A red flag with a black square in its center is a storm warning flag. It’s part of the National Weather Service’s Coastal Warning Display Program, flown at harbors, marinas, and coastal stations to alert boaters that sustained winds between 48 and 63 knots (55 to 73 mph) are expected in the area. If you see two of these flags stacked vertically, the warning is even more severe: that signals a hurricane, with winds of 64 knots (74 mph) or higher.
How Storm Warning Flags Work
The flag itself is square, bright red, with a solid black square centered inside it. It’s designed to be visible from a distance and unmistakable against the sky. During the day, coastal stations fly these flags on a mast where boaters can spot them before heading out. At night, the same warnings are communicated using colored lights instead of flags.
The storm warning flag sits near the top of a four-tier system. Each level uses a distinct visual signal so there’s no confusion between them:
- Small Craft Advisory: A single red triangular pennant. Winds of 18 to 33 knots (21 to 38 mph) or hazardous wave conditions are expected.
- Gale Warning: Two red triangular pennants, one above the other. Winds of 34 to 47 knots (39 to 54 mph).
- Storm Warning: One square red flag with a black square center. Winds of 48 to 63 knots (55 to 73 mph).
- Hurricane Warning: Two square red flags with black square centers, stacked vertically. Winds of 64 knots (74 mph) or greater.
The shape difference is key. Pennants are triangular, warning flags are square. If you see a square red flag, you’re looking at storm-force or hurricane-force conditions, not a lesser advisory.
What It Means for Boaters
Storm-force winds are dangerous for virtually any recreational vessel. Waves in open water can easily reach 20 feet or more, and even protected harbors experience intense chop and surge. When a storm warning flag is flying, small boats should not leave port, and boats already on the water should seek shelter immediately.
The U.S. Coast Guard recommends that anyone who sees a warning flag tune to NOAA Weather Radio for the latest forecast details, including the timing, direction, and duration of the expected storm. The flags tell you the severity of the threat but not how soon it will arrive or how long it will last, so the radio broadcast fills in those critical details.
For coastal residents who aren’t boaters, a storm warning flag at a nearby marina is still useful information. It means conditions are serious enough that the National Weather Service expects potentially destructive winds in the area, and it’s worth checking the forecast for any inland effects as well.
Where You’ll See These Flags
The Coastal Warning Display Program operates at participating harbors, yacht clubs, marinas, and Coast Guard stations along the U.S. coastline, the Great Lakes, and some major inland waterways. Not every marina participates, but the program has been a fixture of coastal communities for well over a century. The flags follow a standardized design so they look the same whether you’re in Maine, Florida, or California.
Some coastal businesses and beachfront properties also fly these flags informally to alert visitors, though the official program is managed through the National Weather Service and cooperating stations.
Not a Motorsports Flag
If you encountered a red flag with a black element during a race, you’re looking at something different. In auto racing, a solid red flag means the session has been stopped due to dangerous conditions, while a black flag with an orange circle summons a specific car to the pits for mechanical problems. There is no standard motorsports flag that combines a red background with a black square. The red-with-black-square design is specific to maritime weather warnings.