What causes lake-effect snow? What to know ahead of this week's winter storm

The Chicago area will see at least one weather phenomenon this week, and there’s a strong possibility residents could see another.

The first phenomenon will be lake-effect snow, which could dump six or more inches of snow on parts of the region as it moves onshore late Sunday night and into Monday morning.

The second phenomenon that may occur given the intensity of the storm is known as “thundersnow,” a relatively rare occurrence that features intense lightning along with heavy snowfall.

So what causes those two phenomena?

According to the National Weather Service, lake-effect snow occurs when a mass of cold air moves over the relatively warm, unfrozen waters of a large lake, such as Lake Michigan.

That mass of cold air is currently sweeping over the lake, with water temperatures measured in the 50s this weekend, according to NWS data.

As that cold air pushes over the lake, heat and moisture rise off of the water, causing clouds to form. Those clouds continue to grow thanks to that heat and moisture, and eventually snow starts to form.

Finally, wind pushes the burgeoning clouds back over land, leading to heavy snowfall accumulations near the water that can pile up in a short amount of time.

What’s noteworthy about lake-effect snow is that it occurs in bands, meaning that some areas can see heavy snowfall while communities nearby barely see any.

Some of the same fuel for lake-effect snow can lead to thundersnow, especially in a situation where there is ample instability thanks to the rising heat and moisture and the unseasonably cold air.

According to the NOAA, shallow layers of warm air can lift into the atmosphere during a snowstorm, causing an electrical charge that results in the discharge of lightning.

Those shallow layers of warm air also increase snowfall rates, which is why it’s useful to meteorologists, who use it to track the heaviest snowfall, according to the Forest Preserves of Will County.

Both phenomena may be observed by Chicago-area residents on Monday, especially in areas closer to the lake, according to the National Weather Service.

A winter storm warning has been issued for Cook, Will and Kankakee counties until Monday afternoon, along with Lake, Porter and LaPorte counties in northwest Indiana.

Some parts of the area could see six or more inches of snow, with locally heavier totals possible. It’s unclear where the lake-effect snow bands will set up, meaning that there could be wide variances in how much snow is falling in the area.

A winter weather advisory is in effect for parts of DuPage and Will counties, along with Jasper and Newton counties in northwest Indiana.

from NBC Chicago https://ift.tt/Fi8J2Gc