Satellite image of Invest 90L with the area of potential formation outlined in orange. (NOAA/NHC)

Monday evening update: The National Hurricane Center announced Subtropical Storm Andrea has formed as of Monday evening. Called a subtropical storm because it has a blend of both tropical and non-tropical characteristics, it is packing peak winds of 40 mph. The storm could strengthen slightly through Tuesday before starting to weaken and ultimately dissipating later this week.

It becomes the fifth tropical storm to form before the official start of hurricane season in the last five years in the Atlantic. Hurricane season does not begin until June 1.

Original post from Monday afternoon

Hurricane season doesn’t start until June 1, but a tropical or subtropical storm could form in the next few days. If it happens, the storm will be named Andrea.

It does not pose any threat to the United States.

The culprit is an area of disturbed weather centered roughly halfway between the Bahamas and Bermuda. It has slowly become better organized over the past few days. The National Hurricane Center is giving it a 60-percent chance of becoming a named storm by the middle of this week.

While it has been drifting west closer to the East Coast in recent days, an approaching cold front is expected to nudge it north to northeast in the coming days — generally toward Bermuda.

The disturbance has a relatively short window to intensify into a named storm. By Wednesday, wind shear, which tears apart storms, is forecast to become prohibitively strong.


Track and intensity guidance from a series of simulations from the American global model. (weathernerds.org)

If the disturbance does develop into a subtropical or tropical storm, it would make 2019 the fifth consecutive year with a preseason-named storm following Ana in 2015, Bonnie in 2016, Arlene in 2017, and Alberto in 2018.


Date of first named storm formation over the past 50 years. The official start of hurricane season is marked by the horizontal green line, and the median date of first formation is marked by the horizontal dashed black line. (Brian McNoldy)

The trend over the past 50 years has been for the first-named storm to form earlier and earlier, though there is a large spread. The median date for the first-named storm over that period (1969-2018) is June 23, but an objective measure of that spread called the “standard deviation” is a hefty 33 days.

Although the Atlantic hurricane season officially begins on June 1 and ends Nov. 30, storms have historically formed in all months of the year.

The 2019 Atlantic hurricane season is predicted to feature near to slightly below normal activity overall.