SLO County weather: Cool-down on the way as highs fall into the 70s. Here’s what’s ahead

So far this September, the San Luis Obispo County Airport daytime high temperatures have averaged 80 degrees, which is normal at this time of the year.

The overnight low temperatures are running 5 degrees warmer than usual at nearly 60 degrees, primarily due to the abundant amount of subtropical moisture that has been streaming over our area and acting like a blanket during the night.

The Paso Robles Airport has seen slightly cooler daytime highs than typical at 86.5 degrees; usually, this location sees a high of 90 degrees. Similar to San Luis Obispo, Paso Robles has seen the average overnight low running about four degrees warmer than usual.

An upper-level low-pressure system off the Central California coastline will continue to produce gentle to moderate (8 to 18 mph) northwesterly (onshore) winds, extensive night and morning marine low clouds with pockets of fog and mist, clearing back to the ocean during the afternoon. High temperatures on Sunday through Wednesday will reach the low 80s in the inland valleys (Paso Robles) and low to mid-70s in the coastal valleys (San Luis Obispo). The beaches will remain in the 60s.

A weak cold front will move through San Luis Obispo County on Wednesday night into Thursday morning, with drizzle in the coastal regions and cooler conditions. In fact, the inland valleys will need help to reach the high 70s, while the coastal valleys will only reach the high 60s. Due to warmer-than-average seawater temperatures, the beaches will remain in the 60s on Thursday into Friday.

Increasing northwesterly winds on Saturday through next Sunday will produce a more significant amount of clearing and slightly warmer temperatures.

There is still no indication of any significant Santa Lucia (northeasterly/offshore) wind events developing through the end of September.

Surf report

A 3- to 5-foot northwesterly (300-degree, deep-water) swell (with an 8- to 11-second period) is expected along our coastline on Sunday through Monday, increasing to 4 to 6 feet (with an 8- to 15-second period) Tuesday into Thursday. This northwesterly swell will lower to 3 to 5 feet (with an 8- to 12-second period) Friday. Increasing northwesterly winds on Saturday through next Sunday will generate increasing northwesterly sea and swell.

Combined with this northwesterly swell will be a 1- to 2-foot Southern Hemisphere (200-degree, deep-water) swell (with a 13- to 15-second period) Tuesday through Friday.

Seawater temperatures will range between 58 and 60 degrees through Sunday, increasing to 61 to 63 degrees on Monday through Friday.

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.