SLO County weather forecast for the week of July 18: Chance of thunder and lightning

An upper-level high-pressure system over the desert southwest combined with a thermal trough over California’s great Central Valley will produce a classic late-July Central Coast weather pattern with low clouds and mild temperatures in the coastal regions and warm conditions in the North County.

A pattern of moderate to fresh (13 to 24 mph) northwesterly winds developing during the afternoon, decreasing during the night and morning, will continue through Monday. This onshore flow combined with the strong upper-level high will create a shallow but persistent marine layer with pockets of fog and drizzle during the night and morning along the coastline.

Monsoonal moisture is expected to stream into the Central Coast on Sunday into Monday with variable amounts of high-level clouds as the upper-level high-pressure system retrogrades back toward the West Coast. This moisture, coupled with some instability, will lead to an increased chance of thunderstorm activity over the Sierra Nevada and Tehachapi mountains.

While the bulk of activity is favored to remain confined to the Sierra Nevada and the surrounding foothills, there is a slight chance of thunderstorm development along the Central Coast. Rain is possible with any storm that develops, but minimal accumulation is currently forecasted.

Unfortunately, these storms could produce dry lightning. This will increase fire potential in areas that these storms move through; the forecast will be monitored closely for any changes.

Sunday’s high temperatures will range from the mid-90s in the inland valleys (Paso Robles) and the high 70s in the coastal valleys (San Luis Obispo). The beaches will remain in the 60s.

Fresh to strong (19 to 31 mph) northwesterly winds along the coastline will develop on Tuesday and continue through the rest of the week. These winds will produce mostly clear skies during the afternoon and warmer temperatures along the beaches, especially in Cayucas, Avila Beach, and Shell Beach.

The inland valleys will reach the low 80s, while the inland valleys and the low 100s. Later this week, the inland and coastal valleys will gradually cool as high-pressure decreases.

The long-range models show fair and dry conditions persisting into the following week, with no indications of precipitation.

Surf report

A 3- to 5-foot northwesterly (295-degree, deep-water) swell (with an 8- to 11-second period) will develop along our coastline on Sunday and continue through Monday.

Increasing northwesterly winds will generate a 5- to 7-foot northwesterly (315-degree, deep-water) sea and swell (with a 5- to 9-second period) on Tuesday through Saturday.

Seawater temperatures will range between 55- and 58-degrees through Monday, decreasing to 53 to 56 degrees on Tuesday through Saturday.

This week’s temperatures

LOWS AND HIGHS, PASO ROBLES

LOWS AND HIGHS, SAN LUIS OBISPO AND COASTAL VALLEYS

PG&E safety tip

“When thunder roars, go indoors!” Find a safe, enclosed shelter when you hear thunder.

Safe shelters include homes, offices, shopping centers, and hard-top vehicles with the windows rolled up.

If you are caught in an open area, act quickly to find adequate shelter.

John Lindsey’s is PG&E’s Diablo Canyon marine meteorologist and a media relations representative. Email him at pgeweather@pge.com or follow him on Twitter @PGE_John.