© 2019 Terry Swails

ELEVATED RISK FOR SPRING FLOODING...

March 23, 2019

River flooding continues for much of the region. Rain, warmer temperatures and rapid snow melt the last couple of weeks has led to flooding along the Cedar, Iowa, Wapsi and others. However, with no snow left in the local area many have already reached their crest and are falling except the Mississippi which is slowly rising from snow melt up north making its way down stream.

 

NOAA (National Oceanic and Atmospheric Administration) released its 2019 Spring Flood Outlook and it's definitely concerning especially along the Mississippi River. They suggest nearly two-thirds of the lower 48 states face an elevated risk for flooding through May. 

 

The potential is there for moderate to major flooding in at least 25 states. The areas of greatest risk include the upper, middle and lower Mississippi River basins including the mainstream Mississippi River, the eastern Missouri River, and Red River of the North. On the map below, moderate flooding is indicated by the red color and major by the purple which highlights much of the Mississippi River basin especially from southeast Minnesota south to the Quad Cities. 

Record snow across a large area of the country has set the stage for the elevated flood risk. In fact, the upper Mississippi River basin has received rain and snow more than 200% above normal. 

 

The outlook is based on several factors: current snowpack, drought, soil moisture, frost depth, streamflow and precipitation. 

 

A wet fall for many areas saturated the soil heading into winter. Deep frost depth limits water infiltration thereby generating more runoff from rain and snow melt than soil that is not frozen. 

 

The image below is the latest frost depth from the North Central River Forecast Center where a few inches to as much as 36 inches or more remain in parts of Minnesota. That is aided by several inches of snow still on the ground.

 

 

Snow cover is still extensive in the upper Midwest. However, warmer temperatures and rain lately have melted much of it away in northern Iowa and southern Minnesota where a few inches are still left. Further north, into Minnesota and central/northern Wisconsin up to a foot or more still blanket the frozen ground.

 

 

Take a look at the snow cover from just one week ago on the image below. Big difference. 

 

A considerable amount of water remains in the left over snow all throughout the Mississippi River watershed as you can see below. As much as 2-6" of water is waiting to get released into the river system. 

 

 

The snowpack will take a big hit with some rapid melt this weekend as temperatures warm above normal into the 40s and 50s. Below is the GFS model and its forecasted highs for both Saturday and Sunday.

 

First, Saturday.

 

 

Now, Sunday.

 

 

Then, it cools off. The Climate Prediction Center has a greater than 40% probability of below normal temperatures heading into April. That is good news because the cooler temperatures will slow the melting process.

 

A big driver and something that could complicate the flooding situation for the worse is spring rainfall. That combined with snow melt will only increase the chance for significant flooding. The latest precipitation outlook from the Climate Prediction Center from March 30th to April 5th has a normal chance of precipitation in the upper Midwest which is encouraging news. 

 

The EPS Euro ensemble model which is comprised of 51 different members places the heaviest precipitation well south of the area and into the mid-to-lower Mississippi River valley. Below is the mean total precipitation of all its members valid through April 5th. 

 

 

However, it is just a models interpretation of the current trends. Storms are going to go where they want. Any shift in developing storm systems or tracks, and where heavy rainfall lays out could change things so it's a situation that continuously needs to be watched. 

 

The Mississippi River will likely remain high and above flood stage at many sites well into May. Below is a hydrograph of the river level at Dubuque. The blue line indicates the observed level of the river with the purple dots and line the forecast. You can clearly see the river is expected to slowly rise into next week to 19.1'.

 

 

Below is the hydrograph for the river at Rock Island. It's expected to reach major flood stage (18') late this weekend and remain high well into next week as snow melt makes it way down stream.

 

According to the National Weather Service, there will likely be yet another crest along the Mississippi River and its tributaries when all of the snow up north melts. While the timing is uncertain, the most likely time frame is April 10th through 25th. That could change depending on temperatures and rainfall. 

 

Bottom line, flooding is a high concern especially along the Mississippi River. There are still many uncertainties especially when it comes to rainfall and the extent of the flooding which will need to be closely watched as we move further into Spring.

 

 

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