Lake Effect Rain Defined: A warm season phenomenon

Weather Glossary

Western New York is no stranger to lake effect snow, but did you know about lake effect rain?

It’s a phenomenon that exists and happens under the same mechanism as lake effect snow, only with much warmer temperatures towards the surface.

The Great Lakes are located in a very unique spot where cold air frequently travels over them. It’s the perfect place for lake effect precipitation no matter the type to occur climatologically in the United States.

The phenomenon known as lake effect occurs when you have cold air moving over a warm body of water. As a general rule of thumb a temperature difference between the air and body of water at around 850 millibars (about a mile above the surface) is more than 10 degrees Celsius, you can get lake effect. Large temperature differences can create what’s called steep lapse rates, or areas of lift. This can lead to strong bands of rain or snow.

Similar to lake effect snow, lake effect rain can be very localized and drop heavy amounts of it at times depending on how much cold air and lift is involved. Lake effect rain bands have the potential to produce thunder, lightning and even waterspouts if the temperature difference is great enough.

There are different times throughout the year where you can get one type of precipitation over the other, and different times of the year where lake effect is able to be produced in the first place.

Unstable Season vs. Stable Season

Take a look at the graph below showing the calendar months of the year on the bottom (x-axis) and temperature in Fahrenheit is on the left side of the graph (y-axis). Air temperature is plotted on the red line and lake temperature is plotted on the yellow line.

The graph is the divided up into two categories: the unstable season which is carried over from mid July to mid March, and the stable season that runs from mid March to mid July. This graph shows on average when you could get lake effect snow versus rain, and when lake effect is typically more “suppressed”.

A lake’s stable season is when the water temperature is lower than the average air temperature. That means the likelihood of getting convection of any kind whether it’s rain or snow is significantly lower. Warm air over cold air is a stable air mass.

A lake’s unstable season is when the water temperatures are higher than the average air temperature. This means you can get rising motion and convection to occur over the lakes in the form of cloud cover and precipitation. Cold air over warm air is an unstable air mass.

Here’s an example of lake effect rain being produced in a forecast model during late July. You can get a good idea that this is lake effect rain simply by looking at the colder temperatures flowing over an assumed warm lake, and the nature of the banding going on with the light green “blobs” showing rain.

Luckily, this type of lake effect does not involve shoveling immense amounts of snow our of our driveways. Which one would you prefer, lake snow or lake rain?

~Meteorologist Christine Gregory

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