San Diego County's coast faces a 75% to 85% chance of rain the weekend of Oct. 31, the National Weather Service said.
A weather pattern known as the "Pineapple Express" is pushing tropical moisture toward Southern California, threatening to dampen Halloween trick-or-treating in Del Mar, Solana Beach and communities across the region. The storm could bring the first wet Halloween in decades for some Southern California cities. The NWS in San Diego said many cities in the region have gone 30 to 40 years without rain on Oct. 31, Patch reported.
Light showers could develop as early as Friday, Oct. 30, across San Diego County, according to the NWS San Diego area forecast discussion issued Thursday afternoon. Rain chances increase Saturday afternoon and evening, with the heaviest rainfall arriving late Saturday night into early Sunday morning.
The weather service put the chance of at least a tenth of an inch of rain from Saturday morning through Sunday night at 75% to 85% for coastal and mountain areas. The chance of an inch or more ranges from 35% to 65%, depending on location. San Diego County has the highest confidence for precipitation overall, according to the NWS forecast discussion.
Coastal flooding risk for Del Mar and Solana Beach
Del Mar and Solana Beach fall within the San Diego County coastal flood advisory zone. That advisory remains in effect until 2 p.m. Friday, Oct. 30, following days of unusually high tides that have already flooded low-lying areas across Southern California.
The weather service warned of flooding in beach parking lots, bike paths and harbor walkways. The storm could also drive a surge in sea levels the weekend of Oct. 31, renewing coastal flooding during high tides, according to AccuWeather. Winds and seas could reach hazardous levels by Sunday morning, the NWS said, possibly lingering into Monday, Nov. 2.
Erosion is already a concern. The San Diego Union-Tribune reported that several beaches, including Tamarack State Beach, have suffered major erosion from the high tides.
What's driving the storm
Two Pacific storms are pulling tropical moisture toward the western United States, with a third weather system near Hawaii adding to the pattern, according to AccuWeather. When that moisture corridor stretches from near Hawaii to the West Coast, meteorologists call it a Pineapple Express, a type of atmospheric river.
The Weather Channel described it as a low-end version of a Pineapple Express but said the moisture could be record-setting for this time of year.
Under the more likely forecast scenario, most major Southern California metro areas would receive less than an inch of rain, according to AccuWeather. Mountain slopes could see up to 3 inches. Heavy downpours could trigger localized flash flooding and debris flows in areas burned by recent wildfires.
Cooler Halloween night
Adam Roser, a meteorologist with the NWS office in San Diego, told the Desert Sun in an Oct. 27 interview that the chance of rain "really starts to increase on Saturday during the day." Areas west of the mountains could see Halloween night temperatures dip into the 50s, Roser said.
The NWS cautioned that exact timing and rainfall totals remain uncertain, according to Patch. Some projections suggest the heaviest rain could arrive later than expected.
Rain is expected to taper off Sunday, Nov. 1, with drier and warmer weather returning by Monday.
