On charts that use this convention, a purple line with triangles and semicircles on the same side marks an occluded front. It often lies close to the centre of a low. To work out when it might rain where you are, start with the front and then look for the band of precipitation in the forecast.
How an occlusion forms
In the classic model of a low-pressure system, the cold front catches up with the warm front. The warm air between them is lifted, bringing cooler air masses together near the ground. Which air mass is colder affects the structure of the front. The line on the chart traces the surface front. Its clouds and precipitation can extend across a much wider area.
What weather can it bring?
Rising air can cool and form cloud. With enough moisture, an occlusion can bring steady rain or more intermittent precipitation. Terrain and the track of the low influence how much falls in any one place. The symbol alone tells you little about gust speeds or whether precipitation will fall as rain or snow.
Use the chart to plan your day
Compare two or more charts and check the time each is valid for. See how the front and the low are moving, then open the hourly forecast for your location. If rain is already nearby, look at a radar loop. Radar shows where precipitation is being detected now, while the forecast helps you judge what may happen next. If the line is still to your west but the rain band already covers your route, use the band to decide when to set off.