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August 2020 has been a devastating month across large swaths of the United States: As powerful Hurricane Laura barreled into the U.S. Gulf Coast on August 27, fires continued to blaze in California. Meanwhile, farmers are still assessing widespread damage to crops in the Midwest following an Aug. 10 “derecho,” a sudden, hurricane-force windstorm.
Each of these extreme weather events was the result of a particular set of atmospheric — and in the case of Laura, oceanic — conditions. In part, it’s just bad luck that the United States is being slammed with these events back-to-back-to-back. But for some of these events, such as intense hurricanes and more frequent wildfires, scientists have long warned that climate change has been setting the stage for disaster.
Science News takes a closer look at what causes these kinds of extreme weather events, and the extent to which human-caused climate change may be playing a role in each of them.
On August 25, NASA’s GOES-West satellite watched as hazy gray smoke emanating from hundreds of wildfires in California drifted eastward, while Hurricane Laura barreled toward Louisiana and Texas. Farther south and east are the wispy remnants of Tropical Storm Marco. Laura made landfall on August 27 as a Category 4 hurricane.NOAA
California wildfires
A “dry lightning” storm, which produced nearly 11,000 bursts of lightning between August 15 and August 19, set off devastating wildfires in across California. To date, these fires have burned more than 520,000 hectares.
That is “an unbelievable number to say out loud, even in the last few years,” says climate scientist Daniel Swain, of the Institute of the Environment and Sustainability at UCLA.
Lightning crackles over Mitchell’s Cove in Santa Cruz, Calif., on August 16, part of a rare and severe storm system that triggered wildfires across the state.Shmuel Thaler/The Santa Cruz Sentinel via AP
The storm itself was the result of a particular, unusual set of circumstances. But the region was already primed for fires, the stage set by a prolonged and record-breaking heat wave in the western United States — including one of the hottest temperatures ever measured on Earth, at Death Valley, Calif. — as well as extreme dryness in the region (SN: 8/17/20). And those conditions bear the fingerprints of climate change, Swain says.
The extreme dryness is particularly key, he adds. “It’s not just incremental; it absolutely matters how dry it is. You don’t just flip a switch from dry enough to burn to not dry enough to burn. There’s a wide gradient up to dry enough to burn explosively.”
Both California’s average heat and dryness have become more severe due to climate change, dramatically increasing the likelihood of extreme wildfires. In an Aug. 20 study in Environmental Research Letters, Swain and colleagues noted that over the last 40 years, average autumn temperatures increased across the state by about 1 degree Celsius, and statewide precipitation dropped by about 30 percent. That, in turn, has more than doubled the number of autumn days with extreme fire weather conditions since the early 1980s, they found.
An unusual dry lightning storm combined with very dry vegetation and a record-breaking heat wave to spark hundreds of wildfires across California between August 15 and August 19. One group of these fires, collectively referred to as the LNU Lightning Complex, blazed through Napa, Sonoma, Solano, Yolo and Lake counties. Firefighters continued to battle the LNU complex fires on August 23, including in unincorporated Lake County, Calif. (shown).AP Photo/Noah Berger
Although fall fires in California tend to be more wind-driven, and summertime fires more heat-driven, studies show that the fingerprint of climate change is present in both, Swain says. “A lot of it is very consistent with the long-term picture that scientists were suggesting would evolve.”
Though the stage had been set by the climate, the particular trigger for the latest fires was a “dry lightning” storm that resulted from a strange confluence of two key conditions, each in itself rare for the region and time of year. “’Freak storm’ would not be too far off,” Swain says.
