Spring flooding concerns rising; a look back at 1965

Heavy winter precipitation and a slow thaw so far are increasing the odds of significant flooding this spring. What happens in the next several weeks will be critical.

More rain and snow this week

Our latest storm system brought yet more rain and snow to Minnesota. The swath of snowfall was pretty narrow from southwestern Minnesota through the central portions of the state into northeastern Minnesota.

The additional precipitation, however, was more widespread with many areas picking up anywhere from one-tenth to one-half inch or more on top of an already moisture-laden snowpack.

The recipe for rising waters

The slow pace of thawing this March doesn’t just induce spring fever, but it also delays the inevitable flow of snow melt into our lakes and rivers which will likely come more abruptly the longer we put it off.

Tracking snow depth is important, but the most critical figure is how much water equivalent is in it. Most of Minnesota has 3 to 6 inches of stored water in the snow just waiting to be unleashed by warmer temperatures.

That amount of moisture is in the top 10 percent of water content for this time of the year, not something you want heading into spring.

Despite the high water content, we do have a couple factors in our favor. The frost depth this winter is less than in a normal year due to the early and abrupt start to winter. Consistent snowpack since November has insulated the ground and kept the frost from developing as deep as it would otherwise.

This means that the ground will be able to absorb moisture sooner once that frost thaws. Lake and river levels are also low still from last summer and fall’s drought.

Increased odds of flooding this spring

What happens in the next several weeks will be critical to just how severe flooding is this spring. We have so much moisture in our snowpack that significant flooding is likely, it’s just a matter of how much worse could it be?

We’re looking at equal chances of a warmer or colder than normal April right now.

We may also be favoring slightly higher chances of wetter-than-normal conditions in April.

A wet late March and April would exacerbate the flooding scenario as would an abrupt warm up, melting the deep snow in a short period of time rather than allowing it to be absorbed some into the ground.

With all of these factors in mind, the National Weather Service issues probabilities of flooding based on modeling of rivers and current weather data.

For the Mississippi River at St. Cloud the forecast calls for a 50 percent chance of moderate flooding, which is three to four times the average spring chance.

In St. Paul, the Mississippi is forecast to have a 61 percent chance of major flooding which is four to five times the average value. For the St. Croix River at Stillwater, it’s even worse: a 73 percent chance of major flooding which is six to seven times higher than the average spring probability.

Flood history: 1965 remains one of the worst

Understanding how the spring sets up often requires looking at history. 1965 was a notoriously bad year for flooding, especially on the Mississippi River. It remains one of the worst flood years for many places in Minnesota.

That year saw a very snowy March with 37.1 inches — the second most ever recorded in the Twin Cities in March — and a wet April. March was also very cold with temperatures averaging 13 degrees below normal, keeping the snow pack solidly in place very late in the season.

On March 29, 1965, the snow depth in the Twin Cities was a whopping 27 inches. All of that had melted by April 10. This all led to catastrophic flooding in the region.