The arrival of monsoonal moisture has advanced the chances of thunderstorms in San Diego County's mountains and deserts for Friday afternoon, while a new warming trend will settle in over the weekend.

Since Wednesday morning, a mix of clouds has been observed, with low clouds clinging to the coast and altocumulus clouds appearing over the deserts. There have been a few isolated cells near Mount Laguna, bringing brief periods of rain, but monsoonal moisture is expected to increase on Thursday afternoon. At that time, we expect more thunderstorm activity rather than over the weekend. The weekend is expected to be mostly dry for those areas with only slight chances of thunderstorms.

A high-pressure ridge will expand from the Four Corners region and warm us up all the way to the coast during the weekend and early next week. Inland areas could reach up to 10 degrees above normal in the afternoons, while beaches could be a few degrees above the seasonal average.

Thursday's daytime highs are expected to top out in the 70s along the coast, the inland valleys in the 80s, the mountains in the low 90s, and the deserts around 110 degrees. An Excessive Heat Watch will go into effect for all deserts this weekend, with Borrego Springs on alert for Friday and Saturday.

The marine layer will shrink with the strengthening ridge but won't disappear completely. We will still have low clouds in the morning and evening along the coast, although they won't stick around as long during those periods.