ISO-NE, the folks who run the six-state power grid, say the wildfire smoke that makes the air unpleasant on some days makes forecasting power demand more complicated. (The whole release is here)
Smoke from wildfires in Canada has traveled to New England, significantly lowering production from solar resources in the region compared to what ISO New England would expect absent the smoke.
The smoke has also lowered actual temperatures in New England compared to what weather models are forecasting. This leads to lower demand on the regional grid, as there is less need for things like air conditioning.
These two factors—decreased production from solar resources and decreased consumer demand due to lower temperatures—has made forecasting demand for grid electricity challenging.