The Chicago Yacht Club’s Race to Mackinac is a little over two weeks away. It’s too early to begin working on your weather forecast. However, reviewing the long-term average conditions on Lake Michigan during July is great way to set the stage for a weather forecast, particularly for if you are participating in the Race for the first time.
Wind and Wave Observations
The National Data Buoy Center (NDBC) maintains two floating discus bouys in Lake Michigan. Buoy 45002 is located in the northern basin north of the Manitou islands, while buoy 45007 is located in southern Lake Michigan approximately 43 nautical miles southeast of Milwaukee, Wisconsion (click here for a map of Lake Michigan).
During the summer, large areas of high barometric pressure are a regular feature over the Great Lakes. In response to the light pressure gradient that accompanies these highs, the long-term average wind speed at both buoys reaches an annual minimum of approximately eight knots during June and July (figures 1 and 2, below). The average wind gusts aren’t much higher, with both locations having a long-term average of approximately 10 knots (view 45007 graph) (view 45002 graph).
The wind roses (figures 3 and 4, below) show the distribution of observed wind direction and wind speed at the buoys for the month of July from 2008 to 2013. The predominant winds at buoy 45007 are southerly, with south to west-southwesterly winds occurring nearly 30% of the time. Although the buoys are only 162 nautical miles apart, the wind rose for buoy 45002 (figure 4) shows a markedly different wind pattern from its southerly neighbor. The southeasterly and strong southerly wind at 45007 is essentially absent at 45002. Instead, southwesterly winds occur far more frequently — approximately 35% of the time. In addition, northerly winds appear to be slightly less frequent at buoy 45002.
Figures 5 and 6 (below) show the distribution of observed wind speeds at the buoys for the same period covered by the wind roses. The winds at buoy 45007 were 10 knots or less 56.6% of the time. Winds from 10 to 15 knots were observed 30.1% of the time. The winds at buoy 45002 were slightly stronger than those at buoy 45007, with winds less than 10 knots occurring less often and comprising 53.9% of the observations. Winds from 10 to 15 knots occurred more frequently at buoy 45007 than 45002 — 30.1% versus 28.8%. Winds over 15 knots occurred more frequently at the northern buoy — 17.2% at buoy 45002 and 13.3% at 45007. Based solely upon the long-term averages, the Chicago Mac is a light to moderate air race.
It should be no surprise that the wind speed minimum in July is accompanied by an annual minimum in wave heights. The average wave height at both buoys in July (figures 7 and 8, below) is approximately .5 feet.
The long and tremendously cold winter of 2013-2014 will be remembered by the residents of the Great Lakes for a long time.
The lakes remained ice-covered much later than normal, resulting in current water temperatures that are lagging behind the long-term average. The long-term average surface Lake Michigan water temperature for late June is 63F, but this average is calculated using the entire surface of the Lake and therefore moderates the cold temperatures observed by the off-shore buoys. As of June 30, the water temperature was 46.0F at 45007 and 43.7F at 45002.
While the average surface temperature of Lake Michigan is about 5° F degrees cooler than the long-term average (click here), it is significantly colder than the past several years (figure 9). In particular, the water temperatures in 2009, 2012 and 2013 were well above the long-term average, and more than 10° F warmer than the current average Lake temperature.
Below normal water temperatures will have an impact on the weather. Overnight watches are going to be unusually chilly. And has been experienced on Lake Michigan over the past several days, during the right conditions cold off-shore waters can produce expansive and dense fog banks.
But of primary importance to Chicago Mac sailors, colder than normal lake temperatures are likely to have an impact on the wind. A cold lake often creates a layer of cold air several hundred feet thick just above the surface, resulting in a low-level temperature inversion. A temperature inversion is an atmospheric condition where the temperature in an air column warms instead of cooling with increasing height. Low-level temperature inversions can have a significant effect the wind, most often by suppressing wind gusts.
Let’s look at a recent example (June 13, 2014) from southern Lake Huron using data from NDBC station HRBM4 and buoy 45008 to illustrate the impact that a low-level temperature inversion can have on the wind. HRBM4 is located on the western shore of Lake Huron near Harbor Beach, Michigan approximately 28 nautical miles from buoy 45008.
Both stations were located in an area of building high pressure following the passage of a cold front. The sustained winds at HRBM4 (figure 10) ranged from 10 to 15 knots, while gusts peaked slightly above 20 knots. During the same period, the sustained winds at 45008 (figure 11) reached only 11 knots and gusts peaked at 13 knots. A comparison of the wind gust observations (click here) shows they were subdued at the offshore station.
Both stations were located in a similar surface pressure gradient, therefore the difference in wind observations was due to the nature of the temperature profile of the air column above each station. Figure 12 shows the temperature profile of the air column above HRBM4, with the red line representing the temperature and the green line showing the dew point. At HRBM4, the warmest temperature was located at the surface with steadily decreasing temperatures aloft, represented by the steady shift of the temperature trace (in red) to the left. A low-level temperature inversion was not present above HRBM4.
The surface air temperature was much colder at 45008 due to the cooling influence of Lake Huron. In contrast to the air column above HRBM4, the warmest temperature in the column above 45008 (figure 13) was not at the surface, but nearly a thousand feet above. The path of the temperature trace to the right (annotated profile) is the telltale signature of the low-level temperature. This cold layer of air near the surface suppressed wind gusts by preventing momentum from stronger winds aloft from reaching the surface.
A comparison of the change in temperature in the air columns above HRBM4 and 45008 is shown in figure 14. The dramatic warming — instead of cooling — with height above 45008 is easy to recognize. Above the inversion (at nearly 1,000 feet), the temperature profiles at both locations were nearly identical.
The strength of the low-level temperature inversion created by the cold lake isn’t consistent across the entire lake. It is typically strongest near the center of the lake where the water is coldest and diminishes near the shore where water temperatures are much warmer. The wind direction also plays a role. In a brisk southwesterly or westerly wind, the creation of a low-level temperature inversion in the near-shore waters along the western shore of Lake Michigan would be inhibited by the transport of warm air from the land out over the lake. Similarly, a strong easterly or northeasterly wind would produce a weaker low-level inversion near the eastern shore of the lake. The bottom line is that competitors favoring an offshore route to Mackinac will be more affected by low-level temperature inversions than those staying closer to either the western or eastern shore.
Lake and Land Breezes
Since lake and land breezes form in response to temperature differences between the air over the land and the lake, colder than average water temperatures will have an impact on their formation and strength. If the large-scale weather pattern is supportive of the development of subtle wind patterns, the colder water temperatures should increase the speed of the daytime lake breeze. In contrast, the typically weaker nighttime land breeze will be further weakened by the below-normal off-shore water temperatures.
Because the subtle thermally-driven winds develop within the context of the larger-scale wind, potentially stronger lake breezes and weaker land breezes aren’t inherently good or bad. It all depends upon the direction of the large-scale wind. For example, if the large-scale winds are offshore, stronger lake breeze dynamics may result in weaker winds in the near shore waters. In contrast, if the large-scale wind is onshore, better lake breeze dynamics will result in stronger onshore winds.
Marine Weather Forecasts
While a climatological analysis provides competitors with an awareness of the average conditions on Lake Michigan during July, it is weather — not climatology — that rules a long-distance sailboat race. For the latest marine weather forecasts and links to a wide range of marine forecasting resources, please visit the LakeErieWX Chicago Yacht Club’s Race To Mackinac Weather page or my Lake Michigan Marine Weather Dashboard.