To make a successful winter forecast for the U.S., meteorologists must examine a dizzying array of factors, from fickle fluctuations in ocean conditions in the tropical Pacific Ocean to the decline of Arctic sea ice and related snow cover in Siberia.
Each forecasting group, including the National Oceanic and Atmospheric Administration (NOAA), which produces the official outlook for the U.S., has their own biases that can determine whether their projection proves prescient or wildly off target.?
One of the most accurate winter outlooks during the past several years has been issued by a forecast group led by Judah Cohen, a meteorologist and devout snow-lover who works in the private sector at AER in Massachusetts.?
A division of Verisk Analytics, AER forecasts climate and weather conditions for clients in the energy industry, among others, and Cohen has received funding from the National Science Foundation to study possible ties between Arctic sea ice melt, fall snow cover in Siberia, and the behavior of the stratospheric polar vortex during the winter.?
Cohen released his 2017-18 winter outlook to the public last week on the NSF website and published it in a lengthy, geeky blog post on Monday. His forecast differs from NOAA's outlook and that of other groups in some significant respects.?
For example, Cohen thinks that overall, colder-than-average conditions will dominate a large portion of the lower 48 states, with areas from the Pacific Northwest to the Central states and much of the East seeing colder-than-average air temperatures.?
Meanwhile, the Southwest and southern states are likely to see milder-than-average conditions, largely due to the influence of a La Ni?a event that has developed in the tropical Pacific Ocean. Such events, which features cooler-than-average waters along the equator, tend to lead to mild winters across the Southwest and southern tier of the U.S.?
However, La Ni?a is not a particularly strong predictor of how much snow will fall in the big cities of the Northeast. Some La Ni?a winters have been extremely snowy, such as 1995-96 and 2008-9, while others have seen little in the way of blockbuster storms.?
The reasoning behind his new outlook, Cohen said in an interview, is three-fold. ?
First, October snowfall in Eurasia was above normal, which can encourage particular weather patterns to develop that reverberate both downwind and upward into the upper atmosphere.?
Second, Arctic sea ice extent is at its second-lowest ?value on record for this time of year, and should continue to be well below average for the entire winter. This will influence conditions in the Far North and beyond, since open water alters the exchange of heat and moisture with the atmosphere, chartph.compared to areas of the ocean covered by sea ice.?
Third, strong "blocking patterns" have been observed at high latitudes, including over Greenland. Such weather systems can act as stop signs in the sky, forcing storms to linger over particular regions or even in some cases allowing intense storms to form in the first place. Along the East Coast of the U.S., for example, major snowstorms tend to take place when a blocking pattern is in place over Greenland.?
"All three factors favor colder temperatures across at least parts of the Northern Hemisphere mid-latitude continents during the winter months," Cohen wrote.?
Want to know where this cool weather is chartph.coming from? We have a Greenland Block north and Rex block west, forcing arctic air into Canada and the northern US! #USwx #Energy #AGwx
Map provided by @RyanMaue and @WeatherdotUS pic.twitter.chartph.com/PhZKoecNnK
— Beth Carpenter (@B_Carp01) November 22, 2017
Cohen's methods work to try to predict the dominant phase of the Arctic Oscillation during the winter, which is a key factor influencing the weather in the northern midlatitudes, particularly in the Midwest and eastern U.S. and western Europe.?
When the Arctic Oscillation is negative, cold and snowy conditions are favored (though not guaranteed) in the eastern U.S.
Cohen's predictions have been quite accurate for several of the winters during the past decade, warning of unusually cold and snowy weather in the East when others did not pick up on this in advance. However, he called for a colder-than-average winter in the Midwest and East last year, and the opposite occurred, rendering the forecast somewhat of a bust.
Unlike past years, when all signs pointed to a cold winter in the U.S., this year is not a slam dunk forecast in Cohen's view. This is partly because his snow advance index, which looks at how quickly snow cover increases during October across a portion of Eurasia, was below average.?
“[I'm] not terribly confident in the forecast, but I decided to lean more on snow cover extent which would lead to the colder solution,” he said.?
Cohen said that since 1990, there have been four colder-than-average La Ni?a a winters in the East, and five milder-than-average La Ni?a winters. “I think the La Ni?a is maybe a little more robust further west than what a lot of people show just based on the statistics,” he said.
One "wild card" in the winter weather forecast this year, according to Cohen, is the tendency for a strong area of high pressure in the Central Pacific Ocean, which, like a large boulder in a river, affects the airflow around it. If this weather feature shifts east or west it could influence where the coldest air sets up across the U.S., he said.?
A move to the east from its current location, for example, would enhance the risk for a cold winter in the East. “That could be a very dominant player," Cohen said of the ridge of high pressure area.?
The Climate Prediction Center (CPC), which is a unit within NOAA that issues seasonal forecasts, expects a much different winter than Cohen's group does. The leading predictor in CPC's outlook is La Ni?a, which prompted the center to go for a greater likelihood of colder-than-average conditions in the Pacific Northwest and Upper Midwest, and a milder-than-average winter almost everywhere else.?
The CPC has examined the techniques that Cohen's group uses, but hasn't yet decided to incorporate those methods into its work, treating it as emerging science that's still being evaluated.?
Of CPC's outlook, Cohen said it looks remarkably similar to last year's winter forecast, which was predicated on a La Ni?a that never fully developed.?
"It’s hard to have two winters in a row repeat each other," Cohen said. "I guess they kind of shoe-horned this into kind of La Ni?a paradigm. We’ll see how it works out.”
The Weather Company's winter forecast is also similar to CPC, since some of the same signals present leading into last winter are there again.?
Last winter was one of the warmest on record in much of the Midwest, East, and South, whereas the Pacific Northwest was colder-than-average. Todd Crawford, chief meteorologist for The Weather Company, which IBM owns, also cautioned that blocking patterns could be the difference between a milder-than-average winter in the East and a frigid one.?
Where to plan that ski vacation
One chartph.common thread through all these outlooks is that ski areas in the Pacific Northwest as well as northern Rockies could do quite well, snowfall-wise, this winter. This means that resorts in British Columbia, Oregon, Washington, Montana, Idaho and other states could be good bets to book your ski vacation.?
But even ski resorts in the Northeast could end up doing well, snow-wise, this winter, Cohen said, particularly if the Arctic Oscillation frequently goes negative and cold air spills into that region.
On the other hand, ski resorts in the Southwest, including parts of Colorado, New Mexico, and Utah, could be negatively affected by warmer-than-average conditions and below average snowfall. So, perhaps hold off on that Taos, New Mexico booking.
However, last winter should serve as a cautionary tale. The official NOAA forecast going into the winter was similar to this one, and all the snow wound up falling in central and northern California. Some resorts in the Lake Tahoe region had such a deep base of snow that they stayed open through July 4, 2017.?
So use these forecasts as a guide, but know that mother nature always has a few tricks up her sleeve to keep forecasters humble. ?