Here's the article: http://www.avweb.com/avwebflash/news/GPS-Interference-NOTAM-For-Southwest-226365-1.html

Excerpt:

Although the FAA doesn’t go into detail, it seems the military is testing something that can disrupt GPS over a huge area, centered on China Lake, California, home of the Navy’s China Lake Naval Weapons Center. On Tuesday, June 7, the FAA is warning that GPS signals down to 50 feet AGL could be “unreliable or unavailable” between 9:30 a.m. and 3:30 p.m. local time over a radius of 253 nautical miles, which includes the L.A. Basin, Bay area and Las Vegas.

And here's the official FAA notice https://www.faasafety.gov/files/notices/2016/Jun/CHLK_16-08_GPS_Flight_Advisory.pdf (so this is not just another wild internet rumor).

I haven't seen any major stories in the news about GPS not working in California today - so I'm assuming they just wanted to be extra cautious about anything potentially involving flying safety.

Still, this is yet another reminder that a GPS is not a substitute for a map & compass...