Monday night’s flash flooding event, how did it happen?

DULUTH, MN. (Northern News Now) - On Monday night, an interesting mesoscale phenomenon resulted in a localized stationary downpour over the East Hillside neighborhood of Duluth, resulting in over 4″ of rain over a short 3 hour timeframe and intense flash flooding. So, how did it happen?

Below is the scene prior to the extreme event at 5:35 PM. Station plots show a NE wind in the area, nothing out of the ordinary with a thunderstorm west of Duluth. All was going as forecasted, with a steady line of rain moving from west to east on Thursday evening which would drop about 0.5″ to 1.5″ of rain across the Northland. However, a stationary pocket of heavy rain would form right over East Hillside shortly after this. Here are the three main reasons for this bizarre occurrence.

Reason Number 1: Low Level Convergence

Below is the scene at 6:55 PM. Notice how following the passage of the thunderstorm the Park Point weather station shifts to a SE wind, and the NWS Duluth ASOS shifts to an ESE wind, while Two Harbors weather station remains with a NE wind. It appears outflow from the dying thunderstorms may have resulted in a low-level wind shift, leading to low level convergence. What this means is thunderstorm caused winds to shift in the Twin Ports region, while winds remained the same elsewhere, leading to winds to converge over the area near where flash flooding occurred. This convergence causes air to move upward as it has nowhere else to go when converging. Upward motion fuels cloud formation, which can cause heavy precipitation. Greater upward motion results in greater precipitation rates. Overall, this wind shifts seems to have been enough to cause significant upward motion in a small area close to the flood zone.

Reason Number 2: Elevated Instability

Taking a look at a model-generated sounding we can see how the atmosphere likely looked during the event. A sounding is essentially a temperature and moisture profile of our atmosphere from the surface all the way up to high altitudes, about 50,000 feet in this case. The sounding may be hard to read from the naked eye, but essentially it shows that our environment was moist and likely had some limiting factors such as a low level inversion near the surface that kept upward motion at bay. An inversion in the atmosphere is an area where our temperature gets warmer as we go up in altitude, rather than the typical cooling, which can keep rising air from moving further up in our atmosphere. However, it also shows that if some lift were to overcome those limiting factors, then some instability above the inversion could be accessed. This is known as elevated instability, which is instability (formally known as Convective Available Potential Energy, or CAPE for short) in the atmosphere that is located above the earth’s surface. This was likely a contributing factor to our flash flood, as the downpour of east hillside would have required a steady amount of instability to cause such an extended period of heavy rain.

Reason Number 3: Orographic Lifting

Below is a topographic map showing the steep incline we are all aware of that likely contributed to some orographic lift as well. Orographic lift is rising air caused simply by air moving up terrain. As the combination of converging SE and NE winds ran into the East Hillside neighborhood, the terrain would also help lift the already converging air, adding an extra mode of upward motion that fueled the heavy area of precipitation.

Results:

These three factors result in the beginning of the flair up can be seen below at 7:35 PM just east of the Duluth Airport weather station (the station plot with the red “57″ and green “54″), with low-level convergence seemingly still in place.

As time goes on, the flair up of heavy rain would grow and remain rather stationary over the East Hillside Neighborhood, and was much more noticeable to the east of the main line of rain. Here’s the radar at 8:05 PM.

Low level convergence seemingly remained in place even as the main line of rain moved through, keeping heavy rain in place over the East Hillside neighborhood of Duluth while the rest of the Twin Ports area saw only moderate rainfall. The East Hillside Neighborhood shows the heavier red and orange rainfall rates over it, while the rest of the region has the moderate green and yellow rainfall rates.

Finally, we see the heavy rain end simultaneously with the low-level convergence following the passage of the main line, which likely disrupted the mesoscale event allowing things to dry out around 10:30 PM.

The result of this very localized weather event was incredible. Over 4″ of rain in just about 3 hours for one neighborhood of Duluth! Here were the rainfall results.

Summary:

The combination of low level convergence caused by thunderstorm outflow, along with elevated instability, and some orographic lifting resulted in a very rare super localized flooding event that saw streams of water pouring down the East Hillside neighborhood. This resulted in some damage, erosion, and a brief shutdown of the I-35 tunnel. Here is a collection of photo’s we received from the flooding event!

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