Modesto’s summer has been cooler than usual. Here’s why and the long-range forecast
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- Modesto's July averaged mid-90s, with only one day surpassing 100 degrees.
- High pressure shift westward strengthened coastal winds and inland upwelling.
- Delta breeze drove cooler air inland, moderating Central Valley temperatures.
For an area that typically reaches triple-digit temperatures in the summer, Modesto has stayed unexpectedly mild this July, with the daily high temperature averaging in the mid-90s and many days reaching into only the 80s.
Compared to last summer, when the temperature reached over 100 degrees more than 20 times during June and July alone, Modesto has only seen one day over 100 degrees in the past two months.
Part of this shift can be linked to perception, according to Noah Hughes, professor of earth sciences at Modesto Junior College. This concept is referred to as “shifting baseline syndrome,” where people’s expectations of environmental conditions gradually change due to the loss of memory or experience.
“The perception that it’s been a cool summer is partly about the weather, but it’s also partly because it’s been so hot the last 10 years or so,” Hughes said. “People’s baseline has shifted in terms of what they expect.”
Still, with a shifted perception of what summer usually feels like, there are scientific reasons relating to atmospheric pressure and wind to explain the milder climate in Modesto.
“If you can understand the patterns of atmospheric pressure, then you can understand the wind patterns,” Hughes said. “And if you can understand the wind patterns, you can understand the weather.”
This year in particular, an area of high pressure that used to be closer to California has now moved west over the Pacific Ocean. This shift in pressure creates stronger, more consistent winds coming from the north along California’s coast.
These winds then interact with the ocean through a process called upwelling in which cold water moves closer to the surface. Upwelling explains why coastal regions, like the San Francisco Bay Area, typically experience cool summers, while more inland areas experience higher temperatures due to their distance from the coast.
“But this year, that temperature difference has been stronger than normal,” Hughes said. “The water temperatures off the coast have been stronger than normal, and it’s been quite hot inland, especially over the Sierras. We’ve had this really strange dichotomy between cold along the coast and unusually warm in the mountains, and Modesto’s kind of in the boundary zone.”
These temperature differences create what’s called a delta breeze or onshore flow, where cool air from the coast is brought to Modesto. This week, when the temperature has remained steadily in the 80s, the delta breeze has been especially strong and cool air has been poured through and over the mountains, providing the Central Valley with a milder climate.
“You can sort of think of the delta breeze as the natural air conditioner for the valley, but it only works when you’re close to the delta,” Hughes said.
As for the upcoming months, long-range forecasts predict these coolingpatterns will continue. When it comes to predicting patterns year by year, as for the summer, there is no clear way to create relationships between one year and the next, according to Hughes.
“What happens in one year doesn’t really seem to inform what happens in the following year,” Hughes said. “However, there are these cycles that can impact our weather, like El Nino and La Nina, but those things are more important in the winter climate in California.”
This story was originally published July 25, 2025 at 1:30 PM.