Timeline: What to expect and when as ‘bursts of heavy, wet snow' to fall across Chicago

While Wednesday morning started out rainy for some, the Chicago area’s first snowflakes of the season are expected to fall later in the day, with “intense bursts of heavy, wet snow” expected by Thursday morning, the NBC 5 Storm Team said.

The snow comes with big temperature dip and the potential for wind chills overnight to drop into the teens.

As you dust off your snow boots, here’s a Chicago weather timeline of what to expect and when as the first flakes arrive.

Wednesday morning

Wednesday morning started off rainy for some, NBC 5 Meteorologist Alicia Roman said, with heavy downpours in the 6 a.m. hour in Will, Kankakee and DuPage Counties in Illinois, and Lake County in northwest Indiana.

By mid-morning hours, the rain was expected to move out, Roman said. After that, several hours of dry time along with some bits of sunshine was expected, Roman said, along with highs in the mid-to-upper 40s.

Wednesday afternoon

By 2 p.m. a few spotty showers were expected to return, Roman said, starting in the northern counties. Between 5 p.m. and 6 p.m., those showers were expected to change over to snow, Roman said.

Wednesday night

The rain-snow mix Wednesday afternoon was expected to transition into light snow flurries between 8 p.m. and 9 p.m. Wednesday night, Roman said, with snow staying scattered overnight.

Thursday morning

Those hitting the road around Thursday morning can expect to see more snow for the rush hour commute.

“As we approach 7a .m., we may start out with snow,” Roman said. “Perhaps light, steady snow at times.”

Between 8 a.m. and 10 a.m. that snow could become more intense, Roman said, with bursts of wet, heavy snow across the area.

According to the National Weather Service, wet snow could fall Thursday morning at rates of one inch per hour, with highest snowfall rates in the late morning.

The NWS also warned of reduced visibility, slush-covered roads and slippery travel at that time.

“Thursday morning could be tricky,” Roman said, of what the rush hour commute could look like.

In parts of Southeast Wisconsin, a winter weather advisory will go into effect at 6 a.m. Thursday through 12 p.m., with between two and four inches of snow possible.

Thursday afternoon and evening

Any snow or slush that does accumulate in Illinois is expected to be washed away, Roman said, as snow was expected to transition back to rain between around 1 p.m. or 2 p.m. Thursday.

Showers were expected to continue in Northeast Illinois Thursday night, Roman said, with temperatures staying in the 30s and 40s.

Winter weather, ‘bomb cyclone’ in other parts

Chicago’s first snowflakes of the season come as a major storm swept across the northwest U.S., battering the region with strong winds and rain, causing widespread power outages and downing trees that killed at least one person.

The Weather Prediction Center issued excessive rainfall risks through Friday and hurricane-force wind warnings were in effect as the strongest atmospheric river — a large plume of moisture — that California and the Pacific Northwest has seen this season overwhelmed the region. The storm system that hit starting Tuesday is considered a “ bomb cyclone,” which occurs when a cyclone intensifies rapidly.

The areas that could see particularly severe rainfall will likely reach from the south of Portland, Oregon, to the north of the San Francisco area, said Richard Bann, a meteorologist with the National Weather Service Weather Prediction Center.

“Be aware of the risk of flash flooding at lower elevations and winter storms at higher elevations. This is going to be an impactful event,” he said.

What is a bomb cyclone?

A bomb cyclone occurs during the rapid intensification of a cyclone located between the tropics and the polar regions, according to the National Oceanic and Atmospheric Administration. It can happen when a cold air mass collides with a warm air mass, which is something that can occur over ocean waters, the agency says.

The measurement needed to determine whether a cyclone can be classified a bomb cyclone can be tricky, but it largely concerns a swift drop in pressure. Atmospheric pressure is measured in millibars by the National Weather Service. If a storm decreases 24 millibars or more in 24 hours or less, it can be considered a bomb cyclone, said Stephen Baron, a forecaster with the weather service in Gray, Maine.

“I would say rapid intensification of hurricanes is one of the more common times we see it,” Baron said. “We do see it with Nor’easters occasionally.”

Why is it happening on the West Coast?

The National Weather Service Weather Prediction Center has issued excessive rainfall risks starting Tuesday and running through Friday because of the powerful storm expected in northern California and the Pacific Northwest. The storm is arriving as the region experiences an atmospheric river, which is a long plume of moisture, over the Pacific Ocean.

The Weather Prediction Center said the storm intensified swiftly enough that it’s considered a bomb cyclone.

Bomb cyclones can happen in many places, and aren’t unique to the West Coast. They can occur in several parts of the world’s oceans, including the Northwest Pacific and North Atlantic.