As we move into fall, the days get shorter and shorter, with less sunlight and cooler temperatures.
On a chronological basis during fall, an area of high pressure builds at the surface over the Great Basin — the area between the Sierra Nevada range to the west and the Rocky Mountains to the east — and begins to dominate our coastal weather.
This condition usually produces northeasterly (offshore) winds, especially during the night and morning hours.
In other words, the winds flow from the land out to the Pacific.
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These winds bring the relatively dry air to our shoreline, pushing the marine layer far out to sea, which produces sparkling clear visibility along our beaches.
During the fall, coastal temperatures can actually exceed those in the interior.
In the northern part of our county, the cooler temperatures have already caused some leaves to change colors.
Another sure sign of fall is the increase in the number of people walking around in coats during late-night and morning hours.
Just the other day, someone mentioned to me that it sure feels more like fall even though the temperatures remain in the 70s.
He said that the air felt different, dryer and more like what he would experience in the Sierra Mountains than a summer along the Central Coast, even though the temperatures were about the same.
He’s right! The air is dryer because of the lower humidity levels. Humidity is the amount of water vapor in the air.
The “relative humidity” is the amount of water vapor the air could hold at a particular temperature.
Warm air can hold more water vapor than cold. When air with a given amount of water vapor cools, its relative humidity goes up; when the air is warmed, its relative humidity drops.
When the relative humidity reaches 100 percent, it has reached its dew point. This phenomena causes clouds, fog, rain, snow or some other type of precipitation to develop.
When the air mass moves in an offshore direction, it descends from the Santa Lucia Mountains and then warms from compressional heating.
As a result, the relative humidity drops. This lower humidity level makes it feel dry and crisp, more like fall than the heavier air of summer.
This week’s forecast
Marine low clouds and a few areas of fog will burn off from the interior and coastal valleys later this morning, giving way to hazy sunshine this afternoon.
The marine overcast will be slow to clear from the coast today.
Today’s high temperatures will range from the mid- to high 70s in the interior, while the coastal valleys will mostly be in the high 60s.
Temperatures along the coastline will range from the high 50s to the mid-60s.
Coastal low clouds will move back inland tonight through Monday morning, burning off by Monday afternoon.
The last time that we received over an inch of rain in October occurred on Oct. 19, 2004, when a cold front passed over San Luis Obispo county.
The Diablo Canyon Ocean Lab rain gauge recorded 1.5 inches of precipitation in just six hours.
That rain season produced 35.71 inches of rain.
The storm system approaching California is displaying many of the same characteristics of the October 2004 system.
This system will give increasing southerly winds and clouds Monday.
The southerly winds will build to moderate gale to fresh gale (32- to 46-mph) levels along the coast on Tuesday.
Rain will increase through the day on Tuesday, becoming moderate to heavy Tuesday evening into Tuesday night as a vigorous cold front passes over San Luis Obispo County.
The rain will taper off through the day Wednesday with clearing across most locations by Wednesday night.
Estimated rain totals will range between 1 and 2 inches along the coast and in the coastal valleys while the coastal mountains should range between 2 to 3 inches.
At this time, Thursday looks dry with fair and warmer weather expected next weekend.
Surf and sea report
This morning’s 2- to 3-foot northwesterly (315-degree deep-water) swell (with a 7- to 11-second period) will continue at this height and period through tonight.
A 2- to 4-foot west-northwesterly (285-degree deep-water) swell (with a 16- to 18-second period) will arrive along our coastline on Monday morning, increasing to 3- to 5-feet (with a 15- to 17-second period) on Monday afternoon and night.
Combined with this west-northwesterly swell will be increasing southerly seas late Monday afternoon and night.
Increasing southerly (prefrontal) winds will generate 6- to 8-foot southerly (185-degree shallow-water) seas on Tuesday morning, increasing to 11- to 13-feet (with a 4- to 8-second period) on Tuesday afternoon and will continue at this height through Wednesday morning.
These southerly seas will be followed by a 9- to 11-foot westerly (270-degree deep-water) swell (with an 11- to 14-second period) on Wednesday afternoon through Wednesday night.
The westerly (270-degree deep-water) swell will lower to 8- to 10-feet on Thursday, further decreasing to 5- to 7-feet on Friday.
A safety tip
At Pacific Gas and Electric Company, the safety of our customers and crews in the field is priority.
So, if you see a downed power line — whether it was by lightning or other cause, assume it is energized and keep yourself and others away.
Call 911 immediately to report the location of the downed line, and then call PG&E’s 24-Hour Emergency and Customer Service Line at 1-800-743-5002.
John Lindsey is a media relations and nuclear communications representative for Pacific Gas and Electric Company. Got a weather question? Email John at email@example.com.