Tropical storm could form before start of Atlantic hurricane season for sixth straight year

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The Atlantic hurricane season doesn’t officially start until June 1, but the National Hurricane Center is tracking a disturbance off the East Coast that could become a named storm this weekend.

If the storm forms, earning the name Arthur, it will mark the sixth year this has happened before hurricane season.

The weather system might be a harbinger of an active Atlantic hurricane season. Preseason forecasts are almost all predicting a substantially busier than normal season, with one forecast out of Pennsylvania State University calling for one of the stormiest seasons on record.

On Tuesday, the Hurricane Center issued a special statement noting an area of low pressure is expected to develop “a couple hundred miles northeast of the Bahamas” around the weekend. Given favorable environmental conditions, the Hurricane Center stated it has a 50% chance of developing into a depression or storm.

If the low pressure system develops and gets a name, it would likely be considered a “subtropical” storm, containing features of both tropical and nontropical (or mid-latitude) weather systems. Storms with subtropical characteristics are most common among those that form unusually early or late relative to hurricane season dates (June 1 to Nov. 30).

The American (GFS), Canadian, UKMet, and European (ECMWF) computer models all simulate the development of a subtropical weather system over the weekend, but they mostly forecast it to remain safely offshore of the East Coast.

Just the Canadian model forecasts that its western periphery might graze the North Carolina Outer Banks with some gusty showers Sunday night.

By early next week, forecast models generally predict the weather system to be drawn into a cold front pushing off the East Coast toward the open Atlantic. Before doing so, it might generate some rough surf for the Southeast and Mid-Atlantic coastlines.

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Preseason storms have been a staple in the tropical Atlantic in recent years, forming in each of the past five.

Andrea, which formed on May 20 last year, became the seventh preseason storm to develop in 10 years. It followed Alberto in 2012 (May 19), Beryl in 2012 (May 25), Ana in 2015 (May 8), Bonnie in 2016 (May 28), Arlene in 2017 (April 20) and Alberto in 2018 (May 26).

This list does not include Alex, which formed on Jan. 16, 2016. While falling in the 2016 calendar year, Alex was meteorologically a remnant of the 2015 season.

There is clearly a trend toward earlier instances of first storm formation over the past five decades, with a range spanning from April 20 to Aug. 30 and a median date of June 20.

Jim Kossin, a climate scientist at the University of Wisconsin, published a study in 2008 indicating the Atlantic hurricane season has grown longer due to the warming of the ocean. And the flurry of May storms over the past decade would only seem to bolster that study’s finding.

The official start and end dates of June 1 to Nov. 30, set by the Hurricane Center, were never intended to capture all of the tropical storm and hurricane activity, just the vast majority. The dates have been changed over time and could certainly be adjusted again.

When an official “hurricane season” was first defined about 86 years ago, it spanned June 15 to Oct. 31. Since then, it was changed to June 15 through Nov. 15, then to June 1 through Nov. 15, and, in 1965, to its current dates (June 1 through Nov. 30).

If it is next expanded to May 15 through Nov. 30, it would conveniently match the bounds of hurricane season in the eastern Pacific Ocean.

This year, the eastern Pacific Ocean saw its earliest start to hurricane season on record. On April 25, the National Hurricane Center announced that a minor disturbance southwest of the Baja Peninsula had become a tropical depression. The depression formed two weeks earlier in the year than the previous first-forming tropical system on record in that ocean basin.