Moisture from once-Hurricane Dolores will spread rare July showers and thunderstorms into Southern California through Monday. While the rainfall will be welcome, flash flooding is a concern.
Dolores weakened below tropical depression status late Saturday, but its moisture is set to survive the journey into Southern California.
The combination of this moisture and monsoonal moisture will lead to another round of showers and thunderstorms to end the weekend with the mountains being most active.
More numerous showers and thunderstorms will then develop Sunday night and persist into Monday from around Los Angeles to Santa Barbara to San Luis Obispo. There will even be a steadier period of rain for a time west of Los Angeles.
San Diego will miss out on the wettest weather, but there will still be a shower or thunderstorm grazing the city. A couple of showers and thunderstorms will also rumble northward to Merced and Fresno, California.
The prospect of rain across drought-stricken Southern California will definitely be welcome, especially during a time of year when rainfall is rare. Any rainfall will further help firefighters battling the destructive North Fire.
Downtown Los Angeles and San Diego only average 0.01 of an inch and 0.03 of an inch, respectively, of rain each July.
The 1.03 inches of rain that fell alone on Saturday at San Diego caused the city to break its long-standing July rainfall record of 0.92 of an inch from 1902.
However, too much rain falling too quickly will create issues. Downpours with rainfall rates approaching or exceeding an inch per hour could trigger flash flooding or mudslides in the recently burned areas.
The flash flood risk will be greatest in the Southern California mountains through Sunday evening before expanding to Los Angeles, Santa Barbara and San Luis Obispo overnight and into Monday.
The thunderstorm activity will gradually wane around Los Angeles during the second half of Monday.
Even if flooding does not ensue, the downpours will reduce visibility for motorists and heighten the risk of vehicles hydroplaning at highway speeds. Where rain has yet to wash away oil residue, roads can turn slick when the rain initially falls.
Any thunderstorm will also spark lightning. The danger of being struck by lightning is present as soon as thunder is heard. Lightning strikes occurring well away from where it is raining could touch off new wildfires.
A double hazard exists for surfers since crashing ocean waves will make thunder difficult to hear and swells generated by Dolores will create rough surf and a high rip current threat through Monday. That is especially true on south-facing beaches.
The danger of flooding downpours will not be confined to Southern California, also persisting across southeastern California, southern Nevada and Arizona to end the weekend.
This includes Palm Springs, California; Las Vegas; and Phoenix and Flagstaff, Arizona. The danger will diminish in Palm Springs and Phoenix on Monday as Dolores' moisture shifts northward.
More typical dry July weather will return to Southern California on Tuesday as Dolores' moisture further fizzles to the north and the weather pattern of monsoonal thunderstorms in the Four Corners region resumes.