Much of Minnesota made it into at least their top 20 seasonal snowfalls with many more in their top 10. That, combined with a slow start to spring and lots of winter rain means we have a lot of water stored on top of the ground just waiting to be unleashed.
The recent big snows and a March that’s slow to warm up are reasons why the National Weather Service has upgraded the flood outlook to a well above normal risk.
Purple points below are spots where major flooding is forecast this spring. Anything that’s not green means that at least flood stage is expected to be reached.
We’ve had a lot of moisture this winter. Much of central and eastern Minnesota has seen double to triple the normal precipitation the past 90 days.
The unusually deep snow pack for this late in the season ranks pretty high when we look at historical records. For much of central Minnesota, which will feed into the Mississippi eventually, we’re in the top 20% of mid March snow packs.
Not only have we had a lot of snow but we’ve had a lot of winter rain. Unlike in the warmer months, winter rain has no where to go but be absorbed into the snow pack or run off. With the deep snow we’ve had, that snow just became a giant sponge that then froze. The 1.5+ inches of water we received in just rain in January and February is basically still sitting in the snow pack… waiting. This is why we not only look at snow depth but the water equivalent in that snow.
The above map means that most of Minnesota has 3 to 6 inches of water waiting to be set free. Unlike in a thunderstorm event, where that much rainfall would be localized, this will be across most of the state. For more than half of Minnesota this water content in the snow pack is in the top 10% for this time of year.
What determines the extent of flooding will be how fast that snow melts and HOW it melts. Of course temperature and future spring rainfall matter quite a bit, and we’ll get into the forecast scenarios in a minute, but we also need to look at the two factors that will be beneficial to us this spring:
The first, is that, because winter struck so fast and early this year, the ground never really froze to depths it typically does. Snow that’s been consistently on top of the ground all winter helps to insulate it, so frost depths are pretty shallow.
Shallow frost depths mean that when the big thaw comes, we won’t have to wait as long as normal for the ground to be able to absorb snow melt and precipitation. Which brings us to those soils. If the soil was bone dry like it was in the fall, it would absorb more melt like a sponge.
The top image was soil moisture content in early November compared to early March. We’ve made up a lot of ground, but for most of us, including much of the Mississippi River basin, the soil at least isn’t saturated… it has some room to absorb water.
To put these factors into context, let’s look at the spring floods of 1965. These were the worst in history for much of Minnesota- specifically the Mississippi. My late grandfather, Gordy Sundgaard snapped many photos of a Saint Paul underwater that April. See if you can recognize the spots:
The March and April of 1965 illustrate the worst case, ‘everything, everywhere, all at once.’
For less impactful flooding you want a nice, slow thaw in March. The problem this year has been a thaw that’s been TOO slow, and then we just add more snow to the snowpack. March 1965 saw 37.1 inches of snow- the second most ever recorded in the Twin Cities in March and a lot of it came late in the month, combined with temperatures that were a staggering -13 degrees below normal. This meant the snow pack peaked at a mind boggling 27 inches, more than 2 feet by March 29th. ALL of that snow was gone by April 10th. April was a wetter than normal month also that year.
We all know that eventually the snow will melt, no matter how cold the start to spring is. The longer we put off the big thaw, the more likely it will be very abrupt, like in 1965.
That brings us to forecasts. The two variables that will really matter are temperatures the next several weeks and additional precipitation.
Most of our longer range models are forecasting a continuation of at least above normal precipitation heading into April. Here’s just one example from the Canadian model through April 15th of precipitation anomalies (difference from normal).
As far as temperatures are concerned, most models are forecasting below normal temperatures to continue the next four weeks at least, this may help to mitigate a huge-all-at-once melt. BUT, a lot will depend on if the temperature pattern is consistently below normal or just averages out to be below normal with radical swings from cold to warm (which is possible in April we know). Here’s the European model forecast for the next 30 days for temperature anomalies (difference from normal):
Regardless of the future weather however, we have SO much moisture locked up sitting on the ground that significant flooding is still likely. Luckily, lake and river levels are lower than average thanks to the drought last year, giving us more wiggle room. It’s just that temperature and precipitation could make a bad situation a much worse situation.
Take the probability forecasts for some rivers as an example. These are from the National Weather Service:
The Mississippi river at Saint Cloud has a 50% chance of reaching moderate flood state and a 19% chance of hitting major flood stage. Those probabilities are 3 to 4 times greater than an average spring.
The Mississippi river at Saint Paul has a 61% chance- a likelihood- of reaching major flood stage this spring. That’s 4 to 5 times greater than average!
The Saint Croix at Stillwater has a 73% chance of reaching major flood stage, a whopping 6 to 7 times greater than normal chance.
So while we’re all itching for spring, let’s not wish real warm, stormy weather too soon or the results could be a big problem.
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