Reflecting on Tropical Storm Bill

After impacting coastal Texas and portions of the Plains and Midwest with rain, wind, and flooding for nearly a week, Tropical Storm Bill has dissipated, leaving the industry plenty to think about.

The storm organized quickly in the Gulf of Mexico and intensified to tropical storm status before making landfall in southeast Texas on June 16, bringing torrential rain, flash flooding, and riverine flooding to the region, including areas still trying to recover from record rainfall in May. Many surrounding towns and cities experienced heavy rain over the next few days, including some that recorded as much as 12 inches (30 cm). Thankfully though, most high exposure areas like Houston, TX, were spared of significant flooding.

Still, as damage is assessed and losses are totaled, Tropical Storm Bill reminds us of the material hazard associated with tropical cyclone (TC)-induced precipitation, and the importance of capturing its impacts in order to obtain a comprehensive view of the flood risk landscape. Without understanding all sources of flood hazard or their corresponding spatial and temporal correlation, one may severely underestimate or inadequately price a structure’s true exposure to flooding.

Of the $40 billion+ USD in total National Flood Insurance Program claims paid since 1978, more than 85% has been driven by tropical-cyclone induced flooding, approximately a third of which has come from TC-induced rainfall.

The most significant TC-rain event during this time was Tropical Storm Allison (2001), which pummeled southeast Texas with extremely heavy rain for nearly two weeks in June 2001. Parts of the region, including the Houston metropolitan area, experienced more than 30 inches (76 cm) of rain, resulting in extensive flooding to residential and commercial properties, as well as overtopped flood control systems. All in all, Allison caused insured losses of $2.5 billion (2001 USD), making it the costliest tropical storm in U.S. history.

Other notable TC-rain events include Hurricane Dora (1964), Tropical Storm Alberto (1994), Hurricane Irene (2011). In the case of Irene, the severity of inland flooding was exacerbated by saturated antecedent conditions. Similar conditions and impacts occurred in southeast Texas and parts of Oklahoma ahead of Tropical Storm Bill (2015).

Looking ahead, what does the occurrence of two early-season storms mean in terms of hurricane activity for the rest of the season? In short: not much, yet. Tropical Storms Ana and Bill each formed in areas that are most commonly associated with early-season tropical cyclone formation. In addition, the latest forecasts are still predicting a moderate El Nino to persist and strengthen throughout the rest of the year, which would likely suppress overall hurricane activity, particularly in the Main Development Region. However, with more than five months remaining in the season, we have plenty of time to wait and see.