If you noticed a nip in the air Sunday morning, it wasn't your imagination. Fall-like weather is slowly descending over the Bay Area, delivering a chance of light rainfall, isolated thunderstorms and temperatures that are forecast to dip about 10 to 15 degrees below normal across the region.

A few raindrops appeared on the lenses of the National Weather Service's alert cameras in Mendocino County early Sunday morning, but it may take a little while longer for the system to make its way further south, Nicole Sarment, a meteorologist for the agency's Bay Area office, told SFGATE over the phone Sunday.

"This is not a typical cold front we're seeing," Sarment said. "It's a bit of a two-parter. With the orientation of the system, we'll see some gusty winds, but moisture doesn't come into play until later tonight and early tomorrow morning."

Rain might be a generous term for the precipitation that's yet to come, she said. It's expected to be so light that some people may only see drizzle. Most parts of the region are still likely to get a few hundredths of an inch - including downtown San Francisco and inland portions of the East Bay like Livermore - and up to two-tenths of an inch in higher elevation areas and parts of the South Bay, including San Jose Airport. Some coastal locations like Big Sur may also see more because of the way the topography forces cool air to rise, producing clouds and condensation. Still, Sarment said there is little to no rain to speak of in the North Bay, which tends to see rainfall soonest and typically gets the brunt of the system.

"This map is very dismal that I'm looking at," she said. "It speaks to the nontraditional nature - it's not sweeping through, but wrapping around a rotating low-pressure system. It's something we're focusing on a lot because it's September and it's the first real system."

The timing of the precipitation, she added, is "kind of all over the place." If the North Bay does see drizzle or rain, it likely won't arrive until midnight and is expected to linger through early Monday afternoon, with a 10% chance of thunderstorms for that portion of the region and 5% for the rest of the Bay Area. Small hail, gusty winds and fire starts are also a possibility, the weather service said.

Notably, Sarment said the weather service's balloons - two of which are launched out of Oakland each day to measure temperature, dew point, wind and air pressure in the upper atmosphere - are showing potential to drop to a height of 500 millibars, which has never been recorded for the month of September. It would be the lowest a weather balloon has ever been, compared to 4,500 other launches dating all the way back to 1948.

"What it means is we are seeing conditions in the upper atmosphere that would be typical for January, and they're happening in September," Sarment said. "It's abnormal for this time of year, and it's going to drive those below-average temperatures."

For the most part, the Bay Area will see daytime highs hovering in the 70s, with overnight lows sticking around the 50s and upper 40s in the North Bay and higher elevation spots. A warmup is slated to arrive by Thursday.

Sarment said people shouldn't worry too much about how the weather might affect their Monday morning commute, but that they should keep an eye out for some slick roads.

"It's a reminder that we are in the shoulder season," she said. "If people are looking for an early onset of fall, it's going to feel like it. This is one of the first systems that has made it this low to impact us in some respect."