SLO County forecast calls for foggy beaches, warm days — but a clear Fourth of July

A series of late-season storms will move through the Pacific Northwest this weekend into Monday, bringing widespread rain to the region.

While the Central Coast will remain dry, these systems will deepen the marine layer and usher in well below seasonal temperatures through Thursday. A warming trend is expected to begin Friday and continue into the Fourth of July weekend.

A late-season, upper-level low-pressure system (555 decameters) centered over the Pacific Northwest will send a series of cold fronts southward, producing rainfall as far south as Mendocino County through Monday.

Meanwhile, upper-level northwesterly winds will transport a cool air mass from the Gulf of Alaska into California, keeping temperatures across the Central Coast well below normal through Thursday.

The inland valleys, including Paso Robles, will see afternoon highs mainly in the mid-to-upper 70s. Coastal valleys, such as San Luis Obispo, will remain in the mid-to-upper 60s, while most beaches will stay in the low 60s. The exception will be Cayucos, Avila Beach and Shell Beach, where highs will reach the upper 60s.

Marine stratus clouds will continue to develop each night and morning, bringing areas of fog, mist and occasional drizzle. Fresh-to-strong (19 to 31 mph) northwesterly winds during the afternoon will help erode the marine layer, allowing skies to become mostly sunny along the beaches from late morning into the afternoon through Monday.

One fly in the ointment is the possibility of localized Santa Lucia (northeasterly/offshore) winds developing Sunday morning. If these winds materialize, they could suppress marine cloud formation in coastal areas through much of the day.

From Tuesday into Wednesday morning, a weak cold front will move across the Central Coast. Gentle to moderate (8 to 18 mph) westerly winds will accompany a deeper and more persistent marine layer, with little afternoon clearing expected along the beaches.

In other words, we’ll transition from “June Gloom” to “No Sky July.”

By Wednesday afternoon, fresh to strong (19 to 31 mph) northwesterly winds will redevelop and persist through next Sunday.

These stronger winds should once again scour out the marine layer, allowing for increasing sunshine along much of the coastline during the late morning and afternoon.

At the same time, high pressure will begin strengthening over California, bringing a significant warming trend beginning Friday and continuing into the following week.

By next Monday, July 6, inland valleys, including Paso Robles, are expected to reach triple-digits temperatures.

Coastal valleys, such as San Luis Obispo, will warm into the low-to-mid 80s, while northwesterly facing beaches will remain comfortably cool in the mid-60s. Southerly facing beaches, including Cayucos, Avila Beach and Shell Beach, should warm into the mid-to-upper 70s.

Looking ahead to the Fourth of July weekend, a plume of subtropical moisture is expected to bring periods of variable high cloudiness.

These clouds may help weaken the marine layer, while persistent fresh-to-strong northwesterly winds should keep coastal fog from redeveloping during the evening. If this pattern holds, viewing conditions for Independence Day fireworks in Cayucos and Pismo Beach should be quite favorable.

Surf report

Gale-force northwesterly winds off the Northern and Central California coastline will generate a 6- to 8-foot northwesterly (300-degree, deep water) sea and swell (with a 5-to 11-second period) this Saturday through next Saturday, decreasing to 3- to 5-feet (with an 8- to 14-second period) next Sunday and Monday.

A 2- to 3-foot Southern Hemisphere (210-degree, deep water) swell (with a 15- to 17-second period) is forecast along our coastline this weekend, decreasing on Monday into Tuesday.

Another 2- to 3-foot Southern Hemisphere (215-degree, deep water) swell (with a 15- to 18-second period) is forecast along our coastline on Thursday and will remain at this level through next Sunday.

Seawater temperatures along the immediate coastline will range between 56 and 59 degrees through next Saturday.

On this date in weather history

1975: Lee Trevino and two other golfers were struck by lightning at the Western Open golf tournament in Oak Brook, Illinois.

1980: The temperature at Wichita Falls, Texas, soared to 117 degrees, the hottest reading of record. Daily highs were 110 degrees or above between June 24 and July 3.

2016: The famous microclimates of the Central Coast were certainly on display on June 28. In Paso Robles, the mercury soared to 109 degrees at 5 p.m., nearly smashing a record of 110 degrees set in 1956. At the same time, the temperature at the Diablo Canyon nuclear power plant was just 55 degrees. That’s a 54-degree temperature differential between the Pecho Coast and Paso Robles. PG&E’s Energy Education Center in the Avila Valley reached 105 degrees. Elsewhere in the county, Humbug Vineyards in Creston recorded a whopping 111-degree high, while nearby Windrose Farm experienced 109-degree heat. Atascadero had a high of 108, while the Lopez Lake Ranger Station in Arroyo Grande had a high of 107. Suey Creek Road reached 112 degrees.

2021: Lytton in British Colombia reached 116 degrees, the hottest temperature ever recorded in Canada. Meanwhile, the Portland Airport in Oregon broke its all-time temperature record with a high of 108 degrees. That new all-time record survived all of 20 hours before being eclipsed by the 2 p.m. observation of 109 degrees. Portland went on to peak at 112 degrees with an overnight low of 76 degrees.

Closer to home, a remarkable 51-degree temperature gradient developed between the coast and the inland valleys. The Paso Robles Municipal Airport hit 108 degrees at 2:10 p.m., and at the same time, the Diablo Canyon nuclear power plant reported 57 degrees as gale-force northwesterly winds brought in a cool air mass from the Pacific Ocean.

2022: The Paso Robles Municipal Airport reached 109 degrees. The record for June 28 is 110 degrees set in 1956. The high at the Diablo Canyon nuclear power plant was 57 degrees, a 52-degree temperature gradient between the coast and the inland valleys.

This week’s temperatures

LOWS AND HIGHS, PASO ROBLES

SUNDAY: 45, 79

MONDAY: 50, 78

TUESDAY: 49, 75

WEDNESDAY: 47, 73

THURSDAY: 48, 77

FRIDAY: 51, 85

SATURDAY: 58, 93

SUNDAY: 61, 94

LOWS AND HIGHS, SAN LUIS OBISPO AND COASTAL VALLEYS

SUNDAY: 50, 68

MONDAY: 52, 68

TUESDAY: 51, 66

WEDNESDAY: 51, 67

THURSDAY: 51, 68

FRIDAY: 52, 74

SATURDAY: 55, 79

SUNDAY: 56, 80

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