As the final 48 hours of October began to melt away Wednesday, the Bay Area skies turned darker amid chilly temperatures and an expected onset of rain.

What a difference a month makes.

“We have a couple of systems that are coming through the area,” NWS meteorologist Dial Hoang said early Wednesday. “The second system might be even colder than it is right now.”

The first system, a low-pressure trough that Hoang said is pushing its way down from the northern Pacific Ocean, was expected to bring rain to much of the region by early today, according to the weather service. Hoang said it could release as much as a half-inch of precipitation in the higher elevations of the region but that no more than a couple tenths of an inch of rain would fall elsewhere. In the East Bay and South Bay, the first storm may bring a few hundredths of an inch, he said.

The same system also is expected to provoke the first significant snowfall of the season. Forecasters said about 3-6 inches of snowfall was expected for the Sierra Nevada in elevations above 6,000 feet starting Wednesday afternoon.

The weather service said the “snow will come in waves” and issued a winter weather advisory for elevations above 6,000 feet, lasting through 5 p.m. today.

“There may be periods of little to no snow with heavier snow later in the day,” the weather service wrote on social media. They added that “travel delays and slick roads are expected.”

How quickly the first system moves through the region likely will determine conditions when Bay Area trick-or-treaters set out tonight.

The weather service said the start of some showers on Wednesday morning made it more likely that the system would depart the region by the time trick-or-treaters set out tonight.

Those going out should dress warm; forecasters said temperatures were expected to be in the mid- to low 60s tonight.

“Then, behind that first system, is another one,” Hoang said. “We expect that one to come late Friday and move into Saturday.”

That storm front is expected to bring a bit more intensity, Hoang said. The weather service anticipates the second system will drop at least a half-inch of rain throughout the region, with the higher elevations receiving as much as a full inch.

Temperatures in the coldest spots again will dip into the low and mid-40s overnight.

All of it is a far cry from the blistering hot conditions that dotted the Bay Area with temperatures in the 100s at the start of October. The heat wave was the m

onth’s longest and hottest since 1980, according to the weather service.

Temperatures will begin to move up again on Sunday, when the last remnants of the second storm system likely have departed the region, Hoang said.

Temperatures in some area may climb into the low 70s again starting Monday.