This storm began as an amplifying shortwave embedded in generally zonal flow across the Rocky Mountains. At 00 UTC 17 October, the 500 mb absolute vorticity maximum is located over Utah and Wyoming with a minimum 500 mb height of 552 dm. The shortwave progresses eastward and by 12 UTC 17 October, the vorticity maximum has increased and the heights have dropped to 546 dm with the main shortwave located over Nebraska and South Dakota. A cutoff low forms and the heights continue to fall over the next 12 hours as it tracks further northeastward over Minnesota. After 00 UTC 18 October, the upstream ridge amplifies and hinders further eastward progression. Subsequently the storm stretches meridionally and develops two vorticity centers: a northern one over northern Minnesota and Canada that tracks slowly northward and decreases in amplitude and a southern one over Illinois and Indiana that tracks slowly eastward. This southern branch eventually forms a semi-stationary cutoff low over Ohio and is associated with substantial rainfall and some flooding over Ohio and the northeastern US on the 20th of October, 1996.
At 00 UTC 17 October, the surface low pressure center is located over western Kansas with a central pressure of 995 mb. The strongest thermal gradients associated with this storm lie along the cold front that extends from the low through northern New Mexico and along the warm front that extends from the low through central and eastern Nebraska. The thermal gradients along the rest of the warm front, through Iowa and Wisconsin, are much weaker. By 12 UTC 17 October, the surface low has progressed northeastward and although the central pressure has only deepened to 992 mb, the pressure gradients around the low have strengthened and the shape of the thermal, wind and pressure patterns are classical at this time. The surface low continues to deepen over the next 12 hours (00 UTC 18 October)and the thermal structure indicates that the storm has begun to occlude. The surface cold front remains strong throughout the lifetime of the storm dropping the temperature well over 20F(or C??) in.....By 12 UTC 18 October, the surface low begins to fill and the storm structure weakens with time.
During the early stages of the this storm, the majority of the clouds and precipitation lie north of the surface low and warm front and extend west of this region behind the low and cold front. Within this large cloud mass, there is embedded convection, especially over Wisconsin and northern Minnesota at 12 UTC 17 October. In addition, there is a significant dry slot that forms over the surface low and northeast of it over the period 06 UTC to 18 UTC 17 October. It is associated with a descending upper level airstream. There are only low level clouds along the surface cold front until 00 UTC 18 October when substantial convection breaks out along the front.
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For more information, contact Lynn McMurdie at Department of Atmospheric Sciences, University of Washington, Box 351640, Seattle, WA 98195-1640; (phone)206-685-9405 mcmurdie@atmos.washington.edu (Updated: 28 October 1997)