Smoke still engulfed California on August 24, as more than 650 wildfires continued to blaze across the state (red dots indicate likely fire areas). The two largest fires, both in Northern California, were named for the lightning storm that sparked them: the LNU Lightning Complex and the SCU Lightning Complex. They are now second and third on the list of California’s largest wildfires.NASA Worldview, Earth Observing System Data and Information System (EOSDIS)
The first was a plume of moisture from Tropical Storm Fausto, far to the south, which managed to travel north to California on the wind and provide just enough moisture to form clouds. The second was a small atmospheric ripple, the remnants of an old thunderstorm complex in the Sonoran Desert. That ripple, Swain says, was just enough to kick-start mixing in the atmosphere; such vertical motion is the key to thunderstorms. The resulting clouds were stormy but very high, their bases at least 3,000 meters aboveground. They produced plenty of lightning, but most rain would have evaporated during the long dry journey down.
Possible links between climate change and the conditions that led to such a dry lightning storm would be “very hard to disentangle,” Swain says. “The conditions are rare to begin with, and not well modeled from a weather perspective.”
But, he adds, “we know there’s a climate signal in the background conditions that allowed that rare event to have the outcome it did.”
Midwest derecho
On August 10, a powerful windstorm with the ferocity of a hurricane traveled over 1,200 kilometers in just 14 hours, leaving a path of destruction from eastern South Dakota to western Ohio.
The storm was what’s known as a derecho, roughly translating to “straight ahead.” These storms have winds rivaling the strength of a hurricane or tornado, but push forward in one direction instead of rotating. By definition, a derecho produces sustained winds of at least 93 kilometers per hour (similar to the fury of tropical storm-force winds), nearly continuously, for at least 400 kilometers. Their power is equally devastating: The August derecho flattened millions of hectares of crops, uprooted trees, damaged homes, flipped trucks and left hundreds of thousands of people without power.
A powerful derecho on August 10 twisted these corn and soybean grain bins in Luther, Iowa. The storm-force winds swept 1,200 kilometers across the U.S. Midwest, from South Dakota to Ohio, damaging homes and croplands and leaving hundreds of thousands of people without power.Daniel Acker/Getty Images
The Midwest has had many derechos before, says Alan Czarnetzki, a meteorologist at the University of Northern Iowa in Cedar Falls. What made this one significant and unusual was its intensity and scale — and, Czarnetzki notes, the fact that it took even researchers by surprise.
Derechos originate within a mesoscale convective system — a vast, organized system of thunderclouds that are the basic building block for many different kinds of storms, including hurricanes and tornadoes. Unlike the better-known rotating supercells, however, derechos form from long bands of swiftly moving thunderstorms, sometimes called squall lines. In hindsight, derechos are easy to recognize. In addition to the length and strength conditions, derechos acquire a distinctive bowlike shape on radar images; this one appeared as though the storm was aiming its arrow eastward.
But the storms are much more difficult to forecast, because the conditions that can lead them to form can be very subtle. And there’s overall less research on these storms than on their more dramatic cousins, tornadoes. “We have to rely on situational awareness,” Czarnetzki says. “Like people, sometimes you can have an exceptional storm arise from very humble origins.”

The Aug. 10 derecho was particularly long and strong, with sustained winds in some places of up to 160 kilometers per hour (100 miles an hour). Still, such a strong derecho is not unheard of, Czarnetzki says. “It’s probably every 10 years you’d see something this strong.”
Whether such strong derechos might become more, or less, common due to climate change is difficult to say, however. Some anticipated effects of climate change, such as warming at the planet’s surface, could increase the likelihood of more and stronger derechos by increasing atmospheric instability. But warming higher in the atmosphere, also a possible result of climate change, could similarly increase atmospheric stability, Czarnetzki says. “It’s a straightforward question with an uncertain answer.”
Atlantic hurricanes
Hurricane Laura roared ashore in Louisiana in the early morning hours of August 27 as a Category 4 hurricane, with sustained winds of about 240 kilometers per hour (150 miles per hour). Just two days earlier, the storm had been a Category 1. But in the mere 24 hours from August 25 to August 26, the storm rapidly intensified, supercharged by warm waters in the Gulf of Mexico.
