  During the outbursts of black hole X-ray binaries (BHXRBs), their accretion
flows transition through several states. The source luminosity rises in the
hard state, dominated by non-thermal emission, before transitioning to the
blackbody-dominated soft state. As the luminosity decreases, the source
transitions back into the hard state and fades to quiescence. This picture does
not always hold, as $\approx$ 40$\%$ of the outbursts never leave the hard
state. Identifying the physics that govern state transitions remains one of the
outstanding open questions in black hole astrophysics. In this paper we present
an analysis of archival RXTE data of multiple outbursts of GX 339-4. We compare
the properties of the X-ray variability and time-averaged energy spectrum and
demonstrate that the variability (quantified by the power spectral hue)
systematically evolves $\approx$ 10-40 days ahead of the canonical state
transition (quantified by a change in spectral hardness); no such evolution is
found in hard state only outbursts. This indicates that the X-ray variability
can be used to predict if and when the hard-to-soft state transition will
occur. Finally, we find a similar behavior in ten outbursts of four additional
BHXRBs with more sparse observational coverage. Based on these findings, we
suggest that state transitions in BHXRBs might be driven by a change in the
turbulence in the outer regions of the disk, leading to a dramatic change in
variability. This change is only seen in the spectrum days to weeks later, as
the fluctuations propagate inwards towards the corona.
