e-mail addresses: bradley.bramer@noaa.gov
dean.melde@noaa.gov
edward.shimon@noaa.gov
gary.austin@noaa.gov
An unusual late-season tornado outbreak produced 23 confirmed tornadoes across northeastern Nebraska, eastern South Dakota and the southern two-thirds of Minnesota on 26 October 1996. Nine tornadoes in South Dakota and eleven touchdowns in Minnesota resulted in October outbreak records in both states. Three of the Minnesota tornadoes were ranked as F2 strength on the Fujita intensity scale. In addition to occurring so late in the calendar year, this progressive tornado outbreak was anomalous in a number of other respects for a severe weather event in the Northern Plains.
The outbreak occurred while the associated surface cyclone was undergoing significant cyclolysis. The central pressure of the cyclone increased from 991 hPa at 1200 UTC 26 October to 1004 hPa by 0000 UTC 27 October. A dryline, as described by the forecasters at SELS, was associated with the event. Surface dewpoint depressions of up to 25°C were observed behind the dryline in southwestern Minnesota at 2200 UTC 26 October. Ahead of the dryline, Minnesota soundings indicated precipitable water values in excess of 200% of normal for late October. This low-level moisture plume produced a mean surface-850hPa equivalent potential temperature ridge of 328K across the southern half of Minnesota by 0000 UTC 27 October.
The outbreak occurred in a highly sheared environment which yielded extreme values of storm relative helicity values across the region. The sounding profile from Chanhassen, MN (MPX) had 0-3 km helicity values increasing from 277 J/kg at 1200 UTC 26 October to an impressive 917 J/kg by 0000 UTC 27 October. This appeared to be due to a rapid intensification of the southerly low-level jet stream east of the surface cyclone. The MPX soundings showed the 850 hPa winds increased from 9 m/s at 1200 UTC 26 October to 31 m/s by 0000 UTC 27 October. Values of surface-based convective available potential energy (CAPE) during the outbreak remained in the modest 500-1000 J/kg range. Resultant calculations of EHI based on high helicity and modest CAPE were anomalously high, in excess of 4, based on sounding values for MPX at 0000 UTC 27 October. These suggest that the EHI may not adequately represent late-season tornado outbreaks in the Northern Plains.
Due to large amount of speed and directional wind shear, many of
the supercells remained relatively shallow. Echo tops, as measured by
regional WSR-88D sites, were generally below 12 kilometers. In
addition, WSR-88D Algorithm Testing And Display System (WATADS)
software showed a distinct tilted structure to many of the convective
cells. This morphology was also observed by a number of severe
weather spotters in the field.