Hurricane Laura intensified rapidly due to the warm waters of the Gulf of Mexico, strengthening from a Category 1 hurricane on August 25 to a Category 4 on August 26 (shown). The U.S. National Hurricane Center warned coastal residents of Louisiana and Texas to expect a storm surge — ocean waters elevated by the storm above the normal tide level — of as much as five meters.NOAA
The Atlantic hurricane season is already setting several new records, with the National Oceanographic and Atmospheric Administration predicting as many as 25 named storms, the most the agency has ever anticipated (SN: 8/7/20).
At present, 2005 still holds the record for the most named storms to actually form in the Atlantic in a given season, at 28 (SN: 8/22/18). But 2020 may yet surpass that record. By August 26, 13 named storms had already formed in the Atlantic, the most ever before September.
The previous week, researchers pondered whether another highly unusual set of circumstances might be in the offing. As Laura’s track shifted southward, away from Florida, tropical storm Marco appeared to be on track to enter the Gulf of Mexico right behind it. That might have induced a type of physical interaction known as a Fujiwhara effect, in which a strong storm might strengthen further as it absorbs the energy of a lesser storm. In perhaps a stroke of good luck in the midst of this string of weather extremes, Marco dissipated instead.
As Hurricane Laura approached landfall, the U.S. National Hurricane Center warned that “unsurvivable” storm surges of up to five meters could inundate the Gulf Coast in parts of Texas and Louisiana. Storm surge is the height to which the seawater level rises as a result of a storm, on top of the normal tidal level.
Debris litters Lake Charles, La., in the aftermath of Hurricane Laura’s landfall August 27.AP Photo/Gerald Herbert
It’s impossible to attribute the fury of any one storm to climate change, but scientists have observed a statistically significant link between warmer waters and hurricane intensity. Warm waters in the Atlantic Ocean, the result of climate change, juiced up 2017’s hurricanes, including Irma and Maria, researchers have found (SN: 9/28/18).
And the Gulf of Mexico’s bathlike waters have notably supercharged several hurricanes in recent years. In 2018, for example, Hurricane Michael intensified rapidly before slamming into the Florida panhandle (SN: 10/10/18). And in 2005, hurricanes Katrina and Rita did the same before making landfall (SN: 9/13/05).
As for Laura, one contributing factor to its rapid intensification was a drop in wind shear as it spun through the Gulf.  Wind shear, a change in the speed and/or direction of winds with height, can disrupt a storm’s structure, robbing it of some of its power.  But the Gulf’s warmer-than-average waters, which in some locations approached 32.2° C (90° Fahrenheit), were also key to the storm’s sudden strength. And, by warming the oceans, climate change is also setting the stage for supercharged storms, scientists say. 

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Weather forecasters in the Philippines got the tip-off in the second week of November 2019. A precipitation forecast that peered further into the future than usual warned that the islands faced torrential rains more than three weeks away. The meteorologists alerted local and national governments, which sprang into action. Mobile phone and broadcast alerts advised people to prepare to evacuate.
By the time the Category 4 Typhoon Kammuri lashed the Philippines with heavy rains in early December, the damage was much less than it could have been. Having so much time to prepare was key, says Andrew Robertson, a climate scientist at Columbia University’s International Research Institute for Climate and Society in Palisades, N.Y. “It’s a great example of how far we’ve come” in weather forecasting, he says. “But we still need to go further.”
Such efforts, known as “subseasonal forecasting,” aim to fill a crucial gap in weather prediction. The approach fits between short-term forecasts that are good out to about 10 days in the future and seasonal forecasts that look months ahead.
A subseasonal forecast predicts average weather conditions three to four weeks away. Each day of additional warning gives emergency managers that much more time to prepare for incoming heat waves, cold snaps, tornadoes or other wild weather. Groups such as the Red Cross are starting to use subseasonal forecasts to strategize for weather disasters, such as figuring out where to move emergency supplies when it looks like a tropical cyclone might hit a region. Farmers look to subseasonal forecasts to better plan when to plant and irrigate crops. And operators of dams and hydropower plants could use the information to get ready for extra water that may soon tax the systems.
