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Severe weather awareness week: looking into caused the Merrill tornado

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6-P-10 YR TORNADO ANNY

(WAOW) —  It's severe weather awareness week, and today News 9 takes a look back at a tornado that hit the North Central Wisconsin area 10 years ago.

This past weekend was the 10th anniversary of the Merrill tornado. It is not very common to have tornadoes in April in the region so you might be wondering what led to the tornadic conditions and how many tornadoes formed on that day.

Stormtrack 9 Chief Meteorologist Justin Loew explains what led to the rare event.

Warm and humid conditions are the primary ingredients to generate stormy conditions and it was well above normal on April 10th of 2011. High temperatures reached the low to mid 70's across the area, including 74 in Wausau and 72 in Rhinelander. Dewpoints, a measure of moisture in the air, rose up to around 60 degrees, which is also well above normal for April.

Wind shear, which is a change of the speed or direction of the wind in different levels of the atmosphere, is also important for the formation of supercell thunderstorms that can generate tornadoes and it was in a higher range. As a cold front moved in from the west, it triggered several thunderstorm cells that produced a record number of tornadoes for any single day in April in Wisconsin's history.

In addition to the Merrill tornado there were 14 other tornadoes in the state. In our area, the longest tornado of the day occurred in Adams county and struck the Cottonville area.

Another two tornadoes touched down in Forest county, three different tornadoes left their mark in Waushara county and one tornado hit both Langlade and Juneau counties.

The Merrill tornado was rated ef3 and was the strongest of the day. The Adams county Cottonville tornado was an ef2, as was the one that blew through central Forest county. All of the other tornadoes were rated ef0 and ef1 but still caused plenty of damage.

Strong tornadoes in April are not too common, but they do happen. It's good to remember that tornadoes have occurred in every month of the year in Wisconsin except for February.

Justin Loew

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