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Kevin Jeanes explains the systems that have been bringing heavy rain, flooding to metro Detroit

Posted at 6:05 AM, Jul 30, 2021
and last updated 2021-07-30 07:24:48-04

Metro Detroit has been hit with historic rain over the past two months in metro Detroit, leading to massive flooding throughout the area.

These are the rainfall totals for three Michigan areas for June and July over the past three years.

Grosse Pointe Farms
2021 – 17.42"
2021 – 6.08"
2019 – 5.70"

Ann Arbor
2021 – 13.03"
2020 – 6.61"
2019 – 10.66"

Detroit Metro Airport
2021 – 10.10"
2020 – 7.42"
2019 – 5.49"

There are some climate factors mixed in with why we've had some flooding, but here's the flooding scenario.

Oftentimes, we get these stalled-out frontal boundaries, called stationary fronts, and when we get those, we usually get multiple rounds of rain.

Low-level jet transports deep, tropical moisture from the Gulf of Mexico and that's how we get our heavy rain. It can also come from the Atlantic Ocean.

So, a stalled front is a bad thing, but based on typical storm system paths, it happens frequently. It's just a question of where the low lines up and where the front lines up. If it tracks just south of us, it can create a bullseye.

The area of rain around the area of low-pressure pivots and creates a bullseye area where the most rain falls as the low moves from the southwest to the northeast. That's the worst scenario for us in southeast Michigan.

With the drought conditions going on out west, we've had several times this summer where there's been a very big ridge of high pressure farther to the north and out to the west. That can create a blocking pattern.

Basically, a blocking pattern means there's a higher chance to get these stationary fronts and that could lead to the possibility of these types of scenarios.

Other factors: Climate change. It's warming up, and warmer air holds more water, we can reach record values of just the amount of water vapor in the air.

Also, urban expansion. If this really heavy rain falls in a really densely-populated area, lots of concrete rather than land to absorb the water, you get more runoff and that's how you get flash flooding.