No report is timelier than the one made from the flight deck of aircraft in flight. In fact, aircraft in flight are the only means of observing actual icing and turbulence conditions. Pilots welcome pilot weather reports (PIREPs) as well as pilot weather briefers and forecasters. Pilots should report any observation, good or bad, to assist other pilots with flight planning and preparation. If conditions were forecasted to occur but not encountered, a pilot should also report this inaccuracy. This will help the NWS verify forecast products and create more accurate products for the aviation community. Pilots should help themselves, the aviation public, and the aviation weather forecasters by providing PIREPs.
AC 00-45G - Aviation Weather Services (3.2 Pilot Weather Reports (PIREP) - Page 3-34) - https://www.faa.gov/regulations_policies/advisory_circulars/index.cfm/go/document.information/documentID/215166
Also see Aeronautical Information Manual (AIM) 7−1−20. Pilot Weather Reports (PIREPs) (page 7-1-43) - https://www.faa.gov/air_traffic/publications/#manuals