The impacts of the Great Lakes on springtime frontal systems in the region are examined. The study is motivated by climatological evidence that indicates reduced precipitation in the region during April-June, relative to surrounding areas.

Numerical simulations were conducted using the PSU/NCAR mesoscale model MM5V3 to examine impacts of the Great Lakes on a variety of mesoscale precipitating weather system types in late spring, when lake temperatures are climatologically lower than surface air temperatures. For each type, simulations with and without the lakes were compared to evaluate the extent to which and the ways in which the Great Lakes influence spring weather.

Overall results indicate that negative fluxes of heat and moisture (e.g., transfer of heat and moisture from the atmosphere into the water) result in a slight cooling, hydrostatically induced higher pressure, and anticyclonic flow near the surface. While these characteristics result in reduced precipitation over the center portion of the region, they also seem to enhance or reduce the strengths of approaching fronts on the periphery of the region depending on the type of front and the direction of approach. For example, warm fronts approaching from the south are intensified and their northward progression slowed as a result of lake aggregate enhanced northeasterly flow and lake aggregate cooled air on the cold side of the front so that precipitation amounts are enhanced across central and northern Indiana and Ohio. Cold fronts, however, approaching from the west, can be either enhanced or weakened depending on the strength of the front and the lake air temperature differences on the warm side of the front.