200-year flood event: Wettest year on record so far

Paul HuttnerPaul Huttner Sep 22, 2016
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The Severe Summer of 2016 came to a merciful end at precisely 9:21 am CDT this morning. Astronomical autumn couldn’t arrive soon enough for flood-weary Minnesota.

Summer’s final act? Yet another torrential multi-inch rain storm. Flooded basements. Submerged cars. Washed out roads. Instant rivers where streets used to be.

Scenes from around the metro last night bordered on post-apocalyptic. When MNDOT calls out the snow plows to push floodwaters from the freeway, you know you’re dealing with an extreme weather event.

Numerous roads are underwater or in wash-out mode south of the Twin Cities from Waseca to Kenyon.

Meteorologists preach “turn around don’t drown” during flash floods. This is why. That road you drive everyday and think is under a foot of water? It may not be there anymore.

Record rainfall totals

Rain gauges and basements overflowed in Maple Grove and other north metro suburbs with 6″ to 8″ rainfall totals. An unofficial report of 9.86″ came in from Maple Grove last night.

Twin Cities NWS

A big chunk of real estate saw several inches of rain in the latest deluge.

Waseca saw a summer’s worth of rain in less than 48 hours. The Waseca Cooperative Observer station set a record two day total since data collection began in 1914.

100 to 200+ year flood event?

Keeping score on the so called ‘return interval’ for flash floods is tricky. Like many things in life, it depends on how you measure. I spoke with Pete Boulay from the Minnesota DNR State Climatology Office today about where the Maple Grove rains fall in the historical record.

Pete says using the more comparable 12-hour MN COCORAHS total of 8.11″ reported in Maple Grove would yield a 100 to 200-year flood event on NOAA ATLAS 14 point precipitation frequency estimates. (In blue below)

1 SSW Maple Grove [Hennepin Co, MN] COCORAHSreports HEAVY RAIN of 8.11 INCH at 7:00 AM CDT — COCORAHS STATION MAPLE GROVE 1SSW /MN-HN-171/.

NOAA

But if Maple Grove’s unofficial MESONET report of 9.86″ were deemed accurate and used, (in red above) we’d be looking at something closer to a 500+year flood event.

1 SSW Maple Grove [Hennepin Co, MN] MESONET reports HEAVY RAIN of 9.76 INCH at 8:14 AM CDT — MESONET STATION MAPLE GROVE /E8235/.

So how you keep score matters, but it’s also somewhat semantics. The practical effect of that much water is still largely the same.

According to Pete Wednesday night’s 6″+ metro rain event covered about 360 square miles, well short of the 1,000 square miles needed to qualify for a “mega-rain event.”

Minnesota DNR Climate Working Group

Here’s more on Wednesday night’s flood event from the Minnesota DNR.

The finale of the Wet Summer 2016 was one more heavy rain event on the day before Autumn.

A slow-moving weather pattern set the stage for more heavy rains. As the sun set on September 20, thunderstorms developed along a warm front that was from Nebraska through far Southern Minnesota. Heavy rains fell at Waseca and Steele Counties and across the southern Twin Cities metro area. These storms developed north of a warm front that stretched from a weak area of low pressure over southeast South Dakota eastward across southwest and south central Minnesota. Over the next 24 hours the front had become more or less stationary parked across southern Minnesota. During the late afternoon and evening of September 21, strong thunderstorms formed in two places, a narrow band across the northern Twin Cities and over south central and southeast Minnesota. Heavy rains fell over the northern Twin Cities over the span of about seven hours from about 5pm to around midnight, with lighter rains after that.

The highest total found so far with a manual rain gage in the northern Twin Cities was 8.11 inches from a Hennepin County CoCoRaHS Observer in Maple Grove. Some other higher totals found across the northern Twin Cities included 7.65 inches in northern Fridley in Anoka County. Some Hennepin County totals include: 6.27 inches at Maple Grove, 6.15 inches at Brooklyn Park and 5.88 inches at Brooklyn Center. 5.22 inches fell at Lexington in northern Ramsey County. There were sixty abandoned cars with water least to their doors in northwest metro suburbs during the height of the storm.

Another area of heavy rains fell in an area from Mankato eastward to the Mississippi River and then into Wisconsin. The highest 24-hour total ending on the morning of September 22 was 7.64 inches at the U of M Southern Research and Outreach Center in Waseca. This brought the two-day total at that location to 10.16 inches, including the 2.52 inches of rain that fell with the first wave on September 20-21. This is the highest two-day total recorded in Waseca. The ground was already saturated at places like Waseca and this exacerbated the flooding situation. There were many reports of basements flooded, closed roads and standing water in Waseca and other locations as well. I-90 was closed for a time in Austin.

Twin Cities: On pace for wettest year on record

Tell the (great) grand-kids. This is how it used to be back in 1911. With 30.81″ of precip so far this year MSP Airport is still on pace for the wettest year on record. It’s amazing some of us are all still above water.

Forecast: Coming up for air

A few scattered showers linger through Friday, but at least we catch a break form significant rain for about the next 48 hours. The next front (and there’s always a next front) blows into Minnesota Saturday afternoon with more rain.

NOAA

Mercifully Saturday’s weather system is more ‘progressive’ than last night’s monsoon. Rainfall totals should be much lower, but still may top an inch in some towns Saturday night.

NOAA

Ryder Cup forecast: Improving

You’ll need you favorite sweatshirt for the early practice rounds at Hazeltine next Tuesday and Wednesday. Temperatures in the 50s and 60s and spotty showers Tuesday and Wednesday will provide a cool weather challenge for players.

The latest GFS  and Euro runs are looking better for tournament play Friday through Sunday. More sunshine and afternoon temperatures in the upper 60s to near 70 degrees would be almost ideal.

Norwegian Met Institute (Temps in Celsius)

If current forecasts verify, we could be looking at some beautiful classic early fall weather to showcase Minnesota for the Ryder Cup next weekend.

Stay tuned.