Introduction
After a challenging 2016 Chicago Mac (click here for a summary), this year’s competitors were likely hoping for an easier trip to the island. But as is often the case, Mother Nature wasn’t inclined to cooperate. Although the race started in pleasant sailing conditions, a rare weather phenomenon known as a heat burst, or dry microburst, caused two separate and frightening incidents late on Saturday night. The one-two punch of Saturday’s heat burst, combined with brisk northerly winds following the passage of a cold front on Sunday, caused nearly 30% of the fleet to retire from the race.
The Forecast
At 10:00 am on Saturday morning July 15, 2017, a cold front extending from a low in northeastern Canada was located just north of Lake Superior (Figure 1). This frontal boundary was expected to sweep down Lake Michigan, reaching its southern shore by mid-morning on Sunday (click here for forecast).
Figure 1: Surface analysis valid at 7:00 am on Saturday, July 15, 2017.
Typical of cold fronts, its passage down the lake was expected to produce a veering of the wind from southwesterly to northerly, increase wind speed and wave heights, and raise the potential for thunderstorms. The Open Lake Forecast from the Chicago National Weather Service office published at 9:00 am on Saturday morning stated:
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.
SUNDAY NIGHT…North winds 15 to 25 kt becoming northeast 10 to 15 kt by overnight. Waves 3 to 5 ft subsiding to 2 to 4 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.
SUNDAY NIGHT…North winds to 30 kt becoming northeast 10 to 20 kt overnight. Waves 4 to 7 ft occasionally to 9 ft subsiding to 3 to 6 ft occasionally to 8 ft overnight.
The wind forecast from the North American Forecast Model (NAM) valid at 1:00 pm on Saturday afternoon (Figure 2 supported the NWS marine forecast of west to southwest winds across the southern portion of Lake Michigan and southwesterly winds further north. The forecasts from 1:00 pm on Saturday through 7:00 am on Sunday morning show the converging wind pattern marking the passage of the cold front down the lake.
Figure 2: Wind forecast from the NAM valid at 1:00 pm on Saturday, July 15, 2017.
Thunderstorm Forecast
The Storm Prediction Center’s (SPC) Convective Outlook (Figure 3) placed most of Lake Michigan at a Marginal risk of severe thunderstorms (wind gust > 50 knots, 1″ diameter hail, or a tornado) on Saturday with a bulls-eye of Slight risk encompassing most of Wisconsin and a small portion of western Lake Michigan. (Click here for an explanation of the SPC’s risk categories.)
Figure 3: SPC Day One Convective Outlook for Saturday, July 15, 2017.
In addition to the risk of severe thunderstorms conveyed in the Convective Outlook, the SPC publishes Thunderstorm Outlooks showing the probability of thunderstorm development (either severe or non-severe) in four-hour intervals (Figures 4 to 7). The Thunderstorm Outlooks for Saturday indicated the probability of thunderstorms on Lake Michigan was highest (40%) from 3:00 pm to 7:00 pm across the northwest corner of the Lake.
The Hazardous Weather Outlook (HWO) for Lake Michigan issued by the Chicago NWS at 11:04 am on Saturday mentioned “a chance of thunderstorms this afternoon and tonight. Some of these thunderstorms may be severe.” Examined together, the forecasts suggested that thunderstorm development would be associated with maximum daytime heating, and were likely to diminish during the evening hours.
Thunderstorms Reach the Fleet
Sailing downwind in modest southerly and southwesterly breezes (10 to 15 knots), the racing fleet made good progress along the rhumbline and was tightly clustered in the center of the lake just north of Milwaukee by 11:30 pm on Saturday night. (Figure 8). Meanwhile, the steadily-moving cold front was stretched across Lake Michigan just north of the fleet (Figure 9) and the line of thunderstorms along its leading edge stretched from western Wisconsin east across Lake Michigan (Figure 10).
On Doppler radar imagery, the precipitation rate of thunderstorms is measured in decibels of Z (dBZ), which is more commonly known as reflectivity. Since the strongest storms tend to produce the highest precipitation rates, reflectivity can be used a proxy for a storm’s intensity. Color scales vary, but on most radar reflectivity imagery, the brightest colors are associated with the highest values of dBZ. (Click here for a primer on Doppler Weather Radar).
Figure 10: Base reflectivity radar image at 11:31 pm on July 15, 2017. From NWS Milwaukee.
The reflectivity image from 11:30 pm (Figure 10) indicates the strongest storms (based on dBZ values) were located over inland Wisconsin and well west of the fleet. Based on their appearance on radar alone, the thunderstorms over Lake Michigan seemed much weaker and far less threatening than the storms to the west. Further, a series of radar images from 11:00 pm to 11:30 pm suggest these storms had weakened as they moved south. But as is sometimes the case, radar doesn’t always tell the whole story.
Anatomy of a Heat Burst
Not long after 11:30 pm, Meridian X, a Farr 400, was hit with a southwesterly wind of 30 knots. A few minutes later, the wind increased to 40 knots. As the boat rocketed along at nearly 18 knots, a crew member fell overboard. At nearly the same time, High Priority 2, a Corsair 31 trimaran, was traveling at 16 knots when it was hit by a 40-knot gust. The burst of wind caused High Priority 2, with a crew of four, to suddenly capsize. Both boats were relatively close to the “weakening thunderstorm” over central Lake Michigan (Figure 11). Fortunately, thanks to the assistance of the Coast Guard and other Mac participants, all the sailors were rescued. High Priority 2 was recovered several days later. The culprit for both incidents was a rare type of strong thunderstorm downdraft known as a heat burst. Heat bursts usually occur at night during the late spring or summer and are often associated with a dissipating thunderstorm.
