Snow in the Great Lakes: Past, Present, and Future

Summary

  • Snowfall in the Great Lakes region can be categorized as lake-effect snow and non-lake-effect snow.
  • Observations show lake-effect snowfall is increasing around the regions of Lake Michigan and Lake Superior.
  • Snowfall amounts will vary in amounts depending on lake ice cover amounts and atmospheric conditions, and the snow can transition to freezing rain or rain in much warmer conditions.  

The description of snow (including snow cover, snow depth, and snow density) is a complicated variable in the Great Lakes region due to the influence of the lakes on local climate.  Great Lakes snow can be partitioned into two main categories – lake-effect snow and non-lake-effect snow.  The regions where lake-effect snow occurs are well defined, however, some are expanding in a warming climate.  Lake-effect regions receive greater amounts of snowfall because the lakes enhance precipitation in those areas.  Lake-effect snowfall only occurs when the lakes are not completely frozen over.

While snowstorms that impact the entire region are decreasing, lake-effect snowfall is increasing around Lakes Superior and Michigan.  Snow depths going into spring are decreasing as warming occurs, and earlier spring snowmelt is occurring. While observations suggest that, on average,  there is more snow during snow events, faster melting means that snow cover is less in late winter and early spring.

Projections of future climate in the Great lakes, and especially future snow, have a lot of uncertainty.  Global climate models are not a reliable source of information for lake-effect snowfall, and regional climate and weather models play a role in filling that gap.  High resolution models can be used to study the interactions between changing air and water temperatures, and their relation to lake ice cover, which are the primary variables for understanding how snow may change in the future.      

Lake ice cover plays a major role in the development and distribution of lake-effect snowfall.  When the lakes are completely frozen over there is essentially no lake-effect snowfall, because the moisture supply for snow, via evaporation from the lake, has been cut off.  Lake ice cover has been decreasing in recent years, which has been accompanied by increases in lake water temperature (due to increasing air temperatures).  Models run under these conditions predict increasing lake-effect snowfall and an expansion of the lake-effect zone, which is consistent with what has already been observed.

What Influences Snow Formation?

Snowfall in the Great Lakes region is primarily from synoptic (i.e., large-scale) weather systems moving across the region (Figure 1), lake-effects, or a combination of both.  When large air masses pass over the Great Lakes region they are modified by the presence of the lakes. 1 2 Cold air from the north, such as storm Type 1 in Figure 1, enter the Great Lakes region as a low-pressure system and pick up moisture as they pass over the relatively warm surface waters of the lakes.  The additional moisture contributes to increased precipitation (snowfall when temperatures are cold enough) downwind (generally on the eastern side) of the lakes.  Some snow events are initiated by synoptic systems and transition to lake-effect snow.

Lake-effect zones (Figure 2) can receive contributions of snowfall from synoptic-scale snowfall events  and lake-effect events.  Synoptic-scale weather systems passing over the Great Lakes pick up moisture from the lakes, which in turn enhances snowfall downwind.  In a typical scenario, cold air advection behind a departing synoptic system will result in cold air moving over the lakes leading to the onset of lake-effect snow events.  However, cold air moves farther inland,  its moisture loss is not replenished and snowfall can diminish.  Unlike large-scale snowstorms that impact great portions of the region, lake-effect snowfall is localized.

Lake-effect zones are typically located directly downwind of the lakes and in a fairly narrow band following the lakeshore.   Several factors are responsible for determining the amount of lake-effect snow at a given location. The surface area, depth, and orientation of the lake plays a large role in determining the severity of lake-effect snowfall. 3 The surface area of the lake combined with its orientation influences the extent two which the lake modifies and air mass passing over it. Lakes that are oriented parallel to the direction of the winds, have the greatest spatial influence on passing air masses because the winds are over them for longer periods of time. Lake surface area and depth are measures for the size of a lake, and larger lakes (i.e., Lake Superior) generally have a greater influence on local and regional weather.  Lake Superior has the greatest impact on local snowfall amounts with 100% more winter precipitation falling downwind compared to Lakes Erie and Ontario that only have precipitation increases of 15% from the lake-effects. 4 The primary reason larger lakes have greater influence is because they hold their warmth from summertime longer into winter delaying ice formation, which maintains a moisture flux to the atmosphere that can support snowfall. In addition to maintaining the moisture flux, the heating from the lakes influences the buoyancy of the air that contributes to the over-lake convection.

The major low pressure systems which lead to winter weather storm tracks. Image provided by the Illinois State Water Survey.

