Many southern Wisconsin and northern Illinois residents may have experienced something other than rain falling from the skies early Thursday afternoon. It wasn't snow, although wasn't too far from it, and it wasn't hail or sleet. It was actually graupel. Graupel isn't a new weather phenomenon, but it isn't one that we experience all too often either.
A strong, and cold, low-pressure system over the northern Great Lakes will continue to filter in an unseasonably cold air mass across the Midwest. This was felt Thursday afternoon when temperatures stayed in the 40s and wind chills were down in the 30s! Scattered rain showers developed as the cloud cover moved in during the late morning and early afternoon, but it wasn't all rain that occurred. Temperatures were cold enough just a few thousand feet above the surface to allow for the development of graupel. Graupel is different from hail in both the way it forms and looks.
Snowflakes that formed in the clouds way above the surface fell through a layer in the atmosphere where there were supercooled water droplets. These are water droplets that are in an environment where the air temperature is below freezing, but the droplets haven't frozen - they are in a pure form, not freezing onto any dust or other particles in the atmosphere (known as condensation nuclei). When the snowflakes are introduced into this layer the water droplets freeze, or rime, on to the snowflakes forming tiny, white pellets that often times resemble small hail. However, graupel is softer than hail.
The scattered rain showers from earlier in the day have ended, but a chilly rain is set to move in Friday evening. Most of the weekend is looking pretty dry.
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