NOAA Winter Outlook: El Niño a Dominant Player, but Wild Cards Still Possible

Figure 1. NOAA’s outlook for winter temperatures (top) and precipitation (bottom) for the three-month period from December 2015 to February 2016. NOAA outlooks are expressed as probabilities for above- or below-average conditions. In the three-class system used by NOAA, an area labeled “equal chances” means that there’s roughly a 33% chance each of below-, near-, or above-average outcomes. If a location is shown with higher odds of above-average conditions, then the probability for below-average outcomes goes down proportionally (e.g., 50% above-average, 33% near-average, and 17% below-average). See NOAA’s online reference guide for more details.

In a nutshell: Wet and cool South, mild and dry North
The enhanced subtropical jet streams common during El Niño tend to boost precipitation across the U.S. Sunbelt and decrease it toward the northern tier of states, as reflected in Figure 1. The same dynamics act to “smoosh out” temperature contrasts across the nation: the cloudy, wet conditions across the South are often accompanied by chilly temperatures, while the drier conditions toward the Northern Rockies are often joined by relatively mild air. NOAA’s Mike Halpert said at a Thursday-morning teleconference that the forecast implies about 2% fewer heating degree days than average. This would also be about 6% fewer days than last winter, he added.

A couple of key caveats:

—NOAA’s probabilities are not meant to imply any judgment on how intense an outcome might be. They’re simply showing where unusually cool, mild, wet, or dry conditions may prevail. Higher odds for those outcomes don’t necessarily mean that the results will be more dramatic than in other areas.

—As the name implies, the seasonal outlooks are meant to convey conditions for the three-month winter period as a whole. They aren’t designed to show how much variability there could be across those three months, and of course weather can vary a great deal within a 90-day period.

With that in mind, let’s look at a few potential regional wild cards around the contiguous 48 states. (Warmer- and drier-than-average conditions are good bets for both Hawaii and Alaska.) For more detail on how El Niño affects various parts of the nation, see our roundups published on July 28 and July 30.

California
The strongest El Niño events—like the one now in place–are closely linked to wet winter conditions, especially over Southern California. In both 1982-83 and 1997-98, California arguably got too much of a good thing, with mudslides and floods causing millions in damage. In his October blog post, WU weather historian Chris Burt takes a close look at how those two seasons panned out. One important element will be the temperatures that accompany any big winter storms. If they’re on the warm side—a big problem in recent years—then the snowpack accumulating over the Sierra Nevada could end up disappointingly low. Regardless, aquifers and ecosystems stand to benefit big time if El Niño produces as expected. Overall, this winter offers the best chance in years for California to make up some (though not all) of the hydrologic ground it’s lost during the severe drought in place since 2011. Residents will need to keep calm and carry on for a while longer, though, as the parade of storms common during strong El Niños often doesn’t arrive until December or even January. And crucially, even high odds aren’t the same as a guarantee. While the mega-El Niños of 1982-83 and 1997-98 were both very good to California in terms of precipitation, one of the three next-strongest events (1965-66) fell below average in winter precipitation for all but southern California. You can see how El Niños of various strengths performed at Jan Null’s excellent website on El Niño and California precipitation.

Pacific Northwest
This region is heading into the El Niño of 2015-16 after a dry winter and a very warm, dry summer. Unfortunately, one of the most dependable outcomes of a strong El Niño is winter warmth and dryness from Oregon and Washington into Montana. So the region could go into spring and summer 2016 with even more water worries than last year.

The South
Drab winter weather—chilly and damp—is likely to prevail from Texas to the Southeast coast in 2015-16. The risk of severe weather may be boosted along the immediate Gulf and southeast Atlantic coastal areas. Florida, in particular, needs to watch the skies this winter, as strong El Niño events are associated with a heightened risk of tornado outbreaks, as in the deadly Kissimmee outbreak of February 1998.

The Midwest and Northeast
Tucked inside the somewhat equivocal NOAA outlook for this region is some important nuance. The 1982-83 and 1997-98 El Niños both led to a vast swath of warm winter conditions covering much of Canada and the northern United States, all the way from the Northern Plains to New England. Given the long-term trend toward warmer global temperatures, some truly impressive “warm waves” seem likely to take shape in this area. At the same time, the last few winters have been surprisingly cold and snowy over parts of the Midwest and Northeast. Various experts attribute this to the reverberations of unusually warm water in parts of the tropical Atlantic, the presence in some years of a negative North Atlantic Oscillation (NAO), and/or the loss of Arctic sea ice. We don’t yet know how all of these factors will line up for 2015-16, but I would cast my lot on a mixed-bag winter from the mid-Atlantic to New England, with periods of marked warmth punctuated by occasional sharp but transient cold blasts. Those could end up producing at least one big snowstorm if a negative NAO enters the picture. A good case in point is the winter of 1982-83, when a comparably strong El Niño was in place. Though the winter of 1982-83 averaged quite mild in the Northeast, it also produced the crippling Megapolitan snowstorm of February 10-12, 1983, which dumped 20” – 30” in northwestern suburbs from Washington to Boston. Below is a “blast from the past” YouTube audio clip of a KYW radio newscast from the Philadelphia area during the height of the storm.

Bob Henson

from:    http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=3155