To portend means to be a sign or warning that something (usually something important or serious) is going to happen. When dark clouds portend a storm, they signal that rain is coming. When your dog suddenly acts nervous and hides under the bed, that behavior might portend an approaching thunderstorm that you haven't heard yet.

The word often suggests something significant or dramatic ahead. A sudden drop in temperature might portend the first snowfall of winter. When a character in a story discovers a mysterious symbol carved into a tree, that detail might portend danger or adventure to come. Scientists might say that certain geological signs portend a volcanic eruption.

Portend works like a forecast, but it's more ominous or serious than simply predicting what's next. You probably wouldn't say that your rumbling stomach portends lunch, but you might say that your best friend's unusual silence portends an uncomfortable conversation. The related noun is portent, meaning a sign of something coming: “The earthquake was a portent of the eruption that followed.”