An Examination of Two Atypical Severe Weather Episodes in the Northern Plains: The Role of Jet-"Front" Interaction

Five severe weather episodes over the 2000 and 2001 convective seasons were identified to occur with weak surface trough features in the Sioux Falls forecast area. A common element of these cases was that the operational models underforecast the low-level convergence and instability along the surface trough. In most instances, this information resulted in an underforecast degree and/or nature of the convective threat, sometimes within a few hours of the event. Of concern is that many of these cases were associated with tornadic episodes, with 3 of the cases producing a tornado with F2 damage.

Detailed analysis is presented for two cases which affected portions of northeast Nebraska, extreme southeast South Dakota, northwest Iowa and southwest Minnesota. The first case occurred on the evening of 17 August 2001. During a five-hour time span, NWS Sioux Falls issued 19 severe weather warnings affecting 17 counties. Ten confirmed tornados occurred that evening, including one F2 tornado which caused extensive damage in Jackson, Nebraska. The second case occurred on 30 April 2001, with over 30 reports of severe weather, including 3 tornadoes.

Synoptic and mesoscale evolution for each case is initially presented through multiple observational platforms, using satellite, radar, and surface observations. This analysis revealed that the pre-storm environment underwent significant and rapid modification in each case, featuring development and enhancement of a prefrontal trough. The prefrontal trough served as the primary focus for low-level moisture convergence. Strong differential thermal advection aloft acted to intensify mid-level destabilization. In each case, a fairly compact potential vorticity (PV) anomaly was tracked on satellite, which induced development along the prefrontal trough after the time of maximum surface heating. While upper-level features were adequately predicted, there were significant shortcomings to predicted low-level fields. This included a much weaker and less convergent prefrontal trough, resulting in little vertical motion and no QPF. Recognition of the interaction of upper-level waves with low-level features can lead to better anticipation and predictability of the potential and mode for severe convection.