Competing in the Chicago-Mac is never easy – after all, it is at least 333 statute miles to Mackinac Island. And by all accounts, the 108th running of Mac was unusually challenging. Light easterly winds on Saturday afternoon made progress toward the Island difficult for the racing fleet. The most significant challenge, however, appeared on Saturday evening, delivered by prolonged periods of thunderstorm activity. The storms repeatedly battered the fleet, hampering progress and prompting the withdrawal of several competitors due to minor crew injuries and equipment issues.
Light Winds Hinder Progress
By early Saturday afternoon, Lake Michigan was under the influence of a large area of high pressure centered just north of Lake Superior. This high was bisected by a stationary front originating from an area of low pressure near North Dakota and extending east across southern Lake Michigan (click here for surface analysis). This pattern resulted in light easterly winds across the southern half of Lake Michigan, impeding the progress of the racing and cruising fleets.
Observations from automated weather stations indicated that winds across southern Lake Michigan were generally less than 10 knots from late morning through mid-afternoon on Saturday (Figures 1-4). Weak winds were also observed at the offshore buoy southeast of Milwaukee (click here for observations). As a result, at 4:30 pm on Saturday afternoon the majority of the racing fleet was only 25 miles from the start, and still south of the Wisconsin border (click here for tracking image).
The Forecast Calls for Thunderstorms
The Day One Convective Outlook (Figure 5) issued by the Storm Prediction Center (SPC) on Saturday morning suggested there was a risk of severe thunderstorms developing near a warm front draped across the upper Midwest. This warm front was expected to drift south towards the southern tip of Lake Michigan and transition to a stationary front by Saturday evening (click here the surface analysis). While the bullseye of Enhanced risk was confined to central Minnesota, the area of Slight and Marginal risk extended east across Lake Michigan. By 9:00 am on Saturday morning, a cluster of thunderstorms was already moving east across southern Wisconsin toward southern Lake Michigan (click here for radar image).
In addition to the Convective Outlook, the SPC issued Thunderstorm Outlooks which displayed the probability of both severe and non-severe thunderstorms occurring on Saturday in four-hour increments (Figures 6-8). The Outlooks indicated thunderstorms were possible (a 10% chance) from 3:00 pm until midnight.
A Stormy Evening
Throughout the day, the SPC continued to monitor the potential for severe weather across southern Wisconsin and northern Illinois. Shortly before 3:00 pm CDT, the office issued Mesoscale Discussion #1376 (click here for the discussion details). In the discussion, the SPC expressed concern that the potential for severe thunderstorms with damaging wind gusts was increasing across the region. The warm/stationary front across the area was expected to act as both a focal point for thunderstorm development and as an eastward track for those storms to travel toward southern Lake Michigan. In a process called “training”, fronts often promote the formation of thunderstorms that pass repeatedly over the same area.
These ongoing concerns led the SPC to issue Severe Thunderstorm Watch 412 at 3:20 pm CDT (click here for watch details). The Watch area included southern Wisconsin, northern Illinois and the Illinois waters of Lake Michigan. The Watch, in effect until 10:00 pm CDT, warned of damaging wind gusts to 70 mph and hail greater than 2 inches in diameter.
At 4:30 pm CDT, a large cluster of thunderstorms 25 nautical miles west of Lake Michigan was moving east at approximately 20 knots. The cluster was readily apparent on radar. Far subtler on the radar image was evidence of the lake breeze front running parallel to and several miles inland from the Lake Michigan shore (Figure 10). During the next hour, the lake breeze front was the focal point for the development of several thunderstorms which subsequently matured and merged into the cluster approaching from northern Illinois (click here for a radar loop).
The leading edge of the thunderstorm cluster reached the Lake Michigan shore around 5:30 pm (Figure 11). Thanks to the persistent light winds, the racing fleet was directly in the path of the approaching storms (click here for the race tracking image). Just as the thunderstorms reached the Lake, NWS Chicago issued a Special Marine Warning for the nearshore and open waters from Winthrop Harbor to Wilmette. The Warning mentioned the potential for frequent lightning and wind gusts greater than 34 knots. NWS Chicago refreshed the Special Marine Warning for this area of Lake Michigan at 5:51 pm CDT and again at 6:11 pm CDT, increasing the potential wind gust speed to 40 knots and 50 knots respectively.
This initial group of thunderstorms plagued the racing fleet with strong winds and lightning until a little after 7:00 pm CDT. The respite following these initial thunderstorms was short-lived, however, as the second wave of storms reached the fleet around 9:00 pm CDT and persisted until after 11:00 pm CDT. (Click here for a radar loop of the thunderstorms on Saturday evening.) The storms produced 40 to 50 knot gusts in the proximity of the fleet (figures 12 to 15).
As is often the case with severe thunderstorms, observations of peak sustained winds and peak gusts varied considerably in the area near the fleet. Below are the key observations from a few automated weather stations (click here for a map showing their location):
- NDBC CHII2
- Maximum sustained of 40.3 knots at 6:40 pm CDT
- Maximum gust of 48.3 knots at 6:30 pm CDT
- NDBC CNII2
- Maximum sustained of 24.2 knots at 6:30 pm CDT
- Maximum gust of 36.9 knots at 8:00 pm CDT
- NDBC 45174
- Maximum sustained of 29.1 knots at 6:30 pm CDT
- Maximum gust of 44.7 knots at 6:30 pm CDT
- NDBC KNSW3
- Maximum sustained of 36.9 at 5:20 pm CDT
- Maximum gust of 47.2 knots at 5:20 pm CDT
On Sunday morning, the fleet encountered additional thunderstorms followed by much stronger southerly winds. These developments will be featured in a future blog.