Cloudy, muggy weather could bring chance of rain and thunderstorms to SLO County

An upper-level low-pressure system off the Pacific Northwest will continue to produce near-seasonal temperatures throughout the Central Coast through Friday.

But then, a significant amount of monsoon moisture is forecast to move into the desert southwest next weekend (Aug. 24-25).

Some of this subtropical moisture is expected to make its way over the Central Coast with variable mid- to high-level clouds, higher humidity levels, and a chance of rain showers and thunderstorms.

A pattern of gentle to moderate (8 to 18 mph) Santa Lucia (northeasterly) winds developing during the overnight and early morning will keep the marine stratus clouds out to sea. However, as the winds shift out of the northwest later in the morning, the low marine clouds will develop along the shoreline.

As the northwesterly winds build to fresh to strong (25 to 31 mph) levels in the afternoon, they will mix out the temperature inversion layer, leaving behind windy and clear skies. This pattern will continue through Monday.

The inland valleys (Paso Robles) will warm to the mid to high 90s, quickly cooling during the evening. The coastal valleys (San Luis Obispo) will range between the high 70s and low 80s. Temperatures along the beaches will reach the 60s, except for Cayucos, Avila Beach, and Shell Beach, which will get to the low 70s. It appears the hottest day of the week will be Tuesday, with the inland valleys reaching the triple-digit mark and the coastal valleys in the mid-80s.

Strong to gale-force (25 to 31 mph) northwesterly winds will develop along the coastline Tuesday into Thursday. These winds will produce cooler temperatures throughout the Central Coast by Wednesday and keep the marine layer to a minimum, with mostly sunny skies forecast throughout the region.

The northwesterly winds will decrease on Friday, allowing the marine layer with pockets of fog and mist to redevelop in the coastal region and surge inland overnight, clearing to the coastline during the afternoon.

The humid, cloudy weather is expected to reach the Central Coast on Saturday and Sunday.

Surf report

A 3- to 5-foot northwesterly (300-degree deep-water) sea and swell (with a 5- to 8-second period) is forecast along our coastline through Monday, increasing to 5 to 7 feet with the same period on Tuesday into Thursday. This northwesterly sea and swell will lower to 3 to 5 feet (with 5- to 18-second period) on Friday into Saturday.

Combined with this northwesterly swell, will be a 1- to 2-foot southern hemisphere (225-degree deep-water) swell (with a 14- to 16-second period) on Monday through Thursday.

Seawater temperatures

Surface seawater temperatures will range between 53 and 55 degrees through Monday, lowering to 51 to 54 degrees on Tuesday through Friday.

On this date in weather history (August 18)

1788: A small but powerful hurricane inflicted great havoc upon forests along a narrow track from New Jersey to Maine. A similar storm track today would cause extreme disaster in the now populated area. (David Ludlum)

2016: Calm winds allowed varying amounts of smoke from the 8,300-acre Chimney Fire to spread throughout San Luis Obispo County.

2020: Paso Robles Airport reached 111 degrees, breaking the old daily record of 110 degrees set back in 1992. Paso Robles had broken a daily record two days in a roll. The Santa Maria Airport reached 89 degrees, besting the old record of 88 degrees set in 1984. The Santa Maria Airport had seen five days in a roll of record-breaking temperatures. Most California heat waves last around three days, beyond that is highly unusual.

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.