Subseasonal forecasting is improving slowly but steadily, thanks to better computer models and new insights about the atmospheric and oceanic patterns that drive weather over the long term. “This is a new frontier,” says Frédéric Vitart, a meteorologist at the European Centre for Medium-Range Weather Forecasts in Reading, England.

The in-between
Weather forecasters are always pushing to do better. They feed weather observations from around the world into the latest computer models, then wait to see what the models spit out as the most likely weather in the coming days. Then the researchers tweak the model and feed it more data, repeating the process again and again until the forecasts improve.
But anyone who tells you it will be 73° Fahrenheit and sunny at 3 p.m. four weeks from Monday is lying. That’s just too far out in time to be accurate. Short-term forecasts like those in your smartphone’s weather app are based on the observations that feed into them, such as whether it is currently rainy in Northern California or whether there are strong winds over central Alaska. For forecasting further into the future, what the rain or winds were like many days ago becomes less and less relevant. Most operational weather forecasts are good to about 10 to 14 days but no further.
Early warnings of Typhoon Kammuri’s approach enabled safe evacuations of many thousands of residents of the Philippines in early December 2019.Ezra Acayan/Stringer/Getty Images News
A few times a year, forecasters draw up seasonal predictions, which rely on very different types of information than the current weather conditions that feed short-term forecasts. The long-term seasonal outlooks predict whether it will be hotter or colder, or wetter or drier, than normal over the next three months. Those broad-brush perspectives on how regional climate is expected to vary are based on slowly evolving planetary patterns that drive weather over the scale of months. Such patterns include the intermittent oceanic warming known as El Niño, the extent of sea ice in the Arctic Ocean and the amounts of moisture in soils across the continents.
Between short-term and seasonal prediction lies the realm of subseasonal prediction. Making such forecasts is hard because the initial information that drives short-term forecasts is no longer useful, but the longer-term trends that drive seasonal forecasts have not yet become apparent. “That’s one of the reasons there’s so much work on this right now,” says Emily Becker, a climate scientist at the University of Miami in Florida. “We just ignored it for decades because it was so difficult.”

A global impact
Part of the challenge stems from the fact that many patterns influence weather on the subseasonal scale — and some of them aren’t predictable. One pattern that scientists have been targeting lately, hoping to improve predictions of it, is a phenomenon known as the Madden-Julian Oscillation, or MJO.
The MJO isn’t as well-known as El Niño, but it is just as important in driving global weather. A belt of thunderstorms that typically starts in the Indian Ocean and travels eastward, the MJO can happen several times a year.
An active MJO influences weather around the globe, including storminess in North America and Europe. Subseasonal forecasts are more likely to be accurate when an MJO is happening because there is a major global weather pattern that will affect weather elsewhere in the coming weeks.
But there’s still a lot of room for prediction improvement. The computer models that simulate weather and climate aren’t very good at capturing all aspects of an MJO. In particular, models have a hard time reproducing what happens to an MJO when it hits Southeast Asia’s mix of islands and ocean known as the Maritime Continent. This realm — which includes Indonesia, the Philippines and New Guinea — is a complex interplay of land and sea that meteorologists struggle to understand. Models typically show an MJO stalling out there rather than continuing to travel eastward, when in reality, the storms usually keep going.

At Stony Brook University in New York, meteorologist Hyemi Kim has been trying to understand why models fail around the Maritime Continent. Many of the models simulate too much light precipitation in the tropics, she found. That light drizzle dries out the lower atmosphere, contributing to the overly dry conditions favored in these models. As a result, when the MJO reaches the Maritime Continent, the dryness of most models prevents the system from marching eastward, Kim and colleagues reported in August 2019 in the Journal of Geophysical Research: Atmospheres. In real life, that rain doesn’t happen. With this better understanding of the difference between models and observations in this region, researchers hope to build better forecasts for how a particular MJO might influence weather around the world.
“If you can predict the MJO better, then you can predict the weather better,” Becker says. Fortunately, scientists are already making those tweaks, by developing finer-grained computer models that do a better job capturing how the atmosphere churns in real life.
