A few weeks ago, the Australian Bureau of Meteorology declared that the Pacific Ocean is no longer in an El Niño state and has returned to “neutral.” American scientists at the National Oceanic and Atmospheric Administration have been more hesitant, but they estimate that there is an 85% chance that the Pacific will enter a neutral state in the next two months and a 60% chance that a La Niña event will begin by August.

After an El Niño that was one of the three strongest in the last 40 years and that brought a wet winter to the U.S. — and California, in particular — this transition could mean a dramatic shift in weather as we enter the summer.

The progression from El Niño to La Niña, which is part of a broad system called the “El Niño Southern Oscillation,” or ENSO, is the result of conditions in the tropical Pacific. During the neutral phase, which is or soon will be in effect, the so-called trade winds rush from east to west along the equator. These winds push warm surface water with them, bathing Indonesia and New Guinea in the balmy waters of the “Pacific Warm Pool” and forcing cold water to rise from the deep ocean along the coast of South America.

As an El Niño phase begins, these winds weaken, so that warm sea surface temperatures move east toward South America. This can cause climatic shifts across the globe: landslides in Peru, drought in Australia, fish die-offs in the eastern Pacific and more frequent atmospheric rivers in Southern California. These changing weather patterns also weaken the trade winds further, leading to more warm water off the coast of South America, which in turn weakens the winds, and so on.

So what prevents El Niño events from continuing to strengthen forever?

Well, it turns out you can think of the Pacific Ocean sort of like one enormous bathtub, and El Niño like a wave of warm water sloshing from one end of the bathtub to another. When that wave reaches the Ecuadorean coast, it bounces back, carrying the warm water back toward Asia and Oceania, which strengthens the trade winds, which push the warm water faster, until the wave reaches the other end of the “bathtub” — this is a La Niña phase, when the west Pacific is especially warm and the east Pacific especially cold — at which point the process repeats. This is the “oscillation” that gives ENSO its name, and it is why a strong La Niña event often follows a strong El Niño.

This winter’s El Niño event had sea surface temperature anomalies of 3.6 degrees (2 degrees Celsius), which qualifies it for the unofficial status of “very strong El Niño.” As is typical, the warm waters of El Niño led to high global temperatures, but because of the unprecedented effects of climate change, these temperatures were anything but typical. In December, when El Niño was at its peak, global surface temperatures were 0.45 degrees (0.25 degrees Celsius) above the next hottest December on record.

This increase may not seem so unusual given the current era of ever-climbing temperatures, but when you consider that the difference between the coldest December on record (back in 1916) and the second-hottest (in 2016) is less than 3.6 degrees, it is far more shocking — so surprising that prominent climate scientists have begun to publicly wonder whether there are elements missing from our understanding of climate change.

Fortunately, the onset of neutral ENSO conditions, followed by the likely La Niña, should begin to bring global temperatures down, at least temporarily. This will be little consolation for the U.S., as the National Weather Service predicts above-average summer temperatures for virtually the entire country. Moreover, La Niña events are associated with drier conditions across the southwestern U.S. that could persist into next winter. While this year’s generous Sierra snowpack should insulate California from the effects of a scorching summer, the state is never more than one below-average winter from a drought.

There are also potential implications for the rest of the country — La Niña has been linked to higher hail and tornado activity in the Southeast and an increase in hurricanes in the Atlantic and Gulf of Mexico. In fact, many experts are predicting a “hyperactive” hurricane season in the tropical Atlantic, with one forecast going as high as an unprecedented 33 named storms. On the flip side, however, there will probably be a slow hurricane season in the east Pacific, with little chance of a reprise of Hurricane Hilary’s passage over Southern California last August.

Of course, all of these forecasts — that La Niña tends to cause dry conditions in Southern California, that this location will get more hurricanes while that region gets more hail, and even how strong an El Niño or La Niña event can become — are based on correlations and theories that researchers have rigorously developed using data from the last half century.

