I discussed ‘mega-rainfalls’ (defined by 6 to 8+ inches over a 1,000+ square mile area) a few weeks ago and they just keep coming for parts of the U.S.. The latest big rain fell last night into this morning in central Mississippi. Radar estimates show 10 to 12 inches of rain falling in some places.
Local National Weather Service offices will do the ‘deep-dive’ statistical analysis, but broad analysis shows that it’s at least a 200 year flooding event.
(Above: ‘return period’ or occurrence in years, probabilistically speaking, for the rainfall in central Mississippi over the past 24 hours from NOAA)
You can imagine these mega-rainfall events are extremely rare occurrences, statistically. After the most recent deluge in the Dallas-Ft. Worth, Texas area the term ‘1,000 year flood’ has appeared. It’s actually the FIFTH 1,000 year flooding event (at least) in the last five weeks in the United States.
How can we say something is a 1,000 year event if our rainfall records only go back 100 or 200 years? It’s all about probabilities and statistics.
If we have enough data we can do a pretty thorough statistical analysis on it. You’ll recall when I’ve discussed extreme temperatures the good old ‘bell curve.’ When data is normalized (i.e. the mean or average and the median, the middle number are the same) it generally fits or is distributed under a bell shaped curve or plot.
When we say an event is a 1,000 year event, that means it has a 1 in 1,000 (1/1000) = or 0.1% chance of happening on a given time period in a given place. Every place is going to have a different amount of rainfall that would equate to a 1,000 year flood based on their local climate. For example, places along the Gulf Coast have a higher threshold because tropical systems can dump lots of rain there sometimes, even if it is rare. To get a foot of rain in North Dakota for example is going to be much, much more rare as it would take a large amount of moisture and the confluence of atmospheric events coming together at once.
That’s why, one of the recent 1,000 year floods is actually not a lot of rain (to a Texan or Minnesotan). Death Valley, a notoriously dry place, received 1.46 inches of rainfall earlier just a couple weeks ago in a 3-hour period: that’s more than a whole year’s worth of rainfall there.
[https://www.nps.gov/deva/learn/news/death-valley-experiences-1-000-year-rain-event.htm]
For the Twin Cities area, a 1,000 year flood event would have to be over at least 6 inches based on my rough estimation. The 1987 ‘super storm’ event which dumped 8 to 11 inches of rain between July 23-24th, 1987 almost certainly qualified to be even more rare than that.
[https://www.dnr.state.mn.us/climate/journal/870723_24_superstorm.html]
Climate change undoubtedly is playing a role in increasing the chances of such events occurring. That’s because for every 1 degree (F) of warming, the atmosphere holds 4% more water vapor. The relationship between temperature and water vapor is exponential as well.
That’s why for example, a dew point of 72 (think our nice humid July days) holds 4.5 times more water content than a dew point around freezing.
Indeed in Minnesota we’re seeing our state not only become warmer but also wetter. Despite a drought for some this year and most of us last year, the overall trend is undoubtedly wetter. We still live in an extreme place so droughts and floods will be the norm, but at least the wet extremes are becoming more extreme.
If we look at the number of days with heavy rains, say 2”+, we’re seeing more of them. While one extra day of 2”+ rain may not seem like a lot, that’s almost 10% of our annual rainfall.
Our annual precipitation is increasing too, from about 26 inches to now 30 inches. That’s like getting an extra June every year of rainfall, which is our wettest month of the year.
With a couple more months left of the ‘warm season’ and a lot of hurricane season to get through yet, get used to hearing more about mega-rains and 1,000 year rainfall events. We can only hope that in the Twin Cities at least our drought departs quietly rather than with a deluge.
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