A nationwide split between colder- and wetter-than-average conditions in parts of the northern states and warmer- and drier-than-average conditions overall in the southern tier states is on tap for the 2017-18 winter, according to an updated outlook released by The Weather Co.
The most recent National Weather Service Climate Prediction Center (CPC) update reported that weak La Niña conditions emerged during October and now have a 65-75% chance of persisting at least through the Northern Hemisphere winter of 2017-18. In October, CPC had predicted a 55-65% chance for La Niña conditions during the winter months.
“La Niña is likely to affect temperature and precipitation across the United States during the upcoming months,” CPC noted. “The outlooks generally favor above-average temperatures and below-median precipitation across the southern tier of the United States and below-average temperatures and above-median precipitation across the northern tier of the United States.”
The Weather Co. said signs of La Niña's influence may already be in play as November has been delivering colder-than-average temperatures in the North Central and Northwest states.
Temperature outlooks for the winter time frame of December through February from both The Weather Co. and the National Oceanic & Atmospheric Administration feature some similarities in what is typically expected in a La Niña winter.
“As is typical in La Niña base state winters, we expect the greatest risk of cold early in the winter in the eastern U.S., with the cold retreating towards the Pacific Northwest as the winter progresses,” said Dr. Todd Crawford, chief meteorologist with The Weather Co.
According to The Weather Co., the highest odds of below-average temperatures are from the Pacific Northwest to the upper Mississippi Valley but also extend into the Northeast due to increased risks of "blocking" weather patterns featuring dips in the jet stream getting stuck over the eastern states.
“The rest of the U.S. is expected to see near- or above-average temperatures this winter,” the report noted. “The warmest temperatures, relative to average, will likely be found over the South Central states and parts of the desert Southwest. A mild winter is typical throughout the South during La Niña winters."
Since last winter featured a weak La Niña, Crawford said it’s reasonable to use it as an analog for this winter’s forecast. Last winter, much of the East into the South and Midwest had one of their warmest winters, while the Northwest was colder than average.
However, Crawford cautioned against simply assuming that this winter will be an exact repeat of last winter, especially if it has more blocking weather patterns in the upper atmosphere, as mentioned above.
One particular blocking pattern, he said, would feature a southward dip in the jet stream east of the Rockies that could “lock in” for extended periods of time, leading to shivering results.
“Analysis of other analog sets and statistical models suggest increased blocking risks relative to last year, which isn't saying much but should result in a colder winter,” Crawford added.
On the other hand, if a scenario with less blocking panned out, he said conditions would be similar to last year, with below-average temperatures confined to the Pacific Northwest again.
An individual cold front or an upper ridge of high pressure can lead to a period of colder or warmer weather, respectively, that bucks the overall three-month trend, the report explained, adding, “The same front or area of high pressure can bring a brief period of enhanced precipitation or dry spell that may or may not be indicative of the overall trend that is forecast.”
As for precipitation, The Weather Co. suggested that portions of the Ohio Valley, Great Lakes and northern Rockies have the highest odds of above-average precipitation this winter. “Depending on temperatures at any given time this winter, we could see increased odds of snow in those regions,” it said.
Meanwhile, the South could be in for a dry winter, which is typical during La Niña.
“If this pans out as forecast, there is the possibility of growing drought conditions, also giving a boost to wildfire danger in this region next spring,” the report warned.