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Warm streak ends, thunderstorms possible

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San Diego’s long streak of warmer-than-normal days, which hit 101 on Monday, ended today.

As if to put an exclamation point on the change, the city could get thunderstorms and rain Wednesday.

Every day from May 10 through Aug. 18, the average temperature in San Diego was above the climatological norm for the date. A day’s average temperature is calculated by adding the high and the low, and dividing by two.

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The 101-day stretch was the fourth longest in city history and the longest in 30 years.

Phil Gonsalves, a National Weather Service forecaster, said an unusually strong low-pressure system for August has contributed to both the cool-down and the thunderstorm chances. The low moved down the coast today and is expected to be over Southern California Wednesday. Thunderstorms with brief heavy rain, gusty winds or dry lightning are possible anywhere in the region Wednesday. The mountains and deserts could have another round of storms on Thursday.

“I can count on one hand the times I’ve seen this scenario in August,” Gonsalves said, referring to the strength and latitude of the low-pressure system.

Gonsalves said atmospheric changes over the eastern Pacific in recent days helped set up wind patterns that caused upwelling of cooler waters off the Southern California coast. That upwelling, in turn, created cooler air near the surface early today and helped establish a thick marine layer, which pushed deep into the inland valleys this morning. The cooling marine layer burned off by late morning but was back at the coast late this afternoon.

The average temperature in San Diego today through 6 p.m. was 71.5 degrees, after a high of 74 and a low of 69. Average for the date is 72.

San Diego’s longest streak of warmer-than-normal days was 168, in 1984.

rob.krier@utsandiego.com; (619) 293-2241; Twitter: @sdutKrier

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