August 20 marked the beginning of what is typically the most active portion of the Atlantic hurricane season, and NOAA satellites are closely monitoring the action. Historically, more than 85 percent of all major (Category 3, 4, and 5) Atlantic hurricanes form after this date. In fact, hurricane experts at Colorado State University even ring a bell every year to mark the occasion.
In the eastern Pacific, there also tends to be a peak in activity during late August, but it’s less pronounced than in the Atlantic. Here, relatively high levels of activity tend to be spread out over a longer portion of the season than in the Atlantic, with most tropical cyclones occurring between late June and early October.
This proved to be true this year when on August 14, a low-pressure system developed off the southern coasts of Mexico, Guatemala, and El Salvador. This disturbance strengthened into Tropical Storm Hilary on August 16 and became a hurricane the following day. Hilary continued to rapidly intensify, reaching Category 4 strength by August 18 with sustained winds of 145 mph. The same day, the National Hurricane Center issued the first-ever tropical storm watch for Southern California, while the Weather Prediction Center issued the first-ever level 4 (high risk) for flash flooding in the desert valleys of Southern California.
As Hilary moved north-northwestward, it began to weaken and made landfall as a tropical storm in San Quintín, Baja California on August 20. It later crossed into California near Palm Springs, bringing torrential rainfall, flooding, and mudslides. Up to 10.5 inches of rain fell in Southern California, including around 2 to 3 inches in Los Angeles and San Diego, which set summer records.
Hillary also brought heavy rainfall and flooding to parts of Arizona and Nevada as it continued on its path. It weakened into a post-tropical cyclone as it crossed the border into Nevada early on Aug. 21. That same day, the National Weather Service of Los Angeles said totals for Hilary had broken “virtually all rainfall daily records.”
Numerous storms have also been forming over the Atlantic Ocean. Within the span of 18 hours, three tropical storms formed—Emily, Franklin, and Gert. Tropical Storm Emily formed on August 20, 100 miles west-northwest of the Cabo Verde Islands, and transitioned into a post-tropical cyclone on August 21. The same day that Tropical Storm Gert formed about 485 miles east of the Leeward Islands.
While Emily and Gert were relatively short-lived and dissipated over the ocean, Franklin, which formed east of the Leeward Islands, made landfall in the Dominican Republic on August 23, bringing heavy rains to Hispaniola. This rainfall produced some flash and urban flooding as well as mudslides across the region, and as Franklin moved north, it also brought tropical storm conditions to the Turks and Caicos islands.
By August 22, another tropical storm, Harold, formed in the western Gulf of Mexico, making it the fourth Atlantic named storm to form within 39 hours. According to Meteorologist Philip Klotzbach, this is the fastest time on record for four Atlantic named storms to form, breaking an old record of 48 hours that was set in 1893 and matched in 1980. However, he noted that the observational network was much less robust in 1893, so there is some degree of uncertainty with the reported timing that year.
Harold made landfall on San Padre Island, Texas on August 22, and was the first Atlantic storm this season to do so in the U.S.
As the Atlantic and Pacific hurricane seasons continue, NOAA satellites remain our watchful eyes in the sky, providing critical information for hurricane forecasting, tracking, and intensity estimation.