By John C. Toman. Posted August 7, 2012, 7:09 AM.

Temperatures soared Monday in Southern California' inland valleys in what is shaping up as the strongest heat wave of the year to date.

And it's going to get even hotter. High pressure over the southwest continues to expand westward. An excessive heat warning has been issued for Wednesday though Friday, when temperatures are expected to peak. They could be five degrees or more higher than Monday's tally.

A sampling of Monday's temperatures at WeatherCurrents stations: 110 in Lake Elsinore, 109 at Wildomar, 108 at Menifee, 107 in Moreno Valley, 106 in Hemet, Perris and San Bernardino, 104 in Riverside (Canyon Crest) and Jurupa Valley, 103 in Temecula, and 99 in Simi Valley.

Fallbrook was the cool spot at 87°F, and towards the coast, San Diego's City Heights recorded 84°F.

In Western Riverside and San Bernardino counties, Monday was ten to fifteen degrees above normal.

The San Jacinto Valley experienced thunderstorms in the 4pm hour Monday, as moisture spilled over from the mountains. WeatherCurrents' station in Hemet recorded 0.24", and other stations at Cranston (0.29") and Keenwild (0.25") also recorded rainfall.

The thunderstorm threat is expected to continue through the week over the mountains and deserts.

Relief may finally come over the weekend with minor cooling. Greater cooling is possible next week.