National Weather Service offices in northern Illinois and western Pennsylvania reported unexpected accumulating snows generated from nuclear power plants in their forecast coverage areas as the arctic airmass settled over the region on Tuesday.
NWS Chicago said radar depicted two very small and narrow bands of snow moving across the Will and Kankakee county border with another area streaming northwest to southeast on cold winds near the Grundy and Livingston county border around 5 a.m. CST Tuesday.
Highly localized snowfall accumulations of one to as much as three inches fell with these bands.
NWS Pittsburgh reported a similar phenomenon later Tuesday evening around 8 p.m. EST across parts of Beaver and Allegheny counties, where a band of accumulating snow formed from the Beaver Valley Nuclear Generating Station near Shippingport.
Steam billowing from stacks at the plant, NWS said, contributed to at least one inch of snow here.
"The warm moisture source from those cooling towers interacting with the cold artic air that’s in place across Western Pennsylvania generated this snow shower, this snow band that lasts for about three hours,” said Fred McMullen, a NWS meteorologist.
Temperatures across northern Illinois and western Pennsylvania were in the single digits to around zero at the time the nuclear snow bands occurred.
Low temperatures and concentrated moisture in the air are two key ingredients in getting such events to occur, according to AccuWeather Expert Senior Meteorologist Bernie Rayno.
Rayno said temperatures first need to drop to at least five degrees for a cloud to form. Then a warm moisture source is needed, which came from the cooling ponds and steam.
These elements, he said, combined in the low levels of the atmosphere to create an inversion, which kept the rising warmer air from the cooling ponds and steam from dispersing into the atmosphere to result in the freak snowfall.
This is similar to lake effect snow. Lake effect snows occur when a mass of sufficiently cold air moves over a body of warmer water, creating an unstable temperature profile in the atmosphere.
As a result, clouds build over the lake and eventually develop into snow showers and squalls as they move downwind.
McMullen says if you saw the snow generated from the power plant; consider yourself lucky as this specific kind of event only happens every three to five years.