When you think of lake effect, snow flakes will undoubtedly be dancing through your mind. But lake effect precipitation produced over the Great Lakes is not all snow — much of it, especially before November, falls as rain.

The lake effect season has been off to a slow start this year as temperatures around the Great Lakes and Northeast have generally been well above average during September and early October. But the atmosphere is undergoing a change and the term “lake effect” will be climbing the charts soon.

The process for creating lake effect rain is similar to what produces lake effect snow. As cold air from the higher latitudes sweeps across the warmer Great Lakes, heat and moisture are added to the air mass. It requires at least a 23-degree difference between the lake temperature and the air temperature in the mid-levels of the atmosphere. As air rises over the relatively warm lakes, clouds and precipitation form.

During the late summer and early autumn, temperatures are well above freezing from ground-level well up into the atmosphere. As a result, precipitation forms as rain, or if it is sub-freezing aloft, snowflakes melt into raindrops on the way down.

Although lake effect rain has been observed around the Great Lakes as early as late July, the main season for this precipitation occurs from September and October into very early November. Lake effect rain occurs at least once a week during this period. After that, the main precipitation type becomes mainly snow as air masses descending into the Lower 48 become colder.

As with lake effect snow, lake effect rain mainly occurs after the passage of a cold front, as cold air is advected across the Great Lakes. Unlike winter events, the air can be much more unstable with higher cloud tops. The average lake surface temperature on Lake Erie, for example, is 67 degrees around Sept. 20, and falls slowly to 59 degrees by Oct. 15.

Lake effect rain is sometimes accompanied by thunder during this period. Although thunder events are more common during October, September has the most thunder with lake effect rain (around 40 percent of the time). Also, with higher relative humidity levels than winter, cloud cover can be more widespread and bands of very heavy rain can develop, especially if the vertical wind shear is low.

Although most early season lake effect events are rain, there are exceptions. Sometimes, rain can mix with or change to graupel. Graupel forms when precipitation begins as snow, melts on the way down to Earth, and refreezes in the very low levels of the atmosphere before hitting the ground. Because Great Lakes temperatures fall slowly through October and much of November, graupel can easily form, especially close to the lake shores where more heat is added to the lower atmosphere.

But there is another big exception to the rule. On Oct. 12, 2006 a cold front passed through the Great Lakes in the early morning. Some very cold air swept across a warm Lake Erie. Days ahead of time, forecasters expected a lake effect rain event. The water temperature of Lake Erie was 62 degrees. An updated forecast indicated rain to mix with or even change to wet snow for a time.

On the morning of the event, the National Weather Service in Buffalo was forecasting a snowfall total of 1 to 3 inches. Shortly after Noon, the temperature was dropping down into the 30s and a mixture of rain and graupel was falling. The air aloft was colder than the models had forecast, and instability was extreme.

By mid-afternoon precipitation had changed to all snow and the forecast was updated to 3 to 6 inches. Snow formed into a heavy band and it was relentless. Lightning and thunder accompanied the snow. Heavy wet snow was falling with leaves on the trees. Cloud tops exceeded 25,000 feet. By early evening tree and power line damage was extensive and there was massive loss of power across the metro area.

Tom Niziol, the Weather Service’s meteorologist in charge at the time, called it “the night that the trees cried”. By the next morning, snow totals up to three feet were reported (officially 22.6 inches at the Weather Service office in Buffalo). An estimated 400,000 customers were without power and schools were closed for a week.

So, most early season lake effect precipitation events across the Great Lakes are rain, but we’re dealing with weather, so there can also be those (darned) exceptions.