Temperatures have dropped from the triple digits into the 90s, but Sacramento-area residents may not feel much relief.
For that you can blame monsoonal moisture, according to the National Weather Service.
A weak low pressure system off the coast has funneled moisture from the Colorado River Valley, Baja California and Arizona into Northern California, said Craig Shoemaker, a meteorologist with the weather service.
Most of the moisture is at altitudes of 10,000 to 20,000 feet, but “we have had some low-level dew points,” Shoemaker said.
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That has resulted in weather conditions not typically seen in the Sacramento region during the summer. Between midnight and about 6 a.m. Friday, numerous lightning strikes were reported from south Sacramento to Lodi, Shoemaker noted.
The weather service also reported a phenomenon known as a “heat burst” around midnight in Vacaville.
Nighttime temperatures were hovering around 80 degrees, when suddenly temperatures shot up 15 degrees, hitting a high of 95 degrees right before midnight.
Although heat bursts are “not out of the question,” they are not something you “see often, especially around here. Usually, they are more common in the Midwest and the Plains,” according to Courtney Obergfell, a meteorologist with the weather service.
A heat burst occurs as a thunderstorm begins to die away. The cold air once found on top of the thunderstorm begins to sink to the surface as the thunderstorm collapses, said Obergfell.
As the air descends, it warms up, and the water within it evaporates. This causes a burst of warm air over a localized area. After the initial heat and winds, the burst usually dissipates within the next hour, which is what occurred in the skies over Vacaville.
Along with warming air temperature, heat bursts most often bring about short-lasting, strong winds. The Vacaville Nut Tree Airport reported wind speeds rising to 22 mph during the night.
Although temperatures were lower Friday due to cloud cover – it was 91 degrees in downtown Sacramento at 2:30 p.m. – the relative humidity was 40 percent.
“That’s high for that time of day,” Shoemaker said.
Monsoonal conditions, he said, occur in the Sacramento region about once every three or four years.
“It’s not that uncommon if you get a pattern set up,” Shoemaker said.
That pattern is expected to continue into late next week. The Sacramento-area forecast through Thursday is for high temperatures in the mid- to upper 90s and overnight lows in the low to mid-60s.
Laura Sussman: 916-321-1611