This is related to the probability density of the <A HREF="https://en.wikipedia.org/wiki/Quantum_harmonic_oscillator">quantum harmonic oscillator.</A> (The Hermite polynomials are the eigenstates of that problem.) For large $k$ the density
$$P_k(x)=\frac{1}{\sqrt{2\pi}k!} [h_k(x)]^2 \, e^{-x^2/2},$$
normalized to unity, has the approximation
$$p_k(x)=\frac{1}{\pi}(4k-x^2)^{-1/2},\;\;|x|<2\sqrt{k}.$$
The boundaries $\pm 2\sqrt{k}$ are the classical turning points of the harmonic oscillator. In that context $E=4k$ is the energy of the particle and $V(x)=x^2$ is the harmonic potential in which it moves. The large-$k$ approximation is known as the <A HREF="https://en.wikipedia.org/wiki/WKB_approximation">WKB approximation</A>. More generally, for an arbitrary potential $V(x)$ the density varies as $[E-V(x)]^{-1/2}$.

The plot compares $P_k$ and $p_k$ for $k=100$. The function $p_k$ is a smoothed version of $P_k$, suitable for averaging quantities that vary little on the scale of $1/\sqrt{k}$.

<IMG SRC="https://i.sstatic.net/g0TN6.png" WIDTH="400"/>

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Alternatively, one can use the asymptotic expansion of the <A HREF="https://en.wikipedia.org/wiki/Hermite_polynomials">Hermite polynomials,</A>
$$e^{-x^2/4}h_k(x) \propto \cos \left(x \sqrt{ k}- \pi k/2 \right)\left(4k-x^2\right)^{-1/4}.$$
Squaring and averaging over the rapidly oscillating $\cos^2$ gives $p_k(x)$.