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Offshore Low Keeps Northern California in the 70s This Week

Persistent Delta breezes lock in valley highs at least 10 degrees below normal until high pressure returns late weekend.

A wavy jet stream loops across the Northern Hemisphere. These locked-in configurations can trap heat domes over one continent while steering cooler air into others.
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A few clouds and Delta breezes are present this morning and while afternoon temperatures will be warmer than Tuesday, they will still be below seasonal normals.

Overview

  • A cutoff low-pressure system stalled off the West Coast since Monday has driven an unseasonable cool spell across Northern California.
  • Gusty Delta breezes continue to generate sustained winds of 10–20 mph in the valley and up to 35 mph in the Sierra, creating choppy lake conditions and heightened fire-weather risk.
  • Valley daytime highs have lingered in the mid-70s to low-80s, about 10 degrees below the typical mid-July average of around 93 °F.
  • Isolated thunderstorms and scattered mountain showers are forecast in the Sierra through Friday, bringing a risk of dry lightning where vegetation is driest.
  • Forecast models show high pressure rebuilding late this weekend, with valley temperatures rebounding into the 90s by next Sunday.