2018 started hot, literally.

It brought the highest February temperature ever recorded in Harrisburg weather history: a toasty 79 degrees.

Winter weather fought back though in March, bringing a record late-season snowstorm, with nearly a foot of snow recorded on March 21st.

Severe weather and flash flooding then became a repeating theme for spring and summer.

May 15 brought destructive thunderstorms as a wet microburst and a weak tornado both formed in parts of Cumberland County. 

July and August both featured flooding rain across Dauphin, York, and Lancaster counties.

Adding insult to injury, more flash flooding hit Franklin County in mid-September, plus wild temperature swings in October and early November led to more heavy rain. 

Early November tornadoes were not far from the Midstate.

How can this weather be explained?

The swings in temperature and rain can be a result of slow-moving, stagnant weather patterns.

This spring, summer and fall featured long periods of warm air sitting over the East Coast. Warmer air can hold more moisture, and so when it rains, it pours.

These patterns are driven by the jet stream, which is seeing major changes due to climate change.

“The jet stream owes its existence to the contrast in temperature between the warm equator and the cold polar region,” said Dr. Michael Mann, a climate research professor at Penn State. “If you warm the Arctic more than the rest of the planet, you decrease that contrast, you weaken the jet stream, and you cause it to exhibit far more prominent north and south meanders. It’s that combination that gives you extreme weather that is very persistent.”

A weak jet stream sagging to the south can help transport a lot of moisture from the Atlantic Ocean, leading to extreme rains.

We aren’t the only ones seeing extreme weather and record rain. Hurricane Harvey and Hurricane Florence are examples of when heavy rain and stalling patterns create a disaster.

“The warmer the ocean, the more moisture there is in the atmosphere. The more moisture in the atmosphere, the more rainfall you can get when those storms make landfall,” Mann said. “It stayed in place, it didn’t move on. It sat there with record levels of moisture dumping it out on the same location day after day after day. And that was the worst flooding event on record.

“With Florence, similar story: very warm ocean temperatures, very slow-moving storm — second worst flooding event on record. First and second most rainfall accumulation events have happened both in the last two years with those two events,” Mann said.