Sitting down? A winter storm is heading this way

After about 150 days of dry weather that has frustrated firefighters, devastated crops and contributed to Southern California water woes, the region is expected to see its first significant rains beginning today.

A storm moving into the Southland from British Columbia is bringing unseasonably cold temperatures, snow at high elevations and steady precipitation, a welcome sight for many on the heels of a 2006-07 weather year that was the driest on record in Los Angeles.

The cold front arrived in Northern California on Wednesday, months ahead of schedule, bringing temperatures 10 to 15 degrees below normal.

Generally, average September temperatures hover at 74 degrees in downtown Los Angeles.

“The storm is pretty unusual. It’s pretty much our first winter storm of the season, and it’s barely fall,” said Oxnard-based meteorologist Edan Lindaman of the National Weather Service.

The storm, which was over Oregon on Wednesday morning and had moved into Northern California by late afternoon, was expected to stop over the Central Coast for 12 to 18 hours before continuing toward Los Angeles.

Meteorologists predict that the storm will reach Point Conception, near Santa Barbara, by tonight.

A deep marine layer preceding the storm is expected to produce widespread drizzle.

The real activity, Lindaman said, will be late Friday, when the storm moves inland toward Los Angeles. It could produce half an inch to an inch of rain.

The last measurable rain in downtown Los Angeles was April 22.

John Abatzoglou, a researcher with the Desert Research Institute in Reno, said the storm was carrying cold air aloft and traveling over warm land and has the potential to create nasty thunderstorms.

The storm was part of a jet stream but broke off, making its way south, and will now probably pick up moisture from the tropics, Abatzoglou said.

“It’s almost like a storm you would expect in winter. It’s a very deep storm, it has cold air aloft and will tap into some subtropical moisture,” he said.

“Cold air is essentially up north of us, and I believe what is happening is this system is bringing a lot of cold air down and getting pinched off,” he said. “This is the type of air mass that should be up around 50 degrees latitude but will really be at about 35 degrees latitude.”

Bill Patzert, a climatologist with the Jet Propulsion Laboratory in La Cañada Flintridge, said the storm Wednesday was responsible for temperatures in the 60s in Fresno, about 20 degrees below normal for September.

Patzert cautioned that the expected precipitation in the Southland over the next few days will not be an indicator of a wet winter or even a full break in the dry spell that has plagued Southern California.

Patzert, Abatzoglou and other weather experts have predicted a dry winter, saying enhanced upwelling in the eastern Pacific indicates a La Niña condition.

Normally, La Niña conditions mean a dry winter and dry year, Abatzoglou said, noting that 10 of the 12 La Niñas since 1895 have produced an average of about 85% of the region’s normal levels of rain.

Widespread rain in the Southland over the next two days, especially precipitation greater than an inch, could bring relief to firefighters battling blazes fueled by the dry weather.

“A good dousing of water on those fuels might be really beneficial for the fire season,” Abatzoglou said.

Firefighters across Southern California have been hampered by extremely dry conditions. Los Angeles County Fire Department spokesman Tom Richards said rain was welcome during fire season.

Rain is “always a benefit,” Richards said.

“But on the converse side of that is: Too much rain in fire stricken areas can cause mudslides. Once an area is burned out, it has nothing to hold the soil in place. All the foliage is burned up, so all the moisture sinks right to the soil, which causes it to slide down the hills.”

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ari.bloomekatz@latimes.com