Corey T. Guastini, Lance F. Bosart and Ross A. Lazear
Department of Atmospheric and Environmental Sciences The University at Albany/SUNY 1400 Washington Avenue Albany, NY 12222
On 29-30 June 2012, a severe mesoscale convective system (MCS) known as a derecho impacted a large area from northern Illinois to the Mid-Atlantic coast directly killing 13 people, leaving millions without power, and causing over one billion dollars in damage. This derecho exhibited a classic progressive derecho signature whereby elevated convection first initiates north of an east/west oriented surface thermal boundary then transitions to a severe wind-producing, surface-based MCS when the cold pool associated with the elevated convection intersects a very unstable air mass on the southern side of the boundary. This ~1500 km long-track derecho was notable for the lack of predictability associated with its initiation, organization and maturation. This lack of predictability was amplified by the failure of the operational numerical weather prediction models to forecast the derecho's extension to the Atlantic coast, an event seldom seen in available derecho climatologies.
The focus of this presentation will be on elucidating the environmental and mesoscale dynamical and thermodynamical processes that governed the structure and evolution of the derecho during its ~18 h life cycle. Although it has been shown previously that transient midlevel disturbances propagating eastward along the poleward periphery of a subtropical continental anticyclone can initiate MCS formation, no such disturbance was evident in this case despite the presence of a robust subtropical continental anticyclone. Instead, the initial elevated convection appears to have been triggered by weak low-level warm-air advection over the surface thermal boundary. After convection initiation, strong cold pool formation enabled the elevated convection to become surface-based and long-lived all the way to the Atlantic coast as will be demonstrated from a detailed radar analysis.
A noteworthy forecast challenge in this case was the unexpected eastward extension of the derecho across the Appalachians to the Atlantic coast. It is hypothesized that the derecho was able to reach the Atlantic coast because a warm, moist southerly flow ahead of an Appalachian lee trough enabled a mesoscale area of extreme instability to form along and to the east of the trough axis. Operational numerical weather prediction models that incorrectly failed to forecast the observed warming and moistening along and east of a lee trough east of the Appalachians may have misled forecasters into thinking the derecho would weaken and dissipate while crossing the Appalachians. Time permitting, we will report on the results of a climatological and composite analysis of derecho environments with the goal of facilitating the identification of mesoscale-environmental interactions associated with long-lived destructive derechos.