In Norfolk, Va., there are several days every year when locals have to drive alternate routes and homeowners watch as rising waters come within inches of their doors because a storm or particularly high tide has sent the sea into the streets.

The number of such days is going steadily upward thanks to local land subsidence and global sea level rise. This year, a strong El Niño could provide a further boost to the number of such “nuisance floods” in Norfolk and other low-lying coastal areas on both the West and East coasts, a new report and outlook from the National Oceanic and Atmospheric Administration says.

Nuisance flooding, or “sunny day” flooding, can happen when storms offshore whip up ocean waters or when a high tide — especially those called King Tides — raise water levels enough to send them flowing into roads, sewers and basements. While the flooding is tame compared to the destruction of storm surge from Hurricane Katrina or Sandy, it still comes with real economic costs in water damage.

King Tide raises sea levels in San Francisco.

King Tide raises sea levels in San Francisco.

Along the West Coast, El Niño amps up nuisance flooding by leading to warmer sea surface temperatures and higher sea levels (water expands as it warms), while on the East Coast its effect is more indirect. The changes it causes in the circulation of the atmosphere tends to lead to more winter storms that head toward the mid-Atlantic, William Sweet, a NOAA oceanographer who led this and previous reports, said.

Areas like Norfolk, Atlantic City, N.J., and San Francisco have seen an increase in nuisance flooding over recent decades thanks to the rise in sea levels as the oceans absorb excess heat from the atmosphere and land ice melts into the sea. Since the 1960s, coastal communities have seen a 300 to 925 percent increase in such events, depending on location, NOAA has found.

With further sea level rise, nuisance flooding could reach a tipping point, defined by NOAA scientists as the point when there are 30 days of such floods a year. For most locations, this is expected to happen by mid-century if greenhouse gas emissions aren’t slowed, but could be sooner for some locations, like Boston or New York.

The new report finds that this year coastal cities could see anywhere from a 33 to a 125 percent increase in the number of nuisance flood days because of the strong El Niño predicted to persist through the winter.

An El Niño of the strength forecast for the coming months “definitely has a factor on compounding the effect of nuisance flooding,” Sweet said. It makes such events more frequent, but also likely increases their intensity, or amplitude, he said.

Being aware of both the long-term trends as well as more transient influences like El Niño is important for coastal residents in terms of planning for a future of more and more flooding. Even within NOAA itself, this outlook has caused folks to take notice, Sweet said, with some employees contacting him to say that the outlook was helping them plan to protect NOAA’s coastal facilities.