Can forecasters predict weather 90 days out? Why long-term outlooks are so complicated

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

The National Oceanic and Atmospheric Administration’s Climate Prediction Center issues three-month outlooks, also known as seasonal outlooks, up to a year out.

After one of the hottest and driest summers on record across the Southwest, for example, scientists expect more of the same as fall turns to winter, with continued warm and dry conditions overall.

“Outlooks are first and foremost probabilistic,” said 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 can’t predict the temperature or chance of rainfall months out, but they can make general predictions on temperature and precipitation trends in a given season.

Scientists base their outlooks on numerous climatological observations, overarching trends and computerized models. Changes in actual conditions can change the outlook. The Climate Prediction Center has issued a La Niña watch — a climate pattern triggered by cooler Pacific waters — which heavily influences the center’s seasonal outlooks.

There is a 60% chance La Niña conditions will be met, and scientists predict it will emerge in October or November. It could continue through January to March 2025, impacting weather and ocean temperatures around the world.

“During the winter months — December, January and February — the Southwest is still leaning warmer and drier than normal,” said Jon Gottschalk, chief of the operational prediction branch at NOAA’s Climate Predication Center. “Right now, our outlooks are primarily the way they are because of the expectation that La Niña will develop.”

Winter outlook: La Niña could turn Arizona's hot, dry summer into a warm, dry winter. What to know

How do meteorologists make seasonal outlooks?

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

Among the most reliable sources for a seasonal outlook are physical drivers and climate patterns that are easily observable with consistent outcomes. El Niño Southern Oscillation, or ENSO, is one of the best tools for long-term forecasts.

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 trade winds that blow east to west along the equator intensify. These stronger winds push warm water from the eastern Pacific to Asia, warming the water in the western Pacific.

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

“When that happens, changes in tropical rainfall along the equator change the jet streams across the Pacific and North America,” Gottschalk said. “Those changes are what causes temperature and precipitation changes over a season.”

This tends to trigger warmer and drier winters throughout the Southwest, the central Rockies, the Gulf Coast and the eastern seaboard. Surface temperatures in the Atlantic increase in response, which can create more activity during the Atlantic hurricane season.

El Niño is the opposite, when trade winds weaken or occasionally reverse completely, increasing oceanic temperatures. Warmer waters push 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 tend to be closer to normal without extremes triggered by El Niño or La Niña.

ENSO is one of the strongest climate signals and a major driver for seasonal outlooks.

“If there’s not a strong El Niño or La Niña present, the outlooks are going to be based on different things,” Crimmins said.

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

“Those factors can play a significant role in the outlook,” Gottschalk said. “If we’re entering a period in the summer and soil conditions are very dry, that can have a positive feedback on temperature, meaning it will be warmer.”

The opposite can occur if there is more snow cover coming out of winter, as moist soils can provide cooling.

They also use long-term climate trends over a 30-year window. The current timeframe is from 1991 to 2020, and scientists can use data trends in that window to predict if those conditions will continue.

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

“Positive or negative temperature trends are a component to our outlooks. It would be silly for us to not utilize those because, for certain parts of the country in certain seasons, they are quite strong,” Gottschalk said.

“For example, in the Southwest during the spring months, warmer temperature trends are quite strong in the last 10 to 15 years compared to the climatological 30-year period,” he added.

Meteorologists also use climate prediction models, entering data on current conditions and historical trends to simulate the evolution of atmospheric and ocean conditions in the future.

“You get what’s called an ensemble or multiple realizations for the same future season,” Gottschalk said. “The more those models are in agreement, the more confidence we have in the forecast.”

How reliable are long-term outlooks?

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

“Our outlooks are probabilistic, and they’re not certainties, but they can utilize some of our information,” Gottschalk said.

Crimmins simplifies seasonal outlooks to assigning a probability on the map of whether an area is going to have above-average, below-average or normal precipitation and temperatures.

An outlook’s ability to accurately predict the weather one way or another will be weak if there is an equal 33% chance for each event occurring. This is represented through white spots on prediction mapping .

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

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

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

If farmers know it’s going to be a snowy winter, they’ll get into their fields later in the spring. If it’s drier, they can better manage their water supplies to ensure their crops are watered.

The energy sector can prepare energy resources if they know it will be a warmer summer or colder winter. Officials can prepare cooling centers or shelters for extreme cold ahead of time 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’re going to get right now,” Crimmins said. “That’s why you get this probabilistic forecast where it leans dry but leaves the door open for average to above-average conditions. I know it’s maddening to people because they just want to know if it’s going to be dry or not.”

Freeways under water: How a Pacific hurricane flooded Phoenix and why it could happen again

What does this mean for Arizona?

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

“La Niña is forming quite slowly, and the slower it forms, the less time it has to actually peak. The probabilities 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 can be dangerous for the drought-stricken region.

“We’re always in a drought,” Crimmins said. “A La Niña event on the horizon doesn’t spell any relief, and it most likely spells some continued deterioration or sliding towards short-term drought.”

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

“(Drought) is probably going to continue and worsen, and we’ve got to be thinking next spring about the fire season and water situation,” Crimmins said. “We have to be vigilant all the time, 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 hayleigh.evans@arizonarepublic.com .

This article originally appeared on Arizona Republic: Can forecasters predict weather 90 days out? Why long-term outlooks are so complicated