The Eastern Pacific Has Spawned a Tropical Cyclone Train

The first few weeks of the hurricane season in the eastern Pacific Ocean were so quiet, you could hear crickets. Then on July 2, what would become a flurry of tropical storms began, with one following right after the other like the cars of a train.

Tropical Storm Celia and Hurricane Darby, the third and fourth storms of the season, are churning off the west coast of Mexico, and while they pose no threat to land, the frequency at which these multiple storms have developed is eye-opening — and potentially record-breaking.

According to Eric Blake at the National Hurricane Center, the record for the most tropical cyclones to form in July in the eastern Pacific is seven, set in 1985. (Tropical cyclones are the general term for hurricanes and typhoons.)

A couple of global forecast models indicate two new tropical systems could form over the same parts of the eastern Pacific in the next seven days. While it’s too early to count on the record being broken, it is certainly worth keeping an eye on the basin. The next three storms to form would be named Estelle, Frank, and Georgette.

In contrast to the spate of Pacific storms, the weather in the tropical Atlantic has been fairly quiet over the past few weeks. This is due to a large area of dust and dry air which has been pushed over the Atlantic from the Sahara.

The storm development in both basins is opposite of what forecasters normally expect during the current transition from El Niño to La Niña, as water temperatures near the equator in the central and eastern Pacific Ocean are cool rapidly.

That transition from El Niño to La Niña means that activity in the eastern Pacific is expected to be below-average, as there is stronger vertical wind shear, meaning there is a more of a change in wind speed vertically through the atmosphere. The changing wind speeds make it more difficult for thunderstorms that develop in the tropics and concentrate into a circulation that becomes a tropical cyclone.

Credit: NOAA

None of the current eastern Pacific tropical cyclones will pose a threat to the U.S., as steering winds usually keep these storms on a westward path over the open water of the Pacific. On occasion, the remnants of these systems can get pulled to the north and bring heavy rain to the southwestern U.S. This was the case last year, as the remnants of Hurricane Linda brought flooding rain in July to Southern California, which frequently goes through July with no rain at all.

Hurricane season in the eastern Pacific starts May 15, about two weeks earlier than the Atlantic. The eastern Pacific also does not have the sharp peak in activity like the Atlantic. The busy time starts in July in the eastern Pacific, whereas it typically waits until August in the Atlantic.

Atlantic hurricane season and Eastern Pacific hurricane season.

Credit: Climate Central

The sudden increase in tropical development may be related to the phase of Madden-Julian Oscillation (MJO). The MJO is a pulse of enhanced showers and thunderstorms that circles the tropical part of globe about every 30-60 days. In its current phase, the MJO tends to increase the moisture content in the middle part of the atmosphere over the eastern Pacific.

As the world warms from climate change, its effect on tropical cyclones is still under study. According to the IPCC Fifth Assessment Report, there is the expectation that tropical cyclones will last longer and produce more rain. And while the frequency of the most intense storms may increase, the overall frequency of storms would remain unchanged, or perhaps even decrease.

The jump in storm development over the past several days is a reminder that even when seasonal trends in tropical storm frequency are well known, there will always be exceptions and variations within a season — both in the eastern Pacific as well as the Atlantic