up to three (3) feet of snowfall, primarily from lake-effect snow, and there is plenty more to come over the next several weeks!
Figure 1: Seasonal snowfall accumulation from 12Z 30 September 2021 to 12Z 2 December 2021, centered over a) Midwest United States, and b) Northeast United States. Source: Pivotal Weather, with derived data plotted from the National Operational Hydrologic Remote Sensing Center (NOHRSC).
Behind a pair of cold fronts associated with a nearby clipper system, lake-effect snow began to fall over portions of northern Michigan on 2 December 2021 and has consistently produced light to moderate snowfall across this region as of 06Z 3 December 2021 (Figure 2). Although this event pales in comparison to the feet of snow that lake-effect events have been known to produce, this example forms under the same conditions. As depicted in Figure 3, lake-effect precipitation, either in the form of rain or snow depending on the temperature, forms when colder air is advected over a relatively warm body of water. With a greater temperature contrast between the surface and air in the lower parts of the atmosphere, the air becomes more unstable, and convection is created. During late winter, lake-effect actually comes to a halt as ice begins to form on the lakes, which prevents evaporation from occurring to the degree needed to cause condensation from lake-effect alone. For lake-effect events, convection manifests itself in the form of horizontal convective rolls (HCRs), or “cloud streets”, as shown on the radar imagery, where small-scale rising and sinking motions along the boundary layer produces rows of clouds and precipitation as opposed to a single convective line.
Figure 2: NEXRAD Base Reflectivity (0.5 deg.) centered over Gaylord, Michigan from 1239Z 2 December 2021 to 0141Z 3 December 2021. Source: College of DuPage.
Figure 3: Conceptual model of lake effect snow over an idealized lake. Source: The COMET Program.
While forecasters use a wide array of tools when predicting winter weather, meteorologists use two main tools aside from the wind direction to determine if and where lake-effect snow will occur: lapse rates and frictional convergence. As previously mentioned, instability arises where there is a sharp temperature contrast between the surface and the air close above. Lapse rates are the perfect diagnostic tool for looking at instability in the lower parts of the atmosphere, as it measures how quickly temperature decreases with respect to height. In Figure 4, lapse rates greater than 5C/km over northern Lake Michigan on 3 December 2021 suggests that colder air was present above the warmer lake surface, which plays a large role in promoting instability. Through an evening sounding on 3 December 2021 in Alpena, Michigan (Figure 5), strong cooling with height are evident in the red temperature profile, in which the air within the lowest kilometer of the atmosphere cools at a rate close to the dry adiabatic lapse rate.
Figure 4: 0-3 km lapse rate (contours, C/km) from 1200 UTC 02 December 2021 to 0900 UTC 03 December 2021, taken in three-hour intervals. Source: NOAA/NWS Storm Prediction Center.
Figure 5: Observed sounding (top left panel) and hodograph (top right panel) data from Alpena (APX), Michigan at 0000 UTC 3 December 2021. Source: NOAA/NWS Storm Prediction Center.
In tandem with strong lapse rates, forecasters also examine convergence along the lakeshores as wind speeds decrease during the transition from a frictionless lake surface to a friction-filled landmass with trees and other obstructions. Similar to the development of thunderstorms, convergence at the surface allows for air to rise, which can then form clouds and precipitation. The fill pattern on Figure 6 shows where the greatest surface convergence occurs, which is also very strong along the northwestern shores of Michigan’s Lower Peninsula. With strong lapse rates and strong frictional convergence across this region, lake-effect snow survived through 3 December 2021 in a pattern that seeks to repeat itself until the lakes freeze over later this winter.
Figure 6: Surface-based total deformation (shading, meters), axes of dilatation (blue bars) and theta (red contours, K) from 1500 UTC 02 December 2021 to 0000 UTC 03 December 2021, taken in three-hour intervals. Source: NOAA/NWS Storm Prediction Center.