Click here for the series of slides compiled by Mike Baker of the National Weather Service in Boulder. Here’s the summary:
– An elusive El Niño continues to challenge seasonal climate forecasters with NOAA’s Climate Prediction Center. Forecasters say a wavering El Niño, expected to have developed by now, makes this year’s winter outlook less certain than previous years. “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 Mike Halpert, deputy of NOAA’s Climate Prediction Center.
– When El Niño is present, warmer than normal ocean water in the equatorial Pacific Ocean shifts the patterns of tropical rainfall eastward that in turn influence the strength and position of the Pacific and Polar jet streams and storms over the eastern Pacific Ocean and United States. This climate pattern gives seasonal forecasters confidence in how the U.S. winter will unfold. For that reason, “an El Niño watch remains in effect because there’s still a window for it to emerge,” according to Deputy Director Halpert.
– Other climate factors can also influence winter weather across the country. Some of these factors, such as the North Atlantic Oscillation (NAO), the Pacific Decadal Oscillation (PDO), and Pacific Inter- Decadal Oscillation (IPO), are also prominent large scale climate patterns that influence our winter weather to varying degrees. The NAO, in particular, is difficult to predict more than one to two weeks in advance, and adds uncertainty to the winter outlook for the Northeast and Mid-Atlantic portions of the country.
– According to Climate Prediction Center’s U.S. Winter Outlook, issued October 18, 2012, odds favor warmer-than-average temperatures and an “equal chance” for above-, near-, or below-average precipitation across Colorado during the November 2012 to January 2013 climate season. [ed. emphasis mine]
– This precipitation outlook does not project where and when snowstorms may hit or provide total seasonal snowfall accumulations. Snow forecasts are dependent upon the strength and track of winter storms, which are generally not predictable more than a week in advance.
– Based on climate records, storm track and jet stream composites, a high wind storm frequency study for the Boulder area, and seasonal climate and ENSO outlooks prepared by NOAA’s Climate Prediction Center and The International Research Institute for Climate and Society (IRI), the following weather conditions are more likely to occur in Colorado this winter season during weak El Niño and/or neutral ENSO conditions:
1. Above-average temperatures statewide, with odds favoring northwest and west central portions of the state
2. Slightly above-average precipitation for southern portions of Colorado where storm tracks tend to favor during El Niño events, even weak ones. Otherwise, near- to slightly below- average precipitation is anticipated for the remainder of Colorado. Nonetheless, precipitation-snowfall across Colorado should be greater overall this winter season, compared to the meager amounts observed last winter [ed emphasis mine].
3. There is a reasonable chance that Colorado will see more storms this winter season as compared to last winter, although these storms are not expected to as intense or as long-lasting as those more often observed during stronger ENSO episodes.
4. One explanation for these weaker, shorter-lived storms; storm tracks do not normally persist over any one part of the state for days at a time during weak to neutral conditions, such as northern Colorado during moderate to strong La Niñas and southern Colorado during moderate to strong El Niños.
5. Strong, potentially damaging downslope winds (known as Chinook and Bora) in the Boulder area, historically have occurred least often during weak El Niño winter seasons. Their odds of occurrence increase slightly during neutral ENSO conditions, and peak during La Niña and El Niño events of strong intensity.
6. Lastly, according to the U.S. Drought Outlook, issued October 18, 2012, those areas of the United States, including Colorado, that were severely impacted by extreme drought over the past year are unlikely to see much relief from drought conditions this winter [ed emphasis mine].
More coverage from Bob Berwyn writing for the Summit County Citizens Voice. From the article:
There’s a reasonable expectation that the state will see more storms than last winter, but forecasters don’t expect those storms to be as intense or long-‐lasting as those commonly observed during stronger El Niño or La Niña episodes, as the storm track is expected to be inconsistent in what looks to be either a weak El Niño or even neutral Pacific ocean conditions.
What looked to be at least a moderate El Niño developing in June and July started to fade away again, with quick cooling evident across the equatorial Pacific Ocean — which isn’t necessarily a bad thing for western Colorado, which is sometimes left high and dry during El Niños. Some models are now showing an increasing probability of tilting back toward another La Niña next spring, with uncertain consequences, but the trend is a cause of concern, as some of Colorado most severe extended droughts have been associated with extended La Niña periods.
A Statistical Preview of Denver's November Weather: http://t.co/tU2XFGV5— NWS Boulder (@NWSBoulder) October 31, 2012