Hurricane Hilary could bring rain, thunderstorms to SLO County as it hits California

Hurricane Hilary will move northward along the Baja California coastline and weaken to a Tropical Storm as it makes landfall near San Diego on Sunday evening; the last Tropical Storm to reach the Southern California Bight occurred in September 1939.

In 1858, a hurricane tracked from San Diego to Long Beach, the only known hurricane to reach California.

This Tropical Storm Hilary will quickly decrease in strength, becoming a tropical depression as it moves northward through Nevada. This storm will produce thunderstorms and heavy rain throughout Southern California and Nevada.

Along the Central Coast, NOAA’s Global Forecast System (GFS) weather model indicates thunderstorms and rain starting Sunday night and continuing through Monday morning, turning to scattered showers on Monday afternoon through Tuesday morning.

Remember, thunderstorms can produce lightning, strong and gusty winds and hail. If lightning roars, go indoors; there is no safe place outside.

The GFS model is advertising between 0.50 and 1.75 inches of rain for Northern Santa Barbara and San Luis Obispo counties, with the heaviest rainfall in the eastern regions. In fact, the far eastern regions could see as much as 4 inches.

However, the European Centre for Medium-Range Weather Forecasts (ECMWF) model keeps most of the rain to the east of San Luis Obispo County, leaving our area mostly dry.

Nevertheless, this system will create muggy conditions later Sunday into Tuesday and gusty easterly winds on Monday, with warm overnight lows in the high 60s throughout the Central Coast.

During the day, the inland valleys (Paso Robles) will reach the low 90s on Sunday, cooling to the low to mid-80s on Monday and Tuesday, while the coastal valleys (San Luis Obispo) will reach the mid-80s on Sunday, decreasing to the mid- to high 70s on Monday into Tuesday. The beaches will warm up to the 70s over this period.

Gentle to moderate (8 to 18 mph) northwesterly winds will allow the marine layer with pockets of fog and mist to develop along the coastline, moving inland during the night and morning Wednesday into Thursday. This condition will produce much cooler overnight low temperatures.

A significant change in the weather pattern is expected on Friday into next Sunday as strong to gale-force (25 to 38 mph) afternoon northwesterly winds develop along the coastline, producing sunshine along the beaches and mild temperatures across the Central Coast.

Hurricane Hilary is part of a seasonal pattern called the North American monsoon.

If the subtropical moisture source is plentiful — especially when remnants of hurricanes and tropical storms from the eastern Pacific Ocean and the Gulf of Mexico move over our area — periods of rain and thunderstorms can interrupt our Mediterranean climate.

On July 18 to 20, 2015, abundant subtropical moisture from the remnants of Hurricane Dolores produced record amounts of warm July rainfall and lightning. A weather station in Paso Robles reported 3.55 inches, while numerous weather stations around San Luis Obispo reported around an inch on July 20, 2015.

All that rain nearly delayed the start of the Mid-State Fair due to localized flooding.

August 1976 was the wettest August on record.

Two subtropical weather systems produced nearly 1 inch of rain in Santa Maria, while San Luis Obispo saw nearly 1.5 inches between Aug. 15 and Aug. 20.

Later in September, Hurricane Kathleen developed in the eastern Pacific and took an unusual path northward through Baja California. It crossed the U.S.-Mexico border near El Centro, east of San Diego, as a tropical depression.

Kathleen produced gale-force southerly winds and widespread flooding in many parts of the west, especially in California’s Imperial Valley. Subtropical moisture from Kathleen produced 1.74 inches in Santa Maria and 1.72 inches of rain in San Luis Obispo on Sept. 10 and 11.

Surf report

A 4- to 6-foot northwesterly (290-degree, deep-water) swell (with an 8- to 12-second period) is expected along our coastline on Sunday and will remain at this level through Tuesday, decreasing to 3- to 5-feet on Wednesday through Thursday.

Strong the gale-force (25 to 38 mph) northwesterly winds will generate a 4- to 6- foot northwesterly (310-degree, deep-water) sea and swell (with a 5- to 8-second period) on Friday, building to 6- to 8-feet next Saturday and Sunday.

Combined with this northwesterly swell will be a 1- to 2-foot Southern Hemisphere (200-degree, deep-water) swell (with a 13- to 15-second period) on Sunday through Thursday. Note: Swell from hurricane Hilary will be coming from a southeasterly direction; consequently, it will not impact the Central Coast.

Decreasing northwesterly winds along California’s coast have produced less upwelling which in turn has allowed the ocean along our coastline to warm up. Seawater temperatures will range between 61 and 64 degrees, decreasing to 57 to 59 degrees on Friday into next Sunday.

This week’s temperatures

LOWS AND HIGHS, PASO ROBLES

LOWS AND HIGHS, SAN LUIS OBISPO AND COASTAL VALLEYS

John Lindsey is a retired PG&E marine meteorologist. Email him at JohnLindseyLosOsos@gmail.com or follow him on Twitter @PGE_John.