Meteorologist Victor Gensini of Northern Illinois University in DeKalb led a recent project to use the MJO, among other factors, to forecast tornado outbreaks in the central and eastern United States two to three weeks in advance. As the MJO moves across and out of the Maritime Continent, it triggers stronger circulation patterns that push air toward higher latitudes. The jet stream strengthens over the Pacific Ocean, setting up long-range patterns that are ultimately conducive to tornadoes east of the Rocky Mountains. In the June Bulletin of the American Meteorological Society, Gensini’s team showed that it can predict broad patterns of U.S. tornado activity two to three weeks ahead of time.
High above the poles
Another weather pattern that might help improve subseasonal forecasts is a quick rise in temperature in the stratosphere, a layer of the upper atmosphere, above the Arctic or Antarctic. These “sudden stratospheric warming” events happen once every couple of years in the Northern Hemisphere and much less often in the Southern Hemisphere. But when one shows up, it affects weather worldwide. Shortly after a northern stratosphere warming, for instance, extreme storms often arrive in the United States.
In August 2019, one of these rare southern warmings, the largest in 17 years, began over the South Pole. Temperatures soared by nearly 40 degrees Celsius, and wind speeds dropped dramatically. This event shifted lower-level winds around Antarctica toward the north, where they raised temperatures and dried out parts of eastern Australia. That helped set up the tinder-dry conditions that led to the devastating heat and fires across Australia in late 2019 and early 2020 (SN: 2/1/20, p. 8).
Thanks to advanced computer models, forecasters at Australia’s Bureau of Meteorology in Melbourne saw the stratospheric warming coming nearly three weeks in advance. That allowed them to predict warm and dry conditions that were conducive to fire, says Harry Hendon, a meteorologist at the bureau.
Stratospheric warming events last for several months. As with an MJO, a subseasonal forecast made while one of them is happening tends to be more accurate, because the stratospheric warming affects weather on the timescale of weeks to months. Meteorologists call such periods “forecasts of opportunity,” because they represent times when forecasts are likely to be more skillful. It’s like how it’s easier to predict your favorite baseball team’s chances for the season if you know they’ve just hired the best free agent around.

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A clearer picture
Now, researchers are pushing wherever they can to eke out improvements in subseasonal forecasts. The European forecast center where Vitart is based has been issuing subseasonal predictions since 2004, which have been improving with time. The U.S. National Oceanic and Atmospheric Administration began issuing similar predictions in 2017; they are not as accurate as the European forecasts, but have been getting better over time. Meanwhile, scientists have launched two big efforts to compare the various forecasts.
Vitart and Robertson lead one such project, under the auspices of the World Meteorological Organization in Geneva. Known as S2S, the meteorological shorthand for “subseasonal to seasonal,” the project collects subseasonal forecasts from 11 weather prediction agencies around the world, including the European center and NOAA. The forecasts go into an enormous database that researchers can study to see which ones performed well and why. Kim, for instance, used the database, among others, to understand why models have a hard time capturing the MJO’s march across the Maritime Continent.
The second effort, known as SubX, for the Subseasonal Experiment, uses forecasts from seven models produced by U.S. and Canadian research groups. Unlike S2S, SubX operates in nearly real time, allowing forecasters to see how their subseasonal predictions pan out as weather develops.
That proved useful in early 2019, when SubX forecasts foresaw, weeks before it happened, the severe cold snap that hit the United States in late January and early February. Temperatures dropped to the lowest in more than two decades in some places, and more than 20 people died in Wisconsin, Michigan and elsewhere.

Having an extra week’s heads-up that extreme weather is coming can be huge, Robertson says. It gives decision makers the time they need to assess what to do — whether that’s watering crops, moving emergency supplies into place or prepping for disease outbreaks.
In just one example, Robertson and colleagues recently developed detailed subseasonal forecasts of monsoon rains over northern India. He and Nachiketa Acharya, a climate scientist at Columbia University, described the work in January in the Journal of Geophysical Research: Atmospheres.
