MILWAUKEE — The recent sunshine and string of days with highs in the 40s is melting our snowpack. And while you might be ready to say, "goodbye snow," it's better if the snow melts slowly. Why? To help prevent spring flooding.
March and April are the months when our area typically experiences river flooding. A rapid rise in temperatures, or a heavy rain event while the ground is still frozen, would result in the greatest risk of flooding.
According to the local National Weather Service office, the above-average flood risk includes the following rivers: Baraboo, Sugar, Pecatonica, Rock, Lower and Upper Fox, Milwaukee, and Root. The risk is near average for the Wisconsin River.
As of this writing, most stream flows across Southern Wisconsin are in the normal category, but they are increasing slightly due to current warmth and subsequent melting.
Let's take a look back at winter. The season started out slow, we had a warm November with just a trace of snow. And December only brought 5.3" of snow. Winter got going in January, with 23.3" of snow. Through Feb. 26, Milwaukee has recorded 17.5" inches of snow for the month. The Milwaukee season snow total is running about 9" higher than the seasonal norm. Areas southeast of Milwaukee, near the lake, have had even more snow due to the mid-February lake-effect snowfall event.
The current snowpack puts SE Wisconsin at an elevated risk of flooding. The official snow depth at the Milwaukee airport, as of Feb. 26, is 13". Although the snowpack across southern Wisconsin varies from city to city, in general, the snow contains between 2 and 4 inches of liquid.
The frost depth, as of late February, ranges from about 4 to 10 inches, which is considered shallow to moderate. This is good news; a shallow frost will thaw more quickly, allowing the ground to soak in moisture as opposed to creating runoff.
It's also important to look back at the last few years; this can give us more information about ground saturation and water levels. In 2019, Milwaukee had a very wet year, with both a wet spring and fall. May of 2019 had over 6" of rain, September of 2019 brought 7" of rain, followed by about 6.5" of rain in October 2019. 2020 precipitation was near average.
Another possible issue, as temperatures warm, is the potential for ice jams. Ice jams occur when ice on the surface of rivers and streams begins to break apart. The chunks of ice flow downstream and can block the flow of the river. The water then backs up behind the jam, causing flooding.
Make sure you have a working NOAA Weather Radio to be alerted to any potential flood warnings. If you live near a river, or an area that tends to flood, keep up to date with the latest forecast. Also, make sure to have a family communication plan in case of flooding. Don't keep important documents or treasured family photos in the basement, keep them in an elevated, waterproof container. When it comes to driving, remember the saying Turn Around, Don't Drown. Never attempt to drive through a flooded roadway, it's too hard to know how deep the water is, or if the road has been washed out underneath.
Storm Team 4 will be keeping a close eye on the flood potential as we head into spring. Stay with TMJ4 on-air and online for the latest updates.