SLO County weather: Coastal fog, warmer temperatures later this week

The northwesterly (onshore) winds will continue to bring marine clouds and cooler temperatures to the coastal regions and inland valleys through the 4th of July weekend into Thursday.

A pattern of strong to gale-force (25 to 38 mph) afternoon northwesterly winds, decreasing during the night and morning will continue into Thursday.

This condition will allow the marine layer to develop along the coastline and surge into the coastal and inland valleys during the night and morning with areas of fog and pockets of drizzle. However, these winds will produce mostly clear skies during the afternoon over the 4th of July weekend.

High temperatures will range from the mid-80s in the inland valleys (Paso Robles) and mid-70s in the coastal valleys (San Luis Obispo). The beaches will remain in the 60s, except the southerly facing beaches of Cayucos, Avila Beach and Shell Beach which will reach the 70s.

So many times over the years the cloud ceiling on the Fourth of July was lower than 500 feet along the coastline, and consequently the fireworks display looked like flashes of pink or green fog. On some Fourth of July nights the fog would ebb and flow, allowing for good viewing one moment, only to be obscured by ghostly mist a short time later. If you forecast, “Marine low clouds and fog during the night and morning, becoming mostly clear during the afternoon,” you would probably be correct most of the time. But there are nuances in forecasting fog along our shoreline. All it takes is a small change in the surface air temperature, the temperature inversion layer or the direction or speed in the wind to make the difference between a beautiful Fourth of July fireworks display versus a light show in the coastal stratus.

For this 4th of July on Monday, it looks like the most of marine low clouds will remain off the coastline until later in the evening, before moving in at night into Tuesday morning.

Increasing high pressures will produce triple-digit temperatures in the inland valleys late this week into the following weekend. In fact, Paso Robles is expected to reach 102 degrees by Friday.

Surf report

Sunday’s 5- to 7-foot northwesterly (300-degree, deep-water) sea and swell (with a 5- to 8-second period) will decrease to 3- to 5-feet (with a 5- to 8-second period) on Monday into Thursday morning. This northwesterly sea and swell will increase to 5- to 7-feet with the same period on Thursday afternoon through Saturday.

Seawater temperatures will range between 52 to 56 degrees through Saturday.

This week’s temperatures

LOWS AND HIGHS, PASO ROBLES

LOWS AND HIGHS, SAN LUIS OBISPO AND COASTAL VALLEYS

John Lindsey’s is a longtime meteorologist who lives in Los Osos. Email him at JohnLindseyLosOsos@gmail.com.