An early-season tropical storm is bearing down on the Gulf Coast right now, threatening a multi-state region with another potentially devastating flood.
While the center of Cindy's circulation will likely make landfall in eastern Texas on Thursday morning, the vast majority of the risk is associated with the storm's large plume of tropical moisture that's already running ashore in the central Gulf Coast, from Louisiana to Florida. While there's a chance that Cindy's central core could quickly intensify and threaten Houston, truly tremendous rainfall totals are forecast for places like New Orleans, Biloxi, Mississippi, and Mobile, Alabama and that's what we should focus on. Inland flooding from heavy rainfall—not wind or waves—is historically the biggest cause of loss of life in tropical storms and hurricanes.
Along the Gulf Coast, rainfall totals of 10 to 20 inches are expected, enough to max out the color scale on the National Weather Service's Excessive Rainfall Outlook. That much rain falling over a period of two or three days constitutes a one-in-50 to a one-in-200 year event. (As you might remember, Louisiana had a 500- to 1,000-year rainstorm just last year.) This week's rain will be falling on ground that's already soggy—parts of the Gulf Coast have already received about two to four times the normal amount of rainfall over the last 30 days, increasing the likelihood of a repeat disastrous flood.
To prepare, officials in Louisiana are distributing sandbags and making final checks on the extensive pumping network in areas that are below sea level and prone to flooding, even as the rain has already begun to fall.
If you're thinking that it's a bit early for storms like this, you're right: Cindy is already the third storm of this year's Atlantic hurricane season—a milestone that is normally not reached until mid-August.
As with virtually every weather event at this point, there's a clear link between Cindy and climate change. Since a warmer atmosphere can hold more water vapor (thanks to enhanced evaporation and other factors), rainfall rates during extreme events have become more intense. To prove the point, this morning, the atmosphere over New Orleans was the wettest it has ever been for the date. The one-in-200-year or one-in-500-year calculations include only historical rainfall data and don't take into account changing trends resulting from climate change. Weird rainstorms are happening more often now.
On Monday, Cindy made a bit of weather-nerd history: For the first time, the National Hurricane Center began issuing official warnings for a tropical disturbance threatening the United States before it was officially designated as a tropical cyclone. That might seem like an overly pedantic milestone, but this is a major accomplishment for an agency with a history of excessive emphasis on bureaucracy. (Remember, no official hurricane warnings were issued for Hurricane Sandy's disastrous 2012 landfall in New Jersey by the National Hurricane Center because it anticipated, days in advance, that the storm would transition into a post-tropical cyclone—a type of storm that fell under the jurisdiction of another branch of the weather service.) The result is that residents of the Gulf Coast had official warning of Cindy's approach a full 18 hours earlier than they otherwise would have.