Lake Effect Rains

In the past, I have written about lake-effect snowfalls with particular emphasis on the Great Lakes Region. These snowfalls have had major impacts and unbelievable accumulations, particularly around Buffalo and upstate New York. Lake-effect snows, however, have a lesser known sibling: lake-effect rains.

In general, lake-effect precipitation arises when cold air, passing for a sufficient distance over the relatively warm waters of a large lake, picks up moisture and heat and then precipitates the moisture out over the lake or along the downwind shore. Lake-effect rainfalls are spawned in a similar manner to lake-effect snows, with the biggest difference being the temperature of the overriding blast of cold air, which is cold enough to get the process moving, but warm enough in its lower layers so that the precipitation hitting the ground falls as rain rather than as snow.

For the discussion, I will focus here also on the North American Great Lakes. The phenomenon, however, can be found downwind of any lake large enough to retain its summer heat well into the cold days of autumn and early winter.

Each year as the first cold blasts of air descend from the polar regions, they move across waters of the Great Lakes that are substantially warmer than the cold air. When cold air traverses the lake's warm and moist surface-air, the surface air is lighter than the air above it, a condition known to meteorologists as convective instability, and starts to rise. How high it will rise, in part, depends on the temperature contrast between the surface air and that moving over it. Often, the layer above the lake is also conditionally unstable, meaning that a trigger is needed to start the convective process rolling toward precipitation formation. Usually for the rain events, this is provided by an approaching short wave and attendant surface trough of low pressure from the west.

A study of lake-effect rain events for Lake Erie by Pennsylvania State University meteorologists Todd J. Miner and J. M. Fritsch found, in contrast to most lake-effect snow events, the conditionally unstable layer for lake-effect rain events was much thicker, thus allowing for greater convective activity and often thunderstorm formation. As a result, the greatest number of lake-effect days with thunder around Lake Erie occurs from late September to mid-October when the layer of conditionally unstable air is much deeper. While October had the greatest number of lake-effect thunder events, September had the highest percentage of thunder per lake-effect rain event, over 40%.

Higher relative humidity in the overriding air produces a more widespread cloud cover during lake-effect rain events compared to snow events. Clouds are also deeper than during snow events, leading analysts to suggest that the steering of lake-effect rains may come from a higher level than snow events. Miner and Fritsch suggest the mean 850-700 mb wind vector might be used to better determine the precipitation movement than the 850 mb wind used for snow movement.

Whether the precipitation falls as rain or snow, depends on the temperature profile of the lower to middle troposphere. Since all lake-effect precipitation begins in the clouds as snow, the thermal profile between the precipitation layer and the surface ultimately determines whether precipitation reaches the ground as rain or snow. Thus, for rain events, the temperature of the boundary layer must exceed 0^o C (32^o F) through a sufficient depth to melt the snow to liquid rain.

NOAA GOES-8 visible imagery shows a lake-effect cloud plume over Lake Michigan
during the morning hours on 04 October 1999.
Courtesy NOAA

The Penn State researchers found that lake-effect rains over Lake Erie occurred throughout the autumn season. They begin "in earnest by September and reaches a maximum during November." Rain predominates during September and October. Then, around the first week or so of November, there is a period where mixed precipitation dominates. Following this transition, almost all lake-effect precipitation falls as snow. Miner and Fritsch also found, that although lake-effect precipitation (both rain and snow) events are more frequent in number during early winter, the peak in the amount of precipitation comes earlier in the season.

Their findings for Lake Erie can likely be applied to any of the Great Lakes with, perhaps, different timing for the northern lakes, Superior and Huron, in the transition period and onset of lake-effect snows. Lake Ontario likely has similar patterns. Lake Michigan, being north–south oriented rather than east–west like the rest of the Lakes, might have a mixed pattern in its lake-effect precipitation seasonality.

Learn More From These Relevant Books
Chosen by The Weather Doctor

Heidorn, Keith C.:And Now...The Weather, 2005, Fifth House, ISBN 1894856651
Williams, Jack: The Weather Book, 1997, Vintage Books, ISBN 0-679-77665-6.

Written by
Keith C. Heidorn, PhD, THE WEATHER DOCTOR,
September 1, 2007