Parts of far northern San Diego County into Riverside County were placed under a flood advisory Sunday, according to the National Weather Service of San Diego.
The plume of summer monsoon moisture that normally resides over Arizona has migrated farther west over the last few days. That's allowed for a few mountain showers and storms to develop both Saturday and Sunday. There will be another opportunity for a few of those storms to develop Monday afternoon as well.
Some storms could have very heavy rainfall in hyper-localized areas. Given the very dry ground, heavy rain won't soak in. It will runoff and quickly collect in dry river beds. In these situations, flash flooding will be possible.
The chance for storms will dwindle Tuesday and Wednesday. There could still be a stray storm or two, but coverage will be sparse.
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The second half of the week will have a renewed surge of monsoon moisture. Thursday and through the weekend, there will be a chance for scattered mountain storms. Lightning strikes, gusty winds up to 50 mph, and flash flooding will be possible with the stronger cells later in the week.