Summary of the 25-26 June 2010 Severe Weather Events

A weekend of severe weather took place around the Tri-State area on Friday, June 25th and Saturday, June 26th, producing a few tornadoes, hail up to baseball size, damaging straight line wind gusts, and flash flooding. The following article overviews the environmental conditions that supported severe weather and how conditions were different on Friday versus Saturday, leading to distinctly different types of severe weather events.

 Friday, June 25th

A typical early summer synoptic pattern set up over the United States, with a longwave trough over the Rockies and southwest flow aloft over the northern plains. During the afternoon and evening hours, a shortwave trough embedded within the flow traveled northeastward into the eastern Dakotas and western Minnesota. Daytime heating and warm southwesterly winds at the surface supported destabilization during the afternoon hours, while ample local moisture over Nebraska was transported into the area. A flow of warm air at low to mid levels also supported the development of a substantial cap, which prevented thunderstorms from developing. By the late evening, a surface boundary set up along the Iowa and Minnesota border, providing lift to overcome cap and initiate thunderstorms.

 
Surface map from 9 pm on June 25 (click image to enlarge).
 
Bufkit sounding and 0-1 km hodograph from Cherokee, IA at 9 pm on June 25 (click image to enlarge).
 

In addition to an unstable atmosphere, very strong 0-1 km shear (click for explanation) due to a strengthening low-level jet developed across northwest Iowa during the evening hours. Thunderstorms developed along the boundary in southwest Minnesota shortly before 7 pm and began back-building along the front as it propagated southeast. As this line of storms encountered the highly-sheared environment in northwest Iowa, the storm located on the western edge of the line began rapidly rotating. Funnels and brief touchdowns were reported south of the Minnesota border in conjunction with the western-most storm. Just before 10 pm, a very strong circulation developed at low levels and eventually produced a large tornado, which was later rated EF-4 by our local survey team. Please see the official damage survey for more information.

 
Radar reflectivity and storm relative velocity at 10:09 pm on June 25.
 

A 3D image of a rotation parameter at 10:09 pm on June 25.

 

Image of the tornado between Hospers and Sheldon, taken by Marv Siebersma
 

Storm reports from June 25
As in SPC storm reports, red icons = tornado reports, green icons = hail reports, and blue icons = wind reports
 

 Saturday, June 26th

 The overall synoptic pattern was very similar the next day, with southwest to west winds aloft and humid conditions near the surface, as well as large amounts of instability. Low pressure moved eastward across South Dakota during the morning and early afternoon, and by mid afternoon, was located over east central South Dakota. A wind shift/trough axis extended southward from the mesolow, while the primary cold front followed behind the system. To the east, a warm front extended once again across the Iowa and Minnesota border. Aloft, yet another shortwave was traveling out of the eastern Rockies and into the plains.

 
Surface map from 3 pm on June 26
 
Bufkit sounding and 0-6 km hodograph from Sioux Falls, SD at 5 pm on June 26. Click image to enlarge.

Near the surface warm front, easterly surface winds allowed for a local enhancement of wind shear and led to the potential for supercell development. This dynamic setup and unstable airmass lead to a broad area of thunderstorm initiation over central and eastern South Dakota. Unlike the previous day, 0-1 km shear was less than 10 kts, which does not indicate a favorable environment for long-lived tornadoes. Storms located near the boundary rotated briefly, but could not sustain significant coherent low-level rotation. One brief rain-wrapped tornado was reported on the cool (north) side of the boundary where enhanced wind shear was present; however this was largely not a tornadic event. Enhanced wind shear along the boundary did help to promote rotation at the mid-levels of the storm, which enhanced storm updrafts and lead to the production of severe hail in storms traveling near the front. Due to local moisture availability near the surface and drier westerly winds aloft, the atmospheric profile became supportive of wet microbursts. As the storms moved into the I-29 corridor, severe wind gusts due to microbursts produced isolated damage near the Sioux Falls, Tea, and Harrisburg areas.

 Cross-section of DeSmet hailstorm
Cross-section of the supercell as it was over DeSmet, South Dakota.
 

Photo of hail in De Smet, taken by Derek and Jared Hoefert.
 

Photo of straight line wind damage south of Sioux Falls, taken by John Tulloch.
 

As the evening progressed, the storms moved into a more saturated environment aloft and became more supportive of heavy rain. Storms trained along the surface warm front and produced major flash flooding over previously water-logged soil and in urban areas. Over 6 inches of rainfall was reported in Cherokee, Iowa, and doppler radar estimates a widespread area of 5 to 8 inches fell across Cherokee and Ida counties.

 

 
Bufkit sounding and corfidi vectors (click for explanation) from Cherokee, IA at 11 pm on June 26 (click image to enlarge).
 

Photo of flash flooding in Tea, South Dakota, taken by Brandon Howell.
 

Photo of flooding in Cherokee, Iowa on the morning of June 27, taken by Diana Otto.
 
Radar estimated storm total precipitation for June 26
 

Storm reports from June 26
 

In addition to the tornado, hail, and wind icons, purple icons indicate flash flooding reports. The most significant flooding was reported in urban areas of Brookings and Marshall, MN during the afternoon, with more widespread flooding in northwest IA during the evening.


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