SLO weather forecast for the week of Dec. 30: First a little rain, then fog

A storm system reaching the Central Coast Sunday night will spread rain showers, gusty southerly winds and mountain snow into Monday morning. At this time between 0.50 and 1.75 inches of precipitation is expected with snow levels at 4,000 feet.

A large 1,035-millibar Eastern Pacific High will develop off the California coast and will close the storm door for one or two weeks, producing dry weather into the first week of 2020.

Mostly clear skies and warmer weather will develop over that time; however, dense ground fog will continue to develop in the wind-sheltered valleys during the night and morning.

So why the fog? When the winds are light and air temperature decreases, relative-humidity levels rise, and the valleys begin to take on a grayish tint.

Light winds of less than 5 mph promote the formation of radiation fog by bringing more moist air into direct contact with the cold, wet ground. When the air reaches its dew-point temperature, the relative humidity is at 100 percent and water droplets become visible to the naked eye.

Surf report

A 9- to 11-foot northwesterly (310-degree, deep-water) sea and swell (with a 7- to 17-second period) is forecast along our coastline on Monday into Tuesday morning.

A 949-millibar storm with hurricane-force winds that developed near the International Date Line will generate a very long-period west-northwesterly (290-degree, deep-water) swell that will arrive along our coastline on Tuesday afternoon at 7 to 9 feet (with a 25- to 28-second period), increasing to 9 to 11 feet (with a 20- to 22-second period) on Wednesday into Thursday.

This week’s temperatures

LOWS AND HIGHS, PASO ROBLES

LOWS AND HIGHS, SAN LUIS OBISPO AND COASTAL VALLEYS

PG&E safety tip

According to the California Highway Patrol, here are some tips for driving in foggy conditions:

• Drive with lights on low beam. High beams will reflect off the fog, creating a “white wall” effect.
• Reduce your speed and watch your speedometer. Fog creates a visual illusion of slow motion when you might be speeding.
• Avoid crossing traffic lanes.
• Travel with the driver’s window partially open. Listen for traffic.
• Watch for CHP pace cars to guide you.
• If your vehicle is disabled or you can’t continue, pull well onto the shoulder and turn off lights. Move away from your car. Consider postponing your trip until the fog lifts.

This story was originally published December 30, 2019 at 5:00 AM.