At https://www.aviationweather.gov on the Forecasts puldown, then the Aviation Forecasts selection, the chart opens and often shows what looks like a letter R with a tail in the lower right, in different colors. The legend does show the symbol. What does it indicate?
The Info button (top right) takes you to an explanation site, which in turn links you to
Which explains the Symbol (under the section Fog or special weather) as Thunderstorm