Brace yourself: Here comes much cooler weather, and the Chicago area's first snow of the season.
"A colder air mass has arrived," NBC 5 Meteorologist Alicia Roman said Wednesday morning, noting Tuesday had been in the 60s.
Early Wednesday, temperatures were in the 40s, with rain and heavy downpours in some parts, including Will, Kankakee and DuPage Counties. That rain was expected to into Chicago by around 8 a.m., and then quickly moving out by the mid-morning hours, Roman said.
Around 2 p.m., another round of precipitation was expected, starting in the northern counties. Around 5 p.m., just in time for the rush hour commute, that rain was likely to be a rain-snow mix, Roman said.
By Wednesday evening, around 7 p.m. and 8 p.m., light snow flurries were expected to move into Chicago. Overnight, the snow could become more "intense," Roman said, with more snow falling around 7 a.m. Thursday morning.
Timeline: What to expect and when with ‘bursts of heavy, wet snow' coming to Chicago
Things will get even heavier from there, Roman said.
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"More intense bursts of heavy, wet snow by around 8 a.m., 9 a.m., 10 a.m., Thursday morning," Roman said. "We could see slushy accumulations, especially on grassy surfaces."
The snow was expected to transition back over to rain around 1 p.m. or 2 p.m. Roman said.
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"Anything that does accumulate will be washed away by rain," Roman added.
According to Roman, the high temperature Wednesday was expected to be 47 degrees. The high would drop to 42 degrees Thursday, though wind chill temperatures could make things feel colder.
While temperatures will remain in the 40s through the end of the work week, they'll rise back into the 50s for the weekend, Roman said. But the warmer air will be short lived.
For Thanksgiving, a high of 38 degrees was expected, with even cooler temperatures likely for Black Friday.
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.
