Earlier this week, some of the coldest weather Alabama has seen in decades moved into the state, driving temperatures down to well below freezing and causing concerns for the students who had to get back for their first classes of the new semester.

James Spann, chief meteorologist for ABC 33/40, said the sudden cold weather was caused by a disruption in the jet stream.

“There’s a river of air that runs at about 25,000 feet through the northern hemisphere,” Spann said. “The easy explanation is the jet stream – it’s not as easy as that – weather’s a lot more complicated than people make it out to be, but generally speaking you can find maximum wind velocities at about 25,000 feet. The position of that tends to determine what your weather’s like.”

A dip in the jet stream brought air from the North Pole as far south as Alabama.

“We’re at a phase now where there’s a huge, huge trough – a dip if you will – over the Eastern half of North America, and we happen to be in that trough, and it’s a cross-polar flow, so this air actually originated in Siberia,” Spann said. “As it crosses the pole and over Canada, it doesn’t have a lot of time to modify. There’s a lot of snow cover up there and it just kind of comes down without modification, and when you get a cross-polar flow, often it gets pretty darn cold around here.”

So cold, in fact, that many UA students were toddlers the last time temperatures were this low in Alabama.

“This is the coldest since ‘96,” Spann said. “It got down to six degrees, and after that we had ice. Thankfully, in this case, it’s not going to happen. It’s been 18 years since it’s been this cold.”

Spann said it’s not uncommon for the jet stream to dip this far south. What is uncommon is for it to bring frigid Arctic air with it.

“It happens pretty often, but to have the cross-polar connection, that’s the oddity about it,” Spann said. “That doesn’t happen that often. I’d say about every 20 years is correct for this kind of thing.”

According to data from the National Weather Service, temperatures got as low as -3 degrees Fahrenheit in parts of Alabama Monday night and Tuesday morning, with temperatures below freezing all over the state. But the temperatures didn’t get as low as they did in 1985, when the average temperature was 0 and in double-digit negative numbers in parts of the state.

“It’s been colder than this here,” Spann said. “You go back into the mid and late 80s, we had two outbreaks … During those outbreaks we hit zero here. I don’t know if you can tell the difference between five and zero. I don’t know if any human would notice the difference, but it was a little colder during those outbreaks in the 80s.”

What was truly unusual about this cold weather, according to Spann, was how cold temperatures stayed even when the sun was shining.

“The statistical average high today [Monday] is 53, and it’s 14,” Spann said. “So for you to go almost 40 degrees below average Fahrenheit, that’s just stunning. The average statistical low is 32, so you’re looking at 5. So this is highly anomalous.”

A large trough in the jet stream also corresponds to a ridge somewhere else, according to Spann. “Any time you find a big trough like this, there’s also a corresponding ridge,” Spann said. “And if you look under the ridge, it’s a lot warmer than average.”

Alabama might be having some of the coldest weather on record, but thanks to the ridge, Alaska is seeing relatively balmy temperatures.

“They’ve got some places OVERSET FOLLOWS:up there that are in the 30s, and that’s way above average for them,” Spann said. “It all tends to balance out. If you look for super-duper cold, you can always find super-duper warmth, in terms of comparison to average. We just got the bad draw here.”

Temperatures will likely rise throughout the week, but the trough in the jet stream could mean that the rest of winter will remain cold for the eastern United States.

“Often, upper-air patterns tend to be persistent through seasons, and sometimes years,” Spann said. “And it just seems to me that this mean trough position in the Eastern part of the continent wants to stay there … If you look at December and January, our anomalies are very low. December was very cold and obviously January is starting off very cold. It going to take a lot of warmth to get us back to where we should be for February, and I just don’t know that’s going to happen. So it looks like this winter is going to go down as a cold one.”

The recent cold outbreak did not result in snow for most of Alabama, but Spann said he expects to see some snow threats before winter is over. Luckily, a cold winter does not necessarily mean a harsh tornado season later in the year.

“Statistically speaking, I know our luck is going to run out,” Spann said, referring to the relatively calm weather in Alabama since April 27, 2011. “Then again, you can go a decade with hardly any tornadoes here. The 80s? Quiet as a mouse. We had some very harsh winters in the ’80s, but the tornado seasons were very, very tame, with very few problems. So hopefully we’re in one of those long phases where we’re just going to stay quiet, but I sure can’t guarantee that.”

Spann also emphasized that a particularly cold few days does nothing to prove or disprove global climate change.

“This doesn’t disprove anything,” Spann said. “This is just weather. Climate happens over decades and centuries. Weather happens over minutes and hours. This is just a weather issue. For those people that try and say that this is proof that global warming is a scam, that’s ridiculous. It’s just as ridiculous as people saying on a hot day in the summer that this is proof of global warming. It’s just weather.”

For students traveling by car back to campus, roads should be relatively safe and free from ice.

“Roads will be bone dry,” Spann said. “With dew points, and minus numbers, roads will be totally dry.”

According to Patrick Reilly, president of the University of Alabama Meteorological Society, precautions should still be taken when roads have the potential to ice over.

“This system is by no means a ‘snowpocalypse’ as some individuals are reacting to it.” Reilly said. “However, measures must be taken to ensure personal safety and well-being.”

However, out-of-state students who planned to take a plane back to Alabama could face additional problems.

“The greatest danger, even if you’re coming in from Dallas, is what if your airplane’s coming from Chicago or some place where there’s a winter storm,” Spann said.

Thousands of flights across the eastern United States have been delayed or canceled over the past few days, with Hartsfield-Jackson Atlanta International Airport experiencing average departure and arrival delays of nearly an hour, according to FlightAware.com, a website that tracks flight delays.

Classes will start as normal on Wednesday afternoon, but according to an email from UA News, “Faculty will work with students who experience travel delays because of the severe weather.”

For students who are already back in Tuscaloosa, be prepared for the cold weather when classes start Wednesday morning.

“Just don’t wear your flip flops and tank tops the first day of classes,” Spann said.