Local Research Suggests Slightly Colder Than Normal Winter

On October 9th, the Climate Prediction Center (CPC) issued the first official outlook for the winter season (December 2007 through February 2008). Much of the country will likely see above normal temperatures. However a significant section of the Northwest and North Central portion of the country will likely see more normal temperature patterns. The outlook relies heavily on the development of a La Niña, and the affects often associated with it during the winter months. La Niña is defined as a distinct, large scale cooling of the surface waters in the Pacific Ocean from the Peruvian coast west along the Equator. The counterpart of La Niña, El Niño, often brings much milder and drier weather to the northern plains states. In fact, the winter of 2006-2007 was affected by a weak El Niño and temperatures were above normal for much of the winter season.

With a weak La Niña developing, the CPC outlook calls for Equal Chances for Normal, Below Normal or Above Normal temperatures in much of the Grand Forks Weather Forecast Office area of responsibility. In essence, Equal Chances indicate that normal climatic variability is the most likely outcome during the forecast period. Should the La Niña strengthen and reach moderate intensity, as a few forecast indicators suggest, the impacts may be more significant. A stronger La Niña signal would typically suggest colder than normal winter temperatures across our area.

Of greater interest is what happens across the Grand Forks forecast area when El Niño and La Niña develop in certain manners. Last winters El Niño peaked early and weakened quickly during the last half of the 2006-2007 winter season. This caused temperatures across the Northern Plains to cool a bit more than would be expected during an El Niño (See graphic below) Precipitation also increased, resulting in slightly higher than average snow fall for the season.

2006/2007 Fall through Spring Winter Temperatures at UND/NWS Climate Station

Note that during the fall and early winter, temperatures were frequently well above the average for the same time period. However, nearly coincident with the collapse of the El Niño, temperatures fell considerably across the region. Note that late January through much of February was considerably colder than average, with tremendous variability in the temperatures during the March - April time frame.

One technique used to make seasonal predictions is called composite analysis. One form of composite analysis is to look at similar past weather patterns compared to the weather now, and then look at the result for the following couple of seasons. In the example below, we looked at years when a weak to moderate El Niño was followed by a weak to moderate La Niña within a years time span. The years used in our analysis were 1964-65, 1967-68, 1973-74, 1978-79, 1988-89, 1995-96 and 1998-99. The 1978-79 winter was included in a control mode as that year did not quite attain La Niña status, but the Pacific was a little cooler than average. The months chosen in the analysis were October through April.

The result of those composite fall, winter and spring seasons are shown below.

Composite La Nina winter seasons

Another tool used to produce seasonal outlooks is the trend in temperature over the past decade, when compared to the past 30 years. Looking at the Grand Forks data, there has been a distinct 2 to 3 degree rise in the long term average temperature since the late 1970s, with a notable leveling off during the first few years of the 21st century. In the graphic above, the trend is calculated as part of the overall La Nina influence, showing a 2 degree positive offset.

Looking at the above graph, it is apparent that the fall month of October is warmer than the long term average. November and December temperatures cool to around average, with quite a bit of variability. Toward the end of the year there is a noted drop to predominantly below normal temperatures through March, moderating back to near normal in April.

So, will this temperature trend be repeated this winter? There are several key things which must happen, none of which are easily predictable. Please refer back to the Top News of the Day article released on September 20th 2007. There are several other large scale forcing mechanisms in the atmosphere which can warm, or cool the Northern Plains. It is important to remember that each winter season is unique, with tremendous variability being typical in any given year. It can be -40F on Monday, then warm to near freezing by the end of the same week. Mild, dry weather can quickly be replaced by blizzard conditions and frigid cold. The graphic above is only one illustration of how climate outlooks can be produced, using real data and examples. Finally, keep in mind that approximately 70% of La Niña winter seasons favor cooler than average temperatures in our area. There have been La Niña winters where its influence was not strong enough to prevent above normal winter temperatures.

Your NOAA’s National Weather Service will monitor the weather conditions this fall, and compare them to those in the "outlook" presented above. It should be pointed out that the above technique was used with fair success during last winter season, at least in the Grand Forks area.

The table below contains data from the 2006-2007 fall, winter and spring season, the La Niña Averages and the 1971- 2000 Normals. These represent the average maximum and minimum temperatures during the October through April time period. The snowfalls are the total for the 2006/2007 season, with the 30 year average snowfall, as well as the La Niña and trend adjusted totals listed.

2006/2007   Total
Max Temp. Min Temp. Snowfall
34.3 16.2 47.3
Climatology   Avg. Total
Max Temp. Min Temp. Snowfall
33.0 14.5 44.1
La Nina   Avg. Total
Max Temp. Min Temp. Snowfall
28.5 10.5 56.8
Trends Adj.   Avg. Total
Max Temp. Min Temp. Snowfall
31.5 13.5 53.8

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



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