Despite tranquil weather to start this week, flooding has affected coastal communities from the Florida Keys to Maine. Long stretches of shoreline along the East Coast were inundated, water levels running a foot or more above normal. The swollen sea temporarily claimed streets, parking lots and public parks, and even seeped into homes, reminiscent of a storm surge.

But there was no storm to be found. In fact, many places enjoyed pleasant weather and sunshine. Yet coastal flood advisories plastered the coast, forcing road closures and flooding properties.

The culprit? King tides. A name informally attached to extra-high tides spurred by astronomical alignments, king tides often reach their peak in the fall. Decades ago, their impact was minimal. But added to the background of climate-driven sea-level rise, nowadays they are routinely problematic.

“I think of this like a stacking of phenomena,” said William Sweet, an oceanographer with NOAA specializing in sea-level rise and flooding issues. “We didn’t flood 30 or 40 years ago, but since then … sea levels have been a half-foot to a foot higher.”

The rise in water has left thousands of homes and businesses vulnerable to sunny-day flooding, disrupting daily life and undermining property values in some areas.

As greenhouse gases resulting from human activity continue to accumulate in the atmosphere and warm the climate, sea-level rise and coastal inundation will only continue to grow with time, even on sunny days.

What are king tides and why are they greatest in the fall?

October is typically the worst month for king tide flooding, with some of the highest water levels of the year.

King tides are the highest tides that occur and are easily predictable based on the orbits of the moon and the earth about the sun. Both celestial bodies exert a gravitational pull on the oceans, helping generate tides. The closer either body, the greater the tide.

Earth is closest to the sun in January, by a margin of more than 3.1 million miles compared with June. That increases tides in the months leading up to and around the new year.

High tides are then maximized when the moon reaches its closest point to Earth in its orbit, known as perigee, which occurred Friday.

Local weather plays a role in tides, too, and tends to enhance them in the fall. Along the East Coast, one of the biggest influences on tides is the Bermuda High, a high-pressure system anchored west of the Azores over the open Atlantic. Winds swirl in a clockwise pattern around it.

During the summer, flow around the Bermuda High brings heat and humidity to the Gulf and East Coasts and soupy conditions to the eastern United States. But during the winter, the Bermuda High shifts farther south and east, its influence on local winds dwindling. The onshore flow it generates is maximized in the summer and reaches a minimum during winter.

That means that the overlap of onshore winds and sun/moon-enhanced tides is greatest during the fall, particularly in the months of September, October and November. The tides are usually the most severe in October, having the most propensity for widespread impact. Waters are also still warm from summertime, and “thermal expansion” makes sea levels just a tad higher.

King tides flooding the East Coast

On Sunday, Monday and Tuesday, king tide flooding was rampant up and down the East Coast, despite otherwise tepid weather.

In Key West, Fla., the National Weather Service warned that “flooded roads will probably be a mixture of rain and salt water” after heavy downpours moved through the area. The office posted a photo to Twitter showing the overnight flooding.

Flooding was reported in the Coconut Grove neighborhood of Miami, a location known for its susceptibility to even a minor spike in water level. The community sits barely a foot above sea level. Home values in the affluent area have been affected by the frequent flooding. Walkways in public parks sat half a foot or more below water.

Miami, Jacksonville, Charleston and Wilmington, N.C., were all included in coastal flood advisories Tuesday.

Some roads were cut off in Mount Pleasant, S.C., while intersections were flooded in Charleston. Just last month, Charleston suffered a similar episode of king tide flooding, which hit much of the downtown area and medical district.

The shoreline flanking Washington, D.C., was placed under a coastal flood advisory Monday as well, while in Queens, N.Y., water bubbled up out of street drains on submerged streets.

In Boston, minor splashover was forecast and did occur, but issues were minimal since high pressure overhead suppressed the surge. Localized flooding was also noted farther to the north in northern New England.

Supercharged tides because of sea-level rise

King tides are a natural phenomenon, but added to the background of rising seas, their impact and disruptive potential is growing exponentially. In Miami, for example, a roughly six-inch increase in sea level since 1996 has led to a twelvefold spike in action-tier flooding.

Sweet, of the National Oceanic and Atmospheric Administration, noted that the sea level isn’t just rising — the rate of rise is accelerating.

In 2019, “what we found was that 90 percent of the tide gauges that we analyzed on the East Coast or Gulf Coast [had] highest all-time sea levels,” Sweet said. “In many instances we were one or two inches higher than we were at the last record sea level. Sea levels are stacking up, and even when you have a garden variety [assortment] of processes, now it can mean flooding in the streets when the same phenomenon was happening [without flooding] 30 or 40 years ago.”

One of the challenges with communicating sea level rise is that the impacts associated with it are nonlinear. In other words, even if sea level were to rise at a constant rate, the amount of flooding coastal communities experience would jump far more quickly.

“At 75 percent of these East and Gulf Coast locations, [flooding is] now accelerating on an annual basis,” Sweet said. “That’s a very important notion to understand. And once infrastructure becomes compromised, once [those impacts are] noticed, the change is going to be quick rather than a slow process.”

It’s an issue already taking a toll on coastal communities, many of which are scrambling to plan for the future.

“As sea levels continue to rise, the flooding will become less storm surge flooding and more tidal flooding,” Sweet said. “Then it becomes an elevation game. Already communities are using data to see where they are exposed, and when they have opportunities to relocate critical infrastructure using the types of data we collect to make sound decisions.”