These flags indicate gale and hurricane warnings. Gale flags are solid red pennants. Flying one pennant indicates a small craft advisory. Flying two red pennants together means there is a gale warning in effect indicating winds of 39 mph to 54 mph. The other storm flag is a hurricane flag. This is a red square with a black square in the center of the flag. Flying two of the square red flags indicates a hurricane with wind sustained wind speed of 74-156 mph or more. In conjunction with Hurricane and Storm flags there are also a number or weather flags. With the advent of radio, and the internet many of these types of flags are no longer in use. However they were used to communicate a variety of weather changes and were especially helpful as fronts were moving through areas where boats were on the water and could be effect by the change in weather conditions.
Here are the flags that were used to signal these changes.