SAN FRANCISCO (KRON) — Nearly 200 consecutive days have gone by since a significant storm showered San Francisco with more than a tenth of an inch of rain, according to the National Weather Service.

It’s been so long since anything has fallen from the sky that the NWS tweeted a light-hearted graphic explaining what an umbrella is, and how to use it.

The dry streak is ending Tuesday as a south-moving storm showers rain on the region. While wet weather is a welcome sight for drought-prone California, the first significant rainfall of the season holds a “dirty not-so-secret,” according to Save The Bay San Francisco.

It’s called “First Flush.” Rainwater flushes built-up trash, motor oil, cigarette butts, pesticides, and other pollutants that have collected during dry months.

“All of this pollution is picked up by rainwater and carried through the region’s storm drains, which flow into creeks and ultimately into the Bay,” Save The Bay wrote.

Plastic is the most common waste item that ends up in the ocean during “First Flush,” Associate Director of Policy Allison Chan told KRON4.

“Pretty much anything will go. But some of the most common types of trash we see in waterways is plastic. Part of the reason for that is plastic is light-weight, it floats easily, it blows easily, and plastic does not biodegrade so it will persist out in the environment pretty much forever, and break into smaller and smaller pieces,” Chan said.

Image courtesy Save The Bay San Francisco
Image courtesy Save The Bay San Francisco

In Oakland, there are 15 main creeks, over 30 tributaries, Lake Merritt and the Estuary– all of which flow directly to the bay.

But the pollution problem is not confined to Oakland.

“Every city has a trash problem no matter what they tell you. The reality is every city has the same requirement to meet, which is to eliminate trash from their storm water systems by 2022,” Chan said. “Every city has a role to play in keeping the bay clean.”

Some cities have made more progress than others in reaching the 2022 goal. Oakland is faced with a “large challenge” because of illegal dumping, she said, and city leaders have been working to address the issue.