The Met Office Atlantic tropical storm forecast for 2015 is for eight tropical storms between June and November, with a 70% chance (the ‘70% range’) that the number will be in the range six to ten. This is below-normal relative to the 1980–2010 average of 12 tropical storms.

The forecast number of hurricanes — tropical storms with winds of at least 74 mph — is five (70% range three to seven); the average number of hurricanes is six.

Tropical storm Ana on 8 May 2015 as it approached the South Carolina coast. Credit: NASA Earth Observatory

The forecast Accumulated Cyclone Energy (ACE) index — a measure of the strength and duration of storms over the season — is 74 (70% range 40 to 108); the average ACE index is 104.

The North Atlantic hurricane season typically runs from June to November, but has already seen one tropical storm (Ana) make landfall in South Carolina.

The evolution of the El Niño Southern Oscillation (ENSO) over the next few months will play a large part in the North Atlantic hurricane season.

Forecast centres around the world have now declared that an El Niño has begun in the tropical Pacific.

Joanne Camp, climate scientist at the Met Office, said: “El Niño conditions in the Pacific can hinder the development of tropical storms in the Atlantic, so how this develops will be important for the storm season ahead.”

While it is still too early to determine with confidence how strong this El Niño might be, forecast models from centres around the suggest this El Niño is likely to strengthen during the coming few months.

The tropical storm forecast is produced using the Met Office’s seasonal forecast system, GloSea5.

It has higher resolution than its predecessor, with better representation of the complex physical processes that cause tropical storm and hurricane development.

For regular updates on tropical cyclones worldwide follow @metofficestorms on Twitter.