While heat keeps returning to the mid-Atlantic where last winter all-time record snows fell, nothing but cool weather has plagued the west coast. (May 1 to July 27, 2010 anomalies are shown below, enlarged here).

San Diego’s coldest July in 99 years coming to end
San Diego Union Tribune

San Diego hasn’t had a July that’s been this chilly since 1911, the year Chevrolet entered the car market and a pilot named Calbraith Rodgers made the first coast-to-coast flight. At least, that’s how it appears things will turn out. The average monthly temperature at Lindbergh Field stands at 65.9 degrees, almost five degrees below normal, says the National Weather Service. Forecasters say that the marine layer will prevent the airport’s temperatures from rising above the upper 60s today and that the monthly average is likely to finish at 66.0, making this the coldest month since the downtown area recorded a July average of 66.1 in 1911. The July average in 1914 was 65.8.

On average, it’s been about 1.5 degrees cooler this month in San Diego than it has been in Portland, Oregon, according to the NWS. At 8 a.m. today, it was 63 in San Diego and 62 in Oceanside Harbor.

Forecasters say the local weather has been unusually cool because a series of low pressure systems lingered off central and northern California, which in turn caused the marine layer in San Diego County to last longer, and grow thicker, than it usually does in July. Those low pressure troughs are typically off Oregon this time of year—too far away to have a significant impact.

The weather service says coastal areas also were cooled by a stronger than normal sea breeze, and by strong inversion layers, which makes it harder for clouds to dissipate. There have been only four clear days this month at Lindbergh, less than half the normal number. And the high temperature has hit 70 or above on only eight days this month. The average high, in late July, is 77 degrees.

This combination of factors also has been causing cooler temperatures inland. To date, the average monthly temperature at Ramona Airport is 70.7, or 2.3 degrees below normal. Even Campo is off; its average monthly high, so far: 70.3, or 1.9 degrees below normal.