The Avatar Storm is a huge influx of spirit energy which coalesces within the Gauntlet, but its effects can be seen – and sometimes felt – throughout the Umbra. The Technocratic Union refers to it as the Dimensional Anomaly.
The Avatar Storm occurred at the end of the Week of Nightmares in 1999, leaving a disordered and devastated Horizon in its wake. At the same time, the Gauntlet thickened, apparently embedded with jagged shards of Avatars, although no one is sure if this was cause, effect, precursor or aftershock of the Avatar Storm.
The Storm's winds are drawn to Paradox and will. The costs of being caught unprotected in the path of the Avatar winds can potentially be very high. A mage caught undefended might count death a minor penalty compared to some of the effects of the Avatar Storm that have been observed.