This article is to clear common misconceptions about weather briefings. There is not, in fact, such a thing as a "legal weather briefing."
CFR14 §91.103 requires that pilots obtain all available information before beginning a flight. ForeFlight delivers weather, NOTAMs, and airport information that help a pilot meet this requirement. ForeFlight's weather, NOTAM, and airport information comes from federal regulatory agencies.
Pilots want to know that they will have proof or a record of their briefing. This is important so that in the event of an incident, they can prove they met the requirements under §91.103.
ForeFlight stores a record of the briefing you get for 120 days, provided you tap the Briefing button on the Flights page. If you simply look at METARs and TAFs on the Airports or Maps page, we don't keep a record of that.
The FAA has published an advisory circular that provides an educational roadmap for the development and implementation of preflight self-briefings, including planning, weather interpretation, and risk identification/mitigation skills. Pilots performing self-briefings to meet the requirements of §91.103 should review and become familiar with the concepts in Advisory Circular 91-92.