Second spring flood outlook highlights increased Mississippi River risk

CEDAR RAPIDS, Iowa (KCRG) - The second of three spring snowmelt flood outlooks from the National Weather Service has been released, and it includes some significant changes.

The outlook, issued by individual NWS offices that serve eastern Iowa, showed that the probability of flooding occurring on the Mississippi River in the KCRG-TV9 viewing area has notably increased, pushing it into a much above normal category. Some tributary rivers to the Mississippi, which are any other river in eastern Iowa, showed a very slight increase in risk for flooding but maintained near normal spring flood risk.

Contributing factors to the projected flood risk

The most significant factor that caused the jump in flood risk along the Mississippi River has been ongoing precipitation across the region, especially the continued building of the snowpack to our north in upstream drainage areas. That’s also why the risk for tributary rivers has only increased slightly; the snowpack in Iowa is running below normal, easing the risk for local rivers.

The snowpack to our north has increased, with nearly the entire state of Minnesota with at least 8 inches of snow on the ground, with some spots reaching 20 inches or more (note: this outlook does not have the entirety of this week’s storm accounted for). This means there are broad areas of the Mississippi’s watershed where 4 to over 10 inches of liquid water equivalent is locked up in that frozen snow and ice, waiting to melt and run into the river.

The timing and speed of the snowmelt this spring will be critical for the severity of any flooding that could take place. A rapid snowmelt, especially if paired with additional rainfall, would be the worst-case scenario. A more leisurely melt would reduce the risk of high-end flooding, but could still cause minor flooding given the amount of water locked in that snowpack.

All of eastern Iowa is running above normal for precipitation so far in 2023, which has led to a reduction in drought conditions generally. However, a good portion of the viewing area is still considered at least “abnormally dry,” so we still have room to work with. Relatedly, soil moisture levels are also still near to below normal, allowing for space in the ground for melting snow or additional rainfall to go.

One advantage of the early and deep snowpack up north is that it has kept frost depths shallower, meaning that it’s possible the ground could thaw faster as snow melts. If the ground is hard and frozen, water is more prone to runoff instead of soak in, so a deep frost layer in the ground can be an issue. This is generally not present in our area.

River levels are also generally running near or somewhat above normal for this time of year. The NWS believes that this is a neutral factor at this time, with “sufficient storage” still available for future precipitation events.

How did things change between the two outlooks?

Generally, the risk of flooding went up across eastern Iowa, but the tributary rivers only showed modest increases in most cases. The Mississippi is another case, though, with risk probabilities there increasing by quite a bit.

For a select group of the tributaries:

• Cedar River at Cedar Rapids: 21% chance of minor flooding in the second outlook, compared to 18% in the first outlook and a long-term average chance of 27%.
• Iowa River at Marengo: 64% chance of minor flooding in the second outlook, compared to 57% in the first outlook and a long-term average chance of 71%.
• Maquoketa River at Manchester: 18% chance of minor flooding in the second outlook, compared to 15% in the first outlook and a long-term average chance of 23%.
• Wapsipinicon River at Anamosa: 20% chance of minor flooding in the second outlook, compared to 16% chance in the first outlook and a long-term average chance of 26%.

It’s worth emphasizing that all of these selected points above show a near or just below normal risk for even minor flooding. Some of the increase in the risk can also be explained by moving the outlook period two weeks later into the spring; February is not a common month to flood in Iowa, so eliminating the last two weeks from the outlook does push current and historical probabilities a little higher.

That statistical quirk doesn’t explain the change in risk on the Mississippi, which is significantly higher:

• Mississippi River at Dubuque: Greater than 95% chance of minor flooding in the second outlook, compared to 69% in the first outlook and a long-term average chance of 47%.
• Mississippi River at Rock Island: Greater than 95% chance of minor flooding in the second outlook, compared to 73% in the first outlook and a long-term average chance of 59%.
• Mississippi River at Muscatine: Greater than 95% chance of minor flooding in the second outlook, compared to 75% in the first outlook and a long-term average chance of 61%.

For those locations, the risk of reaching major flood stage is also much higher:

• Mississippi River at Dubuque: 55% chance of major flooding in the second outlook, compared to 26% in the first outlook and a long-term average chance of 15%.
• Mississippi River at Rock Island: 69% chance of major flooding in the second outlook, compared to 33% in the first outlook and a long-term average chance of 27%.
• Mississippi River at Muscatine: 67% chance of major flooding in the second outlook, compared to 33% in the first outlook and a long-term average chance of 25%.

Remember that even a high probability of something occurring does not guarantee that it will occur, even at a greater than 95% risk. However, these relatively high probabilities should give people who live along the Mississippi River reason to pay close attention to the evolving situation through the spring.

Are we headed for a wet spring in eastern Iowa?

Currently, mid- to long-range outlooks from the Climate Prediction Center do indicate a continued trend toward wetter than normal conditions through the next few months. This includes the nearer target of March, and overall for the period from March through May. The number of storm systems, and the degree to which we finish above normal for the season, will matter in how things ultimately turn out.

The NWS assesses future precipitation as currently a neutral or slightly increased threat for flooding on the Mississippi River and its system of tributary rivers.

The final spring flood outlook for 2023 will be issued by the NWS on March 9.

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