LITTLE ROCK, Ark.- As the National Weather Service put it this morning, March is going out like a lamb.
Like this video above from Madelaine Priest in Conway, it is one of a dozen reports of sleet being shared on social media. Many are wondering how it is even possible as temperatures are hovering in the 50s!
Keep reading, we will explain why.
Above is a snapshot from the lower end of a sounding from the National Weather Service Little Rock office. Twice a day, a balloon is launched into the air from North Little Rock to get a sample of what the atmosphere looks like in a vertical view.
This is the sounding from seven this morning, March 31. The red line represents the air temperature and the green line is the dewpoint. The closer the red and green lines are together, the more saturated the atmosphere is at that level. The farther away these two lines become, the drier atmosphere is at the layer.
As you increase with height, you can see that the red line goes below freezing and the red and green lines are very close together. It appears that there is enough moisture for snow to form. As it falls it is moving into an environment that is well above freezing so the snow begins to melt.
As the snow (that is melting) continues to fall closer to the surface it begins to move into a layer of very dry air. Other droplets surrounding that melting snow which include rain begin to evaporate in this dry layer. Since evaporation is a cooling process, it can cool a small column of air enough for those partially melted snowflakes to refreeze and become sleet!
Generally, this isn’t a normal way that sleet forms as another thick layer of sub-freezing air is in place at the surface. Today, we had a unique set of conditions in place where a very local occurrence was possible under a shower that could utilize the ingredients.
In addition to sleet, there were one or two images on social media of something that looks a little different. It is called graupel!
Again, graupel normally forms under the conditions explained above but just like the sleet from today, it is forming thanks to that very dry layer of air.
As the snow is falling through that very dry layer of air next to rain droplets, that snow can stop melting and begin colliding with rain droplets that are supercooled (droplets that remain in a liquid state as temps are below freezing).
This collision results in a soft, crushable substance that is opaque and not as clear as sleet.
Since sleet and graupel are considered snow for climatological records, there will be many locations that report a trace of snow today!