May 2020 Extreme Rain Events
Updated: Jun 8, 2020
This year has been one for the records, not just as far as the economic, health, and social issues, but also for the climate and the weather.
Drought & Fire
We began the year with dry conditions and patches of moderate drought conditions, shown to the left. When you fast forward to early may, shown on the right, moderate drought began to spread across large areas of the state and even severe drought.
Heat Records Locally, and World Wide
Florida has been shattering heat records across South Florida this year, including its earliest heatwave on record, when it hit 90 degrees on three consecutive days in early April. For context, in April, South Florida saw 16 days above 90 degrees, where we would normally average two.
Miami had the hottest April on record by far, 6.1°F above average! Fort Lauderdale also had the hottest April on record, Naples and West Palm Beach had the second hottest April on record.
The heat is not an isolated event, 2020 rivaled 016 as the warmest April on record globally. Cuba saw its hottest April on record by far, 5.0°F above average. Havana, the capital of Cuba, which hit its highest-ever reading of 38.5°C (101.3°F) on April 12. That same day brought a new all-time national high for Cuba of 39.7°C (103.5°F), set at Veguitas; it was the third time in three days that the nation topped its previous all-time high.
This heat and drought also led to wildfires in northern Florida. The data below is comprised of a snapshot of rainfall for every day in May across South Florida. We can see the rain came heavily on only a few days, including Memorial Day Weekend when viral videos of stranded vehicles and kayakers and wakeboarders using the flooded streets for recreation.
From Drought to 287% of average May Rainfall
The thirty-year historical average rainfall for May in Miami-Dade is 5.15". This May we saw 14.31", 278% more than the historical average.
Flood Warnings and Property Damage
These intense rains are leading to flooding that is becoming all too commonplace.
Scientists have been warning us of this for decades. An Architectural Design Approach to Climate Adaptation - Part 1- The Only Constant is Change, presented data showing the warming of the atmosphere and oceans, as well as graphs of the significant increase in the number of days with higher amounts of precipitation. Extreme one-day precipitation events have been rising across the South East US steadily since the 1960s.
Why do we have more rain? As the air warms, its water-holding capacity increases, particularly over the oceans. According to the Clausius-Clapeyron equation, the air can generally hold around 7% more moisture for every 1C of temperature rise. As such, a world that is around 4C warmer than the pre-industrial era would have around 28% more water vapor in the atmosphere. This additional moisture will not fall evenly. Some areas will increase while others see less, due to shifting weather patterns. On average, warming is expected to result in dry areas becoming drier and wet areas becoming wetter, especially in mid- and high-latitude areas. (This is not always true over land, however, where the effects of warming are a bit more complex.)These changes tend to increase proportionately with warming; if the Earth warmed only 2C in an aggressive mitigation scenario such as RCP2.6 rather than 4C, the percent change in precipitation would be roughly half as large.
How is the Weather Predicted to Change?
There are 39 different climate models within CMIP5 that provide estimates of precipitation changes in the future. Unlike for temperature, where models show a general degree of agreement about future regional changes, different models may have the same region becoming much wetter or much drier in a warming world. There are some areas where nearly all the models tell the same story about future changes. The figure below shows the same annual average change in precipitation between today and the end of the century and adds dots to indicate areas where at least nine out of 10 models agree on the direction of change.
There is a general agreement that a warming climate will increase the severity of extreme rainfall and snowfall almost everywhere. A warmer world, they project, will also increase soil evaporation and reduce snowpack, exacerbating droughts even in the absence of reduced precipitation.
The Effects on the Built Environment
When extreme rain events occur on multiple days as we saw at the end of May, the ground becomes very saturated. Our infrastructure also was unable to handle the increased rainfall in certain areas leading to property damage. The saturated ground also led to multiple flood warnings since for South Florida since the heavy rains in May.
For this year through April, the planet has seen a total of ten billion-dollar weather disasters, according to the April 2020 Catastrophe Report from insurance broker Aon. Six of those are related to severe weather in the United States, including two billion-dollar-plus severe weather episodes on April 6-9 and April 10-14.
We must listen to the scientists and reduce our Green House gas emissions immediately to reduce the effects of global warming. The cities that will thrive in the twenty-first century will be where governments invested in comprehensive mitigation and resilience strategies.
The urgency of timely, ambitious, coordinated, and transformative actions is imperative.