Supercell case review from July 3rd 2005

Case Review the Cheney Lake Storm

- by Ken Cook, SOO ICT

On the evening of July 03, 2005, strong straight line winds hit the Cheney Lake Reservoir. Hardest hit was the marina, where most of the water craft and marina were destroyed. The storm survey conducted suggested strong straight line winds, possibly near or exceeding 100 mph.

This case is similar to a case presented at the 18th Severe Local Storm Conference in 1996 by Lemon and Parker. The Lemon and Parker storm exhibited a Deep Convergence Zone (termed by Lemon and Burgess, 1992), producing winds in excess of 100 mph, and extremely large hail. Looking over the data suggests there were some commonalties between their case and the Cheney Lake case.

Here is a schematic from Lemon and Parker. Note how deep the DCZ is (10km). The extreme mixing of environmental air is confined to this narrow area. This acts to shield the high theta-e updraft from the low theta-e downdraft through a very deep layer. PGF winds behind the pressure trough in conjunction with negatively buoyant air and mass convergence over a considerable depth cause an incredible amount of descending air in a confined area (e.g. The RFD). These storms can sustain themselves for in excess of an hour. This requires an equal amount of inflow to be present, or it will gust out.
Here are some radar images from the storm. I will start from 6 km first. Here is an animation of the 6 km reflectivity. Notice how the BWER is noticeable even at this level (and higher), just as the schematic above and paper indicates (deep convergence).
Here is the 6 km SRM data that shows the increasing Storm Relative Inflow and Sustained convergence at this level.
Here is a loop of the velocity data 6 km showing an increase in the rear inflow, increasing to more than 80 knots.
Here is the reflectivity data at 4 km. Notice the WER and how it is orientated. You can visualize the depth and position and assimilate it to the schematic above.
4 km SRM
4 km Velocity
At the 0.5 degrees, notice how the HP Supercell exhibits a bow echo type feature that was found to exist in the Lemon and Parker case. This is the area that observed the highest winds and associated wind damage.
Here is the 0.5 degree SRM data. Notice the Storm Relative Inflow is in excess of 80 knots at only 1300 ft AGL.
The 0.5 degree Velocity data shows 95 knots at 1250 feet AGL.