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Looking for a signal of man-made climate change? Look no further than the deluge of mega-rains and even more rare 1,000-year rains that have hammered the U.S. over the past month. The latest mega-rain happened this week in central Mississippi, where radar estimates show 10-12 inches of rain that led to flash flooding. 

Radar estimated rainfall August 23-24th, central Mississippi

Radar estimated rainfall August 23-24th, central Mississippi

National Weather Service offices will do the deep statistical analysis, but broad analysis shows that the Mississippi mega-rain was at least a 200-year flooding event – and that's just in the bullseye, while surrounding areas only get a massive rain like that once every 20-75 years on average. 

The "return period" or occurrence in years, probabilistically speaking, for the rainfall in central Mississippi over the 24-hour period Aug. 23-24. 

The "return period" or occurrence in years, probabilistically speaking, for the rainfall in central Mississippi over the 24-hour period Aug. 23-24. 

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These mega-rains and 1 in 1,000-year rains are extremely rare occurrences, statistically. And the Mississippi event doesn't for sure qualify as a 1,000-year rain (not until the NWS says so), but the past month has seen Mother Nature birth at least six mega-rains and five 1 in 1,000-year rainfall events. 

  • July 26: St. Louis – 1,000-year rain & mega-rain
  • July 26-30: Eastern Kentucky – 1,000-year rain & mega-rain
  • Aug. 2: Southern Illinois – 1,000-year rain & mega-rain
  • Aug. 5-7 Death Valley – 1,000-year rain (not a mega-rain)
  • Aug. 6: Northern Illinois – mega-rain
  • Aug. 21-22: Dallas – 1,000-year rain & mega-rain
  • Aug. 24-25: Mississippi – mega-rain

How can we say something is a 1 in 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 – the mean (average) and the median (the middle number) are the same – it generally fits or is distributed under a bell-shaped curve or plot.

bell curve

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 over 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 the local climate. For example, places along the Gulf Coast have a higher threshold because tropical systems can dump lots of rain, making those extremely rare instances slightly less rare. 

To get a foot of rain in North Dakota is 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, at least not to a Texan or Minnesotan. Death Valley, a notoriously dry place, received 1.46 inches of rain just a few weeks ago. But it all fell in a 3-hour period. That’s more than an entire year’s worth of rain there.

For the Twin Cities area, a 1,000-year flood event would require at least 6 inches, according to my rough estimate. The 1987 "super storm" that dumped 8-11 inches of rain July 23-24 almost certainly qualified as even more rare than 1 in 1,000. 

Incredible rain totals from the 1987 "super storm" in Minnesota. 

Incredible rain totals from the 1987 "super storm" in Minnesota. 

Climate change undoubtedly is playing a role in increasing the chances of such events. 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.

2017Hurricanes_Graph_en_title_lg1

That’s why, for example, a dew point of 72 (think of Minnesota's 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 the wet extremes are becoming more extreme.

2"+ RAINS

If we look at the number of days with heavy rains, say 2+ inches, we’re seeing more of them. While one extra day of at least 2 inches of 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's worth of rain every year, which is our wettest month in Minnesota. 

ANNUAL

With a couple more months left in 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 our drought departs quietly rather than being wiped out in a day. 

BMTN Note: Weather events in isolation can't always be pinned on climate change, but the broader trend of increasingly severe weather and record-breaking extremes seen in Minnesota and across the globe can be attributed directly to the rapidly warming climate caused by human activity. The IPCC has warned that Earth is "firmly on track toward an unlivable world," and says greenhouse gas emissions must be halved by 2030 in order to limit warming to 1.5C, which would prevent the most catastrophic effects on humankind. You can read more here.

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