Most of La Palma has good weather nearly all year. But the observatory is up at 2400 m (almost 8,000 ft) and occasionally in winter dramatic storms come howling in, dump half a metre of snow overnight, and disappear almost as fast as they came.
If you’re planning a trip to the observatory between November and March, be sure to check the road conditions before you leave. You can do this at the Isaac Newton Group weather station or the Galileo weather station.