A foot to three feet of snow currently covers the 7,700-square mile region that drains into the St. Croix River. A deep frost has frozen soils full of rain from last fall. Now, meteorologists say those factors and more might mean flooding along the lower river once melting begins.
The National Weather Service released its first flood forecast of the season this week, saying Stillwater could see fairly high water.
“For spring 2019, the snowmelt flood threat is higher than it has been for a few years,” the agency reported.
The Twin Cities has seen its fourth snowiest February on record, with 39 inches of snowfall. It has been the seventh snowiest meteorological winter (December through February) at 52.5 inches. The total snowfall for the season was nearly 57 inches as of Feb. 28, already exceeding the average for the whole season of 54.4 inches.
How much of the snow sticks around for how long, and how much water it contains, will decide the downstream impact. So far, cold temperatures have prevented much melting, and snow depth is significant. There are two feet on the ground in St. Croix Falls.
There is also relatively deep frost in the ground throughout the region, reaching down two to four feet. Last autumn saw significant rainfall right before freeze-up, meaning saturated soils were frozen in place before they could drain.
This could mean more meltwater will run over the ground and toward streams and rivers.
St. Croix River flood forecast
The graph below shows the probability of hitting various flood stages throughout the spring in black, and the historic average threat in blue. As NWS reports, if the black line is above or to the left of the blue line, the flood threat is above normal.
“The forecast from FEMA & NOAA is not good but it’s still early and lot’s can change climate-wise,” Stillwater mayor Ted Kozlowski said. There’s a 40 percent chance of major flooding, which he says is the level when the city might “Call in the guard and get the kids out of school, this is all hands on deck.”
There is an approximately 15 percent chance the river could hit 692 feet, which is when it saw some of the worst flooding in recent memory, in 1991. In 1965, it set a record at 694.1 feet. There is about a 10 percent chance of hitting that mark.
The current forecast calls for the highest chance of flooding in late March and early April. But the threat at this point remains pretty consistent through late May.
When warm-up occurs, and how fast, will decide when the crest comes.
Watching the weather
What will happen with the weather during March and April will have a big impact on how severe the melt season is for the St. Croix.
“Near to above normal precipitation, and/or a period of very warm temperatures in March or early April would increase the threat,” the Weather Service says.
Long-range models anticipate the first couple weeks of March will be cold, with a chance of snow.
“If anything we’ll be keeping the snow we have on the ground and maybe even increase it a little bit,” the agency reported in a video. “Long range models indicate some moderating temperatures and precipitation later in the month. The main point is: Watch the forecasts as we move through March, and be prepared should there be big changes.”
View the Weather Service’s full discussion of the flood forecast:
The Weather Service also noted that there is still time for homeowners to buy flood insurance that will cover them this spring. But, because it takes a month for insurance to go into effect, authorities are urging people to act now.
“Don’t gamble. Take action as soon as possible: Call your insurance agent and get a good flood insurance policy,” the MN Department of Public Safety wrote. “So when all of this record-breaking snow melts and the rains come, you’ll have the peace of mind that comes with knowing that your home and your possessions are protected.”
And, as Stillwater’s mayor reminded his constituents, all this also means that boating season is not so far away.