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    • La Niña conditions could be weaker later, new forecast says
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    La Niña conditions could be weaker later, new forecast says

    Harry Posted on

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    National Weather Service expanded coverage Watch La NiñaIt now predicts a weak and brief event expected to occur within the next two months.

    La Niña conditions could herald hot, dry winters for the Southwest, and the weather service initially predicted such a cycle would occur between September and November. It has now lowered its forecast to 57% by the end of the year.

    “It’s hard to say exactly what a weaker start to La Niña might be, but statistically we can expect an overall drier period from January through March,” said Arizona state climatologist Erinanne Saffell.

    While ocean conditions reflect an impending La Niña event, sea surface temperatures are generally near average and have not yet reached the La Niña threshold.

    Here’s the latest on this year’s La Niña watch:

    What is La Niña?

    La Niña and its counterpart El Niño are climatological conditions that are part of a natural cycle called the El Niño-Southern Oscillation, or ENSO. The cycle represents the connection between ocean temperatures and atmospheric conditions occurring in the Pacific Ocean.

    ENSO has three phases: La Niña, El Niño, and the neutral phase.

    During La Niña conditions, trade winds blowing from east to west intensify near the equator. Winds push warm waters from the Eastern Pacific towards Asia, increasing surface temperatures in the Western Pacific. This shift calls for deep, cold waters in the central and eastern Pacific.

    This triggers a chain reaction of weather events. More tropical rain tends to fall in Asia and Australia as the atmosphere responds to changes in ocean temperature. This shifts jet streams across the Pacific and North America, affecting weather across the continent.

    The Atlantic Ocean experiences a more intense hurricane season, while the Pacific experiences a milder hurricane season.

    El Niño is the opposite: Trade winds weaken or even reverse completely, increasing ocean temperatures. Warmer waters send more moisture and energy into the atmosphere, triggering wetter conditions in many regions and accelerating the Pacific hurricane season.

    The third ENSO pattern, the neutral phase, tends to produce closer to average trade wind and ocean temperatures without the extremes triggered by La Niña or El Niño. Sea surface temperatures are currently near average, meaning ENSO neutral conditions remain for now.

    Dry conditions continue: What will it take to end Arizona’s 30-year drought?

    What is the threshold for a La Niña event?

    Climate scientists monitor sea surface temperatures in the central and eastern equatorial Pacific to predict ENSO events. They look for below-average sea surface temperatures for La Niña.

    The official threshold is 0.5 degrees Celsius or 0.9 degrees Fahrenheit below normal.

    As of November 14, the last temperature change from normal in the Pacific was 0.2 to minus 0.3 degrees Celsius below normal, remaining below the threshold.

    “La Niña forms quite slowly, and the slower it forms, the less time it takes for it to peak,” University of Arizona climate scientist Michael Crimmins said in an October interview. “These events have historically presented us with mixed conditions for the Southwest.”

    What is a La Niña winter like?

    Because La Niña conditions develop slowly, climate scientists expect a weak event.

    La Niña affects regions differently. The Southwest, Central Rockies, Gulf Coast, and East Coast experience drier, warmer winters, while the Pacific Northwest, New England, and the Great Lakes generally experience wetter winters.

    Even with a mild La Niña, Arizona will likely experience below-average winter precipitation.

    “This doesn’t mean we won’t get any precipitation, but it does mean winter precipitation in general may be below average from January through March,” Saffell said.

    “Arizona has been lucky with above-average precipitation the last two winters, but unfortunately three wet winters in a row don’t happen very often in Arizona,” he added. “The last time it happened was in 1991, 1992 and 1993.”

    Heat off: Here is AZ’s record summer according to the numbers

    Hayleigh Evans writes about extreme weather and related topics for the Arizona Republic and azcentral.com. Email him story tips: [email protected].

    Harry
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