https://news.yahoo.com/bizarre-snow-strip-forms-over-174214522.html

Integral to its formation was convergence, or the gentle collision
of two air masses at the surface. The "pileup" of air causes it to rise,
blossoming into a band of heavy snow showers. That can also cut back on
precipitation outside the band, since air exiting the clouds is likely to sink
and choke off any snowfall on either side of the axis of best convergence.

Thanks to Brad Kushner.