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Credit a buckled jet stream for the strange weather in San Luis Obispo County

Tribune file photo by Joe Johnston

Windrose Farm is in a little valley along the Huer Huero Creek near Creston, and this summer, owners Barbara and Bill Spencer have been able to raise spinach, various lettuces and Agretti, an Italian green. Barbara will ship an order of Agretti to one of the finest restaurants in New York City today.

That would be out of the question during a normal summer. But this is definitely not a typical summer.

Windrose Farm reported a shivering low of 39 degrees Fahrenheit with dense fog on the morning of July 28. San Luis Obispo reached a high of only 69 degrees that day, a record for the day. To the south, Los Angeles International Airport recorded a high temperature of only 68 degrees on that day, also breaking the previous record for coolest high temperature for the day of 70 degrees, set in 1991.

While areas east of the Rockies sweltered in record-breaking heat, this July was cool up and down the California coastline. San Francisco had its coolest July since 1971, with an average monthly maximum temperature of only 63.1 degrees. From July 20 through July 31, the air temperature at the Diablo Canyon nuclear power plant never even reached the 60-degree mark, remaining in the 50s under mostly overcast skies. This year’s Mid-State Fair was one of the coolest in recent memory, reaching a high of only 75 degrees July 26. Paso Robles normally has a monthly maximum average temperature of 93.9 but averaged only 89.7 degrees this July. The cause of this weather pattern is the location of the jet stream, a tubular ribbon of high-speed winds some 18,000 to 40,000 feet up, flowing in wavelike patterns from the west to the east for thousands of miles.

The jet stream has buckled over the West Coast, producing a persistent upper-level, low-pressure trough. This elongated area of low atmospheric pressure in the upper reaches of the atmosphere stretches from Washington state southward down the California coastline.

This trough has decreased the amount of subsidence, or sinking of the air mass, that normally occurs during the summer. That, in turn, has allowed a deeper marine layer to develop and persist. This layer of cool and moist air from the Pacific has penetrated farther into the coastal valleys and the North County than normal. There have been abundant night and morning low clouds, drizzle and fog, and cooler daytime temperatures.

However, the marine layer has not been deep enough to penetrate into the far eastern regions of San Luis Obispo County or the Central Valley. Temperatures there were near to above normal with readings reaching the low to mid-100s during most of July.

The omega-shaped jet stream has produced a ridge of high pressure east of the Rocky Mountains, causing record-breaking temperatures throughout the rest of the country for the month of July. Given a choice, I’d much rather have our weather pattern than swelter in the heat.

This stable pattern could very well last for the rest of August. However, don’t put away your swimsuit quite yet. The jet stream could still shift westward on a dime, putting San Luis Obispo under a ridge of high pressure and producing hot and clear weather.