Weather Watch

Fall will soon be here but warm weather will continue

A weather balloon in Paso Robles last year.
A weather balloon in Paso Robles last year.

At 1:22 a.m. Wednesday, the sun will be directly above the Earth’s equator, bringing the end of summer and the launch of fall.

As we move into autumn, the days will continue to get shorter with fewer hours of sunlight, but not necessarily cooler temperatures. In fact, this could be the warmest fall on record, leaving many Central Coast residents to ask: When will cooler weather arrive?

This past summer wasn’t normal. In fact, it was the warmest summer recorded in San Luis Obispo since 1893, when temperature records started. San Luis Obispo usually averages 66.3 degrees during summer, but this year the preliminary temperature data shows the mean at around 70.5 degrees.

The previous warmest summer on record was last year, which averaged 68.1 degrees. Farther away from the coast, Paso Robles averaged 73.5 degrees, or about 1.5 degrees warmer than the normal mean of 72.

Temperatures were even warmer than normal along the beaches of the Central Coast. So far this summer, the Point San Luis Lighthouse weather station has seen six days reached over 80 degrees. One day in August, the temperature hit 96 degrees.

To put this in perspective: Just a couple of miles up the coastline back in 2010, the air temperature at the Diablo Canyon Power Plant never even reached the 60-degree mark, remaining in the 50s under mostly overcast skies that summer.

So why is year so much warmer? Well it’s a combination of several factors.

The reduced speed and persistence of northwesterly winds along the coast this summer decreased the amount of ocean upwelling, which brings cooler water from the ocean depths to the surface. Less upwelling helps to produce warmer than normal seawater temperatures.

The reduced amount of upwelling, coupled with the ongoing El Niño event continues to create elevated seawater temperatures along the coast. These seawater temperatures have also produced warmer air temperatures along the shoreline. That, combined with the reduced amount of northwesterly winds blowing across the Pacific, has resulted in far less marine low clouds and fog in the coastal regions.

Another factor was greater than normal amounts of monsoon moisture. This summer, I’ve seen the cycle of low coastal clouds broken when subtropical moisture from the south migrates across our area. This moisture often mixes out the marine layer and can cause our Mediterranean climate to change to a humid, subtropical one reminiscent of Florida. Of course, climate change is also contributing to warmer conditions.

Unfortunately, all these factors combined with another one described below will probably keep temperatures above normal levels in the coastal regions through fall.

Typically during the 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 Santa Lucia (northeasterly) winds, especially during the night and morning hours. These winds bring the relatively dry inland air to our shoreline, pushing the marine layer far out to sea. This produces sparkling, clear visibility along our beaches and warmer temperatures. These winds also produce compressional heating as they descend down the Santa Lucia Mountains.

In October 1987, a strong high-pressure ridge combined with Santa Lucia winds produced a scorching heat wave across California; San Luis Obispo was the warmest location for the nation, with 111 degree temperatures recorded at the airport for two days in a row.

Because of the warm overnight temperatures, many Cal Poly students living in the red brick dorms slept outside, including me. The dorms warmed up like an enormous brick oven.

Thankfully after three days of heat, the high-pressure ridge weakened and the coastal low clouds fog rolled in through the Los Osos and Chorro Valleys with cooler marine air.

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