* WHAT. Highs of 90 to 100 degrees. * WHERE. San Diego County Valleys. * WHEN. From 10 AM this morning to 8 PM PDT Sunday. * IMPACTS. Hot temperatures may cause heat illnesses.

* WHAT. Dangerously hot with high from 110 to 115. * WHERE. San Diego County Deserts. * WHEN. From 10 AM this morning to 8 PM PDT Thursday. * IMPACTS. Heat related illnesses increase during extreme heat events.

* WHAT. Highs from 90 to 98. * WHERE. Riverside County Mountains, San Bernardino County Mountains, and San Diego County Mountains. * WHEN. From 10 AM this morning to 8 PM PDT this evening. * IMPACTS. Hot temperatures may cause heat illnesses.

Monsoonal flow and residual tropical moisture will arrive from the south tonight through Sunday morning. This will bring a slight chance of a few thunderstorms over the local waters this weekend. The chances are currently less than 20 percent at any one location. The most likely timing of this activity will be late tonight through Sunday morning. Any thunderstorms that develop would be capable of producing dangerous cloud-to-water lightning, gusty winds, small hail, and brief heavy rain. Mariners are encouraged to closely monitor the situation and consider making alternative plans if the chances for thunderstorms were to increase.

Monsoonal flow and residual tropical moisture will arrive from the south from Saturday night through Sunday morning. This will bring a slight chance of a few thunderstorms over the local waters this weekend. The chances are currently less than 20 percent at any one location. The most likely timing of this activity will be late Saturday night through Sunday morning. Any thunderstorms that develop would be capable of producing dangerous cloud-to-water lightning, gusty winds, small hail, and brief heavy rain. Mariners are encouraged to closely monitor the situation and consider making alternative plans if the chances for thunderstorms were to increase.