A Deep Analysis Of The South Florida Floods And 4 Key Takeaways

This week something extraordinary happened. Almost two feet of rainfall fell in parts of south Florida. It rained so much that the Fort Lauderdale-Hollywood International airport was closed due to flooded runways. At the time of writing onFriday morning, the sixth largest school system in the nation, Broward County Public Schools, remained closed for a second day. Typically, I use around 800 words in these essays. However, there are four key takeaways from this historic rainfall event that require a bit more text this time around. Let’s dig in.

Tornadic supercells storm were stuck in “park”

A supercell thunderstorm is a storm the has a rotating updraft. Typically when meteorologists speak of supercells, we are on guard for possible tornadoes (more on that in a moment). The south Florida event on April 12th was caused by a supercell thunderstorm that decided to put itself into “park.” The Tweet posted by the National Weather Service discussed why so much rain fell (around 2 feet in some places) in a relatively short period of time.

Fellow meteorologists even dropped their jaws. National Hurricane Center meteorologist Eric Blake tweeted, “25.91” is a lot of rain for a season, much less one day. 👀 #FtLauderdale.” Ed Mansouri is the Founder and CEO of WeatherSTEM, a network of weather observation systems. He told me Thursday, “Our WeatherSTEM unit at Fort Lauderdale International Airport measured a staggering close to 26 inches of rain yesterday….At first I was dubious when I saw it.” He went on to ponder whether it would end up being the single largest daily rainfall total in Florida history eclipsing the 23 inches that fell in the Florida Keys (1980).

Ok, let me put on my meteorologist hat and break down what happened. A low pressure system develop in the northern Gulf of Mexico this week and a warm front drifted north across the southern part of the state. Rainfall was associated with this system. My Florida State University classmate Todd Kimberlain, a meteorologist with the South Florida Water Management District, tweeted a key point that the region already endured a significant amount of rainfall in previous days.

As these systems evolved, several superstorm thunderstorms moved into the region. This event featured storms that became relatively stationary over Ft. Lauderdale. Meteorologist Alex Lamars equated the scenario to placing a faucet over the region and “turning it on and walking away.” He almost made an important point about how efficiently these storms were converting available water vapor to cloud water and ultimately precipitation. My doctoral dissertation examined precipitation efficiency in Florida storms along convergence boundaries so this brought back scholarly memories. I should mention that convergence was certainly part of the story here too as a frontal system was draped across the state.

The NWS Storm Prediction Center (SPC) mesoscale discussion at 8 pm EDT (April 12th) sounded the alarm with some wording that would get any meteorologist’s attention. They wrote, “The atmosphere remains primed for these storms to contain efficient warm rain processes featuring; a highly saturated profile (>90% RH at low-mid levels), a classic skinny CAPE profile, warm cloud layer depths of 11,000 kft, storm motions <10 kts, and effective bulk wind shear values >30 knots to sustain these cells.” A wind profiler, an instrument system that measures atmospheric winds, in the region also showed a steady fetch of moisture streaming in form the ocean (more on this later). SPC concluded the discussion saying, “With the impressive rainfall totals and significant flooding ongoing for some areas, this will only exacerbate what is a life-threatening situation. Expect more street flooding to ensue, and with the sun setting, areas of flooding will become increasingly more difficult to identify for motorists.”

Oddly, the footnote to the event is that two tornadoes also spun out of this system in Broward County.

Impervious surfaces are usually a part of any urban flooding event

This storm was no different. Intense rainfall is certainly one of the ingredients required for this type of event. Scientists are now estimating this to be a 1 in 1000 year event. I actually hate when storms are referred to in that way. It gives people the impression that this event can only happen every 1000 years. However, Jonathan Erdman, a senior meteorology with IBM/The Weather Company, nails the problem in the Tweet below. According to the U.S. Geological Survey (USGS), “In terms of probability, the 1,000-year flood has a 0.1% chance of happening in any given year.” Let’s start being more descriptive for the public and decisonmakers, please.

However, I digress. Let’s get back to the other ingredients. Impervious surfaces in urban environments fundamentally alter the natural water cycle. They cause increased surface runoff and a reduction of infiltration into the ground. The images below of inundated roads or airport runways (1st image in the article) are stark examples of anthropogenic modification to the water cycle.

In 2022, I opined in Forbes that water cycle diagrams that we learned in school are insufficient because they do not capture the ever-increasing role of human beings in the process. In recent months, the U.S. Geological Survey rolled out a new version of its water cycle diagrams (graphic below). I love it. It includes many processes and factors, including imperviousness, that play direct roles in extreme flooding events like the Ft. Lauderdale disaster.

The storms of 2023 are not like the storms of 1970

Experts have warned us for years that the stormwater engineering for cities was designed under the fatal assumption of “stationarity.” What does that mean? Brian Bledsoe leads the Institute for Resilient Infrastructure Systems (IRIS) at the University of Georgia. He often says that many engineered systems and infrastructure were designed under assumptions that rainstorms of 1970 will be the same in 2023. Nope! They just aren’t. The peer-review literature is bursting at the spine with studies noting that intensity of rainstorms is changing, particularly at the top 1-2 percent level. Unfortunately the assumptions and data used by engineers is outdated in many cases. There are efforts to update such information, and we need to do it fast. Basic physics tells us that a warming climate allows more water vapor availability to storms. The well-known Clausius-Clapeyron relationship tells us there is a 6-7 percent increase in atmospheric water content for every degree C increase in temperature.

Dr. Kieran Bhatia raised another interesting wrinkle in the south Florida floods that could have a climate connection. Bhatia is a Vice President at Guy Carpenter who studies climate and extreme events. His tweet below suggests that warmer sea surface temperatures likely contributed to the water vapor “pump” into these quasi-stationary supercell storms.

Bhatia has been pointing out for weeks that ocean temperatures are quite warm (see map below). By the way, this could have implications for hurricane season as we creep closer to June 1st.

The public and policymakers must adjust reference points

As someone with over two decades of experience at the intersection of meteorology, climate, and societal impact, there are things that have become abundantly clear to me. One of those things is that people have static “mental models” or reference weather events in their minds. People who experienced Hurricane Katrina (2005) or a particular flooding event often anchor to that event as an exemplar of the “worse-case” scenario. We all need to adjust our reference storms. A dynamic approach will needed in order to adequately prepare and adjust to the new realities of weather and climate.