UPDATE 7/13 3:46 p.m.: A flash flood watch that has been in place in the mountains and deserts of the Inland Empire has been expanded to include the inland valleys, the National Weather Service reported Friday afternoon.
The watch remains in effect through this evening.
An upper level low pressure system will remain over Southern California through tonight with a very moist and unstable air mass in place, the NWS reported.
Across the Inland Empire, scattered thunderstorms are forecast.
UPDATE 7/13 12:01 a.m.: The flash flood watch has been extended into Friday evening for the local mountains and deserts as wet weather continues through Saturday.
Although the eastern portion of the county will continue to see thunderstorms and showers into the weekend, Lake Elsinore is forecast to be sunny by Saturday, with a daytime high of 99 degrees. Friday's forecast for the city is partly cloudy skies and a 20 percent chance of rain, with hot temperatures reaching 98 degrees.
ORIGINAL POST 7/12: That far off rumbling noise Thursday morning is thunder.
The crashes will continue in the desert and mountain areas of Riverside County today, as summer thunderstorms threaten the region.
"Increasing moisture and instability will generate thunderstorms over the mountains and deserts during the day today," according to the National Weather Service. "Deep layer moisture will be sufficient for locally heavy rain, leading to the potential for excessive runoff in normally dry creeks and streams."
A flash flood watch for local mountains and Coachella Valley is scheduled from 11 a.m. through this evening.
In Lake Elsinore, the National Weather Service is predicting up to a 35 percent chance of rain through the morning, with a few scattered showers and thunderstorms. Temperatures will rise through the day, with the mercury expected to top near 100 degrees.
An excessive heat warning remains in effect across the region, with the desert areas expected to top at 115 degrees.
"Hot and humid conditions will continue to prevail across the southern deserts today, as a very strong ridge of high pressure remains over the intermountain west," the NWS warned in an advisory.
"A slow cooling trend will develop for Friday through the weekend, as a weak trough of low pressure develops near the west coast," according to the weather service advisory.