Record snowfall may supercharge spring flood threat

The scientists who forecast floods for a living are busy these days.

Record February snowfall across the Upper Midwest is changing the flood equation this spring. Hydrologists and climate experts are busy crunching new numbers that incorporate the water content in all that snow on the ground.  That's what will runoff into area rivers and lakes this spring.

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Winter snow piles up at the Huttner Weather Lab in the southwest Twin Cities. Image: Paul Huttner/MPR news.

Monday I spoke with Minnesota DNR Senior Climatologist Kenny Blumenfeld. He tells me that flood forecasters are growing more concerned about significant flood potential as they begin to update spring flood outlooks. And as the snow keeps falling, that outlook grows more important each week going forward.

Widespread, deep snowpack

Kenny tells me the snowpack across the Upper Midwest is at the 99th-percentile rank across a big area. That means snow cover right now is near historic depth and coverage around the region.

Most of the area has more than a foot of snow on the ground. Many locations have more than 2 feet. Some locations up north and in western Wisconsin have 3 to 4 feet of snow cover.

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Here's the map. I've added individual snow depth reports.

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Snow depth as of February 26, 2019 via NOAA.

How much water?

It's called Snow Water Equivalent.

That's how hydrologists calculate to determine how much liquid is available in the snowpack to melt off into rivers this spring.

Right now there are more than 6-inches of frozen water sitting on the ground in much of southwest Minnesota. Same story in western Wisconsin. There's also more than a foot of water in snowpack across northern Minnesota. The rivers along the North Shore will be running hard and heavy this spring.

Here's the latest Snow Water Equivalent map.

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Snow water equivalent on February 26, 2019, via NOAA.

The water in the snowpack across southern Minnesota will eventually drain into the Minnesota and Des Moines river basins. The deep snow in western Wisconsin will hit the St. Croix. Water from both regions will hit the Mississippi River eventually.

There's growing concern for significant flooding in southern Minnesota and downstream along the Mississippi and tributaries into Iowa and Illinois.

Wild Card: March weather

Keeping tabs on snow water equivalent in a winter's worth of snow is hard enough. Predicting how volatile March weather impact runoff adds a whole new level of difficulty to flood forecasting.

The deck is already stacked to drive significant runoff from existing snowpack into our rivers. March typically adds heavy, high water content snow to the equation. Throw in in the potential for a rapid warm up, and heavy rain and you've got the potential for epic flooding this year.

The temperature outlook almost guarantees sub-freezing temps the first week of March.

Latest indicators suggest days above freezing, and nights below freezing the second week of March. That would be a good way to start releasing some of the water in the snowpack slowly. After that, it's anybody's guess.

The best scenario to minimize spring flooding? A slow warm up in late March with light March snowfall.

The worst case scenario? Heavy, wet snow in March, followed by a rapid warmup, and heavy rain.

Stay tuned.