Historical Climatology of Snow in the Great Lakes Region

The geographical distribution of winter precipitation in the Great Lakes is complex (Figure 3).  Depending on the location, vast differences can occur over relatively short distances.  Take for example the lake-effect zone of Lake Erie.  At its maximum, average winter precipitation is about 300 millimeters in western New York, but decreases by almost 30% just over the New York/Pennsylvania border, likely due to the decreased influence of lake-effect conditions with distance from the lakes. This is in contrast to areas upwind of the Great Lakes such as Wisconsin and Illinois and to a lesser extent the eastern half of Lower Michigan where snowfall is fairly uniform.    

Lake Superior’s lake-effect is the greatest of all of the Great Lakes in both regional extent and magnitude.  Lake Michigan and to a lesser degree, Lake Huron, have fairly widespread regions of influence, but they are not as strong as Lake Superior.  For example, Lake Michigan’s lake-effect zone covers roughly the western third of Lower Michigan but only causes increases in precipitation by about 40%. 5

The timing of when snowstorms occur is important for both preparedness and planning.  Lake-effect zones and regions in the northern most part of the Great Lakes have experienced snowfall as early as October, while the onset of snowfall across the rest of the region is delayed until November. 6 The remaining areas experience the most snowfall events in January.  As winter progresses lake ice cover accumulates and acts as a barrier turning off the moisture supply mechanism for lake-effect snowfall.  Most of the region continues to experience snowstorms, primarily from large-scale atmospheric circulation, through April and as late as May in the far north.

Average precipitation (mm) over the Great Lakes basin in winter: a) using all data and b) showing lake-induced changes. Heavy line on b) represents the 80-km lake-effect boundary. Dots indicate locations of precipitation sites. Values in parentheses are for Canada (Scott and Huff, 1996).

Wet vs. Dry Snow

Terms like “wet” and “dry” are important descriptors of snow because five inches of dry snow can have a very different impact on infrastructure/operations than five inches of wet snow due to differences in the amount of liquid water content that defines them. Wet and dry snows are defined by the density of the snow, which is determined by the amount of equivalent liquid water it holds called the snow-to-liquid equivalent ratio (SLR). A climatology of SLR values for the entire United States was reported by Baxter, Graves, and Moore in 2004 for the purpose of improving the ratios that are used in weather forecasting, given that SLR values are used to calculate the depth of snowfall based on an individual forecast for liquid precipitation. 7 They produced several maps that are useful for identifying snow characteristics over the Great Lakes region. 

The SLR value will vary by snow event, given that it is influenced by the source region of the air mass responsible for the snow.  This is due to the fact that different air masses will have distinct temperature and moisture structures that play a critical role in the nature of ice crystals, and thus now, formation and structure. , Despite these differences, the maps in Figure 4 capture the general picture of how the statistical distribution of snow can be characterized.

Snow Density CharacteristicRange of Snow-to-Liquid Ratio (SLR) Values
Heavy1:1
Average9:1 ratio 15:1
Lightratio > 15:1
Table 1: The snow-to-liquid ratio values with relative density of the snow. 

The Great Lakes region has some of the greatest variability of snow density within the contiguous United States.  SLR values in the Great Lakes range from about 8:1 to 20:1. A weather system passing over the region that produces roughly equal amounts of liquid precipitation will produce greater accumulations of snow in areas with high SLR values compared to low SLR values (ignoring factors such as drifting snow). Lake effect snow is almost always less dense than synoptic-origin snowfall. Snow in the southern Great Lakes region is typically more dense than in the north because colder temperatures in the north prevent the air from holding as much moisture, hence the snow is less dense. Due to the prevalent influence of lake effect snows, snow near the lakes is typically less dense, but there are greater amounts of it. Only one-quarter of SLR values indicate high-density snows over the Great Lakes region, so snow is most often average to light-density.

There is considerable variability in the seasonality of snow density, particularly downwind of the Great Lakes (Figure 5). For a given air mass, the more open a lake is, the more moisture can be supplied to that air mass, resulting in a lower SLR value, and the additional moisture from the lakes in the updraft enhances snow crystal formation leading to more dense snow. 9 However, not all lake-effect processes are terminated once the lakes freeze, so the lake-effect zones can still receive considerable snowfall.  

Information about average winter weather combined with snow density information is useful for describing when there is greatest potential for hazardous conditions. Most snowstorms occur in December and January, which coincides when snow densities are lightest, so the greatest snow depths can be expected during these months. Snow densities are typically smallest near the lakes but snow depths are greatest in the lake-effect zones. Snowstorms in the southern portion of the Great Lakes during early winter pose a greater threat for dense snow, although typically snow depths are less.

