The annual meteor storm is known for sometimes producing as many as a thousand fireballs per minute, as it did in 1966, but astronomers say this year sky-watchers are likely to see maybe 20 per hour.
The peak will begin building late Friday night into Saturday morning and continue through early Sunday morning, says Ben Burress, an astronomer at Chabot Space and Science Center in Oakland.
The Leonid meteors aren't really associated with the constellation Leo, they just appear to come from the same place in the sky. The Leonids are actually tiny pieces of the comet Tempel-Tuttle, which orbits the sun in a large ellipse.
"A comet is a often called a 'dirty snowball,' as it's made up of pieces of rock held together by ice. As a comet orbits the sun, it heats up and some of the ice is vaporized, releasing bits of rock along the orbit," says Rebecca Johnson of StarDate magazine.
Tempel-Tuttle orbits the sun in an ellipse. Each year as the Earth moves around the sun, it encounters the trailing tail of debris the comet leaves in its wake. Once every 33 years, Tempel-Tuttle comes close to the Earth as it whizzes by in its orbit. In those years, the debris trail Earth travels through is especially thick, and the resulting meteor showers can be spectacular.
Burress says his grandfather saw the 1933 shower.
"There were thousands of meteors per hour," he says.
This year's Leonids aren't expected to be that spectacular because the comet last passed close to us in 1999.
The expected 10 to 20 meteors per hour isn't bad, Burress says: "That gives you a good chance of seeing one every five or so minutes."
By comparison, the big Perseid meteor shower in August typically rains about 50 meteors an hour down on the Earth.
This year should be good viewing, because the moon will set around 10:30 p.m. Saturday in each U.S. time zone.
"So Sunday morning anywhere from midnight to 3 or 4 a.m. is the prime window," Burress says.
With the exception of the Southeast coast, most of the USA east of the Rockies should have ideal weather for watching the meteor shower Friday night and Saturday morning, according to AccuWeather meteorologist Kristina Pydynowski. However, she says, clouds could prevent anyone from Florida to the eastern Carolinas from seeing it.
For Saturday night's viewing, though, the clouds will clear for most of the Southeast.
Much of the far West will be poor for Leonid-viewing through the weekend, Pydynowski says, because of a pair of Pacific storm systems that will bring clouds streaming across the West Coast and toward the Rockies Friday and Saturday nights.
Clear skies should make for great viewing in the Northeast, Midwest, lower Mississippi Valley and most of the Plains states.