Subtropical Continental Anticyclones, the “Ring-of-Fire,” and Intense Derechos

Intense derechos such as the one that left a significant wind damage swath from the lower Great Lakes to the Middle Atlantic coast on 29-30 June 2012 continue to represent a forecast challenge for human forecasters and high-resolution convection-permitting numerical models. This forecast challenge likely originates from two factors. First, intense derechos can perhaps be viewed as “black swan” mesoscale convective system (MCS) events that because of their relative rarity can pose a challenge to forecaster situational awareness. Second, high-resolution convection-permitting numerical models can exhibit large run-to-run variability in forecasting MCS structure, location, and intensity due to sensitivity to initial conditions, even in ensemble mode, which can act to diminish forecaster confidence in predicting low probability severe MCS events.

The purpose of this presentation will be to discuss the basic ingredients that can contribute to derecho formation. Emphasis will be placed on: 1) warm-season continental subtropical anticyclones, important for producing heat-wave conditions that can be conducive to favorable environments for severe weather on their poleward flank, 2) migratory middle and upper-tropospheric disturbances (aka “ridge rollers”) that track eastward in anomalously strong westerly flow on the poleward flanks of these strong subtropical continental anticyclones and can trigger MCS development in high shear and high CAPE environments, and 3) surface boundaries that can serve as wave guides (“ring-of-fire”) for MCS organization and intensification. Illustrative examples will be drawn from past intense derecho events as well as the recent 29-30 June 2012 derecho event.