The distribution of snow-to-liquid ratio values for the 25th (left), 50th (center), and 75th (right) percentiles during 1971-2000. (Source: Baxter, Graves, and Moore, 2004)

The seasonal average snow-to-liquid ratio (SLR) values in October and November (left), December, January, and February (center), and March and April (right). (Source: Baxter, Graves, and Moore, 2004)

Intense Snowstorms

The lake-effect zones, which have been identified as regions receiving the most annual average snowfall, are also where the heaviest snowstorms are experienced.  A heavy snowstorm, as defined by (Changnon 2006), is an event when 15.2cm or more occurred in one or two days. Weather station data were used to identify where heavy snowstorms occurred in the United States between 1948-2001. 10 The largest amount of snow that is expected to occur at least one time in a 5- and 10-year period are mapped.

The structure of the lake-effect zones is not as apparent in the maps of intense snowfall, because lake-effect snow bands could occur between observation stations.  However, regions around the lakes (upwind and downwind) receive the most intense snowfall events in the region.  Upwind locations receive slightly less intense snowfall events than downwind, but the lakes’ influence is seen on all sides.

Localized differences emerge between the 5-year and 10-year snowstorm maps in the Great Lakes region.  For example, the range of variability across Lower Michigan in the 5-year snowstorm is greater than what is observed for the 10-year snowstorm.  This would suggest that the most extreme events (around 35 cm) in Lower Michigan occur less frequently (every 10-years instead of 5), but they are widespread.  There is little change in the spatial structure of snowstorms in the far northern and eastern portions of the Great Lakes, and in general the 10-year storms have about 5 to 10 cm more snowfall than the 5-year events at a given location.

The amount of snowfall (cm) expected in a 5-year intense snowfall event based on observations from 1948-2001. (Source: Changnon, 2006)

The amount of snow expected in a 10-year intense snowfall event based on observation from 1948-2001. (Source: Changnon, 2006)

The Future of Snow

Is there a climate change signal in the historical observations?

Snowfall observations are very important for describing how snowfall frequency and amounts are changing in the Great Lakes, but observations are not always reliable.  Several inconsistencies and potential biases were found in the U.S. Cooperative Observer Program (COOP) snowfall record. 11 Kunkel et al. (2007) studied several weather stations, locations where observations are taken, and found that some stations very close to one another showed very different snowfall amounts. Although it is possible for some differences to be attributed to natural climate variations, they found most are likely due to station inhomogeneities (i.e., moving a station to a new location that has different sunlight exposure). To overcome data quality issues Kunkel et al. (2007) recommend careful assessment of station histories with surrounding stations to remove any stations that are unsuitable. Kunkel was a co-author of the Historical Climate Sector Midwest Technical Input Report for the National Climate Assessment, which we use below to show how snow is changing in the Great Lakes region. 

Snowfall observations for the Great Lakes show a signal of climate change. There is a general northward shift in the bands of snowfall amounts, and more snowfall has been observed in the north. Upper Michigan had widespread increases in snowfall along with the northern tip of Lower Michigan. Snow in Lake Superior’s and Lake Michigan’s lake-effect zones had upward trends. 12 Lake Michigan’s lake-effect zone pushes farther inland and the eastern part of Lower Michigan experienced less snowfall during the later period, so the snowfall gradient between western and eastern Lower Michigan is increasing.  No trends were found in this study for the remaining lakes. 13

Additional studies report similar findings for changes in lake-effect snowfall, however, differences emerge in the statistical significance of changes as each study used a different set of quality control measures for their snowfall data.  Burnett et al. (2003) show that there is a significant increasing trend in lake-effect snowfall and no change in non-lake-effect snowfall between 1931 and 2001. 14 Kunkel et al. (2009) imposed stricter data quality standards than the Burnett et al. study and showed significant increases to lake-effect snowfall only for Lakes Superior and Michigan between 1925 and 2007. The remaining lakes had mixed results depending on the time frame selected for the analysis, because there were only a few stations to choose from during the earlier part of the period.

Another signal of change is the earlier timing of spring snowmelt. The largest changes in the timing of snowmelt were found to start in late January and continue through late spring. 15

Observed geographic changes in snowfall from 1961-1990 to 1981-2010. Image provided by the Midwestern Regional Climate Center (MRCC).

