Dejamoura Griffin, 7, of Butler enjoys what might be the last warm day of the year Tuesday at Alameda Park when temperatures reached into the mid-60s. A cold weather system from the west is moving this way and will drop high temperatures into the 30s and 40s the rest of the week.

The warm weather is over.

While the high in Butler County was in the mid 60s Tuesday, temperatures will be 20 to 30 degrees below that now.

A cold weather system from the west is making its way into Western Pennsylvania. High temperatures will drop to the 30s and 40s.

Tonight the low will be about 27. On Thursday the high will be about 36 with slight chance of snow showers. The wind chill will be about 30.

AccuWeather Inc., based in State College, Centre County, is attributing the weather to a polar vortex.

“The polar vortex is a large pocket of very cold air, typically the coldest air in the Northern Hemisphere, which sits over the polar region,” said Alex Sosnowski, a meteorologist with AccuWeather.com.

The arctic air will cover most of the lower 48 states, with the exception of southern Florida and the Southwest.

The Butler area won’t see subzero temperatures that hit parts of the western United States, like in the northern Rockies, but temperatures here are expected to be about 20 degrees below average.

Tim Axford, a meteorologist with the National Weather Service in Moon Township, Allegheny County, said the warmer than usual weather earlier this week was because of the cold front.

“A lot of the warm air that’s across the southeastern part of the United States is actually getting pushed up into our area ahead of it,” Axford said. “And so, we’ve got a collision of air masses that will happen.”

High temperatures this time of year are typically in the lower 50s. The cooler-than-average temperatures likely will last through the weekend and into next week.

“We may see a little improvement toward the weekend, but we’ll still be below average,” Axford said. “We won’t be getting back up to the 60s anytime soon.”