Winter preview: La Niña or not?
What does this winter's weather have in store for the nation's center? As forecasters dust off their crystal balls and look ahead to the winter months, there are few signals to help make a clear-cut determination about whether it'll be mild or rough.
El Niño/Southern Oscillation (ENSO) is commonly watched fairly closely for signs showing where water temperatures in the Pacific Ocean will send the jet stream, thereby influencing weather patterns. Usually, a La Niña pattern steers storms and moisture away from the Midwest, while a neutral or El Niño pattern takes the jet stream on a more normal path, sending moisture through the region at a normal clip. And it's all based on a fluctuation of fewer than a handful of degrees Fahrenheit in surface water temperatures.
That much is known. What remains up in the air, experts say, is which way ENSO will twist in the next few months. On one hand, University of Missouri climate scientist Tony Lupo says a weak La Niña pattern will likely take hold. That's based on current surface temperatures in the Pacific Ocean, according to a university report.
"It should be generally quite pleasant. The weather should be particularly good for farmers to get their crops harvested," Lupo says. "Most areas may see the colder weather come earlier than last year, when we were under the influence of La Niña. Also, we should see a normal amount of snow. Last year, the Midwest had little snow well into the winter, and then we got a lot of snow late. I expect a more normal distribution of snow this year."
But at this point, that's a lot to base on ENSO, which is essentially in a neutral position and will likely stay that way, and therefore not have much influence on the winter weather, says MDA Weather Services senior ag meteorologist Don Keeney.
"We are very close to neutral right now, and the vast majority of models are showing a slow warming of the Pacific to positive neutral conditions over the winter. Thus, we will not be in La Niña," Keeney says. "Also, the signal will be so weak that it will not have a major, long-term effect on U.S. weather. Our current outlook does show the warmth continuing through the fall, but it then cools a bit over the winter."
Another key indicator of how winter unfolds, Keeney says, is September temperatures. This month, temperatures have been above normal -- some to the tune of 10 to 15 degrees warmer -- in an area stretching from eastern Washington state southeast to western Mississippi, from northeastern Minnesota to west Texas and far eastern New Mexico. That's in stark contrast to the "ENSO analogs" for temperature prediction that previously predicted slightly lower to only slightly higher temperatures for about the same region.
"Clearly not a good signal correlation in the Midwest," Keeney says.
This creates something of a quandary for forecasters, since ENSO has to date been the most reliable pattern of its kind for projecting seasonal trends, adds Illinois state climatologist Jim Angel.
"It's pretty much been missing-in-action so far. It was not really a big factor in how things played out this summer," Angel says. He agrees with Keeney that an ENSO-neutral pattern is in place right now and will likely remain that way through the winter. "The problem is when you take ENSO off the table, you don't have much to work with. It's the Achilles' heel of long-term forecasting if you don't have those big factors."
That being the case, Angel says he expects more trend-based forecasting to become the guiding feature of longer term outlooks like winter temperatures and precipitation. Right now, he says NOAA's Climate Prediction Center has called for a wetter month of September, and though that's yet to reach fruition, it's the kind of variable on which forecasters might base trend-centered outlooks down the road.
"For a long time, [NOAA Climate Prediction Center officials] were insisting we were going to have a wet fall starting in September through November. Now, they've kind of backed off, and it's a little more neutral for Illinois in the September-October time frame," Angel says. "I think they're kind of backing away from the wet fall forecast. They do show it being wet in December, which for us could be a mixed bag with rain or snow. For October, the models are pointing toward possibly still on the warm side. November may be on the cool side, then warmer for December and January."