Napa County firefighters were battling a large grass fire late Thursday night at the north end of Lake Berryessa.

Fire dispatch said crews were still responding to a 200-acre blaze, near Knoxville-Berryessa Road and the East Side Access Road in the Putah Canyon Recreation Area.

The website reported the fire was 500 acres and threatening five ranches.

Winds were pushing the flames north, dispatchers said.

Napa police warned of possible road closures in the area.

The fire comes at a time fire agencies were watchful of gusty winds.

The National Weather Service issued an alert this week warning of critical fire-weather conditions. Strong and gusty northerly winds will peak Friday morning, the alert says, and could last through Monday. An offshore flow will result lower humidity, especially at or above 1,000 feet elevation.