Preliminary Winter Outlook - September 20 2007

The NOAA’s Climate Prediction Center (CPC) issued its regular monthly and seasonal outlooks for the United States on September 20 2007.  These outlooks are issued on the third Thursday of every month, and include probabilities of above normal or below normal temperatures and precipitation, or areas where normal climate variability is expected.

This winter season is expected to be dominated by a developing La Nina, a pattern of below normal temperatures in the equatorial Pacific. La Nina, and its warm counterpart El Nino are regular fluctuations in the Pacific that affect the weather on a global scale. Historically, La Nina conditions in the Pacific tend to produce colder and wetter winters across the Northern Plains. Several other large scale climate factors come into play as well, including the North Atlantic Oscillation and the Pacific-North America patterns. These patterns can intensify or weaken the affects of the La Nina or El Nino signal, and are often the cause of tremendous variability during the winter season.

The CPC also takes other factors into consideration such as the trends in temperature and precipitation patterns the past 10 to 15 years, as well as a variety of numerical and statistical computer models. Based on these factors, the CPC expects a slightly better than average chance for warmer than normal tempertures during the winter months of December, January and February.

In addition to the larger, national guidance the CPC produces there are "downscaled" climate outlooks available. These downscaled outlooks take the local temperature trends into account to better refine the expected average conditions during a particular season. These downscaled climate outlooks are available by clicking here.

Local research supports the CPC outlooks, and can also help refine the predicted patterns of temperature and precipitation. Looking at the Red River Valley region during La Nina winters, the trends typically point to colder and snowier than the average. However, climate trends during the past 3 decades show a slightly different picture than the long term historical record. The graphic below shows the average temperatures during the winter months of December, January and February from 1970 to 2007, with the years in which La Nina was occurring annotated near the bottom. The winter seasons with a La Nina present tend to be normal, or colder than the long term average. This is not always the case, as is evidenced in the 1999 to 2001 winter seasons when warmer than average temperatures were felt despite a persistent weak La Nina.

Winter Average Temps and La Nina years

As stated earlier, one tool used by CPC is the trend in temperature patterns the past 10 to 15 years. Looking at the figure above, we see that there was a general warming trend in the winter season, especially from the early 1970’s to the mid 1980s. Since the mid 1980s, it would appear that the rate of warming has slowed quite a bit, at least in the Grand Forks record. This trend may have helped keep winter temperatures warmer than normal in the 1999 to 2001 time period, despite a persistent La Nina.

Winter season precipitation may also be affected during La Nina, typically producing more precipitation across the region. The graphic below shows the winter season total precipitation from 1970 to 2007, with the years in which La Nina was occurring.

Winter Season Precipitation 1970 to 2007

As with trends in temperature, the above graphic shows that winter seasons with a La Nina tend to have more precipitation than the average. In addition, there has been an overall and gradual decrease in winter precipitation since the late 1990s, with normal or below normal precipitation being common.

Other large scale atmospheric signals which affect the Northern Plains weather are discussed below.

Madden-Julian Oscillation (MJO)

The Madden-Julian Oscillation is a relatively short term climatological event on the order of 45 to 60 days in length. An MJO is essentially a wave of relatively warm water that translates from the Indian Ocean, across the tropical Pacific to the west coast of South America. There is typically an upper level wave of energy associated with the MJO that has an effect on tropical weather in the Atlantic basin as well. Recent research indicates that these upper level waves dampen hurricane activity in the Atlantic Basin. For our weather, a strong MJO serves to modulate the strength of a preexisting El Nino or La Nina. At this time there is limited skill at predicting the MJO strength, therefore it does not have a significant impact on the seasonal outlooks. However, shorter term forecasts such as the 8 to 14 day outlook consider MJO influences.

Pacific Decadal Oscillation (PDO)

The Pacific Decadal Oscillation (PDO) is a larger, longer time scale oscillation than ENSO. The PDO affects the ocean temperature anomalies over the Pacific Ocean, with a time span of decades.  Since the PDO affects most of the Pacific Ocean, and ENSO is primarily a tropical Pacific Ocean temperature anomaly, the PDO modulates the strength of ENSO.  When the PDO is in the positive phase, as it was from 1978 to 1998, El Nino conditions dominate the tropical Pacific Ocean and strong El Nino conditions like the 1982-83 and 1997-98 events are more likely. La Nina events, the negative phase of ENSO, are weaker and less frequent. During the negative phase of the PDO, the opposite is true. The PDO was in the negative phase from around 1947 until 1977.  During that time, the Valley Region saw generally colder than normal winters. Drier than normal weather is also more prevalent over the Northern Plains, with greater than 60% of our dry years occurring since 1950 occurring during the negative phase of the PDO. Given the 20 to 30 year cycle of the PDO, we would expect the PDO to trend towards the negative phase over the next decade.  The PDO was mostly in the negative phase from 1998 to 2002, but has been mostly in the positive phase since the fall of 2002. So what phase will it be in this winter? Based on the current trend, the negative phase seems likely. Based on that alone, it would seem that a colder and drier than normal winter would be indicated for the Valley Region.

NAO

One of the most significant atmospheric variable to affect the winter weather over the northeastern third of the United States is the phase of the North Atlantic Oscillation (NAO). The negative phase of the NAO results in cold and snowy weather, as was the case during December of 2000, when Grand Forks had 19 inches of snow and averaged 11.8 degrees F below normal. The following month the NAO went into the positive phase, which typically results in warm and dry weather over our region. January 2001 in Grand Forks was 9.1degrees F above normal with only 3.3 inches of snow! Unfortunately, it is difficult to predict changes in the phase of the NAO, meaning that sharp changes in the winter weather pattern like those of December 2000 to January 2001 can occur without much warning. Recently the NAO has been strongly negative which favors colder than normal weather across the Northern Plains.

Sun Spot Cycle at minimum

Local research indicates that there is a weak to modest correlation to the waning leg of the 11 year Sun Spot  Cycle and temperatures patterns in the Northern Plains. While not as strong a signal as the ENSO, it is more common to see the tendency for colder than normal winters in years when the cycle was nearing its minimum. According to the NOAA Space Environment Center, the current cycle (#23)  is near its minimum. If sunspot cycle history is a guide, this winter would favor colder than seasonal temperatures.

Putting all this together, it would seem that the 2007/2008 winter season has the potential for tremendous variability from week to week and month to month, with the overall winter temperatures averaging near to or a tab below normal. Snowfall should be, on average normal to a bit below normal. As this La Nina develops, your NOAA’s National Weather Service will monitor the situation and update the outlook as conditions change.

For more information please contact Mark Ewens, Climate Services Focal Point at Mark.Ewens@noaa.gov or call Mark at 701.772.0720 x327.



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