What is graupel and how does it form?

 A pair of low-pressure systems Friday and Saturday will help to usher in a cooler pocket of air aloft. This is called a "cold core" low because temperatures several miles up will be well below zero! Temperatures will be near or below freezing from the surface to around 1 mile up.

This type of "cold core" low this time of year often produces spotty rain showers, but occasionally when temperatures are just right, graupel can form. Graupel is a type of wintry precipitation that resembles sleet or hail, but forms in a different way from either of those.

When temperatures are cold above the surface like is often the case this time of year, precipitation always starts as snow. Usually by the time it reaches the ground, the air is warm enough to melt it into rain. In unique conditions like we might see on Saturday, the layer of air it falls through is supercooled water droplets. These are called supercooled because they are tiny liquid water droplets that are not attached to any other particles in the atmosphere like dust (referred to as condensation nuclei).

As the snow falls through the layer of supercooled water, the droplets freeze, or rime, onto the snowflakes. This is a very similar process to how raindrops or snowflakes grow larger, but the sub-freezing liquid droplets freeze onto the snowflake, forming a soft white pellet. They resemble small hail but are soft and crushable. Often, graupel looks like tiny Styrofoam pellets with similar texture. Occasionally referred to as "nature's Dippin' Dots", graupel is a unique form of precipitation that is almost a cross between hail and sleet.

We may see our first taste of the upcoming winter season Saturday morning. Scattered showers will be possible through much of the day, but the morning especially could produce a few instances of graupel. Surface temperatures will start in the 30s and conditions aloft will be favorable for such formation of these soft ice pellets. Afternoon temperatures climb near the 50-degree mark, which may be just warm enough to melt anything back over to rain for the second half of the day.