I've noticed these graphics become more popular recently. I "think" I understand it, but I've been mistaken before.
I grabbed this image from the East Coast Feb thread. I did not put in the brown/black circles. The poster I snagged this image from said they represented good chances for snow.
The way I read this:
Date is clear enough.
Top half on the left represents the Ensemble number; swatches in the center represent the amount of snow that ensemble called for, with color code chart on the right.
Bottom half shows snowfall amount the Mean calls for.
So in this example, the time frame of 21 Feb +/- is promising due to nearly every ensemble showing snow, with many/most 2"+. While some other dates show higher amounts in some ensembles, there are also many with no snow, so those dates are less promising.
Am I correct in my interpretation?