Gray Skies, Drizzle May Be Just a Prelude : Weather: A storm blowing in from the Rockies will linger for several days. The season’s first rainstorm could occur this weekend.

The drizzle and fog that kept most of Orange County blanketed in gloom on Tuesday is likely to continue through the week, possibly developing into the season’s first rainstorm on Saturday or Sunday.

“It’s unusually deep,” meteorologist Curtis Brack said of the marine layer that is extending as high as 5,000 feet and creating soup-like conditions. “With so much moisture, it just has to start precipitating out.”

Unlike most storm systems that assault Southern California from the Pacific Ocean and sweep inland, this one began developing about two days ago over the northern and central Rocky Mountains, said Brack of WeatherData Inc., which supplies forecasts for The Times.

“This is the time of year when you start expecting storm systems to come in,” he said, “but this one’s slightly unusual in that it’s taking a backward route up from the Rockies toward the West Coast.”

On Tuesday, he said, the result was low clouds with patchy early morning fog and drizzle through much of the day. While most areas experienced rain in amounts too small to measure, Brack said, El Toro reported .02 inches of rain as temperatures ranged in the 60s countywide.

Beginning this afternoon, Orange County residents, especially in inland areas, may see some slight clearing, Brack predicted, although he stressed that overall conditions are likely to remain the same, with temperatures in the upper 50s to lower 70s for the rest of the week.

Then on Saturday or Sunday, he said, things could break open with the season’s first measurable rainstorm.

“It’s still kind of iffy,” Brack said, “but it looks like it may happen.”

Even if it does, he said, the storm will not break any records. “Right now,” Brack said, “it doesn’t look like anything heavy. Probably less than a quarter of an inch, but it’s more than you’ve had so far.”