Hurricane Hilary has entered cooler water and is starting to weaken, now a category 3 hurricane with sustained winds reaching 125 mph as it moves north-northwest at 16 mph.

Landfall is expected in Mexico on Sunday. The storm is then expected to be at tropical storm strength as it enters southern California, marking the first tropical storm in Southern California since Nora in 1997, more than 25 years ago.

Catastrophic and life-threatening flooding is expected in Southern California.

Rain from Hilary should reach California later Saturday. Early morning monsoonal showers have lead to flash flooding Saturday morning, but that is not associated with Hilary.

-ABC News' Kenton Gewecke

As of 11 p.m. ET/8 p.m. PT, Hilary remains a tropical storm with maximum sustained winds now down to 45 mph. The center of Hilary is located about 105 miles northwest of San Diego. It is moving at 28 mph.

Widespread flash flood warnings remain in effect across Southern California as areas of heavy rain continue to sweep through the region. The flash flood warnings in Los Angeles and Ventura Counties have been extended until 3 a.m. PT Monday. Flash flooding is ongoing across the region. There are also warnings in many desert areas, including Death Valley.

The widespread heavy rain in Southern California will begin to ease up soon, first in the southeastern part of the state. After 10 p.m. PT Sunday, there is a notable improvement regionwide, with the worst of the heavy rain over and the rain becoming less intense. There is lingering rain overnight in some areas but not as heavy as earlier.

Even though the rain ends, there could still be residual impacts, like landslides or rockslides.

- ABC News Meteorologist Daniel Peck