By any measure, the 2017 Chicago Yacht Club Race to Mackinac was no picnic. The Race started on Saturday under very pleasant southwesterly breezes and the fleet made good progress toward Mackinac Island. However, as the fleet worked north, two weather features delivered a one-two punch that prompted nearly a third of the competitors to withdraw from the Race. (Click here for Matt Gallagher’s thorough analysis of the reasons competitors gave for retiring.)
The first feature was a heat burst, a rare thunderstorm event that ravaged the fleet with 50 knot downburst winds near Milwaukee late on Saturday evening (click here for my previous article). The second feature was the passage of a cold front down Lake Michigan on Sunday. Based on a review of the weather pattern and tracking data from YP Tracking, it appears the brisk northerly winds and the resulting steep waves following the passage of the cold front were responsible for many more retirements than the wind from the rare heat burst on Saturday evening.
As racing started on Saturday morning, the approaching cold front, extending from a low in northeastern Canada, was located just north of Lake Superior (click here). This frontal boundary was expected to reach northern Lake Michigan early on Saturday evening (Figure 1) and then continue moving southward and reach the southern shore of Lake Michigan on Sunday morning (Figure 2).
Typical of cold fronts, the wind was expected to veer from southwesterly to northerly and dramatically increase after the passage of the front. The Open Lake Forecast from the Chicago National Weather Service office published at 9:00 am on Saturday morning included:
Northern Lake Michigan
REST OF TODAY…Southwest winds 10 to 15 kt increasing to 15 to 20 kt. A few afternoon gusts to 25 kt. Chance of showers and thunderstorms this afternoon. Waves 1 to 3 ft building to 3 to 5 ft by mid afternoon.
TONIGHT…Southwest winds 10 to 20 kt veering sharply to north 15 to 25 kt late in the evening. Chance of gusty showers and thunderstorms in the evening…then slight chance of showers and thunderstorms early overnight. Waves 3 to 5 ft.
SUNDAY…North winds 15 to 25 kt. Waves 4 to 6 ft occasionally to 8 ft.
Southern Lake Michigan
REST OF TODAY…Northwest winds around 10 kt becoming west 10 to 15 kt late this morning…becoming southwest 10 to 20 kt early this afternoon…then becoming south late. Waves 1 to 3 ft.
TONIGHT…Southwest winds 10 to 20 kt veering sharply to north 15 to 25 kt late. Chance of gusty showers and thunderstorms late this evening and overnight. Waves 1 to 3 ft this evening building to 2 to 4 ft overnight.
SUNDAY…North winds 15 to 25 kt increasing to 30 kt by late morning. Waves 4 to 7 ft occasionally to 9 ft.
The wind forecast imagery from the GFS forecast model (Figures 3 to 8) has been annotated to show the approximate predicted location of the cold front (blue line) at 3-hours intervals from 10:00 pm on Saturday night to 1:00 pm on Sunday afternoon. A review of the wind barbs ahead of the front (south of the boundary) and behind the front (north of the boundary) show the abrupt veering of the wind from southwesterly to westerly then to northerly as the front passes. The color shading (brighter colors) shows the increase in wind speed behind the front. According to the NWS forecast, the brisk winds were expected to produce four to six-foot waves on northern Lake Michigan. Due to an increase in fetch, waves on the southern portion of the Lake were forecast reach nine feet.
Cold Fronts and the Wind
As mentioned earlier, the passage of a cold front is typically associated with a noticeable veering of the wind. Depending on the overall barometric pressure pattern, this veering can be from southerly to westerly, southwesterly to northwesterly, or westerly to northerly. Veering of 90 degrees or more is common, therefore the abrupt veering from southwesterly to northerly during last year’s race was standard behavior for a cold front.
Strong gusty winds often occur after the passage of a cold front (although not always). The speed of the wind is governed by the magnitude of the change in barometric pressure, and the distance between an area of high pressure and an area of low pressure (wind flows from high to low pressure). The rate of the change in pressure is known as the pressure gradient force (PGF). PGF is high, and the winds are stronger, when the pressure changes significantly over a region. In contrast, when pressure changes across a region are slight, PGF is low and winds are light. Whether or not the winds are strong after the passage of a cold front depends upon the PGF.
The best way to judge the relative strength of the pressure gradient is to examine the isobars – contours of constant barometric pressure — on a surface weather map (Figure 9). When isobars are close together, it indicates that pressure is changing relatively quickly, and higher winds can be expected. Isobars that are far apart indicate weak PGF and light winds.
In the case of last year’s Chicago Mac, the PGF after the passage of the cold front increased largely due to an area of building high pressure approaching Lake Michigan from the northwest (Figure 10). At 12:00 am on Sunday, the area of high pressure with a central pressure of 1020 mb was northwest of Lake Superior.
