There’s at least a 50 percent chance of major flooding on the lower St. Croix River this spring, the National Weather Service announced today. High soil moisture from last fall, significant snowpack, and other factors all contribute to the predictions.
On the other hand, there’s also a good chance there won’t be anything to worry about. The National Weather Service must balance the needs of being prepared and avoiding unnecessary alarm.
The river will rise, like it does every spring — how much is the question. The forecasts provide probabilities of the river reaching different water levels, but much depends on the weather between now and spring runoff.
“Overall, the flood potential for the spring melt remains well above normal for the Upper Mississippi River drainage,” the NWS North Central River Forecast Center stated today.
The forecast shows about a 55 percent chance the river will hit 689 feet above sea level at Stillwater, which is considered “major” flood stage and represents a crest in the top seven since 1952.
There’s a greater than 90 percent chance of reaching “minor” flood stage at 687 feet above sea level, making a top 10 historic crest highly possible.
Last year, after a worrying flood forecast, Stillwater proceeded to construct a significant dike, which was ultimately not needed to protect downtown. The water rose to a “top seven” crest of 688 feet above sea level.
Once again this year, the greatest flood risk will be at the end of March and early April. That means there is probably still time for property-owners to purchase flood insurance and get through the standard 30-day waiting period.
Much left to melt
The snow is still about 8 to 10 inches deep across the region that drains to the St. Croix, with deeper amounts in northern Wisconsin, where it’s two feet or more. This is near or above normal. The amount of water in the snow is above normal in the region.
While last fall and the first part of this winter were relatively wet, the past two weeks have been particularly dry. Western Wisconsin is 25 to 75 percent below normal precipitation amounts between from February 12 and 27.
Temperatures have been seven to 10 degrees below normal across Minnesota and Wisconsin. But the recent roller coaster of high temperatures has repeatedly melted and refrozen, the snow has lost some of its moisture, but can also release the remaining water much more quickly in the future.
As long as overnight temperatures continue to fall below freezing, it will help by allowing the snow to melt slowly and steadily.
Wet fall, shallow frost
Soil conditions are also key to how runoff will happen. The amount of moisture, and the frost depth, determine if meltwater soaks into the ground or flows toward streams — and ultimately, the St. Croix.
Soil moisture is “well above normal,” while frost depth is normal or shallower than normal, at six inches to two feet across much of Minnesota and into Wisconsin. The two are connected, with increased moisture part of the reason for shallow frost.
“This is a result of a combination of above normal wetness in the soil profile and snow insulating the ground prior to the deep freeze,” the NWS says. “There were several reports of farmers struggling to get in a very late harvest from the wet fall because the ground did not freeze, sinking equipment in the fields.”
With the ground still unfrozen until right before the first big snowfalls, the frost remained shallower. It is about 10 inches deep in Spooner, Wis., on St. Croix tributary the Yellow River. It’s about nine inches deep near Hudson.
The Weather Service will release an updated forecast in another two weeks, on March 12.