Despite snowy winter, forecasters see average Minnesota spring flood risk

It’s been a snowy winter across much of Minnesota. And most of that snow is still sitting on the ground waiting to melt this spring.

So how much water is in that snowpack? And what does it mean for our spring flood risk? The North Central River Forecast Center on Wednesday released its first spring flood outlook for 2023.

This group of National Oceanic and Atmospheric Administration hydrologists forecasts river levels for the Upper Midwest. They work out of the Twin Cities National Weather Service office in Chanhassen.

Let’s break down the initial assessment for spring flooding across our region.

Snow cover

There is currently 12 to 18 inches of snow on the ground across most of Minnesota. There is a little less in the west and south, and more than 2 feet in parts of northeastern Minnesota.

I’ve included some commentary from Thursday’s initial flood outlook below in each section.

Compared to normal snow depth, quite a bit above average; higher than the 60-80th percentile. The exception is the Mississippi headwaters, which are close to 50th percentile, or right about the average.

Calculating snow water equivalent

Hydrologists can calculate how much water is in the snow cover across Minnesota. This is done through flights with remote sensing equipment, and additional measurements on the ground.

The water in the snow across our region is in the top 5 to 15 winters on record in Minnesota.

Most of the Minnesota and Mississippi basins have 2 to 3.5 inches of Snow Water Equivalent (SWE) on the ground. There is even more in the Chippewa (WI) and St. Croix basins, as much as 4 to 5 inches.

Comparing this year's SWE to normal: we rank in the 80th to 90th percentile over the entire area. This means we have only seen about 5 to 15 years of this much snow water on the ground over the past 70 years, in early February.

Soil moisture

Soil moisture conditions were extremely dry heading into winter, but they have improved some with the recent wet pattern. In addition, the early and heavy snow cover has insulated soils from extreme cold.

This means frost depth is shallow this winter, and some soils are not completely frozen. That may lead to more water absorption this spring.

Soil moisture has started to recover over much of the area from a couple years of drought conditions; it is still below normal but getting better.

Drought conditions have been improving over the winter. Almost all of the area has seen a one- to two-class improvement.

The early snowpack has kept frost depth to a minimum over most of the area this year. In some cases, the soil is not even completely frozen below the snow.

Thus, once melting starts, we may see water quickly infiltrate the soil rather than running off over a frozen layer.

Low fall water levels

Rivers, lakes, and ponds were very low heading into this winter. That means there is plenty of headroom for runoff to raise water levels before flooding occurs.

River Levels at Freeze Up -- most river levels in the area were near to below normal when ice formed, so there is more room for runoff to be incorporated into the rivers before they would flood.

Lake/Pond/Wetlands/Marshes -- Many of these dropped very low during the late summer and remained low into the winter, and thus could intercept a lot of water that would otherwise contribute to river and stream rises.

Bottom line

Even with above-average snow water equivalent between 2 and 4 inches in our region in the snow cover, the flood risk at this point is average for the spring of 2023.

Right now most outlooks favor near-average precipitation this spring across most of our region.

The initial outlook for spring flooding in the upper Mississippi, Minnesota, and Chippewa River basins is near normal.

Though we have an ample snowpack, drier soils and little frost depth will help alleviate at least some of the snowmelt threat.

Of course, heavy additional snowfall and/or significant rainfall at the wrong time could increase the threat as the spring moves along.

Stay tuned.