SLO County weather forecast for the week of Aug. 7: Clear afternoon skies along the coast

A 570-decameter upper-level low-pressure system centered about 500 miles off the Northern California coastline combined with a thermal low over the Central Valley will produce strong to gale-force (25 to 38 mph) northwesterly winds during the afternoon along the coastline Sunday into Monday. This onshore flow will produce mostly clear afternoons and mild temperatures with night and morning marine low clouds with pockets of fog and mist.

This weekend’s high temperatures will range from the high-70s to the low-80s in the coastal valleys (San Luis Obispo) and the mid-90s in the inland valleys (paso Robles). The beaches will remain in the low to mid-60s, except the southerly facing beaches of Cayucos, Avila Beach and Shell Beach which will reach the mid to high-70s. Overall, these temperatures will continue through the end of the week, with no extreme heat anticipated.

The afternoon northwesterly winds will decrease to fresh to strong (19 to 31 mph) levels on Wednesday into Friday; consequently, the marine layer will be more persistent (Fogust) along the immediate coastline. The marine low clouds will spread inland during the night and morning with areas of fog and pockets of mist, clearing by the afternoon.

Monsoonal moisture will continue the stream over the Central Coast Central with higher relative humidity levels and virga — visible streaks of rain that fall from a cloud but evaporate before reaching the ground. This condition will bring rain and thunderstorms to the Tehachapi and Sierra Nevada Mountains.

Surf report

Sunday’s 5- to 7-foot northwesterly (300-degree, deep-water) sea and swell (with a 5- to 8-second period) will remain at this level into Monday.

A 4- to 6-foot northwesterly (300-degree, deep-water) sea and swell (with a 5- to 11-second period) will develop on Tuesday, decreasing to 3- to 5-feet with the same period on Wednesday through Friday.

Seawater temperatures will range between 57 to 61 degrees through Friday.

This day in weather history, August 7

1904 - A flash flood near Pueblo, CO, washed a train from the tracks killing 89 passengers. A bridge, weakened by the floodwaters sweeping through the valley below, gave way under the weight of the train dashing all but the sleeping cars into the torrent drowning the occupants. Rail service was frequently interrupted in the Rocky Mountain Region and southwestern U.S. that summer due to numerous heavy downpours which washed out the railroad beds delaying trains as much as five days. (David Ludlum)

1918 - Philadelphia, PA, established an all-time record with a high of 106 degrees. New York City experienced its warmest day and night with a low of 82 degrees and a high of 102 degrees. Afternoon highs of 108 degrees at Flemington NJ and Somerville NJ established state records for the month of August. (Sandra and TI Richard Sanders - 1987)

Note: Many of these high temperature records were broken in 2014.

2018 - Shifting wind directions from the northeast to the northwest this morning produced dramatic temperature swings at the PG&E Diablo Canyon Meteorological Tower.

At 4:45 a.m. the air temperature climbed to 85.6 degrees, before lowering to 58.8 degrees by 5:30 a.m. The air temperature shot up to 84.0 degrees at 6:00 a.m., since that time, the air temperature has decreased to 54.7 degrees at 11 a.m.

This week’s temperatures

LOWS AND HIGHS, PASO ROBLES

LOWS AND HIGHS, SAN LUIS OBISPO AND COASTAL VALLEYS

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