A classic "donut hole" in the KENX radar imagery this morning. While it may look like there's a strangely geometrical break in the clouds over western Albany County, what we're actually looking at is fairly common winter occurrence: Cold, dry air pooling near ground level overnight. The radio beam of a weather radar system is angled slightly up1 from the horizon--on the order of a degree--so with a substantial layer of dry air at ground level, the radar beam can travel for many miles before it finally intersects with the saturated clouds of the incoming winter storm, where snow has likely already formed and is circulating at elevation. My guess would be that the shrinking we see here is the downward progress of the snow fall--as snow falls relatively slowly, snow that forms higher aloft often evaporates upon encountering a layer of dry air and never reaches the ground. So, a steady snow fall saturates the dry layer from the top down until that layer is damp enough throughout for snow to survive the trip to terra firma.
1 Partly to avoid receiving returns bounced back from taller objects on the ground, partly to avoid--as the beam widens both side-to-side and up-and-down as it travels away from the radar--one particular object, viz. "the ground."
it is now snowing at ground level