Meteorology behind the Tornado Outbreak June 17, 2010

A historic tornado outbreak occurred over eastern North Dakota and western Minnesota during the afternoon into the early evening hours on June 17, 2010. Three violent EF4 tornadoes occurred in the Grand Forks NWS County Warning Area (CWA), in addition to one EF3 tornado. One EF4 tornado occurred near Holmes, ND, another in Wadena, MN and another in the Almora-Bluffton area. There were also 2 fatalities on this day, one in Mentor, MN from an EF3 tornado, and another in Almora, MN. The death in Mentor, MN was caused by an EF3 tornado as it destroyed a convenient store.  In addition, at least a total of 30 tornadoes occurred in the Grand Forks CWA.  If anyone has pictures, video or other eye witness accounts of any tornadoes this day, please email Gregory.Gust@noaa.gov.  This page is dedicated to the meteorology behind this historic tornado outbreak.

A strong area of low pressure above the surface was approaching from the west, with a surface frontal boundary near the tornado outbreak area.  This strong frontal boundary forced air together and caused low level convergence, which is needed for strong thunderstorm development.  The air near the surface was also very unstable, and air parcels were able to rise freely.  In addition, winds were strong at all levels of the atmosphere, and this caused strong wind shear.  A more detailed meteorological explanation of the event can be found below.

The strong wind shear at all levels near the surface boundary caused storm relative helicities, which indicates the potential for spin in the atmosphere, around 500 across the tornado outbreak area.  The effective deep layered shear (0-6km) was also around 60kt.  For comparison, the Barnes county F4 tornado in July 2004 had 0-6km shear values of around 50kt in addition to other tornado outbreak days in this area.  The low level shear in the 0-1km layer was also very high, with 40-50kt of shear in the lower levels level.  This allowed the storms to rotate and enabled tornadoes to form this day. 

The combination of strong shear, high instability and other ideal tornado parameters came together to cause a tornado outbreak on June 17, 2010.  The STP values from 4-7 are extreme with most EF2 tornadoes associated with values greater than 1.  For comparison, the Northwood, ND EF4 tornado had values of STP around 2.