If you don't have an in-flight weather source, to have the best weather data available to you in flight, you will want to Pack for your flight. In terms of weather-related items, Pack will check for and download the latest METARs, TAFs, and AIR/SIGMETS. If you have a Pro Plus, Performance Plus, or Business Performance subscription plan, Pack will download forecast data for the Icing, Turbulence, and Surface Analysis layers.
If you don't Pack for your flight, you'll only have in-flight access to those items you last viewed on the ground except for Radar and Satellite Maps. Radar maps, Satellite maps, and Imagery are not downloadable.