Source: https://pilotweb.nas.faa.gov/PilotWeb/. To find the NOTAM, type in “KZLA” in the Locations field.
The NOTAMs (Notices to Airmen, generated by the FAA) should provide a clue:
NOTAM for LA.
KZLA LOS ANGELES A2832/10 - THE FOLLOWING RESTRICTIONS ARE REQUIRED DUE TO NAVAL AIR WARFARE CENTER WEAPONS DIVISION ACTIVATION OF W537. IN THE INTEREST OF SAFETY, ALL NON-PARTICIPATING PILOTS ARE ADVISED TO AVOID W537. IFR TRAFFIC UNDER ATC JURISDICTION SHOULD ANTICIPATE CLEARANCE AROUND W537 AND CAE 1176. CAE 1155 WILL NOT BE AVAILABLE FOR OCEANIC TRANSITION. CAE 1316 & CAE 1318 WILL NOT BE AVAILABLE FOR OCEANIC TRANSITION. CAE 1177 WILL BE AVAILABLE FOR OCEANIC TRANSITION. W537 ACTIVE, CAE 1176 CLOSED. SURFACE - FL390, 09 NOV 20:00 2010 UNTIL 10 NOV 01:00 2010. CREATED: 08 NOV 20:52 2010
Translation: The U.S. Navy is most likely conducting wargames in the area. All unauthorized air traffic should remain clear of the area.
Source: https://pilotweb.nas.faa.gov/PilotWeb/. To find the NOTAM, type in “KZLA” in the Locations field.