Chicago just notched its wettest May on record — for the third May in a row. At least 8.37 inches of rain has come down so far this month and, with a third of the month still to go, that tally is expected to grow.

This year’s historic May rainfall broke the record of 8.25 inches set in May 2019, which had surpassed May 2018′s 8.21 inches, the previous record holder.

Three consecutive rainfall records are remarkable when you consider the Windy City’s weather reports date back to 1871.

Each of the past three Mays have seen more than double Chicago’s average May precipitation of 3.7 inches. This May, almost all of the rain has come in just four days.

Downpours tip the scales

The deluges began on Thursday when 3.53 inches fell. Nearly an inch and a half came down over the next 48 hours, before a whopping 3.11 inches on Sunday. Both Thursday and Sunday set daily records.

“On that Thursday event, it was just a lot of storms training over the same area,” said Brian Leatherwood, a meteorologist with the National Weather Service in Chicago.

On Sunday evening, the National Weather Service issued a flash flood warning for Chicago, cautioning “[a] band of rain is producing one to two and a half inches of rain in less than an hour.”

The Chicago River overflowed its banks, flooding the Riverwalk and even forcing water rescues in Lower Wacker.

On Tuesday, a shower secured the third record-wet May in a row as 0.18 inches came down by 8 a.m., with radar showing mist, drizzle and light rain likely to continue.

The rainfall has contributed to flooding of the lower Des Plaines River just west of metro Chicago in Lake and Cook counties

In addition, northeasterly winds are stirring up 4 to 6 foot waves along the eastern shores of Lake Michigan, combining with a storm surge of up to a foot and prompting a lake shore flood advisory.

A waterlogged pattern in the Windy City

The month as a whole has been very favorable for wet weather across the Great Lakes. A pool of cold air ensnared within a dip in the jet stream has hovered over the Northeastern U.S., while warm air to the west has brought moisture to the nation’s Heartland. Chicago has found itself at the intersection of the two air masses, with repeated rounds of storms riding the jet stream into the city.

That parked boundary has kept Chicago mostly on its chillier side, with a few spurts of warmth. Temperatures are running 3.3 degrees below average for the month, the city even hitting 31 degrees on May 9.

Leading up to May, Chicago’s rainfall was running about 1.5 inches above average; only February came in on the drier side. Elsewhere, it has been an even wetter start to the year.

“A lot of locations are at least two to three inches above normal” for the year, Leatherwood said.

Over the past 30 days, some parts of the Midwest are running 4 to 6 inches above normal for precipitation; in fact, the nation’s most anomalously wet places have been in the Midwest. One location along Saginaw Bay in Michigan saw 8.10 inches in 48 hours, spurring major river flooding.

Chicago’s pattern late this spring has been primed to steering heavy rainfall into the city. The pattern seemed as if it might change to a drier one about five days ago, but that did not come to pass.

“What’s interesting is that we were expecting things to improve,” Leatherwood said. “We thought weak [high pressure] would move over the East Coast.”

That would have favored warmer temperatures and more in the way of sunshine. But now Leatherwood says the odds of that happening “don’t look as good.”

Although high pressure is banked up to the north, an errant, rogue low pressure system has hovered over the city and brought about inclement weather.

Fortunately, more pleasant weather is on the horizon — at least briefly. The remainder of the week should feature brighter skies and highs in the 60s. Temperatures for the Memorial Day weekend warm up closer to 80 degrees, but a few scattered thunderstorms are possible.

Record-setting May rain fits into longer-term climate trends

Long-term trends suggest spring is getting wetter in Chicago, with average springtime precipitation nowadays about 10 to 20 percent above that of the 1960s.

The increasing precipitation is probably connected to the region’s warming climate. Springtime temperatures in Chicago have increased an average of two degrees since the 1960s.

According to the U.S. government’s National Climate Assessment, the heaviest precipitation events in the Midwest have increased by 42 percent since 1958.

Consistent with that, 10 of Chicago’s top 20 wettest calendar days since 1960 have occurred since 2000.