First measurable rain since May to follow Rosa into California

By Kristina Pydynowski, AccuWeather senior meteorologist

September 30, 2018, 2:47:49 PM EDT

While most of California will miss out on Rosa's soaking, a storm on its heels will bring the first measurable rain since May to San Francisco, Los Angeles and other cities Tuesday into Wednesday.

In California, the rain and flood risk associated with Rosa is expected to remain confined to the state’s southeastern corner early this week.

Rough seas stirred up by Rosa will still continue to plague Southern California through Monday. Rip currents can endanger surfers or anyone who attempts to enter the water, and minor coastal flooding may occur at high tide.

Residents, including fishermen, who climb onto jetties or rock walls will be at risk of being knocked down and swept into the ocean when sneaker waves (random larger waves) overtop these barriers.

The communities missing out on Rosa's rain will not have to wait too much longer for the typical dry spell that unfolds each summer to end.

A new storm sweeping in from the Pacific Ocean is anticipated to spread rain over Northern and central California on Tuesday before dropping into Southern California at midweek.

The heaviest rainfall is expected along the coast from around San Francisco to Santa Maria and Los Angeles, where the potential exists for 0.50 of an inch to 1 inch of rain. Similar amounts may be registered along the southwestern slopes of the Sierra Nevada.

There can be locally higher totals, mainly in the southwest- and south-facing coastal mountain ranges.

"Any snow will be restricted to the highest peaks and ridges of the Sierra Nevada," according to AccuWeather Senior Meteorologist Alex Sosnowski.

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"This would be the first measurable rain since May 21 for Downtown Los Angeles and May 25 for San Francisco," according to AccuWeather Meteorologist Maggie Samuhel. "Farther inland, Fresno has not received measurable rain since mid-April."

While the rain would be a welcome sight to residents to help water lawns and quell the fire danger, some issues may arise.

The first significant rainfall of autumn in California is notorious for causing slippery roads as the rain mixes with oil residue left behind during the dry months.

Motorists are urged to use caution as the rain initially falls. This includes those planning to travel on stretches of interstates 5, 10 and 15. Air travel may also have some disruptions.

If the rain pours down heavily enough, localized flash flooding and debris flows can occur in the burn scar areas.

Thunderstorms may also accompany the rain across Southern California. Remember that as soon as thunder is heard, the risk of being struck by lightning is present.

The rain will bring another press of cooler air into California. Highs across the Central Valley and coastal plain should be held to the 70s Tuesday into Wednesday. Temperatures in San Francisco and some other beaches may even stop short to reaching 70 F.

Fresno has not had a day with a high in the 70s since mid-June. Most of last week was dominated by highs in the middle to upper 90s.

Later Wednesday into Thursday, the storm is expected to spread showers and a few thunderstorms across the Rockies. Where the ground is left oversaturated in the wake of Rosa, localized flash flooding may occur.

When this storm departs California, a resurgence of heat is not anticipated. The weather pattern may instead keep temperatures near to below normal into the upcoming weekend.

The upcoming pattern will mean AccuWeather meteorologists will have to closely monitor the East Pacific for any more tropical impacts on the Southwest.

"There is the chance of strengthening Sergio being drawn up to the Southwest states with rain during the second week of October," according to AccuWeather Long-Range Meteorologist Jack Boston.

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