Near Record Rainfall Event Saturday Morning - KMSP-TV

Near Record Rainfall Event Saturday Morning

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The rain came down in droves Saturday morning across parts of eastern and southern Minnesota leading to flash flooding up and down I-35 including portions of the metro. Scattered storms were in the forecast, but these were not the typical mundane thunderstorm. They didn't come flying into the area with high winds and large hail either, but those aren't the only way a storm can be defined as severe. After it was all over, I'm sure many in the west and south metro thought these storms packed a pretty good punch. But what makes these different? It all had to do with their moisture content. Let me explain…

We all know that summertime in Minnesota often means heat and humidity (and of course the mosquitos, but we will stay on the topic of weather for now). More humidity means there is more moisture in the atmosphere. The more moisture you have in the atmosphere, the more rainfall your storms can produce. The more rain your storms can produce, the more flooding you will likely have. So how much moisture did it take to produce the storms this morning??

Let's say for example, that the average morning in Minnesota during the summer, the atmosphere is filled with enough moisture to fill up Lake Calhoun. It's a pretty sizeable amount of water, takes a while to walk or run around it, but nothing huge. If a thunderstorm were to develop in this type of atmosphere, they would drop maybe a quarter or a half inch of rain in an hour or so as they roll through the metro; no big deal. We see it dozens of times every year. Well this morning was different because the atmosphere was filled with enough moisture to fill Lake Mill Lacs... a far larger body of water. This allows the thunderstorms to drop tremendous amounts of rain with rainfall rates topping 3 inches an hour. These storms are what meteorologists call efficient rain makers because they can produce a tremendous amount of water and drop it very quickly.  How we measure moisture in the atmosphere (the size of our lake) is called the precipitable water index. This basically means that if you stacked one column of air molecules from the surface all the way up to the top of the atmosphere and then rang them out like a sponge, you would get a specific amount of water. So on a typical summer day, the atmosphere hovering over Minnesota has around an inch of precipitable water in it (Lake Calhoun). But this morning, it officially measured at 2.39"… almost 2 and a half times the normal amount (Lake Mille Lacs). This is just 0.06" lower than the highest precipitable water index ever recorded in the Twin Cities. That's incredible!!! So where did all of this moisture come from?? Well, it came from the low level jet.

The low level jet is a term we use to describe southerly flow at the low levels of the atmosphere that drastically increases the amount of moisture in a specific area. It's basically a fire hose that's filling our lake. You can see it in the image below.

This is an image taken from the current conditions in the Upper Midwest around 6am Saturday morning. The colors indicate where the highest wind speeds and the highest moisture are combined to increase moisture levels the most efficiently… AKA our fire hose. Notice its flowing pretty much straight into the Metro.

Now let me show you the precipitable water index from Saturday morning. The map below is an overall look at the precipitable water content for our area. It's not an exact measurement, but it records all other current conditions and then formulates an estimated value.

Any guess where the Metro is located?? The estimated totals top 2 inches in a bulls eyed area in eastern Minnesota that includes the entire Metro area. Now if we overlay the radar picture from that same time, you get…

a set up for extremely heavy rain across parts of Minnesota as those water logged thunderstorms roll through. Here are some rainfall totals from across the area... your backyard totals will vary.

 

 

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