DISCUSSION: There is a big upper level ridge over the Pacific Coast which is increasing temperatures through Labor Day weekend. In addition to the increasing temperatures, the ridge is bringing smoke from fires in Northern California and Oregon into Central California including the San Francisco Bay Area and the San Joaquin Valley. The heat wave is supposed to last through the entire weekend although Sunday seems to be the start of a cooldown from the heatwave as the ridge would slowly moving out easterly. Much of the heat being brought in comes from the very warm Great Basin by the clockwise circulation of the ridge. In addition, there is subsidence (sinking air) occurring along with the planetary boundary layer (PBL) being around 850 millibars in the San Francisco Bay Area which is helping to drive up the temperatures.
On Friday, many records for the month of September across the San Francisco Bay Area was shattered including in San Francisco where the main city reached 106 degrees while San Francisco International Airport had reached a high of 104 degrees. Some of the highest temperatures in the 110s were recorded in locations away from the San Francisco Bay such as Concord. Meanwhile, in the Sacramento and San Joaquin Valleys, temperatures reached the 100s as well and threatened to climb to 110 especially in Sacramento where the high at Sacramento Executive reached a high of 109 during the day.
Saturday was a continuation of the heat and daily high records. San Jose had topped up at 107 degrees at the airport while Livermore got warmer by a few degrees. However, some locations were cooler by a few degrees along the coastline but most places remained about the same. In addition to the daily high, the lows were very warm as San Jose bottomed out at 74, San Francisco in the low 70s, while Oakland and Livermore got down into the high 60s. All of which are at least 10 degrees above the normal low temperatures meanwhile the highs were between 20 and 30 degrees above the normal highs according to the climatological reports provided by the National Weather Service. The lows on Saturday were much higher than Friday as a some of the heat was trapped by smoke that was aloft as well as the low PBL and subsidence.
The high temperatures and smoke were also factoring into air quality for both areas. In the San Francisco Bay Area, the Bay Area Air Quality Management District issued several consecutive days of “Spare the Air” alerts as well as determining the air quality to be unhealthy especially for sensitive groups. Meanwhile, the air quality district for the Central Valley had also measured the air quality as unhealthy for sensitive groups.
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©2017 Meteorologist JP Kalb