SLO County beaches could get a bit chilly this week — but summer weather won’t be far away

Gale-force northwesterly winds last week produced clear skies throughout the Central Coast with seasonal temperatures.

However, along the coastline, these northwesterly winds triggered substantial upwelling.

The surface seawater temperature at the Diablo Canyon Waverider Buoy dropped as low as 50.2 degrees Fahrenheit last Wednesday and 50.7 on Thursday, the first day of summer. These frigid seawater temperatures combined with the onshore flow kept high temperatures in the 50s along many beaches despite the sunny skies.

For the first part of this week, moderate northwesterly winds, increasing high pressure over the Four Corners Region, and cold seawater temperatures will produce a persistent marine layer along the beaches and large temperature gradients between the coast and inland areas.

The marine layer will develop along the coastline and surge into the coastal valleys during the night and morning with areas of fog and pockets of drizzle through Tuesday as a dry cold front will move slowly southward through Central California. Consequently, many beaches will remain primarily overcast, with highs only in the 50s throughout the day.

Away from the ocean, temperatures will warm to the low to mid-80s in the coastal valleys (San Luis Obispo) and high 90s and perhaps triple-digit levels in the inland valleys (Paso Robles). The Central Valley of California will reach the 100s throughout this week.

Strong to gale-force (25 to 38 mph) northwesterly winds are forecast along the coastline on Wednesday through Friday. These winds will mix out the temperature inversion layer, leaving clear skies throughout San Luis Obispo and Santa Barbara counties.

Over this period, high temperatures will reach the high 90s in the inland valleys (Paso Robles), high 70s in the coastal valleys (San Luis Obispo), and 50s along the beaches, except for Cayucos, Avila Beach and Shell Beach, which will reach the mid-70s.

Decreasing northwesterly winds, a deepening marine layer, and cooler temperatures are forecast for the following weekend.

Surf report

A 3- to 5-foot northwesterly swell (with an 8- to 11-second period) is forecast along our coastline on Saturday through Monday, increasing to 4- to 6-feet with the same period on Tuesday.

Strong to gale-force (25 to 38 mph) northwesterly winds along the Northern and Central California coastline will generate a 7- to 9-foot sea and swell (with a 5- to 9-second period) on Wednesday through Friday.

Combined with these northwesterly seas, will be 1- to 2-foot southern hemisphere swell (with a 15- to 17-second period) Sunday through Wednesday.

Surface seawater temperatures will range between the low to mid-50s through Tuesday, decreasing to the low 50s on Wednesday through Friday.

On this date in weather history (June 23):

1902: The temperature at Volcano Springs, CA, soared to 129 degrees to set a June record for the U.S. (Sandra and TI Richard Sanders)

1987: A massive hailstorm hit eastern Colorado, causing an estimated $60-70 million damage. At La Junta, Colorado, hail as large as softballs caused $37 million damage. (Storm Data)

2008: The Eastern Pacific High returned to its normal position off the California coast; consequently, the northwesterly winds increased, and the marine layer returned. Temperatures were between 25- and 35-degrees cooler in the coastal regions than the previous day’s highs.

2021: A strong northerly flowing ocean current combined with the lack of upwelling brought a warmer body of seawater to the California coastline. A week prior, the sea surface temperature was 51 degrees at Diablo Canyon, but on this day, it was 60 degrees.

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 X @PGE_John.