Fall is upon us, so it's time to take a look at lake effect weather. These weather events occur downwind of the Great Lakes. The simple explanation is that cold air moves across a warm lake. I've been doing research on it all week, and I'm getting a better understanding of it.

How Lake Effect Weather Forms

It starts when cold, dry air comes in from Canada or the upper Midwest and moves across the warm waters of the lake. At first, the air doesn't carry much moisture, but that all changes as it moves over the lake. Most strong events occur after a cold front has passed and a large, cold air mass sits over the region.

As the cold air travels across the warmer lake, it picks up heat and water vapor. The air near the lake becomes warmer and more humid than the air above it. This creates instability and rising motion. Once the rising air cools enough to reach the saturation level, clouds and precipitation begin to form.

If the air at the 850 mb level (about 5,000 feet up) is at least 13° C (23° F) colder than the lake's surface temperature, it is favorable for lake effect weather. The greater the temperature difference, the greater the amount of energy and moisture the lake can produce, intensifying the storm.

The distance that the air travels across the lake is called the fetch. When the fetch is longer, the air picks up more heat and moisture causing the intensity of the precipitation to increase.

Wind Drives It

Once the lake effect gets going, the wind determines where the bands form and where they will hit. These storms are very localized. The lake behaves like its own system. You could have whiteout conditions at your house and travel down the road and have it be clear.

Let's use Lake Erie as an example. When the winds shift to a southwest direction, the bands have a long fetch across the lake. This produces intense, long-lasting snow in Buffalo. When we have west winds there is a long fetch as well, creating the most intense weather in Erie County. When the winds turn northwest out of Canada, the fetch shortens and the bands intensify as they move inland due to the air being forced over the higher elevations.

A small shift in the wind can completely change the trajectory of the storm, which can cause the forecast to change by the minute. This makes a forecaster's job hard, especially in the winter.

The Seasonal Changes

We start to see lake enhanced precipitation in October as the air in the atmosphere cools while the lakes remain warm. This starts as rain. Once the surface temperature is below freezing it becomes snow. As the temperature of the lake cools, the 850mb temperature must be even colder to support a lake effect event. If the lake freezes, there is nothing to fuel the cold air coming across the lake. Lake effect snow is done for the season.

Final Words

Having lived in Erie my whole life, lake effect weather is one of the things that made me become so obsessed with the weather. It is one of the most fascinating things in meteorology to me. The way the lake can have such a strong influence just shows how powerful it is. As another season of lake effect begins, I'm excited to implement what I have learned into forecasting and continuing to share what I learn along the way.