To determine the relative risk of spring flooding, numerous factors are considered including snow cover, soil moisture, and current river conditions. A significant snow cover with high water content can increase the chances of flooding once warmer weather melts the snow. Elevated soil moisture conditions reduce the amount of rainfall that is soaked up by the ground and increase the amount of water that then runs off into area streams. Above average river levels reduce the river rise required to reach flood stage, while below average river levels would require an increased amount of river rise to reach flood stage.
Here is a general overview of spring flood risk factors across northeast Illinois and northwest Indiana river basins.
Risk Factor | Current Condition | Effect on Flood Risk |
Fall/Winter River Levels | Below Average to Above Average | Decrease/Increase |
Fall/Winter Soil Moisture | Below Average to Above Average | Decrease/Increase |
Winter Precipitation (Snow Cover) | Near Average (through Feb 24) |
No effect/Increase |
Winter Temperatures (Frost Depth) | Near Average (through Feb 24) |
No effect/Decrease |
Spring Precipitation | Unknown | - |
Spring Temperatures (Snow Melt) | Unknown | - |
Valid February 25, 2021.
Modeled snow cover (below, left) across area river basins ranges from near 2 inches to near 18 inches, which contains 0.25 inches to 4 inches of water equivalent (below, right). The deepest snow cover was in far northwest Illinois and southwest Wisconsin. The majority of the area had snow water equivalent values of 1.5-3.0 inches.
Valid February 23, 2021. Click here for the latest snow cover information.
Modeled soil moisture values (below) across the area range from slightly below average to slightly above average. Values were the lowest in northern Indiana, at the 10th to 20th percentile for this time of year. Values were highest in southern Wisconsin, at the 80th to 90th percentile.
Frost depth ranged from 4 to 10 inches across area river basins, with the deepest frost depth values reported in southern Wisconsin. Despite very cold weather, heavy snow pack has reduced the frost depth penetration.
Valid February 23, 2021. Click here for the latest soil moisture information from the Climate Prediction Center.
River streamflow across the area ranges from much below average to above average (below). The highest streamflow levels are in southern Wisconsin and northern Illinois.
River ice spotters report generally ice-covered rivers across the area. The most significant river ice cover is likely occurring in the Rock and Fox River Basins. Heavy river ice may lead to ice jams this spring, especially if temperatures and precipitation lead to significant river rises, but confidence in exact timing and extent of possible ice jams is low as of this outlook.
Valid February 23, 2021. Click here for the latest streamflow conditions from the USGS. Click here for the latest river ice spotter reports.
The long term (next couple months) outlook from the Climate Prediction Center indicates above average temperatures favored by mid-March, with above average temperatures slightly favored for the remainder of spring. The outlook indicates above average precipitation slightly favored for spring.
Although snow cover, soil moisture, and recent river levels can provide some indication of the relative risk of spring flooding, any weather system that produces heavy rainfall could cause flooding. Spring flood outlooks are not able to assess the risk of flooding due to heavy rainfall more than a week or so in advance.
Click here for the detailed Spring Flood Outlook which includes tables with probabilities of flooding along area rivers. Relative flood risk can also be viewed on the Advanced Hydrologic Prediction Service (AHPS) page for our area.
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