Why did the forecasts ahead of the Christmas week snowstorm change? A meteorologist explains.

As winter weather approaches northern Illinois – a storm that could bring with it heavy winds and freezing temperatures – the system is expected to bring a little less snow than originally was projected earlier in the week, officials said.

The original models projecting the upcoming weather system are expected to still prove fairly accurate when it comes to the hazards it will pose to the region, National Weather Service Meteorologist Jake Petr said Wednesday. Nevertheless, Petr explained the change in forecasts from the beginning of the week until now: Weather projections are less reliable the further out they are.

“It’s something we struggle with, too, as far as how people interpret our forecasts,” Petr said. “Because they change over time.”

When projecting weather, meteorologists use models to spot a system that is heading in a direction, Petr said. The further away that system is, however, the harder it is to read. As it gets closer, available tools such as weather balloons can track the various features of the impending weather event.

As a result, original forecasts from a week to 10 days out are not very reliable and often can be disregarded, Petr said. However, when long-term models show the weather system having a consistent pattern over and over again, those are times that forecasts even a week out can be trusted.

In the case of the winter weather set to arrive Thursday, multiple long-term models showed a consistent pattern, Petr said. Those models also showed several hazardous factors, namely wind, temperature and snow. Because of that, the weather service began advising people to take precautions.

Petr said those models so far have held up “fairly well” in predicting the upcoming weather. One area where it struggled was figuring out where to place the system, which had a lot of variation. As a result, the weather service stopped short of forecasting what the level of snow could look like, and the results seem to suggest less snow will hit the Chicago area than what was possibly expected.

That weather is expected to move across northern Illinois around the midmorning hours, and get to the Chicago area around noon to early afternoon. High winds and freezing temperatures, which will have a chance for snow on its initial front, are expected.

“The focus of our message has been the weather will create hazardous traveling conditions,” he said. “Low visibility, slick roads. That sort of thing.”

The latest forecasts are calling for between 2 and 5 inches of snow, which is on the lower side of what had been expected. Higher amounts are projected in Indiana, which could get up to 6 or 8 inches, because of a lingering lake effect.

That doesn’t change the risks for those traveling in the weather across the Chicago area, Petr said.

“Travel is not going to be nice,” he said. “When you combine that with the winds and the cold, that’s why we’re still pushing travel is going to be dangerous.”

The worst of the snow is expected to subside by Thursday evening going into Friday, Petr said. Powerful winds, which could hit 35 to 45 mph, and cold temperatures will continue through Friday, with the windchill expected to get into the minus 20s and 30s. Saturday will see the windchill a little warmer, but not by much.

Snow flurries could also continue into Friday as well.