It's the astronomers forecast. It shows at a glance when, in the next 48 hours, we might expect clear and dark skies for one specific observing site. There are many, but the example above is for Cherry Springs State Park, Pennsylvania.
It's is specifically intended for amateur astronomers. The forecast data comes from a numerical weather model run by those very cool guys at the Canadian Meteorological Centre.
Read from left to right. Locate a column of blue blocks. That's when the sky will likely to be clear and dark. A more detailed explanation is here, which also links CMC's original forecast maps.
We had cloud cover here but here is something for the next go around of lunar things...