C. Chang
In 2018, the scientists focused on the Indian state of Bihar, where the regions north of the Ganges River are flood-prone and the regions to the south are drought-prone. Every week from June through September, the team worked with the India Meteorology Department in New Delhi to produce subseasonal rainfall forecasts for each of Bihar’s regions. The forecasts went to the state’s agricultural universities for distribution to local farmers. So when the summer monsoon rains arrived nearly 16 days later than usual, farmers were able to delay planting their rice and other crops until closer to the time of the monsoon, Acharya says. Such subseasonal forecasts can save farmers both time and money, since they don’t need to pay for irrigation when it’s not needed.
Acharya is now working with meteorologists in Bangladesh to develop similar subseasonal forecasts for that country. There the monsoon rains typically start around the second week in June but can fluctuate — creating uncertainty for farmers trying to decide when to plant. “If we can predict the monsoon onset by around the mid or end of May, it will be huge,” Acharya says.
Nachiketa Acharya (front row, white sweater), Andrew Robertson (behind Acharya) and other climate scientists work with farmers and other residents of Bihar, a state in northern India, to develop and disseminate longer-term weather forecasts so that residents can plan when to plant and irrigate their crops.N. Acharya
Subseasonal forecasts can also help farmers improve productivity in regions such as western Africa, says Shraddhanand Shukla, a climate scientist at the University of California, Santa Barbara. He leads a new NASA-funded project that is kicking off to help farmers better time their crop planting and watering. The effort will combine satellite images of agricultural regions with subseasonal forecasts out to 45 days. If farmers in Senegal had such information in hand back in 2002, Shukla says, they could have better managed their plantings in the run-up to a drought that killed many crops.
As global temperatures rise and climate changes, meteorologists need to keep pushing their models to predict weather as accurately as possible as far in advance as possible, Vitart says. He thinks that researchers may eventually be able to issue forecasts 45 to 50 days in the future — but it may take a decade or more to get to that point. New techniques, such as machine learning that can quickly winnow through multiple forecasts and pinpoint the most accurate one, may be able to accelerate that timeline.
“There’s no single breakthrough,” Becker says. “But there are a lot of little breakthroughs to be made, all of which are going to help.” More

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Amid a sweltering heat wave across the western United States, a remote spot in Death Valley, Calif., may have just earned the title of hottest place on Earth in nearly a century.
On August 16, the Death Valley spot — appropriately named Furnace Creek, with a population of 24 — logged a temperature of 130° Fahrenheit (54.4° Celsius). If verified by the World Meteorological Organization, or WMO, that temperature will be the hottest recorded since 1931, and the third hottest since record keeping began.
Furnace Creek also holds the record for hottest recorded temperature on Earth, logged in 1913 at 134° F (56.7° C). In second place is Kebili, Tunisia, with a logged temperature of 55.0° C (131° F) on July 7, 1931.
The  verification process for such global records of weather extremes, which are archived at WMO, may take months, says archive chief Randall Cerveny, a meteorologist at Arizona State University in Tempe (SN: 7/1/20). Substantiating a record involves an international committee of atmospheric scientists poring over the original observations, the equipment used to make it and the calibration practices. But “based on available evidence, we are preliminarily accepting the observation,” Cerveny says.
Some scientists have contested the 1913 observation. In 2016, an analysis posted online at Weather Underground suggested that the logged temperature was “essentially not possible” based on meteorological conditions, including that there was no evidence of a particularly intense heat wave from any other stations in the area at the time. For now, though, the record stands, because “no credible substantial evidence” supporting this claim has been submitted to WMO, Cerveny says.  
There is precedent for previous records being dismissed once disproven. In 2012, WMO determined that what was then thought to be the hottest recorded temperature, a 1912 observation of 57.8° C (136° F) in Libya, was not valid. That was supported by the discovery in 2010 of the original, mislogged observation sheet bearing five separate errors. More