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Spring flooding potential

It is probably the absolute last thing farmers want to think or talk about: flooding. Flooding occurred last spring after a record setting snow season and delayed the harvest season. Now we are already looking forward to the upcoming spring after what seems like an early onset of winter.

There is a checklist of conditions that can lead to flooding:

We can already cross two items off that list starting with high river levels.

For a list of specific river levels near where you live check out this page. While all area rivers are below the action stage with no short term flooding potential forecast, the rivers are running higher that their average. For example, here is a look at the Cedar River in Waterloo. The river is currently sitting above the 75th percentile for cumulative flow, where it has been all year.

There is a lot of moisture in the ground after a wet fall. Across eastern Iowa, soil moisture content ranks in the 90th to 99th percentile, which is highly anomalous.

This means the soil will not accept much more water and may runoff if there is additional precipitation.

Both of the above criteria already signal the potential for rivers freezing at high levels, widespread ice jams (even in places that don’t normally get ice jams), spring flooding, and impacts to agricultural interests.

The rest of the checklist depends on what happens this winter and spring. Any more checks will worsen the probability and severity of spring flooding.

NOAA just recently released their winter outlook with low to medium confidence based on the lack of a El Niño or La Niña in the Pacific. That means precipitation and temperatures will be driven by short-term climate patterns that may result in wide swings of either. That being said, the outlook, which should not be considered a forecast, pegs above average precipitation this winter, which is less than ideal if verified.

This refers to the amount of moisture in the snow itself. Warmer temperatures, close to freezing, will result in a wetter and heavier snow. This is the type of snow that will be slushy on the roads and is good for making snowballs and snowmen. In this scenario, one inch of water may result in around ten inches of snow.

Colder temperatures will result in a drier snow – one with a lesser liquid water equivalent. In this scenario, one inch of water will result in about 20 inches of snow or more as it will be fluffier.

Chances for flooding increase with a wetter snowpack.

A rapid thaw will result in more water being released from the snowpack than the soil can handle, causing runoff and thus spring flooding.

If a deep frost is present in the soil, a melt of the snowpack would result in more runoff and spring flooding since the soil is not ready to take in water. A cold winter would result in deeper frost. We are already off to a very cold start to the year with temperatures getting even colder into the early part of the workweek. The winter outlook (Dec. – Feb.) shows equal chances for below or above average temperatures.

More precipitation in the spring will result in higher chances for flooding.

So far the checklist looks like this:

The NWS will hold their 2020 Probabilistic Spring Flood Outlook on the following dates:

February 13, 2020
February 27, 2020
March 12, 2020

Check back for updates throughout the winter and spring if you are concerned about spring flooding.

For more information, here is the National Weather Service summary.

 

Brandon Libby

Meteorologist

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