Observations of a tornadic and non-tornadic circulation near the KDVN WSR-88D associated with the 19 June 1998 squall line

On 18 June 1998, a squall line raced across Iowa and northern Illinois producing widespread damage from severe winds (>25 ms-1), several tornadoes including two rated F2, and at least two injuries. The squall line occurred with a strong, mid-tropospheric shortwave trough in an atmosphere characterized by moderate shear (15 ms-1) and extreme CAPE (4000 Jkg-1).

Several vortices were apparent in the KDVN WSR-88D storm-relative velocity imagery along the leading edge of the squall line and were typically associated with areas of enhanced damage. A few of these vortices produced documented tornadoes, while other vortices did not. Non-supercell tornadic vortices associated with squall lines pose one of the greatest challenges for operational meteorologists attempting to warn for tornadoes, especially with hope of providing useful lead time.

Both a tornadic and non-tornadic vortex occurred within 60 km of the KDVN RDA. The vortices were only 40 km apart and occurred within 5 minutes of each other. The characteristics of these two vortices will be compared to determine if any signals were present that could lead to discrimination between tornadic and non-tornadic vortices on the leading edge of squall lines. The small scale of the tornadic vortex proved to be especially challenging to identify in real-time using data viewed via the WSR-88D Principal User Processor (PUP). WATADS data available at a higher resolution and with the NSSL Tornado Detection Algorithm will be compared to their operational counterparts. Implications for warning decision-making with respect to all these issues will be addressed.