Heading to the beach? These US beaches will be dangerous or gross this holiday weekend.

Memorial Day weekend may mark the unofficial start to summer, but beachgoers in several states are being warned of heavy rain, windy conditions and unhealthy bacterial levels.

Even if it's safe to kick off summer at your beach of choice, in some places it's not looking as enjoyable as you might like. Some U.S. beaches are likely to be plagued by cloudy weather or gross mats of seaweed this weekend.

More than 42 million Americans are expected to be traveling this Memorial Day weekend, according to AAA, which estimates a 7% increase in travelers compared to last year.

If you're headed to the beach, here's what to know:

Southern California: Cloudy weather, high bacteria and blue jellyfish

From cloudy skies along the coast (not-so) affectionately known as "May Gray" to high bacteria levels, some Southern Californians looking forward to a beach weekend might be in for a disappointment.

The National Weather Service in San Diego said marine layer clouds were covering the region on Saturday morning.

"The sun should make an appearance for most places this afternoon, but it may be pretty brief at the coast," the weather service said in a tweet.

Beaches in the region have also been littered with blue jellyfish-like creatures known as by-the-wind sailors or velella velella, which aren't dangerous to humans, that have washed up in recent weeks. The California Department of Fish and Wildlife says the slimy creatures can't control their direction and "sail" with the wind, and wash up on shore if blown too close.

Thankfully, they're small and unlikely to ruin anyone's trip to the beach.

Velella velella, also known as by-the-wind sailors, seen on a San Diego County beach on May 7. The jellyfish are not dangerous to humans, according to the California Department of Fish and Wildlife. They continued to be observed in Southern and Central California into late May.

Meanwhile, Los Angeles County officials warned against going in the water at a handful of beaches due to levels of bacteria higher than what is considered safe. The Los Angeles County Department of Public Health said people should avoid swimming, surfing and playing in the water at the following beaches this weekend:

  • Mothers Beach in Marina Del Rey

  • Santa Monica Pier in Santa Monica

  • Topanga Canyon Beach in Malibu

  • Malibu Lagoon at Surfrider Beach

  • Latigo Shore Drive in Malibu

  • Marie Canyon Storm Drain at Puerco Beach

  • Las Flores Creek at Las Flores State Beach

Stormy weather at southeastern beaches as Hurricane Center tracks weather system

The National Hurricane Center is monitoring a storm system off the coast of South Carolina this weekend and warned it will cause dangerous riptides and gusty winds along the southeastern U.S. coastline.

"Even though development into a subtropical or tropical cyclone is not expected, the system will produce gusty winds and dangerous surf and rip current conditions along portions of the southeastern United States coast through Sunday," the center said.

STORMY WEATHER: Hurricane Center watching system near Carolinas. Will it ruin Memorial Day weather?

Officials warned about rip currents at beaches including Wilmington and the Outer Banks in North Carolina; Charleston and Myrtle Beach in South Carolina; Savannah, Georgia; and Jacksonville, St. Augustine, and Daytona Beach in Florida.

See the photos of Florida's sargassum seaweed

Southeast Florida beaches have been blanketed with "blobs" of sargassum seaweed, known for being brown and crunchy when it washes up on shore, and floating in island-like masses in the ocean, never touching the floor due to its buoyancy.

When it covers beaches, it can inconvenience visitors as they search for a clear spot to sprawl out or have to move the seaweed out of the way. It also rots after washing ashore and can produce a foul smell and even irritate the eyes, nose and throat.

Here's what it looks like on Treasure Coast beaches ahead of the holiday:

Sargassum seaweed has collected along the shoreline at Jensen Beach Park on Thursday, May, 25, 2023, on Hutchinson Island. The beach is 4191 N.E. Ocean Blvd.
Sargassum seaweed has collected along the shoreline at Jensen Beach Park on Thursday, May, 25, 2023, on Hutchinson Island. The beach is 4191 N.E. Ocean Blvd.
Sargassum seaweed covers the shore north of South Beach Park at 1702 Ocean Drive, Vero Beach  on Wednesday May 24, 2023 around 6 p.m.
A few beach goers on May 25, 2023, didn’t let the seaweed stop them from coming to Waveland Beach, the southernmost guarded beach in St. Lucie County. The beach is at 10350 S. Ocean Drive in Jensen Beach.

WHAT TO KNOW ABOUT DELAWARE BEACHES: Headed to the Delaware beaches this Memorial Day weekend? Here's your guide

Contributing: Treasure Coast Newspapers

This article originally appeared on USA TODAY: Beach plans? Memorial Day to be dangerous or gross on these US beaches