PG&E has warned about 70,000 customers in 29 Northern California counties that they could lose power starting Wednesday evening as strong winds and dry conditions raise wildfire concerns.

The utility says wind could damage power lines and start fires. Its outage maps place more than 4,700 Alameda County customers and another 1,600 in Contra Costa County in potential shutoff areas. The shutoffs have not been confirmed, and any outages could last through Saturday, PG&E says.

In Walnut Creek, Lissa and Mike Biddle received an alert about a possible Wednesday evening shutoff. They have a generator from previous fire-related outages but are preparing for the possibility of several days without electricity. PG&E spokesperson Tamar Sarkissian urged customers to charge phones, other electronics and electric vehicles ahead of time.

Contra Costa fire officials are monitoring the winds and plan to add staff if needed.