CALISTOGA, Calif. (NewsNation Now) — The air in the Napa Valley was so thick with smoke Wednesday that the devastation of the Glass Fire was not visible without driving down side streets in St. Helena and Calistoga. 

At the end of White Sulphur Springs Road, several buildings that were part of a retreat center were left in ruins. 

It was once a peaceful getaway and in one sense it was peaceful still three nights after the fire broke out. There was not even a hint of a breeze. 

That was good news as the fire burned slowly in the hills on both sides of Highway 29 near Calistoga.

But firefighters were concerned about what’s coming. 

The National Weather Service has been warning of another period of strong winds with very low humidity in the area between Thursday afternoon and Friday evening. 

The flames of the now 51,000-acre Glass Fire are likely to pick up both in intensity and forward speed on Thursday.

Meanwhile, flames just off Old Lawley Toll Road in Calistoga forced mandatory evacuations for some residents.