Under a full moon late Monday night, singer-songwriter Reneé Rapp wrapped up performing “Poison Poison” at Merriweather Post Pavilion then addressed the crowd. “What is going on in the harbor, y’all?” she asked.

Rapp was in Baltimore over the weekend and posted a photo of herself pinching her nose. “I’m not even playing like this is kind of an assaulting scent??” she wrote in the caption.

Baltimoreans are too familiar with the stench of the "pistachio tide" that has turned the Inner Harbor bright green over the past two weeks. It occurs when layers of water switch because of temperature changes, bringing green sulfur bacteria to the surface, and with them, the smell of rotten eggs.

Environmental groups say this has been the longest and most widespread pistachio tide — extending slightly outside the Inner Harbor area — in recent memory.

The tide started the week of Sept. 22, said Chris Luckett, an aquatic biologist with the Maryland Department of the Environment, but rain and cooler weather in the forecast this week could bring some relief.

Canadian high pressure will bring cooler temperatures to the Baltimore region Tuesday evening, said Luis Rosa, a meteorologist at the National Weather Service’s Baltimore/Washington forecast office. There’s an 80% chance of showers starting Tuesday afternoon into early Wednesday, and precipitation could amount to between a quarter and half an inch, Rosa said.

High temperatures through Saturday will linger in the mid 60s and low 70s, with evenings as cold as 47 degrees.

“Hopefully it’s enough to make a real difference with what people are seeing and smelling,” Luckett said.

Luckett said this summer was relatively dry and warmer temperatures persisted longer than usual, contributing to low levels of dissolved oxygen in the water. Sulfur bacteria thrive in the absence of oxygen, and the extended length of this tide indicates just how low oxygen levels have been, Luckett said. Rainfall, wind and cooler weather will all help mediate the oxygen levels and clear up the tide.

Low dissolved oxygen also harms aquatic life, Luckett said. There were three fish kill events this summer in July, August and September, and there were elements of sulfur bacteria in all of them, he said.