What’s the forecast for last week of summer in SLO County? Cool temps, maybe even some rain

The last full week of summer along the Central Coast will see well below-seasonal temperatures, with the inland valleys only reaching the 70s.

There is even a chance of a few rain showers Monday into Wednesday as an early-season upper-level low-pressure system moves southward along the California coastline.

Gentle to moderate (8 to 18 mph) northwesterly winds on Sunday will allow low marine clouds to surge inland, producing overcast skies overnight with pockets of mist and drizzle.

By the afternoon, these stratus clouds will clear from the inland valleys (Paso Robles) in the morning and coastal valleys (San Luis Obispo). Many of the beaches will remain partly to mostly cloudy through the day.

The inland valleys’ temperatures will reach the high 70s on Sunday, while the coastal valleys (San Luis Obispo) will range between the mid-60s. The beaches will vary between the high 50s and the low 60s.

A 555-decameter upper-level low-pressure system will wobble southward along the Northern and Central California coastline Monday into Wednesday with fresh to strong (19 to 31 mph) northwesterly winds and partly cloudy and, at times, mostly cloudy skies with a chance of widely scattered rain showers. Rain showers, fog, mist and drizzle may develop overnight.

If your location receives precipitation, rainfall amounts will remain below a tenth of an inch.

This system will create well-below seasonal temperatures, with the inland valleys only reaching the low 70s and mid-60s in the coastal valleys, with the beaches ranging between the high 50s and low 60s.

Fresh to strong (19 to 31 mph) northwesterly winds, low marine clouds in the morning and evening, and slightly warmer temperatures will develop on Thursday through the following Saturday.

Fall rolls in next Sunday, Sept. 22, at 5:44 a.m., which is the autumnal equinox.

Surf report

A 3- to 5-foot northwesterly (295-degree deep-water) swell (with an 8- to 11-second period) is forecast along our coastline through Sunday.

Fresh to strong (19 to 31 mph) northwesterly winds along the California coastline will generate a 4- to 6-foot northwesterly (305-degree deep-water) sea and swell (with a 4- to 9-second period) on Monday, increasing to 6- to 8-feet with the same period on Tuesday into Wednesday.

This northwesterly sea and swell will lower to 4- to 6-feet on Thursday through Saturday.

Combined with this northwesterly sea and swell will be a 1- to 2-foot southern hemisphere (220-degree deep-water) swell (with a 15- to 17-second period) Tuesday into Thursday.

Surface seawater temperatures will range between 51 and 55 degrees through Saturday.

Weather watches and warnings

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One this date in weather history (Sept. 15)

1967: A surprise snow and ice coating paralyzed Boston during the evening rush hour, according to historian and meteorologist David Ludlum.

1987: Thunderstorms spawned 22 tornadoes in eastern Texas, Louisiana and Oklahoma. Severe thunderstorms spawned 18 tornadoes in Mississippi and seven in Georgia the next day. Thunderstorms in southeastern Texas produced wind gusts up to 102 mph at Galveston and gusts up to 110 mph at Bay City. There was a total of 49 tornadoes in the south central U.S. in two days.

2013: The maximum temperature reached 81 degrees at Los Osos and Diablo Canyon due to the Santa Lucia winds. The high temperatures the day before at both of these locations only reached 58 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 Twitter @PGE_John.