An intense cutoff low developed over the Great Lakes during the period 11–15 September 1996. As the low deepened, height falls in the lower troposphere exceeded those at upper levels, the cold-core low evolved into a warm core system, and vertical wind (speed and directional) shear decreased dramatically. The low eventually developed an eye and spiral bands of convective showers. In addition, the cyclone briefly produced tropical storm force winds and excessive rain (>10 cm) that caused flooding. From a satellite perspective, this system bore a striking resemblance to a hurricane. It is believed to be the first time that such a feature has been documented over the Great Lakes.
Because the initially cold-core cyclone moved slowly across the Great Lakes when they were near climatological peak temperature, heat fluxes, particularly latent heat fluxes, were unusually large. For this reason, it is hypothesized that the lakes, especially Lake Huron, played an integral role in the system's development. An analysis of the static stability present during the event suggests that a deep layer of conditional instability allowed lake-modified air parcels to reach altitudes not normally associated with lake-forced convection.
The hypothesis that the heat and moisture fluxes from the Great Lakes played a significant role in the system's development is supported by the following: 1) The cyclone deepened considerably in the presence of very weak baroclinicity, with the most substantial height falls occurring after the system reached Lake Huron. 2) The combination of surface sensible (Fs) and latent (Fh) heat fluxes exceeded 700 W m−2 during the low's development. This value is comparable to flux calculations during wintertime arctic air outbreaks over the Great Lakes as well as for polar low cases and category one hurricanes. 3) The low strengthened considerably more at lower levels than at upper levels. 4) The thermal structure of the cyclone appeared to evolve into a warm-core feature from its original cold-core structure, with a significant positive tropospheric thickness anomaly observed over the system's center.
*Department of Meteorology, The Pennsylvania State University, University Park, Pennsylvania.
+Atmospheric, Oceanic and Space Sciences Department, University of Michigan, Ann Arbor, Michigan.
