Expect balmy weather at the coast and lightning in the mountains this weekend

High clouds will make the San Diego County coastline cooler than normal Thursday. A thickening marine layer might do the same Friday.

But beach cities will warm up this weekend, says the National Weather Service. And the monsoon could trigger lightning in the mountains.

The region is experiencing a brief downturn in the heat. Conditions will change Saturday when high pressure drives temperatures into the 77-to-80-degree range in San Diego. If the marine layer is weak, the temperature could rise even higher. Sea surface temperatures will remain in the 63-to-70-degree range — far below normal, as they’ve been for much of the summer.

Forecasters say the seasonal monsoon will likely strengthen on Saturday, greatly increasing the chance that thunderstorms will rumble through the San Diego County mountains and possibly into the western valleys. Those storms could throw off lots of lightning, elevating the risk of wildfires across much of East County.