Residents of the Great Lakes region may currently be hearing forecasts for “lake-effect rain showers.” Some may already be soaked. Just what are lake-effect rain showers, and why are they happening?

Lake-effect rain showers are the liquid equivalent of the more notorious lake-effect snows that towns from Marquette, Mich., to Buffalo know all too well. Although lake-effect rains don’t pile up mountains of snow, these concentrated bands of rain can release enough water to cause localized flash flooding in more extreme events.

Since Wednesday, heavy lake-effect rain bands with embedded thunder and lightning and even waterspouts have visited the shores downwind of several of the Great Lakes.

The mechanism that produces the localized areas of rainfall is essentially the same as lake-effect snow. Cold air moves across the relatively warmer waters of the lakes and that creates a steep drop in temperature from the lake surface through the first several thousand feet in the atmosphere. The measure of the that temperature decrease with height is referred to as the lapse rate.

Steep lapse rates promote rising air that results in a transfer of warm air and moisture from the lake into the overriding colder air. The end product is a rapid buildup of clouds and precipitation over and downwind of the lakes.

On Tuesday, cold air from Canada began spilling over the Great Lakes. While lake temperatures range from the low 50s on Lake Superior to the mid-60s on Lake Erie, air temperatures have dropped as lows as the 30s in the northern Great Lakes, as a deep layer of cold air overspreads the region. That huge temperature difference is producing steep lapse rates, and, as a result, areas downwind of the lakes are witnessing vigorous lake-effect rain bands and squalls.

The atmosphere has been sufficiently unstable for a swarm of waterspouts over Lake Erie. The International Centre for Waterspout Research tweeted that one observer “has spotted an unbelievable 35 waterspouts” since cold air began pouring over the lake.

Of course, most people think of snow when it comes to the term “lake-effect” weather. In reality, the Great Lakes provides a significant influence on the local weather year-round, as reflected in the yearly air temperature at Buffalo and the water temperature of Lake Erie shown below.

Lake-effect seasons can be defined based on whether the lake temperature is warmer or colder, on average, compared with the air temperature. The “stable” season is defined as the period when the water temperature is lower than the average air temperature. The “unstable” season refers to the period when the water temperature is higher than the average air temperature.

When the lake water is cooler on average than the air temperature, cloud development is suppressed, as shown in the satellite image below on the left. On Lake Erie, that season runs from late March through mid-July. Prevailing cool southwest winds keep clouds from developing during those afternoons, and places like Buffalo bask in sunny skies and a cool breeze that moderates the heat of the summer.

In contrast, by mid-July, the shorter days and advance of cooler air masses from Canada result in the air temperature in the vicinity of the Great Lakes to cool more rapidly than the water, and the lake, on average, becomes warmer than the air. At first those lake temperatures are warmer only at night, and during August and into early September, the lakes occasionally will produce nocturnal convection, or thunderstorms, over the lake. You may also start to see cold air funnels/waterspouts develop as a result in the rapid cooling of the air with height (lapse rate) over and immediately downwind of the lakes.

As we head through fall, lake-effect rain showers show up first and by November, when the air becomes cold enough for snow, the season most people equate to “lake-effect” kicks in, when those notorious lake-effect snowstorms pummel places like Buffalo, as shown in the satellite image on the right in the above image.

In essence, from season to season the roles of the land and lake temperature get reversed, and the outcome, so beautifully depicted on the satellite images, is opposite, as well.

The Great Lakes Region provides one of the unique natural laboratories on earth to study the interactions of these huge inland bodies of water and their impact on the small-scale effects of the weather and climate of the region.

This week’s forecast of lake-effect rain showers also serves as a “heads-up” for residents of the region. It won’t be long before the snow falls and it’s time for everyone from municipal highway departments to ordinary citizens to get the plows ready, make sure the snowblower is gassed up, set the snow stakes along the driveway and get out the winter clothes because Mother Nature is following its annual change of seasons, in this case, lake-effect seasons.

Tom Niziol recently retired as winter weather expert at the Weather Channel after a 32-year career at the National Weather Service office in Buffalo.