Hilary is “highly likely” to come within 100 miles of San Diego as a tropical storm and generate torrential rains that could cause dangerous flooding countywide and freakishly raise nighttime temperatures into the 80s, according to the National Weather Service.
Forecasters said Friday morning that Hilary — which is now a Category 4 hurricane — could drop 2 to 3 inches of rain at the coast, 2 to 4 inches in the valleys and 5 to 10 inches in the mountains and deserts.
The outer bands will begin producing rain countywide on Saturday. The rain will become more severe on Sunday, peaking Sunday night and early Monday, when Hilary is expected to arrive in Southern California as a tropical storm.
Forecasters say Hilary is likely to between the Channel Islands and the California-Arizona border and will directly impact San Diego. It’s possible that the system will edge east and flow ashore in Baja California, which would result in less rain on the San Diego County coast but still produce wild downpours in the eastern half of the county.
The San Diego River in the Fashion Valley section of Mission Valley could reach 7 feet or higher late Sunday night and early Monday.
Computer models indicate that Julian and Borrego Springs could get 4 to 6 inches of rain, while higher amounts fall on the eastern slopes of the desert, especially along Interstate 8, near the boundary of San Diego and Imperial counties.
“The rain could last for many hours in some places,” said Brandt Maxwell, a weather service forecaster. “And we think there’s a greater than 50 percent chance that tropical storm-force winds will hit coastal waters with gusts of 39 mph or higher.”
Maxwell added that the weather service, for the first time in history, was planning to issue a tropical storm watch for San Diego on Friday. A flash-flood watch will be in effect for the area from the coast to the inland valleys from late Saturday until Monday night.
Forecasters said that Hilary is so big, and so filled with warm, moist air, the system could cause the temperatures to rise into the 80s at the coast on Saturday night.
San Diego only averages 0.01 inches of rain in August. The most it has ever received during the month is 2.13 inches, which occurred in 1851. “The rain that’s coming to rival that level,” said Adam Roser, a weather service forecaster.
There is an 80 percent chance that rain will be falling early Sunday afternoon, when the Padres play the Arizona Diamondbacks at Petco Park. It’s possible that the storm could cause the first rainout ever to occur during August. The rain also will be heavy in the morning, when San Diego State University begins fall classes.
The forecast is based on the latest guidance from the National Hurricane Center, which said that Hilary became a Category 4 hurricane late Thursday, with sustained winds of 145 mph and periodically higher gusts. The system will lose strength over cooler waters along northern Baja California.
Such storms are quite rare in San Diego.
The city has been directly hit only once by a hurricane. That event occurred on Oct. 2, 1858, generating winds that gusted upwards of 70 mph. San Diego also was directly hit by a tropical storm on Sept. 25, 1939. It produced winds up to 50 mph.
The two storms originated along the west coast of Mexico, below Baja California, where the water gets warm enough to sustain such activity. The systems rarely travel very far up the Baja California peninsula, where the water is significantly cooler, partly due to the presence of the chilly California Current.
Tropical Storm Kay showed unusual stamina last September, coming within 150 mph of San Diego before it curled off to the west and died. The system produced strong winds and rain in greater San Diego.