If there is a signal in the observations, is it consistent with basic theory?

Snowfall decreases in the south and increases in the lake-effect zones are consistent with observed decreases of synoptic snowfalls and increases in lake-effect snowfalls. 16 Increased lake-effect snowfall is commonly attributed to warmer air temperatures that cause warmer lake surface waters and less ice cover. 17

Is there a trend in the climate models for Great Lakes snow?

There is better agreement amongst the models for projections of wintertime precipitation compared to summertime precipitation. The Midwest Technical Input Report to the National Climate Assessment claims more precipitation can be expected during winter, but more rain and freezing-rain is likely instead of snow. 18 The anticipation of more rain and freezing-rain is consistent with warming winter air temperatures.

Global change model projections are consistent with early trends observed with respect to the influence of changing Great Lakes ice coverage on seasonal snowfall.  Namely, complete ice coverage results in major reductions of lake-effect snowfall along with colder temperatures and an overall more stable atmosphere over the lakes. 19 Unfrozen lakes determine the spatial distribution of lake-effect snowfall compared to completely frozen lakes that have no impact on snow placement. 20 When unfrozen surface waters are warmed, lake-effect snowfall amounts are increased downwind and the lake-effect region expands farther inland. 21  Observations point more toward decreasing ice coverage.22 Therefore, we can expect more precipitation in the lake-effect regions and beyond if ice cover continues to decline.  However, the form that precipitation takes will depend on the extent of warming.  Slight warming will increase snowfall amounts, but if the atmosphere is warmed beyond the freezing threshold more precipitation will fall as rain or freezing-rain.

Are the projections reliable?

Climate model projections (GCM and downscaled) of precipitation, especially in the Great Lakes region, contain a lot of uncertainty.  Most of the uncertainty is related to how well weather is represented in the models for the Great Lakes region, but even the best GCMs have spatial resolutions too coarse to simulate lake-effects and other small-scale dynamics.  Many models do not even include the lakes, which is important for the interpretation of the projections and the description of their uncertainty.  In downscaled data, the representation of the lakes is not necessarily improved,  so it is important to know how each model treats the lakes.  In addition, GCMs use simple snow models that do not capture the complexity of important snow processes. 23

Even though there are several reasons to be skeptical of future snow projections, the models can provide useful information. For example, although the GCMs do not represent the Great Lakes, well they are in fairly good agreement that air temperatures during all seasons will warm. 24 There is a positive feedback between air temperatures, lake water temperatures, and ultimately the amount of winter lake ice cover. 25 As the air temperature warms the lake waters are also warmed, and since lakes have a relatively high heat capacity (they are slower than the land surface to change temperature) their waters stay warmer later into fall. The persistence of relatively warm waters into early winter delays the formation of ice. Additionally,  the period that ice covers the lakes is shortened as springtime temperatures warm. So, with atmospheric warming, we see both a warming of lake waters and a decrease in lake ice cover. This scenario was used to drive the high resolution weather forecasting model (Wright et al., 2013), which simulates lake-effect precipitation, and the result was an increase in the amount and spatial coverage of lake-effect snowfall.  Even though the GCM projections are not the most reliable source of information about snowfall in the Great Lakes region, their information about regional changes can be used to inform localized simulations.

