‘Substantial weather pattern change’ coming to California

Tuesday is the first day of summer — and it will certainly feel like it across Northern California, where temperatures are expected to soar, the National Weather Service said.

After a weekend of mild weather, a “substantial weather pattern change” is coming to California this week, UCLA climate scientist Daniel Swain wrote on Twitter. “A prolonged period of significantly hotter temperatures is expected inland, which will be a dramatic shift from recent cool temperatures up north.”

Swain said another twist is coming in the state’s weather this week: “Additionally, it appears that an early/pre-monsoonal surge will arrive across southern 2/3 of CA by mid-week. This will bring chance of at least mountain/desert t-storms, & perhaps even chance of lightning over lower elevations south of I-80. Details still fuzzy, but could be some dry-ish lightning over peak drought region.”

The Central Valley will see some of the hottest temperatures in the northern part of the state, with triple digits likely to persist in many locations through the weekend. Tuesday and Wednesday are likely to be the hottest days of the week in the Central Valley. Further north, Redding is forecast to record a high of 104 on Tuesday and 107 on Wednesday. Sacramento is predicted to hit 102 on both Tuesday and Wednesday, the weather service said.

“We’re not expecting any records, but we’re going to be about 10 to 15 degrees above normal for this time of year,” said Scott Rowe, a forecaster with the weather service’s Sacramento office.

In the San Francisco Bay Area, the mercury is expected to hit its peak of the week on Tuesday; Discovery Bay projected to reach 105, Concord 103, Santa Rosa 100, San Jose 98, Oakland 93 and San Francisco 83.


“It’s still toasty through Wednesday, Thursday, and then by Friday starting to cool a little bit more,” said Sarah McCorkle, a forecaster with the Bay Area office.

A surge of monsoonal enters the forecast on Wednesday, but forecasters said on Monday that it’s too soon to nail down exactly what this low-pressure system will bring to Northern California. In the Bay Area, forecasters think dry lightning and thunderstorms are highly unlikely, but the weather shift is likely to lead to a slight drop in temperatures.

“Temperatures on Wednesday will continue to be widespread 90s with some low 100s for the interior, but should be a touch cooler than Tuesday,” the weather service said.

The highest chances for thunderstorms and dry lightning is along the Sierra crest, said Rowe.

“There’s very low confidence as to the trajectory of this moisture,” he said. “Right now, we have a 10% to 20% chance for dry lighting along the Sierra crest. It’s one of those things that’s hard to forecast.”

Dry lightning is of concern in summer when it can spark wildfires, but Rowe said the fire risk remains moderate in the Sierra due to recent light rain that’s keeping the vegetation somewhat moist.

The chances for thunderstorms, both dry lightning and light rain, associated with the monsoonal moisture are higher in the desert areas of Southern California than in Northern California.

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