But given the recent rapidity of climate change, there are no guarantees that the trends of the past will continue to hold in the future. In situations such as these, climate scientists generally look to computer models to understand how phenomena such as ENSO might shift over time.

Unfortunately, many climate models have not yet developed the ability to predict ENSO accurately — its complexity and the fact that it requires the ocean and atmosphere to shift in tandem make it particularly challenging to represent. This means that as we move into a new era of accelerating climate change, the future of ENSO remains uncertain.

Ned Kleiner is a scientist and catastrophe modeler at Verisk. He has a doctorate in atmospheric science from Harvard.

A different kind of storm could complicate this weekend’s plans.

For the first time since January 2005, the U.S. National Oceanic and Atmospheric Administration has issued a severe geomagnetic storm watch for Friday evening.

The category G4 watch from NOAA’s Space Weather Prediction Center signals the possibility that a concentration of energy flaring from the Sun could disturb our planet’s electromagnetic field once it reaches Earth.

A geomagnetic storm of this size could disrupt communications, like the 2003 event that caused blackouts in Sweden and damaged South Africa’s power grid.

More promisingly, super-charged collisions of solar energy into the gas of our atmosphere creates the dazzling phenomena of aurora borealis, or the northern lights. Typically confined to polar regions, the colorful display could be visible this weekend as far south as Northern California.

“We have a very rare event on our hands,” space weather forecaster Shawn Dahl of SWPC said during a news conference Friday morning.

A geomagnetic storm happens when energy from solar wind — the high-speed current of atomic particles the sun is constantly flinging into space — is transferred into the electromagnetic field that surrounds Earth.

The winds of a Category 5 hurricane on our puny little planet can exceed 150 miles per hour. In contrast, solar wind averages about 870,000 miles per hour all day, every day according to NASA.

Earth’s electromagnetic field deflects the majority of these particles, save for those interactions in the polar areas that produce the northern lights. But unusually intense or concentrated eruptions of energy on the surface of the Sun can disrupt that equilibrium, causing geomagnetic storms like the one potentially headed our way.

On Wednesday morning, astronomers noted a series of solar flares and coronal mass ejections — essentially, giant explosions of energy — emanating from a massive sunspot more than 15 times the diameter of Earth.

The Space Weather Prediction Center has observed seven coronal mass ejections, or CMEs, heading in Earth’s direction, said Mike Bettwy, operations chief at SWPC.

“Based on the data we have, all seven of these are going to be spewing that energy toward us,” Bettwy said.

The energy in these various eruptions is expected to merge and reach the Earth’s magnetic field late Friday or early Saturday.

“Our level of confidence is high that we will have an arrival of these CMEs as early as this evening,” Dahl said. The precise time they’ll arrive is less certain.

SPWC will have more clear information once that solar energy reaches NASA’s Advanced Composition Explorer spacecraft, a satellite about 1 million miles from Earth.

Yet “even though that sounds like it’s far away . . . it doesn’t necessarily give us a ton of lead time,” Bettwy said. Given that solar wind is moving faster than the speed of sound, it will reach Earth only 20 to 45 minutes after passing that 1-million-mile marker in space.

Scientists have previously warned that the strongest geomagnetic storms could wreak havoc on our power and communication systems. That is not what’s expected in this case.

Those of us here on Earth may experience power outages and minor internet or GPS glitches, Bettwy said.

“Geomagnetic storms can impact infrastructure in near-Earth orbit and on Earth’s surface, potentially disrupting communications, the electric power grid, navigation, radio and satellite operations,” NOAA said in a statement. “SWPC has notified the operators of these systems so they can take protective action.”

Watches for milder geomagnetic storms are fairly common. In March, a geomagnetic storm briefly reached G4 strength for a few hours.

That event was the third geomagnetic storm to reach G4 status during the current 11-year solar cycle, which began in 2019, according to the SWPC. At that time, the agency said the event posed no risk of adverse impacts to the public.

“What’s unique about what is potentially about to hit us is that it’s a much more significant event, in terms of what’s going to be hitting the atmosphere,” Bettwy said.