References

  1. Scott, R. W., and F. A. Huff. “Impacts of the Great Lakes on regional climate conditions.” Journal of Great Lakes Research. 22 (1996): 845-863.
  2. Changnon, Stanley Jr A., and Douglas M. A. Jones. “Review of the influences of the Great Lakes on weather.” WATER RESOURCES RESEARCH. 8 (1972): 360-371.
  3. Scott, R. W., and F. A. Huff. “Impacts of the Great Lakes on regional climate conditions.” Journal of Great Lakes Research. 22 (1996): 845-863.
  4. Scott, R. W., and F. A. Huff. “Impacts of the Great Lakes on regional climate conditions.” Journal of Great Lakes Research. 22 (1996): 845-863.
  5. Scott, R. W., and F. A. Huff. “Impacts of the Great Lakes on regional climate conditions.” Journal of Great Lakes Research. 22 (1996): 845-863.
  6. Scott, R. W., and F. A. Huff. “Impacts of the Great Lakes on regional climate conditions.” Journal of Great Lakes Research. 22 (1996): 845-863.[/fn]  A maximum of storms occurs in December downwind of Lakes Superior, Erie, and Ontario.Changnon, Stanley A., David Changnon, and Thomas R. Karl. “Temporal and Spatial Characteristics of Snowstorms in the Contiguous United States.” Journal of Applied Meteorology and ClimatologyJournal of Applied Meteorology and Climatology. 45.8 (2006): 1141-1155.
  7. Baxter, Martin A., Charles E. Graves, and James T. Moore. “A Climatology of Snow-to-Liquid Ratio for the Contiguous United States.” Weather and Forecasting. 20.5 (2005): 729-744.
  8. Baxter, Martin A., Charles E. Graves, and James T. Moore. “A Climatology of Snow-to-Liquid Ratio for the Contiguous United States.” Weather and Forecasting. 20.5 (2005): 729-744.[/enf_note] This is observed during early winter (October and November) near Lakes Erie and Ontario and is shown by lower SLR values near the lakes (12:1) than farther inland (13:1).  The ability of the lakes to increase snow densities is dampened once the waters are frozen over, which typically occurs throughout December, January, and February. 8Baxter, Martin A., Charles E. Graves, and James T. Moore. “A Climatology of Snow-to-Liquid Ratio for the Contiguous United States.” Weather and Forecasting. 20.5 (2005): 729-744.
  9. Changnon, Stanley A.. “Frequency distributions of heavy snowfall from snowstorms in the United States.” Journal of Hydrologic Engineering. 11 (2006): 427-431.
  10. Kunkel, Kenneth E., et al. “Trend Identification in Twentieth-Century U.S. Snowfall: The Challenges.” Journal of Atmospheric and Oceanic TechnologyJournal of Atmospheric and Oceanic Technology. 24.1 (2007): 64-73.
  11. Kunkel, Kenneth E., et al. “A New Look at Lake-Effect Snowfall Trends in the Laurentian Great Lakes Using a Temporally Homogeneous Data Set.” Journal of Great Lakes ResearchJournal of Great Lakes Research. 35.1 (2009): 23-29.
  12. Kunkel, Kenneth E., et al. “A New Look at Lake-Effect Snowfall Trends in the Laurentian Great Lakes Using a Temporally Homogeneous Data Set.” Journal of Great Lakes ResearchJournal of Great Lakes Research. 35.1 (2009): 23-29.
  13. Burnett, AW, et al. “Increasing Great Lake-effect snowfall during the twentieth century: A regional response to global warming?” JOURNAL OF CLIMATE. 16 (2003): 3535-3542.
  14. Dyer, Jamie L., and Thomas L. Mote. “Spatial variability and trends in observed snow depth over North America.” GEOPHYSICAL RESEARCH LETTERS. 33 (2006).
  15. Andresen, Jeff, Steve Hilberg, and Ken Kunkel. “National Climate Assessment Midwest Historical Climate.” (2013).
  16. Burnett, AW, et al. “Increasing Great Lake-effect snowfall during the twentieth century: A regional response to global warming?” JOURNAL OF CLIMATE. 16 (2003): 3535-3542.
  17. Winkler, Julie, Ray Arritt, and Sara Pryor. “National Climate Assessment Midwest Future Climate.” (2013)
  18. Vavrus, Steve, Michael Notaro, and Azar Zarrin. “The Role of Ice Cover in Heavy Lake-Effect Snowstorms over the Great Lakes Basin as Simulated by RegCM4.” Monthly Weather Review. 141 (2013): 148-165.
  19. Wright, David M., Derek J. Posselt, and Allison L. Steiner. “Sensitivity of Lake-Effect Snowfall to Lake Ice Cover and Temperature in the Great Lakes Region.” Monthly Weather ReviewMonthly Weather Review. 141.2 (2012): 670-689.
  20. Wright, David M., Derek J. Posselt, and Allison L. Steiner. “Sensitivity of Lake-Effect Snowfall to Lake Ice Cover and Temperature in the Great Lakes Region.” Monthly Weather ReviewMonthly Weather Review. 141.2 (2012): 670-689.
  21. Andresen, Jeff, Steve Hilberg, and Ken Kunkel. “National Climate Assessment Midwest Historical Climate.” (2013) 
  22. Yang, Zong-Liang, et al. “Simulation of snow mass and extent in general circulation models.” Hydrological Processes. 13 (1999): 2097-2113.
  23.  Winkler, Julie, Ray Arritt, and Sara Pryor. “National Climate Assessment Midwest Future Climate.” (2013).
  24. Austin, J. A., and S. M. Colman. “Lake Superior summer water temperatures are increasing more rapidly than regional air temperatures: A positive ice-albedo feedback.”