SLO County weather forecast for the week of Sept. 12

Here is this week’s weather forecast by PG&E meteorologist John Lindsey.

An upper-level trough of low pressure is forecast to drop into San Luis Obispo and Santa Barbara counties Monday into Tuesday, producing gentle to moderate (8 to 18 mph) southerly winds, a deep marine layer with extensive coastal low clouds with areas of mist, drizzle and lower temperatures.

In fact, high temperatures may drop as much as 10 degrees below normal by Tuesday, with highs forecast to range between the mid-60s to low 70s in the North County and coastal valleys. The beaches should remain in the high 50s to the low 60s under mostly cloudy skies Tuesday. This system also will produce possible rain showers and isolated thunderstorms in the Sierra Nevada.

Also of note: Moisture and instability will be limited, therefore a major thunderstorm outbreak is not expected. However, given the elevated fire danger in the region, the risk of new wildfire ignitions because of lightning is a concern.

High pressure is forecast to build over California in the wake of the departing upper-level low pressure system Wednesday into Saturday for strong to gale-force (25 to 38 mph) northwesterly winds along the coastline, mostly clear September skies, except for a few areas of overnight marine low clouds along the beaches and coastal valleys, and higher temperatures primarily away from the coast. At this time, rain is not expected through the end of September.

Surf Report

Monday’s 3- to 5-foot northwesterly (310-degree, deep-water) swell (with an 8- to 11-second period) will remain at this height and period through Wednesday morning. Increasing northwesterly winds along the California coastline will generate a 4- to 6-foot northwesterly (320-degree, deep-water) sea and swell (with a 5- to 11-second period) Wednesday afternoon into Friday.

The longer-range models indicate that Hurricane Orlene is developing in the Eastern Pacific. If this tropical cyclone storm develops as advertised, a 3- to 4-foot southerly swell (with a 12- to 14-second period) will arrive along the coastline late Wednesday into Friday.

Monday’s 1- to 2-foot Southern Hemisphere (200-degree, deep-water) swell (with a 12- to 14-second period) will gradually decrease into Tuesday.

Seawater temperatures

Seawater temperatures are projected to range between 55 and 59 degrees through Tuesday, decreasing Wednesday into Saturday.

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This week’s PG&E safety tip: Downed power lines are dangerous. Never touch them. For safety’s sake, always assume that a fallen power line is live. Avoid touching anything, such as a car, objects or equipment, or anyone who is in contact with a fallen power line. Call 911 immediately to report a fallen power line.

This story was originally published September 11, 2016 at 6:49 PM with the headline "SLO County weather forecast for the week of Sept. 12."