Developing Hurricane May Bring Widespread Heavy Rainfall

SAN DIEGO (CNS) - For the weekend into early next week, developing Hurricane Hilary was expected to bring a significant surge of monsoonal and tropical moisture with widespread heavy rainfall in San Diego County, especially for Sunday into Monday, and a high potential for flash flooding in the mountains and deserts, the National Weather Service said today.

Residual moisture could maintain an active monsoon pattern through at least the middle of next week with afternoon and evening thunderstorms in the mountains, deserts, and inland valleys.

A weak low-pressure system moving to near the central California coast may bring drier southwest flow aloft through Friday and begin to spread cooling inland with high temperatures for the coast and valleys a few degrees cooler Thursday and that cooling spreading into the mountains and deserts on Friday.

The marine layer was likely to be near 1,200 to 1,500 feet deep with night and morning coastal low clouds spreading inland in San Diego County into the far western valleys.

Along the coast Thursday, it was expected to be mostly sunny with high temperatures from 74 to 79 degrees, the NWS said. Inland areas were predicted to be mostly sunny with highs from 82 to 86. It should be mostly sunny in the mountains with highs from 89 to 99. The deserts were expected to be mostly sunny with highs from 111 to 115.

A significant moisture surge was expected for Friday night into Saturday with strong southeast winds across the deserts into the mountains, more clouds, much cooler high temperatures for inland areas on Saturday, and increasing chances for precipitation on Saturday for the mountains and deserts.

Tropical Storm Hilary was expected to become a hurricane Thursday and rapidly intensify into a major hurricane as it moves northward off the Pacific coast of southern Baja, eventually tracking near the Pacific coast or inland through northern Baja as it moves northward.

Regardless of the exact track, it was likely to bring a substantial surge of moisture into portions of southern California with a high potential for heavy rainfall and flash flooding, especially for the lower deserts into the adjacent mountains. The latest forecast has rainfall through Monday exceeding 5 inches for the lower deserts into the adjacent mountains with 5 to 10 inches on portions of the east slopes of the mountains.

Once this surge in moisture occurs, the moisture was expected to be slow to dissipate with an active monsoon pattern persisting through at the least the middle of next week.

Northwest wind gusts around or exceeding 20 knots may occur over the outer coastal waters again Thursday afternoon. There is about a 15% to 20% chance of tropical storm force winds over the coastal waters Sunday night through Monday, not counting possible strong winds from thunderstorms.

A south-southeast swell produced by Tropical Cyclone Hilary has some potential to bring high surf to south-facing beaches Sunday and Monday. There was also a chance of lightning late Saturday through Monday.

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CNS-08-17-2023 05:26