Rain Bombs and Thousand Year Floods Catch America’s Attention

The massive August floods in Louisiana have triggered a new high-level examination of the large number of historic inland storms that have clobbered the United States since May 2015. This cluster of dramatic downpours has coincided with 15 consecutive months of global heat records. The national media has grasped that this may not be random coincidence.

The link between extreme precipitation events and accelerating warming of the planet has broken into the mainstream media cycle alongside the presidential campaign and Olympics.

The increase in heavy rainfall and the resultant flooding “is consistent with what we expect to see in the future if you look at climate models,” David Easterling told the New York Times for an August 16 article entitled “Flooding in the South Looks a Lot Like Climate Change.” Easterling is a director at the National Centers for Environmental Information, which is operated by the National Oceanic and Atmospheric Administration.

According to NOAA, parts of Louisiana received up to 79 centimeters of rain in the past week, most of it in a 48-hour period August 11-13. Easterling called the downpour “pretty staggering,” exceeding rainfall levels that his center predicts will occur once every thousand years in the area.

Figure 1: Maximum observed rainfall amounts in relation to corresponding rainfall frequency estimates for the WBHL1 gauge near Zachary, Louisiana. Image: NOAA
Figure 1: Maximum observed rainfall amounts in relation to corresponding rainfall frequency estimates for the WBHL1 gauge near Zachary, Louisiana. Image: NOAA
Image: NOAA
Louisiana 48 hour rainfall, 11 – 13 August. Image: NOAA

But, Easterling cautioned, that NOAA’s exceedance probability analysis is predicated upon the old normal, the idea that the climate is relatively stable. That idea has become outdated. Under the new normal of global warming, it’s likely that a similar storm will inundate central Louisiana sooner than the year 3016.

As Bloomberg News reported a week prior to the Louisiana floods, the physics driving increasingly intense storms is fairly straightforward. A hotter atmosphere holds more water—about 4 percent more for every degree Fahrenheit, or 7 percent for 1 degree Celsius increase in average global temperature. The global energy imbalance caused by anthropogenic greenhouse gases is equivalent to 400,000 Hiroshima atomic bombs, which seems to have given growing currency to the colloquialism “rain bomb” to describe what meteorologists call a “wet microburst.”

Except the intense Arizona downpour highlighted in the August 2 Bloomberg article was more of a “macroburst,” with a radar footprint wider than about 2.5 miles, said Amber Sullins, chief meteorologist at ABC15 News in Phoenix.

“The research showing rain events for (the American Southwest) being less frequent but more intense, due to climate change, seems to be our new reality,” Sullins told Bloomberg.

Nationally, however, NOAA data shows that extreme rain events have become more intense and more frequent since the early 1980s.

Image: NOAA
Image: NOAA

Even though NOAA recognizes that its precipitation frequency estimates don’t reflect the forward-looking realities of a warming planet, the agency is working to complete its most recent update known as NOAA Atlas 14. NOAA’s interactive website provides a user-friendly graphic interface that geographically displays precipitation frequency estimates for durations from 5 minutes to 60 days and average recurrence intervals between 1- and 1,000-years, alongside 95% confidence limits, temporal distributions, and seasonality. Only the Northwest region and Texas have yet to be completed.

While NOAA hasn’t completed the new model for determining annual exceedance probabilities for Texas, the agency used a simplified version to include two Texas storms among eight detailed reports on large, worst-case deluges that exceeded the 1/500 annual probability threshold since May 2015. The reports, available here, also cover storms in Louisiana, Arkansas, Mississippi, West Virginia, Maryland, South Carolina and Oklahoma, along with a few select events dating back to 1913.

While the New York Times and other media have trumpeted NOAA reports of eight 500-year storms since May 2015, NOAA’s chief hydrometeorogical modeler says the United States has probably experienced hundreds of 500-year rainstorms in that period. “If you don’t specify duration or area, then there’s no way to count the number of extreme storms”, said Sanja Perica, who runs NOAA’s Hydrometeorological Design Studies Center. The eight featured by NOAA simply represent the largest or most damaging events.

Meanwhile, the national flood hazard coordinator for the U.S. Geological Survey cautions against equating extreme precipitation events with damaging floods. Citing data through 2014, Robert Holmes says there’s significant regional variation in U.S. flood trends. “We see pockets where flood trends are increasing and pockets where flood trends are decreasing. I think the larger factors where we see increasing flood frequency is that the reporting of events has increased awareness and we are still allowing people to build in the floodplain.”