We've spoken a little about Graupel formation in an earlier post, but I'll add a bit more here. Graupel forms as supercooled cloud-liquid water collides and freezes with on an ice crystal, with the crystal so fully encased in frozen cloud droplets that it is no longer distinguishable.
|This is not your brain, it is graupel (USDA/Wikipedia Commons)|
Graupel formation is favored in storms with high cloud-liquid water content and strong vertical velocities. The former supplies the super-cooled water, and the later enables the hydrometeor to remain suspended as it is rimed. Dry snow might have a fall speed of 1 m/s or less, but the fall speed of snow increases as it is rimed, with graupel attaining fall speeds of up to 3 m/s. Thus, if you want big graupel, you need strong vertical motion.
For graupel to form, we typically want warm storms or at least warm cloud bases so that there is abundant super-cooled water to play with. Further, accretion is more efficient if the cloud liquid water is spread out over a smaller number of cloud droplets (so the droplets are bigger), so it is best if the storm has a smaller number of cloud-condensation nuclei (CCN) on which cloud droplets form. The combination of a high frequency of warm storms and maritime airmasses that are typically cleaner and have fewer CCN is one reason why the Cascades and Sierra see so much graupel. Finally, we want strong rising motion, either produced by convection or direct orographic ascent.
Part of the reason that we've seen alot of graupel so far this year is that our storms have been warm and they have been convective. Once things get colder later in this cycle, we'll see snow that forms primarily from vapor deposition again.