La Niña winter: NOAA’s November update to its 2020-21 Winter Outlook

La Niña developed in late summer, has strengthened, and leaves virtually no doubt that it will last through the winter. A NOAA Climate.gov blog post explains what that usually means for U.S. temperature and precipitation over the next three months, and what the Climate Prediction Center's (CPC) 2020-21 outlook says.

What past La Niñas show

La Niña, like El Niño, often exerts a significant influence on North American winter climate, so winter patterns tend to be more predictable than usual in some regions. Most of the 20 strongest La Niña events of the past 70 years brought drier-than-average conditions across much of the southern tier of the U.S., especially the Gulf Coast.

Winter precipitation patterns during each of the 20 strongest La Niña episodes since 1950, ranked by the December-February Oceanic Niño Index (the three-month average surface temperature departure from normal in the Niño 3.4 region). Dark green means much wetter than normal and dark brown much drier. Maps by NOAA Climate.gov, from NCEI climate division data.

There is also a weaker signal for wetter-than-average conditions around the Great Lakes, in the Ohio Valley and in the Pacific Northwest, extending along the northern tier, but it is less consistent from episode to episode. Even the best signal is far from certain: the Southeast is drier than average about 70-80% of the time during La Niña.

Winter temperature patterns for the same 20 episodes. Dark red means much warmer than normal and dark blue much colder. Maps by NOAA Climate.gov.

Temperature patterns are less consistent than precipitation. There is some tendency toward above-average temperatures across the southern U.S. and milder winters in the northern Plains. Long-term winter warming likely skews the picture: La Niña episodes early in the record tended to be cooler than later ones, so many early winters show larger areas of below-average temperatures.

Why forecasts are odds

Climate forecasts are issued as probabilities because the expected outcome does not always happen, even when ENSO stacks the deck in its favor. Forecasts only name the most likely outcome and give no guarantees. If a map repeatedly shows a 60% chance of above-average temperatures, about 40% of the time the actual temperatures will not be above average.

The outlook

CPC's outlook resembles the average of the individual maps above, and it looks much like the outlook for the winter of 2017-18, the last winter affected by La Niña, which verified fairly well.

The temperature outlook. Blue shows where odds favor a much colder winter, red a much warmer one, and white equal chances. The darker the color, the stronger the chance of that outcome, not the bigger the departure from average. NOAA Climate.gov map, from CPC data.

• Temperature. Above-normal temperatures are favored across the southern and central U.S. and up the East Coast to Maine, and in Hawaii and western and northern Alaska. Chances exceed 50% from the Southwest east across Texas, the Gulf Coast states and into southern South Carolina. Colder-than-normal temperatures are favored along the northern tier from the Pacific Northwest to western Minnesota and in southeastern Alaska, but with lower confidence: no probability there reaches 50%.
• Precipitation. Below-normal precipitation is favored across the entire southern tier and in southern Alaska, with Arizona east to Texas, the Gulf Coast and Florida most likely to be drier than average. Below-average probabilities over parts of New Mexico and Texas touch 60%.

Sources

• NOAA Climate.gov ENSO blog (https://www.climate.gov/news-features/blogs/enso/what-expect-winter-november-update-noaa%E2%80%99s-2020-21-winter-outlook); rewritten in hubnx's own words.