DETROIT – By now you’ve probably seen Local 4′s coverage of the catastrophic flooding in mid-Michigan.
But how much rain fell, and how historic was it?
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A weather observer in Midland reported 4.70" of rain in approximately thirty-six hours from Sunday through Monday night. That amount of rain falling at Midland in a forty-eight hour period is statistically expected to occur only about once every twenty-five years, so the fact that this rain fell in thirty-six hours means that it was a 25-to-50 year event.
Adding insult to injury is that this rain came after another inch to inch-and-a-half of rain fell last Thursday into Friday. So it is no surprise to hear about the enormous rises on the Tittabawassee River, and the resulting dam failures at Edenville and Sanford.
Although rainfall amounts here in the Local 4 area weren’t as high as what fell farther north, we still received significant rainfall since last Thursday. I dug through the observations and added together the various rain totals to come up with this list, and keep in mind that I was not able to get totals from Macomb County, since none of the weather observers reported rain amounts for both rain events to the National Weather Service...at least as far as I could determine from the reported observations available to me online.
- Adrian 3.54"
- Ann Arbor 3.28"
- Detroit 2.42"
- Dundee 2.06"
- Farmington (thanks to Steve Sobel): 2.98"
- Flat Rock 2.68"
- Grand Blanc 3.01"
- Howell 3.41"
- Lapeer 2.25"
- Lexington 2.16"
- Manchester (thanks to Sue Drob): 3.88"
- Milford 3.01"
- Morenci 4.38"
- Owosso 4.66"
- Romulus - Metro Airport 2.78"
- Yale 1.85″
The National Weather Service's Hydrometeorological Design Studies Center has researched rainfall return frequencies and, while they do not provide a statistic for a six-day rainfall period, they do have the stats for a seven-day period.
So, here is the rainfall amount for one week and how often it is statistically expected to occur in the Detroit area:
| One week amount (Inches) | Return Frequency (Years) |
|---|---|
| 3.31 | 1 |
| 3.72 | 2 |
| 4.43 | 5 |
| 5.05 | 10 |
| 5.95 | 25 |
| 6.68 | 50 |
| 7.45 | 100 |
So, did global warming cause this rain event?
I've told you many times in the past that global warming does not CAUSE individual weather events. Rather, the planet's warming climate is increasing the frequency of certain weather events. In this case, extreme precipitation events are definitely occurring more often, and there is a very simple reason why: as Earth warms, more ocean water is evaporating into the atmosphere.
Since it is this atmospheric humidity that storms turn into precipitation, storms now have more “juice” to work with, and are producing more extreme precipitation events worldwide as a result (and yes, that also includes snow).
Simply, human activity has put the atmosphere “on steroids.” So, in the same way that great baseballs players hit home runs before they started taking steroids and then started hitting more of them after starting the steroids, increased water vapor in the atmosphere is increasing the frequency of extreme precipitation events. But again, climate change did not cause the storm which dumped that rain.