There’s that phrase again: Milwaukee, Racine and Kenosha counties could be hit by “lake effect snow” early this week.
So, what exactly is lake effect snow?
We know it’s a big part of living in the Great Lakes, especially in certain areas.
But does it make our weather better? Worse? Different with each mile away from the shoreline?
Here’s a basic primer.
Lake-effect snow typically happens when water isn’t frozen
Lake-effect snow typically happens when the lakes are ice-free and the water is still relatively warm.
As winter drags on, presumably cooling the water and perhaps forming ice sheets, the chances dissipate. However, lake-effect snow can hit as late as February and March if the conditions line up.
Lake-effect snow needs mix of temperature, wind direction
Two conditions need to coincide for lake-effect snow to happen.
First, cold air needs to travel across warm water. The cold, dry air acts like a sponge and sucks up moisture and heat from the lakes. The larger the temperature difference between air and water, the heavier the snowfall. The lake also needs to be ice-free because ice acts as a barrier, preventing moisture from evaporating off the lake. The magic number for the temperature difference is 23 degrees between the water and the air temperature at 5,000 feet.
The second factor is wind direction, or what’s called a lake’s “fetch,” which is the distance that wind travels over open water. The longer the fetch, the more time the cold, dry air mass has to suck up warm water along the way. Depending on what direction the wind is moving, it can spend a lot of time over the open water.
When the moist, warmed air finally hits land, it cools and dumps heavy snow. Cities and industrial areas can intensify these events because they generate additional heat.
Eastern sides of Great Lakes are more susceptible
Lake-effect snow typically falls within 25 miles of the lake, according to the National Oceanic and Atmospheric Administration. Sometimes, though, it can travel as far as 100 miles.
Because jet streams by and large move from west to east, the eastern side of the Great Lakes historically have seen more lake-effect snow. Cities along Lake Erie and Lake Ontario are particularly prone to these intense events because of the long fetches that allow air to pick up a lot of moisture. Places like Michigan’s Upper Peninsula and Grand Rapids also frequently experience these heavy snowfall events.
That’s not to say places along the eastern shoreline of Wisconsin, like Milwaukee, don’t see lake-effect snow. But it’s much less likely, because there would need to be a weather pattern coming from the northeast, Wright said.
Direct hits are hard to forecast due to narrow snow bands
Meteorologists can usually forecast when a lake-effect event will happen days in advance, but predicting what specific city will take the hit can be tricky.
Any changes in the wind direction can drastically change the fetch over the water.
This article originally appeared on Milwaukee Journal Sentinel: Every winter, we use the phrase ‘lake effect snow.’ But what is it?