Mega Rains in Minnesota

In 2016, Minnesota had a record of two mega rains in one year that were only one month apart. Mega rains are defined as a 6 inches of rain covers a span of 1000 square miles. In Minnesota, there have only been 13 of these type of events since the 1800s. The storm on July 11 covered 2000 square miles. These mega rains resulted in 2016 being the wettest year to date in the Twin Cities with a total of 40 inches of rain that year.

Precipitation totals for the two mega rains in 2016

Mega rains have been increasing in number over the last 100 years in Minnesota. Six of the thirteen recorded mega rains in Minnesota happened after 2002. These rains caused up to 10 inches of water which resulted in flash floods that closed southbound I-35 and highway 61 for a period of time. The flooding also resulted water moving downstream into rivers such as Snake, Rum, Mississippi and St. Croix rivers. Some of these rivers rose 10 to 14 feet. The warm front the week before the heavy rainfall was probably the main cause of this 24 hour storm forming in the first place. Thunderstorms can often be paired with flooding, heat waves and tornadoes.

Thunderstorms form when heat and moisture from the ground move up into the sky. When this moisture meets cool air in the sky, cumulus clouds form and collect water through condensation. The air undergoes a cycle as it warms and cools, called a convection cell. Winds that move downward (i.e. downdrafts) into the clouds, causing the rain to fall. A warm environment leads to thunderstorms because of the added moisture in the air. Tornadoes form when the rapidly moving air in a convection cell is combined with other wind. The rising air starts to rotate when other wind pushes on it. The spinning air that moves near the ground is what creates the tornado.

Flooding after the downpour

Heat is one of the essential ingredients that creates a thunderstorm. Hence the reason why a whole week of strong storms can be paired with heat waves beforehand. This particular mega rain was paired with a 25 degree temperature difference across the warm front. Temperatures reached over 100° over the week preceding the thunderstorms. Prolonged exposure to this extreme heat is the cause of over 650 deaths per year.

As if flooding and heat wouldn’t cause enough damage, there were also four tornadoes that took place during the heavy rainstorms. The tornado that hit Litchfield had winds that reached 115 mph and was a EF-2. Although this tornado was the smallest, with a width of 100 yards, it still caused noticeable damage to trees, roofs and houses. The damage to this mobile home that is shown below, proves how devastating this type of natural disaster can be no matter the size. Luckily there were no deaths that were caused by the tornadoes.

Tornado damage to a mobile home in Litchfield

Although thunderstorms are common in Minnesota, they cause significant damage each year. Along with the added factors that come with the rain, there are always surprises as to when a tornado may land or what the weather will be like leading up to a storm. Minnesota weather is full of surprises to say the least.

References

“Central Minnesota Flood: July 11-12, 2016 - Minnesota DNR - MN Department of Natural Resources.” Minnesota Department Of Natural Resources, www.dnr.state.mn.us/climate/journal/160711_12_flood.html.

“How Thunderstorms Form.” How Thunderstorms Form | UCAR Center for Science Education, scied.ucar.edu/shortcontent/how-thunderstorms-form.

Huttner, Paul, et al. “'Mega-Rain' Biggest since Duluth Flood, Extreme Heat next Week.” Updraft, 14 July 2016, blogs.mprnews.org/updraft/2016/07/mega-rain-biggest-since-duluth-flood-extreme-heat-next-week/.