The Forecasts
Surface Weather Pattern
A weather-savvy sailor always checks the surface forecasts from the Weather Prediction Center (WPC) before a race. The maps – available for the next seven days -- show the large-scale features such as areas of low pressure and frontal boundaries that can lead to thunderstorm development.
A key ingredient for thunderstorm development is a source of lift. Even when the air near the surface is warm and moist, thunderstorm formation requires that air to be nudged upwards. The most effective nudging comes from a frontal boundary, such as a cold, warm, or stationary front. The surface forecasts valid from early Saturday, July 23, into Sunday morning (Figures 1 – 4) suggested a warm front would move northeastward toward Lake Michigan during the day on Saturday, followed by the approach of a cold front on Sunday morning. There would be no shortage of lift for thunderstorm development.
NDFD Thunderstorm Forecast
The surface maps also present color shading from the National Digital Forecast Database (NDFD) to show precipitation type (the legend is in the lower left corner). The surface forecast valid at 7:00 am on Saturday (Figure 1) contained an area outlined in red and shaded in light green with red diagonal lines across southern Lake Michigan. This shading strategy indicated a chance of thunderstorms in the outlined area at the time the forecast was valid. The forecasts for 1:00 pm on Saturday (Figure 2), 7:00 pm on Saturday (Figure 3), and 1:00 am on Sunday (Figure 4) show areas shaded in dark red with light red diagonal lines. This type of shading indicates that thunderstorms were likely and that there was a chance of severe thunderstorms. The four surface forecasts suggested the fleet was facing the possibility of a prolonged period of severe thunderstorms from late Saturday afternoon through early Sunday morning.
Milwaukee NWS Forecasts
The Milwaukee National Weather Service (NWS) office is responsible for issuing the offshore marine forecast for all of Lake Michigan. The synopsis of the marine forecast issued at 8:59 AM CDT on Saturday, July 23nd mentioned a cold front would approach Lake Michigan during the evening and “bring with it multiple rounds of intense thunderstorms. The first round of severe thunderstorms is expected … around 6-7 pm Central this evening”.
The Hazardous Weather Outlook (HWO) published at 5:03 am CDT on Saturday morning warned that a “A line of severe storms is expected to move east southeast through the nearshore waters tonight, with damaging winds of 50 knots or greater possible. In addition, embedded waterspouts and large hail are possible.”
The Area Forecast Discussion (AFD) issued at 10:15 am CDT on July 23rd set the tone for the day ahead. "Looking like a troubling forecast for southern Wisconsin for this afternoon and evening with confidence continuing to grow for a significant severe weather event for the area. A lot of ingredients are coming together to put all of the area at risk for later today.”
Checking The Experts
The Storm Prediction Center (SPC) is part of the National Weather Service (NWS) and is home to the nation’s experts on forecasting severe thunderstorms. To be classified as a severe thunderstorm by the NWS, a storm must produce at least one of the following – a wind gust greater than fifty knots, a hailstone greater than one inch in diameter, or at least one tornado or waterspout. Severe thunderstorms pose a considerable threat to racing sailors in the form of damaging wind gusts, locally higher waves, and visibility-limiting rain.
The SPC issues daily Convective Outlooks which show the probability-based forecast where severe weather may develop over the next eight days. Probability categories are displayed on Convective Outlooks using a combination of color shading and abbreviations (the various risk categories are described here).
The Convective Outlook for the 24-hour period beginning at 7:00 am on July 23, 2022 (Figure 5) contained a wide expanse of Enhanced risk stretching from southern Minnesota eastward across Wisconsin and Lake Michigan. While not the highest risk category used by the SPC, an Enhanced risk indicates a significant severe thunderstorm outbreak is possible.
The written discussion supporting the Outlook issued for July 23rd indicated that conditions within the area of Enhanced risk were expected to be favorable for the development of long-lived and fast-moving squall lines. While damaging wind gusts exceeding 70 knots were the primary threat (click here), hail greater than 2 inches in diameter (click here), and a few tornadoes were also possible (click here).
In addition to Convective Outlooks, the SPC publishes Thunderstorm Outlooks, which display the probability-based forecast for the development of both severe and non-severe thunderstorms for consecutive 4-hour periods for the current day. The series of forecasts for July 23rd (Figures 6 to 9) show a bullseye of 70% probability steadily approaching southern Lake Michigan.
The forecasts from the WPC, SPC, and the Milwaukee NWS office reached a consistent conclusion – multiple rounds of fast-moving severe thunderstorms were expected to intercept the fleet from Saturday evening through Sunday morning. The sobering forecast of an unusually prolonged severe weather event prompted many teams to withdraw prior to the start.
