A weak La Niña is likely this winter, and the word doing the work is “weak”

Forecasters put the odds of La Niña conditions developing and persisting through the winter at about 62 percent. Sea surface temperature anomalies in the tracking region are forecast to sit just past the threshold — enough to qualify, not enough to dominate.

That distinction matters more than the headline. The seasonal patterns associated with La Niña — a drier southern tier, a wetter Ohio valley and Pacific northwest, a variable north — come from composites of past events, and those composites are heavily weighted by strong events.

Weak events behave differently

When forecasters separate weak events from strong ones, the picture changes. In weak events, the expected precipitation pattern verified in fewer than half of past winters at most locations. Other drivers — the North Atlantic oscillation, the Madden-Julian oscillation, stratospheric warming events — contribute as much or more, and none of them are predictable at a seasonal range.

The honest summary is that a weak La Niña shifts the odds slightly and leaves most of the winter’s variance unexplained.

A weak event tilts the table. It does not tell you where the ball ends up, and we should stop implying otherwise.— A seasonal forecaster at a climate prediction center

What it is useful for

Seasonal outlooks earn their keep in planning that is sensitive to the average rather than to any single storm: utility fuel procurement, road salt purchasing, reservoir management and agricultural planting decisions.

They are not useful for deciding whether to book a ski trip in February, and every forecaster asked about that question said so in nearly identical words.

• Odds: about 62% for La Niña conditions through the winter
• Strength: anomalies forecast near the qualifying threshold
• Skill: expected pattern verified under half the time in past weak events
• Best use: procurement and resource planning, not individual trip decisions

A further update is due next month. Forecasters noted that weak events are also the hardest to call, and that a shift of a few tenths of a degree in either direction would move this from a weak La Niña to no event at all — which would change the outlook’s wording considerably and its practical value not at all.