Since the summer, NOAA’s Climate Prediction Center (CPC) had been favoring the gradual development of El Niño conditions, going so far as to issue an El Niño watch in recent months. But no more.
[T]he previous El Niño Watch has been discontinued as the chance of El Niño has decreased,” CPC writes in a monthly El Niño discussion released today.
Instead of El Niño, NOAA predicts that the neutral phase of the El Niño Southern Oscillation (ENSO) index will prevail. This neutral phase, sometimes called “La Nada”, is the murky middle ground between El Niño and its sister La Niña (associated with cool water in the equatorial Pacific).
“Relative to last month, the [sea surface temperature] model predictions more strongly favor ENSO-neutral,” CPC writes.
“[T]he tropical ocean and atmosphere may resemble a weak El Niño at times,” CPC writes.
What does all of this mean with respect to winter weather across the U.S. and the Mid-Atlantic, in particular?
That’s anyone’s guess. Recognizing El Niño might be muted, NOAA was decidedly undecided about how the winter would play out in its seasonal outlook released in mid-October. It called for equal chances of above or below average temperatures and precipitation over large parts of the U.S.
“This is one of the most challenging outlooks we’ve produced in recent years because El Niño decided not to show up as expected,” said CPC deputy director Michael Halpert.
When a full-fleged El Niño or La Niña is present, if often serves a useful guide as to the general temperature and precipitation patterns. Without one or the other, these patterns are much less predictable.
Instead, weather conditions become subject to the whims of the North Atlantic Oscillation (NAO), which can change every couple of weeks. When the NAO is negative, the jet stream buckles over Eastern North America, allowing cold air to plunge south, sometimes inciting storminess along the East Coast, like it did often in the Snowmageddon winter of 2009-2010. But when the NAO is positive, the jet stream is displaced far to the north, allowing warm air to flood much of the Eastern half of the country, like last winter.
Some researchers, like Judah Cohen of Atmospheric and Environmental Research (AER) are employing new techniques to predict the prevailing phase of the NAO. Cohen has linked October snow cover in Siberia to the NAO’s dominant winter mode. When there’s high Siberia snow cover and it advances quickly in October, it favors a negative NAO in months that follow.
“Snow cover [in Siberia] is more extensive than last year,” Cohen told the Worcester Telegram.
Capital Weather Gang’s winter outlook will be released on Tuesday, November 13.