SLO County weather forecast: expect rising temperatures inland, mild on the coast

The North County will transition from well below seasonal temperatures during the first part of the week, to triple-digit levels during the latter half, while the temperatures in the coastal regions will remain near normal.

A 1,007 millibar low-pressure system over Northern California will produce moderate to fresh (13 to 24 mph) afternoon northwesterly winds, a deep and persistent marine layer with night and morning mist and drizzle (Fogust) in the coastal regions and below normal seasonal temperatures throughout the Central Coast on Sunday through Tuesday.

Sunday’s high temperatures will range between the low to mid--60s along the beaches. The coastal valleys (San Luis Obispo) will range between the high-60s and low-70s, while low-80s are expected in the inland valleys (Paso Robles).

A ridge of high-pressure over California will produce fresh to strong (19 to 31 mph) northwesterly winds during the afternoon and a thinner marine layer with a greater amount of clearing during the late morning and afternoon along the coastline on Wednesday through Thursday. It will also produce warmer temperatures, especially away from the ocean. In fact, Paso Robles is expected to reach triple digit temperatures on Wednesday and Thursday.

Strong to gale-force (25 to 38 mph) northwesterly winds our forecast along the coastline on Friday into Saturday. These (onshore) winds will keep the coastal regions cooler along the San Luis Obispo County coastline; however, the inland valleys will remain warm.

For the following week (September 4th through the 9th) the long-range numerical models, have been advertising the return of monsoonal moisture to the Central Coast. That phenomenon is part of a seasonal pattern called the North American monsoon. When it occurs, we often experience higher relative humidity levels and plenty of virga — visible streaks of rain that fall from a cloud but evaporate before reaching the ground. However, scattered rain showers, and a chance of thunderstorms are expected.

This day in weather history:

1959 - Lieutenant Colonel William Rankin bailed out of his plane at a height of 46,000 feet into a violent thunderstorm and lived to write about the 45-minute journey (which normally would have been a thirteen-minute descent). He described it as one of the most bizarre and painful experiences imaginable.

2009 - The temperature at San Luis Obispo County Airport rocketed up to 104 degrees at noon. The old record at the airport was 103 degrees back on August 28, 1962. The record for the month of August at Cal Poly was 105 degrees also set on August 28, 1962. Temperatures at the PG&E Energy Education Center was 84 degrees due to southerly winds coming off the Pacific and Diablo Canyon only reached 67 degrees.

2017 - The Paso Robles Airport recorded a high of 110 degrees on Sunday, surpassing the previous record of 106 degrees in 2001. Diablo Canyon’s high temperature only reached 58 degrees, a 52-degree temperature gradient between the coast and the inland valleys.

Surf report

Sunday’s 3- to 5-foot northwesterly (290-degree, deep-water) swell (with an 8- to 11-second period) will decrease to 2- to 4-feet with the same period through Tuesday.

Fresh to strong (19 to 31 mph) afternoon northwesterly winds will generate a 4- to 6-foot northwesterly (300-degree, deep-water) sea and swell (with a 5- to 8-second period) on Wednesday, building to 5- to 7-feet on Thursday. This northwesterly sea and swell is forecast to peak on Friday into Saturday at 6- to 8-feet (with a 5- to 12-second period).

A 1- to 3-foot Southern Hemisphere (210-degree, deep-water) swell (with a 15- to 17-second period) is expected along our coastline on Sunday into Tuesday.

Seawater temperatures will range between 56 to 60 degrees through Tuesday, gradually decreasing on Wednesday through next Saturday.

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.