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IT AIN'T OVER TIL IT'S OVER...

Mid to late March is a time when Midwest hydrologists assess the potential for spring flooding on area rivers and streams. Many factors come into play. Here's a few key indicators.

Frost depth:

When the ground is frozen melting snow or precipitation is not going to be absorbed. Frozen ground increase the chances of spring flooding. The ground has thawed locally but deep frost depths remain in Wisconsin and Minnesota. Melted snow or rain quickly runs off.

Snow cover:

This is important because snow contains moisture. The more snow the higher the moisture content. If the snow melts fast it can be like a massive 3-6" rain over a large area. Throw that in with a couple rounds of heavy rain and frozen ground and the threat escalates. Most of the 1-2 foot snow depths are located in northern Minnesota and Wisconsin.

Here's the current moisture content of the snow that exists to our north. Amounts are generally in the 2-5 range which is near to below normal in most locations.

Temperature are also a big factor. A quick warm-up melts snow and releases the frozen moisture. It can also contribute to convective rain systems containing heavy rain producing thunderstorms. The rest of this month looks colder than normal and this should limit both of those factors until April, especially up north.

Of course, the biggest driver in all is precipitation. A 2-3 week period of heavy rain can be a game changer and even with the proper thaw and melting, flooding can quickly develop, especially if soils are saturated.

I mention this because modeling indicates an active pattern the next 16 days that could impact the flooding potential. Look at what the GFS shows. Pushing 6" in SE Iowa.

The EURO is also bullish showing this for precipitation the next 10 days.

Even more disturbing for many of you is the fact some of this will fall as snow. I don't feel comfortable showing snowfall amounts just yet as temperatures will be marginal and dependent on the intensity and track of the storm. I will say the potential exists for more than 6" of heavy wet snow. The greatest threat currently looks to be near or north of HWY 30 but that is very much subject to change. More to come. Roll weather...TS

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© 2020 Terry Swails