ABERDEEN, S.D.(DRGNews)- A preliminary evaluation from the National Weather Service says a rare and particularly dangerous storm system moved through the Northern Plains during the late afternoon and early evening hours last Thursday.

Kelly Serr is the Warning Coordination Meteorologist for the National Weather Service Office in Aberdeen.

Serr says the NWS uses a combination of radar and on-site evaluation to determine the strength of a storm.

Serr explains how they determine what sites they need to see in person in order to evaluate them effectively.

Seven tornadoes have been confirmed so far.

Derechos feature unusually widespread wind damage, and the NWS says this was one of the most extreme examples on record. It raced north-northeastward at 50 to 70+ mph, and produced measured straight-line winds of between at 60 to over 100 mph. This is about the same as a category 1 hurricane (74 to 95 mph) and an EF0 to EF1 tornado (65 to 110 mph) over a large area. Significant blowing dust was also reported, especially across southeastern South Dakota.