Over the next several hours, the pressure reading of this high would increase to 1022 mb as it moved toward upper Lake Michigan. As the high moved toward the Lake, an area of low pressure remained nearly stationary near southern Lake Michigan. The building high, it’s movement toward Lake Michigan, and the stalled low combined to increase the PGF and wind speeds across the upper portion of the Lake. Click here for an animation of surface weather maps showing the pressure pattern across northern Lake Michigan and the compression of the isobars indicating an increasing PGF. The result – brisk winds buffeting the Chicago-Mac fleet.
At 10:00 pm on Saturday night, the surface analysis map (Figure 11) showed the cold front was draped across the Lake near Sheboygan, Wisconsin. Due to their earlier start, the cruising fleet was well north of the racing fleet and hugging Michigan’s northwest coast. A comparison of the data from YB Tracking (Figure 12) and the surface analysis map shows the cold front had already passed over the cruising fleet.
Six hours later, at 4:00 am on Sunday morning, the cold front had reached Milwaukee (Figure 13).
The southward movement of the front, combined with the northward progression of the racing fleet, now meant all of the Chicago-Mac competitors were exposed to the brisk northerly winds and building waves behind the boundary (Figure 13 and 14).
Based on wind and barometric pressure observations, the cold front passed NDBC Buoy 45002 in northern Lake Michigan at approximately 9:00 pm on Saturday night. The sustained winds and gusts increased over the next several hours reaching a maximum of 20.1 knots and 24.6 knots, respectively at 3:50 am on Sunday morning. A maximum wave height of 3.0 feet was observed at the same time.
Fetch is an important factor in wave heights and Buoy 45002 was located well north of the fleet on Saturday night and Sunday morning. It is likely the longer fetch may explain why many participants reported waves heights greater than 3 feet. For example, Buoy 45024 near Ludington, observed a wave height of 4.6 feet at 8:30 am on Sunday morning.
A similar pattern of increasing wind and wave heights occurred behind the cold front as it progressed down the lake. The highest sustained winds during the race was 26.4 knots which was observed at NDBC MKGM4 (near Muskegon, Michigan) at 3:30 pm on Sunday afternoon. The highest gust of 31.3 knots occurred at NDBC KNSW3 (near Kewaunee, Wisconsin) at 8:20 am on Sunday morning. The highest wave height observation of 6.9 feet was taken at NDBC Buoy 45002 (SE of Milwaukee, Wisconsin) at 10:50 am on Sunday morning. Additional observations from stations across northern Lake Michigan are available below.
The timing of the cold front’s passage, as well as the speed of the wind and wave heights, was well forecast. Over the course of many hours, the combination of strong winds and relentless waves took their toll on boats and crew, and the one-two weather punch won’t soon be forgotten.
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Additional Observations from Around Lake Michigan
- NDBC 45002 – Halfway Between North Manitou & Washington Island
- Highest Wind Observation: 20.1 knots, gusting to 24.6 knots from 350 degrees on Sunday at 3:50 am.
- Highest Wave Observation: 3.0 feet at 3:50 am
- NDBC GTLM4 – Grand Traverse Light, Michigan
- Highest Wind Observation: 19.7 knots, gusting to 24.2 knots from 20 degrees on Sunday at 4:20 am.
- NDBC KNSW3 – Kewaunee, Wisconsin
- Highest Wind Observation: 22.4 knots, gusting to 31.3 knots from 350 degrees on Sunday at 8:20 am.
- NDBC MEEM4 – Manistee Harbor, Michigan
- Highest Wind Observation: 25.1 knots, gusting to 30.0 knots from 6 degrees on Sunday at 6:40 am.
- NDBC 45024 – Ludington, Michigan
- Highest Wind Observation: 22.4 knots, gusting to 29.1 knots from 350 degrees on Sunday at 4:50 am.
- Highest Wave Observation: 4.6 feet at 8:30 am on Sunday morning.
- NDBC SGNW3 – Sheboygan, Wisconsin
- Highest Wind Observation: 25.3 knots, gusting to 28.9 knots from 10 degrees on Sunday at 8:00 am.
- NDBC MKGM4 – Muskegon, Michigan
- Highest Wind Observation: 26.4 knots, gusting to 30.0 knots from 350 degrees on Sunday at 3:30 pm.
- NDBC MLWW3 – Milwaukee, Wisconsin
- Highest Wind Observation: 17.2 knots, gusting to 23.0 knots from 360 degrees on Sunday at 6:20 am.
- NDBC 45029 – Holland, Michigan
- Highest Wind Observation: 22.4 knots, gusting to 29.1 knots from 340 degrees on Sunday at 3:20 pm.
- Highest Wave Observation: 5.9 feet on Sunday at 3:30 pm.
- NDBC 45007 – 43 nm East Southeast of Milwaukee, Wisconsin
- Highest Wind Observation: 26.8 knots, gusting to 31.3 knots from 10 degrees on Sunday at 8:50 am.
- Highest Wave Observation: 6.2 feet on Sunday at 10:50 am