Muggy weather is expected once again Thursday in San Diego County and thunderstorms will be possible everywhere except coastal areas.
The National Weather Service issued a heat advisory for coastal areas and the western valleys that was originally set to expire at 6 p.m. Wednesday, but was extended through 8 p.m. Thursday.
Thunderstorms will be possible Thursday afternoon in the western valleys and the deserts and throughout the day in the county mountains, forecasters said.
The chance of measurable precipitation is 20 percent Thursday in the western valleys and the deserts, while the mountains have a 40 percent chance.
The NWS issued a flash flood watch that will be in effect from noon through Thursday evening in the mountains.
The high pressure system that has been the driving force behind the hot, sticky and unstable conditions this week will make its way east on Friday and give way to a trough of low pressure moving in from northern California, NWS meteorologist Miguel Miller said.
High temperatures Thursday could reach 91 degrees near the coast and inland, 98 in the western valleys, 94 in the mountains and 109 in the deserts.
The NWS advised that those who work outdoors should avoid the midday sun, wear light, loose-fitting clothing and drink plenty of water.
Thunderstorms will be possible again in the mountains Friday afternoon, but conditions will be cooler and drier throughout the county by Sunday, Miller said.
