"Hurricane Huron": An example of an extreme lake-aggregate effect in autumn

An intense cutoff low developed over the Great Lakes during the period 13-1 5 September 1996. The low developed as unseasonably cool air spread over th e relatively warm water of the Great Lakes aggregate (i.e., all the Great L akes). It eventually developed an eye, spiral rainbands, and a warm core, s imilar to those in a hurricane. This event presented some forecast challenges for the Nested Grid Model (NG M) and Eta Model and hence for the National Weather Service. The NGM model forecasted a weaker low (999 vs 993 hPa) to be centered east of the observe d location, over Lake Huron. The Eta Model forecasted a slightly stronger l ow (991 vs 993 hPa) to be centered even farther east than did the NGM, over southern Ontario. As a result of the sea level pressure errors, both model s also forecasted much weaker winds than were observed over the lakes and m uch less precipitation around the lakeshores. The coarse resolution in both models likely contributed significantly to these errors. With-lake (WL) and no-lake (NL) simulations were performed with the Nationa l Center for Atmospheric Research-Pennsylvania State University mesoscale m odel MM5 to determine the impacts of the Great Lakes on development of the low. The WL simulation agreed well with the observations. At the surface, t he intensity and position of the WL low was within 1.7 hPa and 70 km at 30 h into the simulation (1800 UTC 14 September 1996), when the observed low w as most intense. To the extent that the impact of the Great Lakes can be as certained through comparison of the simulations, selected WL-NL differences at the surface revealed that the lakes deepened the WL low by similar to5- 7 hPa and restricted its movement. A comparison of WL and NL simulations at upper levels revealed equally impr essive differences (e.g., lake-induced perturbations). Strong negative (pos itive) height and meso-alpha -scale cyclonic (anticyclonic) wind perturbati ons at 850 (300) hPa support the hypothesis that the Great Lakes were instr umental in generating a warm core and strong winds near the surface. A comp arison of WL-NL differences for this case are compared with those from a mo re typical wintertime case to illustrate that the WL-NL perturbations can b e more intense and can extend to considerably greater depths than in typica l winter cases. Strong latent heat fluxes, low static stability, and slow m ovement (e.g., the cut-off nature) of the synoptic-scale low allowed the st rong heating and moistening from the Great Lakes to extend to midtropospher ic levels for an extended period of time.