During the summer and fall of 2009, your NOAA's National Weather Service watched as the Pacific rapidly changed modes from a La Niña to an El Niño. Based on the historical record, the odds favored a warmer and somewhat drier than normal winter for the northern plains. However, this has yet to be the case. In fact, the winter season so far has been very chilly, and in some respects acting more like the La Niña was still ongoing. What has happened? What will happen?
What happened. One thing that is different with this El Niño event as compared to others is that the area of warmest water in the equatorial Pacific is much farther west. Compared to the 1997-98 El Niño, the convection associated with the warmer water has shifted westward along the equator by 20 to 30 degrees of longitude. Quite frequently, the largest anomalies of sea surface temperature along the equator are located nearly due south of the west coast of the U.S. This year the greatest anomalies are much closer to the International Dateline, or 180 degrees longitude. As a result, the overall weather patterns have been shifted farther west. Instead of a large low pressure system in the Gulf of Alaska pushing warm air into Canada and weakening the Polar Jetstream, the overall pattern has been more "normal" with the Polar Jet remaining relatively strong, allowing the Arctic Regions to supply us with colder than normal air. Also, a large scale atmospheric pattern known as the Arctic Oscillation (AO) has been in a strongly negative phase, which is often associated with colder than normal weather across the plains. Finally, the Pacific Decadal Oscillation (PDO) has been in a negative phase, which is suspected to moderate the impacts of a El Niño.
What will happen? Looking through the historic record, there are several years with similar ocean - atmospheric conditions resulting in colder than normal "weather" during December. The years with an El Niño, a negative AO and a cold phase PDO have had more "normal weather" during the last half of the winter season, with below normal temperatures frequently extending into the early spring. Precipitation tends to average near to or above normal for the late winter into early spring, with much warmer and drier weather by late spring into the start of summer. Local research indicates patterns similar to this were also seen in the spring months of 1959, 1964, 1966, 1969, 1970, 1977, 1988 and 1998.
The trends below are based on a limited statistical analysis of 8 prior years that showed ocean - atmospheric patterns patterns similar to the last few months of 2009. It is important to remember these atmospheric trends can change, resulting in a different outcome than is currently expected. The El Niño is still ongoing and will impact the weather across the U.S. over the next few months. However, El Niño impacts tend to weaken as we get into the spring season.
Expected temperature anomalies for the January - February 2010 period based on El Niño with a negative AO and cold phase PDO.
January of 2010 started out extremely cold across eastern North Dakota and northwestern Minnesota . The cold weather pattern will be replaced by a significantly milder pattern that should last through the middle-latter part of the month. Indications are that a surge in the El Niño will reach the East Pacific during the middle January, and the block in the upper atmospheric pattern in the Atlantic will weaken as well. This will allow milder Pacific air to move across the northern plains. It is probable that the pattern will turn colder again towards the end of January, since the warm pool in the Pacific is not likely to shift east significantly. In addition, although the blocking in the Atlantic will shift east for awhile the underlying factors, such as Atlantic sea surface temperatures, have not changed and a return to a cold pattern is expected. The mild weather in the middle of January will temper the overall cold departures for the month. Eventually, as the El Niño fades, other large scale atmospheric patterns will assert themselves and potentially lead to a cooler than normal spring.
Expected temperature anomalies for the March - April 2010 period based on weakening El Nino with a cold phase PDO.
Precipitation is always much more difficult to predict, due to the large variability common in weather systems. We have all experienced storms that produce heavy snows in one location, then travel 30 or 50 miles to visit someone who escaped the snowfall. This variability makes predicting large scale precipitation patterns more of a challenge. However, based on the historical record and the fact the atmosphere has a "persistence factor", climate outlooks can be made that represent a historical perspective with practical future use.
Expected precipitation anomalies for the January - February 2010 period based on El Nino with a negative AO and cold phase PDO. Overall a more normal precipitation pattern is expected through the rest of the winter season.
There are several large scale ocean-atmospheric patterns (signals) that suggest the potential for increased precipitation during the spring of 2010. These include the very strong, naturally occurring negative AO, cooling waters in both the North Pacific and North Atlantic as a result of the strong 'blocking pattern' that has been in place during December 2009.
Expected precipitation anomalies for the March-April 2010 period based on El Niño with a negative AO and cold phase PDO. There is an increased potential for stormy conditions to develop based on the historical data.
It is critical to remember that these climate outlooks are based on [a limited number of] historical data, and may not accurately reflect the future conditions. Additionally, the differences within each of the individual year's weather used suggests a tremendous amount of variability is likely as we get farther into the spring 2010. Three of the four spring seasons represented here were wetter than the long term average.
Ramifications for the spring 2010 flood season: We are, as of this writing, far better off in some respects than this time in the winter of 2009. The 2009 fall precipitation, while above normal, was much less than the fall of 2008. Frost depths have not been as deep as a year ago. Based on what we have in the river system, the southern Valley again seems most at risk for major flooding the spring of 2010. The AHPS runs from January 4th 2010 have slightly increased the threat for major flooding. As is the case every spring, it will come down to future weather conditions, specifically [any] additional precipitation during the rest of the winter, precipitation at or near the time of spring snow melt, as well as the rate of melt.
Your NOAA's National Weather Service will closely monitor the evolving weather and climate patterns as we go through the rest of the winter season. For additional information, contact Mark Ewens at 701.772.0720x327 or Mark.Ewens@noaa.gov
Images provided by the NOAA/ESRL Physical Sciences Division, Boulder Colorado from their Web site at http://www.esrl.noaa.gov/psd/