Figure 11: Base reflectivity radar image at 11:38 pm on July 15, 2017 showing the location of Meridian X and High Priority 2. Data from NWS Milwaukee.
Even weak thunderstorms produce downdrafts, a storm-scale flow of descending air. Thunderstorm downdrafts form when precipitation particles (water droplets, ice crystals, hail, etc.) become too heavy to be suspended by the storm’s updraft, or when the updraft weakens. As the precipitation particles fall, the melting of ice crystals and the evaporation of water droplets serve to cool the air under the storm. As the descending air cools and becomes denser, it falls faster, increasing the speed of the downdraft. When the downdraft reaches the ground, it spreads out horizontally creating the refreshingly cool wind associated with a thunderstorm.
While all thunderstorms have downdrafts, not all downdrafts are downbursts. Downbursts flow from intense thunderstorms and produce strong damaging straight-line winds. Severe thunderstorms with downburst winds exceeding 60 knots are a regular occurrence during the summer boating season (click here for a summary of the 2011 Chicago-Mac). Meteorologists subdivide downbursts into macrobursts and microbursts depending on the size of the area affected by the destructive winds.
Downbursts are also classified as wet or dry. Both originate in the mid levels of an intense thunderstorm, however wet downbursts are cold and contain rain when they reach the surface and the dry variety are warm and dry. Most downbursts are the wet variety. A heat burst is simply a dry downburst that produces a noticeable increase in temperature and a corresponding decrease in the dew point. They are relatively rare and usually occur at night during the late spring or summer and are often associated with a dissipating intense thunderstorm.
All downbursts – both wet and dry – initially form as a mass of cool and moist air in the mid levels of a strong thunderstorm. It is predominantly the nature of the environment beneath the storm (how moist or dry) that determines if a wet or dry downburst reaches the surface. When the air underneath a thunderstorm is very dry (low relative humidity), most, if not all, of the precipitation particles evaporate well before reaching the ground. As evaporation dramatically slows or ceases, the air no longer cools or cools much more slowly as it falls. Since barometric pressure decreases with altitude, the falling air encounters increasing pressure as it falls towards the surface. The steadily increasing pressure compresses and thereby warms the descending air. In a completely dry environment, warming from compression occurs at a rate of 5.5°F per 1,000 feet of descent. This warming decreases the density of the falling air and thereby slows its rate of descent. If warming is insufficient to overcome the pre-existing downward momentum of the falling air, the warm and dry downdraft reaches the surface as a heat burst (dry downburst).
Figure 12: NWS Heat Burst Explanation.
The tell-tale signature of a heat burst is a sharp increase in air temperature and a corresponding decrease in dew point following the arrival of the thunderstorm’s downdraft. In many instances, a marked drop in barometric pressure is also observed as the blast of warm air arrives.
Conclusion
Observations from several automated weather stations (click here for a map) support the conclusion that a series of heat bursts occurred across southern Lake Michigan late Saturday night and into early Sunday morning as the thunderstorms moved south. In addition, an analysis of the lowest 20,000 feet above Lake Michigan indicate that the atmosphere was relatively dry with a couple of very dry layers (click here for a graph). The highest wind gust of 49.2 knots was observed at station 45007 in southern Lake Michigan at 1:17 am on Sunday, July 16, 2017. The gust was immediately followed by a decrease in barometric pressure (1015.0 mb to1011.4), an increase in temperature (70.2F to 74.5F), and a modest decrease in dew point (67.5F to 65.5F)– all hallmarks of a heat burst.
Southern Lake Michigan Heat Burst Evidence
Note: The observation times on the following graphs are in UTC. Five hours must be subtracted to convert to Central Daylight Saving Time. A short primer on meteorological timekeeping can be found here.
Station 45024 – Ludington, Michigan
Maximum gust of 38 knots from the southwest (200°) was observed at 10:00 pm on Saturday, July 15, 2017 (0300Z on July 16). The temperature increased several degrees. A significant drop in barometric pressure accompanied the gust (click here for graph). Click on either image to open a larger version.
Station PWAW3 – Port Washington, Wisconsin
Maximum gust of 17.2 knots from the southwest (210°) at 11:00 pm on Saturday, July 15, 2017 (0400Z on July 16). The modest gust caused the temperature to quickly increase a few degrees. Click on either image to open a larger version.
Station MKGM4 – Muskegon, Michigan
Maximum gust of 25.3 knots from the southwest (220°) at 11:10 pm on Saturday, July 15, 2017 (0410Z on July 16). The temperature rose several degrees following the gust. A significant drop in barometric pressure accompanied the gust (click here for graph). Click on either image to open a larger version.
Station 45029 – Holland, Michigan
Maximum gust of 24.6 knots from the southwest (220°) at 11:50 pm on Saturday, July 15, 2017 (0450Z on July 16). The temperature rose a few degrees. A significant drop in barometric pressure accompanied the gust (click here for graph). Click on either image to open a larger version.
Station 45007 – Southern Lake Michigan
Maximum peak gust of 49.2 knots from the west (280°) at 1:17 am on Sunday, July 16, 2017 (0617Z on July 16). The strong gust caused the temperature to quickly rise several degrees. A significant drop in barometric pressure accompanied the gust (click here for graph). The station observed a clear sign of a heat — a decrease in dew point as the temperature rose (click here for graph). Click on either image to open a larger version.
Station KNSW3- Kenosha, Wisconsin
Maximum gust of 32.2 knots from the south (170°) at 1:30 am on Sunday, July 16, 2017 (0630Z on July 16). A significant drop in barometric pressure accompanied the gust (click here for graph). Click on either image to open a larger version.
