In the past two weeks, two major hurricanes have impacted the Baja Peninsula in Mexico. Hurricane Norbert bypassed a large portion of the west coast of the peninsula from September 5 to 7, and Hurricane Odile made landfall near Cabo San Lucas on September 14th as a Category 3 hurricane on the Saffir-Simpson Wind Scale. A third system, Hurricane Polo, formed Tuesday, September 16 and is forecasted to follow a similar track to Norbert and Odile, making it the third such tropical cyclone to develop in the region since the beginning of the month.
This serial cluster of storms has been driven primarily by steady, favorable conditions for tropical cyclone development and consistent atmospheric patterns present over the Eastern Pacific. A serial cluster is defined as a set of storms that form in the same part of a basin, and subsequently follow one another in an unbroken sequence over a relatively short period of time. To qualify as a cluster, there needs to be measurable consistency between the tracks. This is typically a result of steady, predominant atmospheric steering currents, which play a major role in influencing the speed and direction of tropical cyclones. One example of a serial cluster is the four major hurricanes (Charley, Francis, Ivan, and Jeanne) that impacted Florida during a six-week period in 2004.
During this recent two-week period, the area off the west coast of Mexico has maintained high sea-surface temperatures near 85.1 degree Fahrenheit and limited vertical wind shear, leading to an active tropical development region. A mid-level atmospheric ridge over northern Mexico has provided a consistent steering pattern towards the north-northwest, producing similar observed tracks for Norbert and Odile and forecasted track for Polo. Devastating amounts of rainfall have occurred with these storms. Hurricane Odile dropped nearly 18 inches of rain in areas around Cabo San Lucas, representing nearly 21 months-worth of typical rainfall. This cluster, while generating significant wind and flood damage along the Baja Peninsula, has also caused torrential rainfall in the southwestern U.S., including Arizona, southern Nevada, and southern California. Last week, Phoenix, AZ, one of the hardest hit areas, experienced over 3 inches of rain in a 7 hour span due to the remnants of Hurricane Norbert. This was the most rainfall to occur in a 24-hour period in the city since 1911, an estimated 1-in-200 year event by the National Oceanic and Atmospheric Administration. Significant rainfall and inland flooding is forecast to continue as the remnants of Odile and Polo move inland, which may lead to widespread flood losses and the potential for compound post-event loss amplification.