Clogged storm sewers, icy culverts, frost pose flood threats for area
Three days after a winter storm dumped seven to 12 inches of snow on the St. Croix Valley, a new system is forecast to bring an icy, wet weekend and the threat of possible flooding to the region.
Weather forecasters at the National Weather Service in Chanhassen said the new system and warm air moving in from the south is setting the stage for rain, freezing rain and snow tonight through Sunday morning.
“Light freezing rain mixed with snow, sleet and rain will spread across the area tonight,” said NWS forecasters. “Although there remains uncertainty on surface temperatures, there is the potential of ice accumulations up to two-tenths of an inch in portions of southern and central Minnesota as well as west-central Wisconsin by Saturday morning.”
The new precipitation, combined with the snow cover from Tuesday’s storm, has NWS forecasters warning of possible flooding from rain. The current forecast calls for a freezing rain-snow mix tonight into Saturday morning.
“However, as warm air surges in from the south, the precipitation will transition to all rain Saturday,” said NWS forecasters, adding that one-quarter- to one-half-inch rainfall is possible in the metro area. “Rain on Saturday across southern Minnesota may cause some minor flooding, especially in urban areas,” forecasters said. “Additionally, snow melt in combination with the rain may lead to runoff issues.”
That rain has few places to drain after the snow earlier this week was cleared from Valley roads, forecasters said.
“Many (storm) sewer grates are snow covered due to the storm earlier this week while many culverts are filled with ice,” forecasters said. “The clogged sewers and frozen culverts will prevent water from effectively running off roads.”
Frost is another factor adding to flood concerns, according to forecasters.
“In addition, much of the rain will fall on areas where we have concerns of concrete frost,” forecasters said. “Some water may run off into area creeks and streams and could lead to localized flooding.”
Concrete frost occurs when the ground is saturated and re-freezes, forecasters said. They added that weekend highs will not be warm enough to thaw concrete frost.
Today will get off to a pleasant start with partly cloudy skies and a high near 37. A rain-snow mix is forecast to move into the Valley around midnight, then change to a snow and freezing rain before transitioning to rain and snow early Saturday, forecasters said.
There is a 90 percent chance of rain and snow Saturday with a high near 37. The rain and snow is forecast to change to freezing rain and snow late Saturday night and become all-snow early Sunday morning. Precipitation is forecast to end late Sunday morning with highs near 32. Sunday night will be partly cloudy and cold with a low near 13.