Ken Pryor Center for Satellite Applications and Research (NOAA/NESDIS) The 25 April 2015 Mobile Bay Regatta Accident
Topics of Discussion • Introduction to thunderstorm downbursts and downburst prediction technique • Mobile Bay Accident: • Timeline • Weather conditions • Comparison to 1 June 2015 Chesapeake Bay thunderstorm event • Conclusions
Thunderstorm Downburst • Strong downdraft produced by a convective storm (or thunderstorm) that causes damaging winds on or near the ground. (Fujita and Wakimoto 1981) • Serious hazard to sailing vessels due to inclined winds that can cause a knock-down. • Hazard to other types of marine craft based on stability criteria.
Downburst Maritme Hazards From Fujita (1981) From Canadian TSB report
Introduction • Downburst prediction products, employing data from geostationary weather satellites (GOES) and numerical weather prediction (NWP) models have been developed to address the need for improved monitoring and warning capability over coastal and open ocean waters. • A downburst index (DI), used with Doppler radar imagery, can effectively indicate potential for strong and hazardous thunderstorm winds. • Downburst prediction products are generated hourly at the NOAA Center for Weather and Climate Prediction (NCWCP).
Downburst Index (DI) • Based on factors that promote thunderstorms with potential for strong winds: • Large storm precipitation content (esp. hail, rain) • Large changes of temperature and moisture (humidity) with height in the lower atmosphere. • Index values are positively correlated with downburst wind strength. • Severe downbursts may occur when theDI > 5.
25 April 2015Mobile Bay Regatta Accident • Dauphin Island Regatta: • Sailing race from north Mobile Bay to Dauphin Island, AL • Race started 90 minutes late (~1100 CDT) due to miscommunication. • 117 vessels and 476 personnel involved in the race. Courtesy Joshua Edwards
25 April 2015Thunderstorm Event Summary During the morning of 25 April 2015, a cluster of thunderstorms developed into a mesoscale convective system (MCS) over eastern Texas and then tracked rapidly eastward along the Gulf of Mexico coast. The MCS intensified rapidly over southern Mississippi and Alabama during the mid-afternoon as the system encountered a very unstable air mass.
Thunderstorm Wind Prediction < 34 kt
Radar Imagery/Wind Potential Bow Echo
Radar Imagery/Wind Potential 2000 UTC 2008 UTC 2014 UTC 2024 UTC 2019 UTC Bow Echo
Storm Impacts on the Regatta Courtesy Joshua Edwards
Storm Impacts on the Regatta • Peak measured wind speed of 64 knots at Middle Bay Lighthouse at 2018 UTC (1518 CDT) • Eight vessels capsized and sank. • 40 people overboard • Six deaths • One of the deadliest sailing accidents in recent history
1 June 2015Thunderstorm Event Summary During the afternoon of 1 June 2015, clusters of thunderstorms developed over the Appalachian Mountains and Piedmont region of Maryland and Virginia. During the late afternoon, clusters of thunderstorms over the Baltimore-Washington, DC corridor developed into a squall line that tracked over the upper Chesapeake Bay into early evening. Air mass over the Chesapeake Bay region was moderately unstable with elevated DI values (2-4).
1 June 2015Thunderstorm Event Summary WSP = Wind Speed Prediction
Radar Imagery/Wind Potential Bow echo
Conclusions • Downbursts are an important component of hazardous winds produced by thunderstorms. • Over 75 confirmed downburst events documented between 2010-2013 thunderstorm seasons over Chesapeake Bay region. • Downburst index indicated severe wind potential (> 50 knots) three hours prior to the Mobile Bay accident, and prior to the issuance of a severe thunderstorm watch and special marine warning for the Mobile Bay region by the National Weather Service.