Does Climate Change Cause More Severe Storms? NOAA Says Yes In Louisiana But Not in Colorado. Canada Mixed?

The 2016 extreme rainfall in Louisiana has been deemed by NOAA to be more likely and severe due to change climate, while the 2013 extreme rainfall in Colorado has been deemed less likely. Why the different behaviour? Like in real estate, its all about location, location, location. Trends are not uniform across the entire continent - Louisiana is influenced by the Gulf of Mexico, and Colorado isn't.

The 2016 Louisiana rainfall event dropped more than six inches of rain in some regions August 11, 2016 and Baton Rouge received over 11 inches of rain August 12. Such an event has been analyzed to be more likely, with a shorter 'return period', meaning a higher probability of occuring. Specifically the 2016 Louisiana storm was assessed to have a 30 year return period today (i.e., 1/30 = 3.3% chance of occurring), while only a 50 year return period in the past (i.e., a lower 1/50 = 2% chance of occurring). Details are here.

Louisiana rain frequency shift - 3 day rainfall.
Details are on the NOAA website, including the graphic above.

But a NOAA-led study published in the Bulletin of the American Meteorological Society indicated that "The probability for an extreme five-day September rainfall event over northeast Colorado, as was observed in early September 2013, has likely decreased due to climate change." That storm dropped 17 inches of rain.

Details of the Colorado rainfall analysis are here, indicating a 12% decrease over the last century:

"Despite a warmer and moister climate, the frequency of September heavy five-day rain events does
not increase in the simulations but substantially declines in northeast Colorado (Fig. 5.2c). Using the model’s 95th percentile of five-day rainfall totals, we find a 12% decline in occurrence during recent decades compared to the late 19th century."

And the following:

"... a slight decline in intensity of the maximum five-day precipitation over the central Great Plains
during summer is also projected (Sillman et al. 2013), emphasizing that global and annual perspectives of climate change may not always pertain to events at a specific place and time."

Research by Peterson shows regional trends in flood magnitude over decades that support the decreasing Colorado rainfall trends:

How to Interpret: "Trend magnitude (triangle size) and direction (green = increasing trend, brown = decreasing trend) of annual flood magnitude from the 1920s through 2008. Flooding in local areas can be affected by multiple factors, including land-use change, dams, and diversions of water for use. Most significant are increasing trends for floods in Midwest and Northeast, and a decreasing trend in the Southwest."

These USA extreme events and trends highlight the difference in regional impacts of a changing climate. In Canada, precipitation analysis by Vincent and Mekis showed less extreme daily rain, although more days with rain:

"The analysis of the precipitation indices for 1950–2003 reveals more days with precipitation, a decrease in daily intensity and a decrease in the maximum number of consecutive dry days."

Research has shown regional trends in annual maximum observed rainfall amounts as well. Environment Canada scientists indicate the following in Atmosphere-Ocean indicating increases and decreases:

"Both the southwest and the east (Newfoundland) coastal regions generally show significant increasing regional trends for 1- and 2-hour extreme rainfall durations. For the shortest durations of 5–15 minutes, the general overall regional trends in the extreme amounts are more variable, with increasing and decreasing trends occurring with similar frequency; however, there is no evidence of statistically significant decreasing regional trends in extreme rainfall amounts. The decreasing regional trends for the 5- to 15-minute duration amounts tend to be located in the St. Lawrence region of southern Quebec and in the Atlantic provinces."

It is important to distinguish between 5-day rainfall trends, noted in the Louisiana and Colorado analyses, 5 minute to 24 hour rainfall trends in the Environment Canada analysis. In the Canadian data the trends can be mixed across a single weather station depending on the duration. This table shows that variability in long term Canadian climate stations:

The data shows, for example, decreasing very short duration and longer duration trends at Toronto Pearson Airport (5 minute, 6, 12, 24 hour), but increasing mid duration trends (10 minute to 2 hour). Ottawa has decreasing short duration trends - suggesting less severe urban flooding potential - and increasing longest duration trends.

Overall, Environment Canada indicates "a general lack of a detectable trend signal" in short duration rainfall (meaning 5 minute to 24 hour) that is important for engineering infrastructure design. In Southern Ontario, there are more statistically significant decreases in annual maximum rainfall than increases:

In Southern Ontario, the increases in extreme rainfall are generally over longest durations (6, 12, and 24 hours) that is not critical for urban flash flooding - in contrast, the shortest duration trends show decreasing extreme rainfall.

Previous posts suggest that the increase in flood damages in Ontario cities can be related to multiple factors such as land use change (also noted by Peterson regarding USA Floods). For example:

Urbanization and runoff explain Ontario flooding

Ontario, Canada city land use change and flood risk

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