If the nose first tilted upwards as some passengers said (and the news report above), is it possible that the aircraft, because of its high altitude and heavy load, stalled, and then descended rapidly because of the stalling?
If that did happen, perhaps its a question of what caused the upward movement of the aircraft to start with...
Also i found
this FAA document regarding operations of aircraft at altitudes over 25,000 feet. It provides a small section about weather and the jet streams, and some info about CAT as well (Starting at page 12 on your PDF reader).
http://www.bom.gov.au/weather/national/charts/UV.shtml - Pretty extreme UV conditions out to the NW - thats todays forecast, but it wouldn't change much between 2 days.