IN SPACE - In this handout photo provided by NASA, Hurricane Patricia is seen from the International Space Station. The hurricane made landfall on the Pacfic coast of Mexico on October 23. (Photo by Scott Kelly/NASA via Getty Images)
Why hurricanes are so hard to predict
01:07 - Source: CNN
CNN  — 

For the past five years, a winter hurricane and a series of tropical storms have formed before the official start of the Atlantic hurricane season. Subtropical Storm Andrea continued the trend this week.

The Atlantic hurricane season doesn’t officially begin until June 1, but meteorologists say the right combination of atmospheric conditions and warmer ocean waters can help form a storm at any given time.

Get prepared: Review CNN’s hurricane checklist

So why do we have a hurricane season?

About 97% of the tropical activity in the Atlantic happens between June 1 and November 30, the National Hurricane Center said.

This includes the majority of tropical storms and minor and major hurricanes that have taken place from August through October – the season’s peak.

The season initially stretched from June 15 through November 15, until weather officials decided in 1965 to expand it.

How common are off-season storms?

It depends.

When you look back to the past five years, it appears that off-season storms are on the rise. Hurricane Alex, an unusual winter hurricane, formed in January 2016 in the northeastern Atlantic Ocean. Meantime, April saw the advent of Tropical Storm Arlene in 2017, while May ushered in Tropical Storm Ana in 2015 and Tropical Storm Alberto in 2018.

But if you go back decades, it’s a slightly different story.

In the past 12 years there have been at least six named storms right before hurricane season officially started – during the second half of May – but in the 31 years before that, there was “a lack of any such activity” in the same time frame, said Dennis Feltgen, a meteorologist and spokesman with the National Hurricane Center.

This year’s Subtropical Storm Andrea, Tropical Storm Alberto last year and Bonnie in 2016 were the most recent. Two storms in 2012, Alberto and Beryl were preceded by Arthur in 2008.

The off-season storms are usually pretty weak and not significant. Like Andrea, they generally don’t hit land, CNN meteorologist Brandon Miller said.

Could the hurricane season dates change?

Maybe, but it could take a while.

The eastern Pacific season, which the US National Hurricane Center also monitors, starts May 15 and, like the Atlantic season, runs through November 30. So why not start the Atlantic season then as well?

“The main practical reason would be to have people get prepared a couple weeks early,” Miller said. “If one of these storms hits land in April or May, coastal residents might not have had a chance to prepare their homes and selves.”

Feltgen says weather officials would need more evidence of a considerable change before making a final decision.

“Given the earlier inactivity, NHC would want to be sure that a definitive change in the long-term climatology had occurred before changing the official start date of the Atlantic hurricane season,” he said.

Is climate change impacting off-season storms?

Warmer ocean temperatures caused by climate change could result in early storms, but that’s just one of many factors that come into play, Miller said.

Wind shear and jet streams could also be facilitating more off-season storms. “It is not a 1-to-1 relationship between warmer temperatures and more earlier or later storms,” he said.

Miller says that storms may be reported earlier now because meteorologists have tools at their disposal that weren’t available in previous decades.

“We have better satellites, we fly planes into storms. We might have missed those storms in previous years.”