The declaration, which Brown is scheduled to announce at 10 a.m. in San Francisco, comes during one of the driest winters on record in California, following two dry years that already have left many reservoirs depleted.
U.S. Sen. Dianne Feinstein, Rep. Jim Costa, D-Fresno and several state lawmakers began urging Brown last month to declare a drought emergency. Brown appointed a committee to review conditions on the ground.
A formal declaration is considered significant as a public relations tool, increasing awareness of residents and, perhaps, federal officials who could accelerate some relief efforts.
Brown's office said Thursday that Brown would "make a major announcement" in an appearance Friday in San Francisco. The administration declined to disclose the nature of the announcement.
But a declaration has been expected, with Brown indicating repeatedly in recent days that he was close to declaring the emergency. Facing calls for a drought declaration while on a two-day swing through inland California this week, Brown said "nobody should discount the seriousness of what we're facing."
Still, Brown has suggested the significance of a formal declaration may be overstated.
"I'm trying to understand what physically we can do in the face of this drought, and then legally what steps can I take," the Democratic governor told reporters in Bakersfield on Tuesday.
Brown said a drought declaration could be helpful, "but at the end of the day, if it doesn't rain, California's in for real trouble. And the governor, through a declaration, can't make it rain."
Brown managed a drought in the late 1970s, when he was governor before. At the time he called for a 25 percent reduction in personal water use statewide and lobbied Washington for federal aid.
PHOTO: Gov. Jerry Brown addresses the media at Fresno City Hall on Tuesday, Jan. 14. The Fresno Bee/Eric Paul Zamora