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How weather forecasters produce forecasts for 90 days into the future

Vaseline October 19, 2024

Meteorologists are usually quite confident in predicting the weather for tomorrow or even next week. But when it comes to predicting the weather next month or the month after that, things can get tricky.

The National Oceanic and Atmospheric Administration’s Climate Prediction Center publishes quarterly outlooks, also called seasonal outlooks, up to a year ahead.

For example, after one of the hottest and driest summers on record in the Southwest, scientists expect more of the same as fall turns to winter, with persistently warm and dry conditions.

“Forecasts are primarily probabilistic,” says Michael Crimmins, a climatologist at the University of Arizona. “They don’t tell you exactly how much it’s going to rain on a given day six months from now.”

Forecasters cannot predict the temperature or likelihood of rainfall for months, but they can make general predictions about the temperature and precipitation trends in a given season.

Scientists base their views on numerous climatological observations, overarching trends and computer models. Changes in actual conditions may alter the outlook. The Climate Prediction Center has issued a La Niña watch — a climate pattern caused by cooler waters in the Pacific Ocean — that strongly affects the center’s seasonal outlook.

There is a 60% chance that La Niña conditions will be met, and scientists predict it will appear in October or November. This could last from January to March 2025 and impact weather and ocean temperatures around the world.

“During the winter months – December, January and February – the Southwest continues to be warmer and drier than normal,” said Jon Gottschalk, chief of operational forecasting at NOAA’s Climate Predication Center. “Right now, our outlook is primarily what it is because of the expectation that La Niña will develop.”

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How do meteorologists create seasonal forecasts?

Meteorologists can use numerous strategies and data sets to create seasonal outlooks and predict future trends.

Among the most reliable sources of seasonal forecasts are physical factors and climate patterns that are easily observable and produce consistent results. El Niño Southern Oscillation, or ENSO, is one of the best tools for long-term forecasting.

ENSO is a natural cycle that represents the relationship between ocean temperatures and atmospheric conditions in the Pacific Ocean. There are three ENSO events: El Niño, La Niña and a neutral phase.

La Niña occurs when the trade winds that blow from east to west along the equator become stronger. These stronger winds push warm water from the eastern Pacific Ocean toward Asia, warming the water in the western Pacific Ocean.

This process brings up deep, cold waters in the central and eastern Pacific Ocean, triggering a chain reaction of weather events.

“When that happens, changes in tropical rainfall along the equator alter the jet streams in the Pacific Ocean and North America,” Gottschalk said. “Those changes are the cause of temperature and precipitation changes during a season.”

This tends to cause warmer and drier winters in the Southwest, central Rockies, Gulf Coast and East Coast. In response, surface temperatures in the Atlantic Ocean are rising, which could increase activity during the Atlantic hurricane season.

With El Niño, the opposite is true: the trade winds weaken or sometimes completely reverse, causing ocean temperatures to rise. Warmer water pushes more moisture and energy into the atmosphere, creating wetter and cooler conditions in many areas and strengthening the Pacific hurricane season.

During the neutral phase, trade winds and ocean temperatures are typically closer to normal, without extremes caused by El Niño or La Niña.

ENSO is one of the strongest climate signals and a key driver of seasonal forecasts.

“If there isn’t a strong El Niño or La Niña present, the outlook will be based on different things,” Crimmins said.

Another way scientists can predict long-term weather is through boundary conditions, or conditions that explain deviations from normal. If there is low soil moisture and drought in a particular area, these conditions can increase temperatures.

“These factors can play an important role in the outlook,” said Gottschalk. “If we go into a period of summer and soil conditions are very dry, that could have a positive impact on temperatures, making it warmer.”

The opposite can happen when more snow falls in winter, because moist soil can provide cooling.

They also use long-term climate trends over a 30-year period. The current time frame is from 1991 to 2020, and scientists can use data trends in that window to predict whether these conditions will persist.

As the effects of climate change intensify and the atmosphere warms from greenhouse gas emissions, many of the overarching trends in this window are warmer, which scientists take into account in seasonal forecasts.

“Positive or negative temperature trends are part of our outlook. It would be foolish not to use them, because they are quite strong for certain parts of the country in certain seasons,” Gottschalk said.

“In the Southwest, for example, during the spring months, warmer temperature trends over the past 10 to 15 years have been quite strong compared to the 30-year climatic period,” he added.

Meteorologists also use climate forecast models, inputting data about current conditions and historical trends to simulate the evolution of atmospheric and ocean conditions in the future.

“You get what is called an ensemble or multiple realizations for the same future season,” Gottschalk said. “The more these models agree with each other, the more confidence we have in the prediction.”

How reliable are the long-term forecasts?

While seasonal forecasts are imperative for scientists, agriculture and other industries across the country, the science is not infallible. Conditions may change or the data may not show a clear trend one way or another.

“Our forecasts are probabilistic and they are not certainties, but they can use some of our information,” Gottschalk said.

Crimmins simplifies seasonal forecasts by assigning a probability on the map of whether an area will have above average, below average or normal precipitation and temperatures.

An Outlook’s ability to somehow accurately predict the weather will be weak if there is an equal 33% chance of each event occurring. This is shown by white spots on the forecast maps.

Right now, much of Arizona is colored orange, with a 40 to 50% chance of above-average temperatures and below-average precipitation in December, January and February.

Crimmins said long-term forecasts tend to be strongest in the fall, with spring being the most difficult time to predict the climatic future.

While the forecasts are never final, they help inform the commercial and energy sectors to prepare for the seasons ahead.

If farmers know it will be a snowy winter, they will enter their fields later in the spring. When it is drier, they can better manage their water supply to ensure their crops are watered.

The energy sector can prepare energy sources if they know it will be a warmer summer or a colder winter. Officials can prepare cooling centers or shelters in advance for extreme cold, or allocate more oil and gas to places like the Northeast and Midwest if they know it will be colder.

“The models are basically as good as they can get right now,” Crimmins said. “That’s why you get this probabilistic forecast where it stays dry, but the door remains open for average to above average conditions. I know it drives people crazy because they just want to know if it’s going to be dry or not.”

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What does this mean for Arizona?

NOAA’s current climate outlook favors warmer and drier conditions in the Southwest, but they do not expect this La Niña event to be particularly strong.

“La Niña occurs quite slowly, and the slower it occurs, the less time it has to actually peak. The chances are highest for a weak event,” Crimmins said.

While it likely won’t be the driest or warmest winter on record, anything less than average precipitation could be dangerous for the drought-stricken region.

“We’re always dealing with drought,” Crimmins said. “A La Niña event on the horizon does not mean relief, and it most likely means continued deterioration or a slide into short-term drought.”

After a weak monsoon without widespread, consistent precipitation, Arizona soils are drier. A dry winter could impact water levels and wildfire season.

“(Drought) will likely continue and worsen, and we need to think about fire season and water situations next spring,” Crimmins said. “We need to be constantly vigilant, but La Niña is always the time to get serious.”

Hayleigh Evans writes about extreme weather and related topics for The Arizona Republic and azcentral.com. Email her with story tips at [email protected].