Nocturnal Tornado Climatology

Abstract

Very few studies on nocturnal tornadoes have been performed, and operational forecasting of nocturnal tornadoes is often guided by the results of studies that are biased toward daytime tornadoes. However, it is likely that tornado environments vary significantly over the diurnal cycle. For example, the depth and nature of storm inflow may change as the daytime boundary layer transitions into a stable nighttime boundary layer, and a low-level jet (LLJ) may form above in the residual layer and free atmosphere. The study performed herein is used to investigate features unique to nocturnal boundary layers and the free atmosphere above that may affect nocturnal tornadoes. A climatology of significant (F2?F5) nocturnal tornadoes in the contiguous United States between 2004 and 2006 shows that environments deemed by previous climatologies as unfavorable for late afternoon/early evening tornadogenesis are in fact conducive to significant nocturnal tornadogenesis. These nocturnal environments may be characterized by marginal convective instability with shallow stable boundary layers. Substantial low-level shear, storm relative helicity (SREH), and exceptionally strong nocturnal low-level jets stand out as the most common features of significant nocturnal tornadoes and have utility in distinguishing environments of weak nocturnal tornadoes from environments of significant nocturnal tornadoes. Analysis of the data gathered in the climatology shows that the suggestions of existing tornado climatologies are inadequate and even misguiding for forecasting nocturnal tornadoes. Several recommendations for operational forecasting of nocturnal tornadoes are made based on the results of this climatology.