Five tropical storms and a summer of frequent thunderstorm activity have propelled parts of the eastern United States to one of its wettest calendar years on record.

Through November, 25 of 344 climate regions nationwide reported precipitation that exceeded 90 percent of years since 1895. All but two of these top-10 percent-rainfall regions were east of the Mississippi River, spread from Louisiana to New England and Illinois to Florida.

At times accompanied by disastrous flooding, the prolific rainfall of 2021 rivals, and in some instances surpasses, infamously wet years for the region. Human-caused climate change probably contributed to this year-long precipitation windfall.

The year began, precipitation-wise, with more of a whimper than a bang. Through March, parts of the Northeast had seen below-average precipitation, while much of the Southeast was around the long-term average.

Spring brought frequent rounds of organized thunderstorms from the Gulf of Mexico to the Mid-Atlantic. By May, the southeastern quarter of the United States was comfortably above average for year-to-date precipitation. The barrage of thunderstorm activity spread north into June and July, as rounds of downpours pushed parts of the Northeast to, or even above, the yearly average.

Thunderstorms through the first half of the year made up for a lackluster winter. But it was a spate of tropical activity from midsummer into early autumn that pushed 2021 to record-challenging levels.

The hydrological windfall began in earnest as Tropical Storm Elsa skirted the East Coast, dispensing a wide swath of heavy rain in a narrow corridor from the Gulf Coast to the Canadian border.

The storm was followed, less than a month later, by Tropical Storm Fred’s large, sluggish circulation, which interacted with a continental storm system to drop significant rain from Louisiana to interior New York.

Only a week after Fred’s deluge, tropical moisture streaming toward a slow-moving front ahead of Tropical Storm Henri allowed up to 10 inches of rain to fall in parts of the New York metro area.

By late August, the sequence of quick-hitting tropical cyclones had dropped considerable rain across much of the East Coast, from the Gulf of Mexico to New England. Flash flooding had struck locations such as New York City, North Carolina, the Finger Lakes region and the Atlanta metro area.

The overlapping swaths of heavy rain were particularly unusual in the Northeast, a region that rarely sees such frequent tropical activity.

As August turned into September, tightly wound and moisture-laden Ida slammed into Louisiana with excessive rainfall that fell atop previously-saturated soils to induce severe flash flooding in the Southeast.

Days later, as the storm’s circulation interacted with a separate storm system above the storm-weary Northeast, a secondary deluge induced a disastrous round of flash flooding that bulls-eyed the northern Interstate-95 corridor.

Days after Ida dissipated, Hurricane Nicholas flung yet another deluge upon the still-reeling Southeast. Up to 20 inches of rain fell from Texas to Mississippi beneath the slow-crawling storm, racking up substantial flood damage because of soils long since rendered saturated.

The nonstop tropical activity, along with a wetter-than-average base state, have propelled rainfall amounts that approach or even break long-standing records up and down the East Coast.

This year’s precipitation output even rivals 2011, one of the most extreme precipitation years on record for the United States. In Ithaca, N.Y., for example, only around 3 inches of rain separated the two years through the end of fall.

Stations near the Gulf Coast are also near the all-time annual precipitation record. For instance, at the end of November, this year’s rain accumulation in Mobile, Ala., was nearly neck-and-neck with the city’s record-setting year in 1881.

A relatively dry December has diminished the chances for record-setting yearly totals in many stations, including Mobile.

The year’s dramatic deluge in the eastern United States was probably exacerbated by the impacts of anthropogenic climate change — from more intense storms to heavier precipitation events.

The ocean has been warming faster in recent years than any other point since the last ice age, helping fuel hurricanes and aiding rapid storm intensification. Tropical storms are also stalling more and dropping more rain in a concentrated area, allowing for more localized flooding.

Records also show the wettest days of each year have been getting wetter over recent decades. A Climate Central analysis showed, nationwide, the events that produce the heaviest precipitation have increased the most east of the Mississippi river over the years. This observation continued to track in 2021, when much of that precipitation fell on a small number of high-accumulation days.

With a warmer atmosphere, evaporation rates increase and make water more available for storms to tap into.

Climate projections show heavy rain events will worsen with additional warming, probably intensifying by 7 percent for every 1.8 degrees of warming. Thus, 2021 could be a preview of extremes to come — heavier precipitation events, more intense storms and extreme flooding.