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    • How weather forecasters make forecasts for 90 days into the future
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    How weather forecasters make forecasts for 90 days into the future

    Caradoc Wyn Posted on

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

    The National Oceanic and Atmospheric Administration’s Climate Prediction Center publishes three-month forecasts, also called seasonal forecasts, covering periods up to a year.

    For example, after one of the hottest, driest summers on record across the Southwest, scientists expect the same thing to happen again as fall turns into winter, with warm and dry conditions continuing overall.

    “Outlooks are primarily probabilistic,” said Michael Crimmins, a climatologist at the University of Arizona. “They don’t tell you exactly how much it will rain on any given day in six months.”

    Meteorologists cannot predict temperature or precipitation chances for months, but they can make general predictions about temperature and precipitation trends in a particular season.

    Scientists base their forecasts on numerous climatological observations, overarching trends and computer-based models. Changes in actual conditions may alter the outlook. The Climate Prediction Center has issued a La Niña watch — a climate pattern triggered by cooler waters in the Pacific — that is heavily influencing the center’s seasonal outlook.

    There is a 60% chance of La Niña conditions occurring, and scientists expect it to occur in October or November. It 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 the operational forecast division at NOAA’s Climate Predication Center. “Right now our outlook is what it is primarily due to the expectation that La Niña will develop.”

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    How do meteorologists make seasonal forecasts?

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

    The most reliable sources of seasonal outlook include physical factors and climate patterns that are easy to observe and provide consistent results. El Niño Southern Oscillation, or ENSO for short, 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 from east to west along the equator become stronger. These stronger winds push warm water from the eastern Pacific into Asia and warm water in the western Pacific.

    This process creates deep, cold water in the central and eastern Pacific and sets off a chain reaction of weather events.

    “When that happens, changes in tropical precipitation along the equator alter jet streams in the Pacific and North America,” Gottschalk said. “These changes cause temperature and precipitation changes over the course of a season.”

    This tends to result in warmer and drier winters throughout the Southwest, central Rocky Mountains, Gulf Coast and East Coast. In response, Atlantic surface temperatures are rising, which may lead to more activity during Atlantic hurricane season.

    El Niño is the opposite, when trade winds weaken or occasionally reverse entirely, causing ocean temperatures to rise. Warmer water pushes more moisture and energy into the atmosphere, leading to wetter and cooler conditions in many areas and intensifying 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 an important driver of seasonal forecasts.

    “If there is no strong El Niño or La Niña, the forecasts will be based on other factors,” Crimmins said.

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

    “These factors can play an important role in the outlook,” said Gottschalk. “As we get into summer and the soil conditions are very dry, that can have a positive impact on the temperature, meaning it will be warmer.”

    The opposite can happen if more snow falls in the winter, as wet soils can cause 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 this time frame to predict whether these conditions will continue.

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

    “Positive or negative temperature trends are part of our forecasts. It would be foolish of us not to take advantage of these because they are quite strong in certain parts of the country at certain times of the year,” Gottschalk said.

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

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

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

    How reliable are long-term forecasts?

    While seasonal forecasts are essential for scientists, agriculture and other industries across the country, 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 not certainties, but they can use some of our information,” Gottschalk said.

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

    A forecast’s ability to accurately predict the weather one way or another will be weak if there is an equal 33% probability of each event. This is represented by white spots on the forecast mapping.

    Currently, much of Arizona is shaded 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 climatological future.

    Although the outlook is never final, it helps the trading and energy sectors prepare for the coming seasons.

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

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

    “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’s 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 dry or not.”

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    What does this mean for Arizona?

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

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

    While it likely won’t be the driest or warmest winter on record, any rainfall below average could be dangerous for the drought-stricken region.

    “We’re always in a drought,” Crimmins said. “A La Niña event on the horizon does not mean relief, but most likely further deterioration or a slide into a short-term drought.”

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

    “(Drought) is likely to continue and worsen, and we need to think about the fire season and water situation next spring,” Crimmins said. “We must always be 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 [email protected].

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