Severe Weather Outlook

  Tuesday June 28, 2005

Tornado outbreak expected Wednesday over the upper Midwest

A strong upper-level trough is moving into the central and northern Rockies tonight and will progress eastward into the northern plains tomorrow. In response, the surface low currently over northeastern Wyoming will propagate eastward into the eastern Dakotas by midday Wednesday. Thus, strong southwesterly winds aloft will superpose strong southeasterlies in the lower troposphere. Moreover, given the strength of the overall storm, a very strong southerly flow will exist above the surface and beneath the mid and upper-level jet stream, resulting in very intense vertical shear of the horizontal wind east of the surface low by mid-afternoon. This intense shear regime, coupled with a very moist lower troposphere and associated significant CAPE, will result in an environment conducive to tornadic supercells east of the dry line/front and south of the warm front/convective outflow boundaries from southeastern/eastern South Dakota by early afternoon into portions of Minnesota by late afternoon. This is a very powerful setup for this time of year and looks to have the hallmark of a potentially significant and widespread severe weather episode. The forecast hodographs feature large curvature/strong shear, leading me to think that this could be a widespread tornado event early before eventually morphing into a large bow-echo/line. I think the area most likely to experience potentially strong/violent tornadoes will materialize east of the surface low generally east of a Benson-Granite Falls-Redwood Falls-Windom-Storm Lake (Iowa) line by 20Z.

David Gold