Tornadoes from elevated convection

On May 21-22, 2004, eighty-four tornadoes swept across Iowa, Nebraska, and Kansas. Twenty-one of these tornadoes occurred on May 21, associated with thunderstorms that formed near a warm front, with 20 of them within 75 km of the front. Of the 21 tornadoes on May 21, 2004, eight were spawned from elevated thunderstorms (using the definition from Colman, 1990). The public generally associates tornadoes with hot, humid weather, so tornadoes from elevated convection, when the surface conditions are relatively cool, can be especially unexpected.

At 00 UTC on May 21, 2004, a stationary front reached from Nevada up into Wyoming through Colorado and into the extreme southeast corner of Nebraska, where the front became a cold front reaching from Iowa into Quebec. A strong temperature and dewpoint gradient existed along the front. The horizontal temperature gradient was 12ºC over 400 km and the dewpoint gradient was more than 9ºC over the same distance. The cold air to the north of the front was characterized by dewpoint temperatures lower than 16°C and light winds from the east and southeast. The warm air to the south of the front was characterized by dewpoint temperatures ranging from 20°C to 23°C and light winds from the south-southwest. By 12 UTC, the front became a warm front, and moved across eastern Nebraska and central Iowa, pushing the warm, moist air into northwestern Iowa and central Nebraska as the winds shifted to the southwest. Surface temperatures had warmed considerably through the day, reaching 25°C to 30°C with dewpoints still ranging from 20°C to 23°C ( See Figure which is the NWS operational analysis for 1200 UTC, May 21, 2004).

The Rapid Update Cycle (RUC) three-dimensional analyses show that the eight tornadoes from elevated convection were clearly located in the cold air to the north of the warm front. We present surface analyses using the available hourly metar data, RUC soundings, and RUC cross sections which document the elevated nature of the convection.

We also will show a high-resolution Weather Research and Forecasting (WRF) model simulation to demonstrate the ability of a mesoscale model to capture the behavior of the front and subsequent convection.

Reference:

Colman, Bradley R., 1990: Thunderstorms above Frontal Surfaces in Environments without Positive CAPE. Part I: A Climatology. Monthly Weather Review 118, 1103-1121.