Expecting the honey flow in this area to end in a few weeks, I was pointed to weather.com's pollen count as a possible indicator. The only problem is, it's the opposite to what I might have expected.
"For the Philadelphia area, the pollen count for Pollen season can vary from year to year depending on a variety of factors, including weather. Here's a snapshot of tree, grass, and weed pollen seasons for this location."
Season --- Onset / Peak / End
Tree --- Mid-March / Late-April / Mid-July
Grass --- Mid-April / Mid-June / Mid-September
Weed --- Early-July / Late-August / Mid-November
I suppose it's not too much different in other areas.
It seems the tree-pollen season matches the spring honeyflow. I don't see bees foraging from grass flowers, so we can disregard that. But what about weeds? What sorts of weeds contribute to the pollen count that people with allergies want to know about? And why don't those weeds carry the bees through the late-summer dearth?