While I was leading a walk along the beautiful Bluff Trail at the Cambria Fiscalini Ranch Preserve last weekend, the winds were out of the northeast — or offshore, as surfers so lovingly refer to them.
The winds were blowing from the coastal mountains toward the ocean and produced perfectly clear and sunny weather with temperatures in the mid-70s.
After all the haze gray and dreary months of Central Coast summer, nature is finally giving us some relief.
A large ridge of high pressure from the Desert Southwest this week has parked itself over California, producing night and morning northeasterly winds.
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These offshore winds along with subsidence, or sinking of the air mass over the county, created some of the warmest beach weather of the year.
When the offshore winds blow, temperatures at our beaches often peak during the late morning hours, before the afternoon northwesterly (onshore) winds cool things off.
Historically, wind data recorded at the Diablo Canyon meteorological tower indicates that offshore winds blow about 12 percent of the time throughout the year, but during the fall this percentage can increase to more than 20 percent.
Autumn is often the warmest season along our beaches because of the greater occurrence of northeasterly (offshore) winds.
At times, you will hear forecasters refer to northeasterly winds as downslope winds.
As the air mass descends down the side of the mountain range, it warms at the rate of about 5.5 degrees Fahrenheit per 1,000 feet of descent. By the time the air reaches the beaches, it has warmed up significantly.
Meteorologists refer to this rate of warming as the dry adiabatic lapse rate.
As the air molecules descend into the higher atmospheric pressure toward the Earth’s surface, they gain kinetic energy as they compress inward.
If you ever filled up a bicycle tire, you probably noticed the tire getting warmer as the pressure increased. This is also referred to as compressional heating.
In Southern California, these downslope winds are called the Santa Anas. Along the Rocky Mountains, these downslope winds are known as Chinook – literally, snow eater – winds.
When the air mass descends and warms, the relative humidity drops. This lower humidity level makes it feel dry and crisp, more like autumn than the heavier air of summer.
After this week’s heat wave, I’m sure a lot of us will be welcoming back the large white blanket of the coastal fog monster and the cooler temperatures it brings.
This week’s forecast
North to northeasterly (offshore) winds at the surface combined with a strengthening upper atmosphere high pressure ridge produced warm to hot temperatures throughout San Luis Obispo County on Saturday.
Cal Poly reached 104 degrees, which was a degree short of the 1978 record high for the day set in 1978. Paso Robles topped at 103, which didn’t break a record.
This late September heat wave will continue through Tuesday.
Today’s temperatures will range from the low to high 80s along the beaches, while the coastal valleys (San Luis Obispo) will reach high 90s with pockets of 100-degree readings.
The North County will hit the low 100s today.
This morning’s gentle to moderate (8 to 18 mph) and at times gusty northeasterly (offshore) winds, especially near the coastal passes and canyons, will turn out of the northwest (onshore) near the coast this afternoon as a weak upper-level trough passes over our area.
These onshore winds will give relief to the beaches and coastal valleys by late this afternoon, however, the North County will remain hot.
The upper-level ridge and northeasterly (offshore) winds will strengthen again on Monday, with warm to hot temperatures across San Luis Obispo County ranging from the 80s at the coast to the mid-90s across coastal valleys to the low 100s across the interior. Paso Robles is forecast to reach 105 degrees on Monday and the California Valley 106 degrees.
The upper-level ridge, responsible for this heat wave, will move slightly eastward Tuesday, bringing some relief to the coast and slight cooling across the interior.
As the ridge moves further east Wednesday, further cooling is expected with the return of the marine layer and fog.
A weak upper-level trough will cross California next weekend, with temperatures near normal for early October along with widespread low clouds along the coast. Still no rain in sight.
Surf and sea report
A series of intense storms in the Gulf of Alaska will produce a 4- to 6-foot northwesterly (290-degree deep-water) swell (with a 17- to 19-second period) late today, increasing to 8 to 10 feet (with a 15- to 17-second period) on Monday.
This northwesterly (290-degree deep-water) swell will decrease to 6 to 8 feet (with a 14- to 18-second period) on Tuesday, then increasing to 7 to 9 feet (with a 14- to 16-second period) on Wednesday. This swell will decrease to 5 to 7 feet on Thursday.
Typhoon Malakas, currently off the east coast of Japan, will move northeastward toward the northern Pacific. As the storm moves into the northern Pacific, it is forecast to become extratropical.
West-northwesterly (285-degree deep-water) swell from the system will arrive on our coastline at 3 to 4 feet (with a 22- to 25-second period) Thursday night and build to 5 to 7 feet (with an 18- to 20-second period) by Friday.
This swell will further increase to 6 to 8 feet on with a shorter period on Saturday.
Arriving from the Southern Hemisphere:
A 1- to 2-foot Southern Hemisphere (225-degree deep-water) swell (with an 18- to 20-second period) is forecast today through Monday. Another low-height, long-period Southern Hemisphere swell is expected on Thursday and Friday.
Seawater temperatures will range between 53 and 56 degrees through Monday, increasing to 54 and 57 degrees on Tuesday through Friday.
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John Lindsey is a media relations representative for PG&E. He is also a local weather expert and has lived along the Central Coast for more than 23 years. To subscribe to his daily forecast or ask a question, send him an e-mail at firstname.lastname@example.org.