SAN DIEGO — A tropical storm rapidly rotating northward off the coast of Mexico intensified to a Category 4 hurricane Thursday night, the National Weather Service announced.

With maximum sustained winds of 140 mph, weather officials say Hilary’s impact is increasing along the west-central coast of the Baja California Peninsula.

Though the U.S. National Hurricane Center (NHC) predicts Hilary to begin weakening by Saturday, there is still one main threat for the San Diego County region.

Shortly before 1 a.m. Friday, NWS announced a High Risk is in effect for Southern California’s Excessive Rainfall Outlook. Valid Sunday, Aug. 20 through Monday. Aug. 21, an increased risk of excessive rainfall is forecast for the county.

Perhaps the biggest threat posed to parts of San Diego County is flash flooding that could be caused by torrential downpours. NWS says this is especially true for the east slopes of the mountains into the adjacent deserts.

Some isolated areas in the mountains could receive over 10 inches of rainfall Saturday through Monday, said NWS. Here’s a look at potential widespread heavy rain for the county during the brunt of Hilary:

For those living in the unincorporated part of the county, Cal Fire officials are offering advice and free sandbags in preparation for Hilary’s stormy debut. Those pickup locations are listed here.

Large swells from Hilary are also spreading northward and are expected to reach the northern portions of the Baja California Peninsular later this weekend. Hilary expected to bring big surf to San Diego beaches.

County officials and road crews are encouraging San Diegans to stay alert and take precautions on the roads this weekend as Hilary makes her stormy visit to the region.