Plane reaches record speed of 1290kmh during flight

Jet stream example from Environment Canada on Feb 23rd 2019

According to the Environment Canada:

On Tuesday, the howling tailwinds for Virgin Atlantic Flight 08 from Los Angeles to London helped it achieve a record ground speed of 1290 km/h and arrive a full 48 minutes ahead of schedule!

This incredible flight speed was caused by a very strong jet stream. Jet streams are wind maxima at high altitudes (roughly airplane height or 250mb), where significant winds are present.

Source: NWS Storm Prediction Center (edited by The Washington Post)

As you can see from the graph above, which was taken from a sounding retrieved above New York, a weather balloon measured a wind speed of 231mph. According to Environment Canada:

This “jet stream”, the high-altitude air current along which storms travel, was blowing in a small area – a “jet streak” - at around 370 km/h.
Source: US National Center for Environmental Predication, Weather Prediction Center

Above is a surface map depicting calm surface conditions across the Continental United States on Tuesday, during the event.