More weather whiplash is on the way to SLO County. It’s about to get a lot cooler

Despite the shorter days of autumn, historically, the warmest month of the year along the beaches and coastal valleys is September and October. In the inland valleys, it is July and August.

This pattern has proven true so far this month, but a significant change in the weather pattern with persistent northwesterly onshore winds, the return of the marine layer (natures air conditioning), and cooler temperatures is expected to start on Sunday as an intense storm moves into the Pacific Northwest.

Gentle to moderate (8 to 18 mph) southerly to northwesterly (onshore) winds and low marine clouds will return to the coastal regions on Sunday. High temperatures on Sunday will reach the low 90s in the inland valleys (Paso Robles) and the low 80s in the coastal valleys (San Luis Obispo). The beaches will cool to the 60s.

A major storm and associated cold front will move into the Pacific Northwest Sunday into Monday. This system will produce fresh to strong (19 to 31 mph) northwesterly winds along the Central Coast, a deepening marine layer, and cooler temperatures throughout the region.

The cold front will move through San Luis Obispo County on Tuesday morning, followed by moderate gale-force to fresh gale-force (32 to 46 mph) northwesterly winds along the coast on Tuesday and Wednesday afternoon.

These winds should be strong enough to mix out the temperature inversion layer, leaving behind mostly clear skies but cooler temperatures. High temperatures on Tuesday and Wednesday will only reach the 70s in the inland valleys (Paso Robles) and 60s in the coastal valleys (San Luis Obispo). The beaches will remain in the 50s.

Warmer temperatures and decreasing winds will develop on Thursday through Friday as a ridge of high pressure strengthens over the Central Coast, weakening on Saturday into Sunday as an upper-level low-pressure moves over the region. Marine stratus clouds with areas of fog and mist will develop overnight, clearing during the late morning and afternoon.

The next chance of rain is forecast on Oct. 15.

Surf report

A 3- to 5-foot northwesterly (295-degree, deep-water) swell (with an 8- to 11-second period) is expected along our coastline on Sunday, turning to 3- to 5-foot northwesterly (305-degree, deep-water) sea and swell (with a 4- to 9-second period) on Monday.

Moderate gale-force to fresh gale-force (32 to 46 mph) northwesterly winds along the coastline on Tuesday and Wednesday will generate an 8- to 10-foot (305-degree, deep-water) sea and swell (with a 5- to 1-second period) will along the Central Coast on Tuesday through Wednesday.

A 5- to 7-foot northwesterly (295-degree, deep-water) swell (with a 7- to 11-second period) is expected along our coastline Thursday, decreasing to 4- to 6-feet on Friday and 3- to 5-feet by Saturday.

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.

This story was originally published October 8, 2023 at 5:00 AM.