A combination of extreme weather conditions — made more possible and frequent by climate change — are coming together to produce potentially record-setting flooding around the Chesapeake Bay on Friday.
It is the sort of flooding that many communities are now considering as they begin confronting the costly investments needed to adjust to new weather extremes and normals.
First off, tides were already rising above average by Thursday evening, something that has always happened around the bay. Astronomical cycles help determine the rise and fall of tides, and their levels naturally vary throughout the month, with the moon pulling some high tides slightly higher than others.
Winds are also a factor. While winds from the east can often push tides higher in places like Baltimore and Annapolis on the western shore of the Chesapeake, for example, in this case the flooding is more widespread. A flow from the southeast helped push water from the Atlantic into the mouth of the Chesapeake and up the length of the bay.
Increasingly strong southeasterly winds were reported in Annapolis and Southern Maryland on Thursday afternoon and evening. On Friday, winds from the east have been gusting up to 40 mph along the western shore of the bay.
But climate change means tidal flooding occurs significantly more often now than it did a decade or two ago. Bay waters have risen by about a foot over the past century, and are now rising three times as fast as they did during colonial times, according to the University of Maryland Center for Environmental Science.
The center’s most recent sea level rise projections, updated at least every five years, found that Chesapeake waters could rise as little as 1 foot or as much as 7 feet by the end of this century, depending on how much carbon the world emits into the atmosphere.
That means the baseline water level from which tides vary is rising.
Days of nuisance flooding, when water spills onto streets or up through storm drains, are increasing as a result. Nuisance flooding can occur more than 40 days a year in Annapolis, a tenfold increase compared to 50 years earlier.
The UMCES scientists predict that such flooding, which now affects Baltimore about 10 days a year, could occur on nearly 100 days a year, on average, by 2050.
In addition, the storm arriving in the Mid-Atlantic on Friday is an unusually potent one. As it neared shore in the Pacific Northwest earlier this week, it set a record for the lowest pressure recorded in that region. As it arrived in the East, it was expected to produce some severe storms and flooding rains from North Carolina into New England.
Climate change makes storms tend to strengthen more, and more quickly, because warmer air holds more water vapor, and warmer water carries more energy to be unleashed.
And that is a factor increasing the intensity of precipitation. Friday’s storm was expected to drop as much as a few inches of rain on parts of Maryland.
From 2000 to 2020, precipitation in Maryland increased by 2.63 inches per decade, and the number of flood days across the Northeast grew by as much as 150% according to recent research by the National Oceanic and Atmospheric Administration.
Chris Strong, warning coordination meteorologist at the National Weather Service’s Baltimore/Washington forecast, said no single weather event can prove or disprove climate change — “climate” refers to the long-term trends and patterns that determine what is considered “normal” weather in any given area. The mix of wind conditions, tide levels and rainfall patterns are creating Friday’s weather threats.
“It’s just the combination of all that coming together to make this a record or near-record event,” Strong said.
But the fact that tides are trending higher and precipitation is becoming more intense means conditions like Friday’s become more possible and probable.