Snow reported at 55 degrees in Kansas City (also; the definition of graupel)

We had surface temperatures in the mid-40s with snow showers in the St. Louis region yesterday (11/12). There was a thin saturated layer within the dendritic growth zone (-10 to -20 C), and with very steep lapse rates beneath that layer, the snow was able to reach the surface before melting. That's one of my favorite types of snow - it could snow its butt off all day, but there's going to be no impacts at the surface ;) .
 

John Farley

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With the downdrafts in these convective snow showers, it is not unusual to get snow and/or graupel to the ground when temperatures in the sunny areas prior to the arrival of the shower are in the 50s. Happens all the time in and near the mountains of Colorado and northern New Mexico, though it is less common in Missouri. But when the conditions are right, i.e. steep lapse rates, it can potentially happen in a lot of places. BTW, the explanation of graupel in the article is wrong - it forms when snowflakes get rimed, due to contact with supercooled water droplets in the cloud, which form rime as they freeze onto the snow in the cloud.
 

Jeff Duda

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BTW, the explanation of graupel in the article is wrong - it forms when snowflakes get rimed, due to contact with supercooled water droplets in the cloud, which form rime as they freeze onto the snow in the cloud.
Actually, both are correct. Graupel can form from multiple processes, including a process involving freezing of liquid water (and also riming). All complex microphysics schemes used in modern NWP models account for both of these processes.
 
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John Farley

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Jeff, you know more than I do about this so I will defer to your knowledge, but my understanding was that if freezing of liquid water is involved, as opposed to riming, the result is hail. That said, I have no doubt that there are instances where both processes are occurring, as nature does not follow our neat classification categories.
 

Jeff Duda

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Well...now that the thread has been derailed into a discussion of the definition of graupel (which I think is warranted)...

The AMS glossary of meteorology's definition of graupel is
AMS glossary of meteorology said:
Heavily rimed snow particles, often called snow pellets; often indistinguishable from very small soft hail except for the size convention that hail must have a diameter greater than 5 mm.

Sometimes distinguished by shape into conical, hexagonal, and lump (irregular) graupel.
I think the main tipping point in the distinction between graupel and hail is the size...that 5 mm threshold. Below that, any frozen particle that isn't clearly just a snow or ice crystal could probably be considered graupel, and that does include hydrometeors that began as liquid and subsequently froze. I do think that some degree of riming is necessary to distinguish graupel from sleet at the ground, though. However, most microphysics schemes do not specify a sleet category for hydrometeors, although that is getting at your last point that hydrometeors exist on a continuum of particle descriptors.

Hugh Morrison at NCAR has been at the forefront of this continuum of particle behaviors by designing what is called the predicted particle properties, or P3, microphysics scheme, that has yet to catch on to regular use in the NWP community, but is a step in the right direction towards abandoning the use of discrete precip types and towards greater realism. Instead of assuming the usual 6 or so hydrometeor forms (rain, snow, cloud water, cloud ice, graupel, and hail [which many schemes lump in with graupel]), superparticles (assumed groups of particles) are prognosticated to have various types of particle properties (mass, mass of rime, volume of rime, and number concentration within a unit volume) which can evolve freely. Particle types can subsequently be diagnosed after the fact from the values of those particle properties.

The paper can be found here: https://doi.org/10.1175/JAS-D-14-0065.1
 
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