Now that we are in heading towards May and spring is on everyone's mind, its worthwhile to look back at the seasonal snowfall in Chicago and Rockford. This past winter will likely be remembered for the blizzard of 2011 back on February 1-2 and the prolonged cold weather that plagued the area. It is interesting that of the 57.9” of snowfall this past winter, 21.2 inches occurred during the blizzard at Chicago-O’Hare airport. So how does this amount of snow rate to years in the past. The figure below shows the distribution of snowfall amounts in Chicago's historic record, which is composed of 124 snowfall seasons dating back to the late 1800s. The most common amounts of seasonal snowfall roughly range from 23 to 44 inches, which straddle the median of 34 inches. So, 57.9 inches is an abnormally high amount of snow for the area. In fact, there are only 13 other seasons in the 124 year record have had more snow. Interestingly, in spite of the abnormality of this years snowfall, the snowfall during the previous three seasons was similar. This marked the fourth consecutive season in which 50 or more inches of snow has fallen; a feat that has never before been archived in the historical record at Chicago.
In Rockford, a total 51.3” of snowfall occurred. So how does this amount of snow rate to years in the past. The figure below shows the distribution of snowfall amounts in Rockford's historic record, which is composed of over 100 snowfall seasons dating back to the early 1900s. The most common amounts of seasonal snowfall roughly range from 17 to 41 inches, which straddle the median of 33 inches. So, 51.3 inches is an abnormally high amount of snow for the area. In fact, there are only 23 other seasons in the 100+ year record that have had more snow. Interestingly, in spite of the abnormality of this years snowfall, the snowfall during the previous three seasons was similar. This marked the fourth consecutive season in which 50 or more inches of snow has fallen; a feat that has never before been archived in the historical record at Rockford.
So why was the weather cold and snowy across the region this past winter? Well there appears to be a couple of reasons for this. First, a moderate La Nina event occurred across the equatorial Pacific. La Nina is simply part of the El Nino Southern Oscillation (ENSO). ENSO is a naturally occurring ocean and atmospheric phenomena that occurs within the equatorial Pacific. La Nina is known for cooler than average sea surface temperatures in the eastern equatorial Pacific. El Nino is the opposite of La Nina. The importance of these sea surface temperatures lie in the fact that they play a large role in the preferred placement for tropical convection in the Pacific. Just like hurricanes in the Atlantic and Gulf of Mexico, tropical thunderstorms thrive over warmer waters. Therefore, during La Nina events little to no tropical thunderstorms develop across the eastern equatorial Pacific during the Northern Hemisphere winter. Thunderstorms can be thought to act as the “bridge” between the ocean and the atmosphere, meaning they alter the atmospheric circulation where they develop. Since the thunderstorm activity is shifted westward during La Nina events, the typical atmospheric circulation pattern is altered. This alteration in the atmospheric circulation has profound impacts on the storm track across much of the United States during the winter and spring.
Across northern Illinois and northwest Indiana, La Nina events are known to increase the likelihood for above average snowfall. However, they are not known to increase the likelihood of producing prolonged cold. Therefore, it appears that another large scale atmospheric phenomenon helped play a part in the weather that we experienced, in which we had sustained cold air for most of December and January, with the exception of a sharp but brief thaw at the end of December. During these two months, the North Atlantic Oscillation (NAO) was in its negative phase. The NAO, like ENSO, can affect the large scale weather pattern, especially across the eastern third of the United States. However, unlike ENSO, the NAO occurs across the North Atlantic Ocean. There are two phases: the positive phase and the negative phase. The negative phase is known for higher than normal atmospheric pressure over the North Atlantic near Greenland and below average pressure across the central latitudes of the Atlantic. The positive phase is the opposite of the negative phase. During the negative phase of the NAO the storm track is diverted farther south across the eastern two thirds of the nation. This allows cold air to infiltrate well south across the lower latitudes of the eastern third of the United States.
The figure below displays the pressure anomalies in the mid and upper levels of the atmosphere during December and January. Notice the blue colors across the eastern United States offshore across the mid latitudes of the Atlantic Ocean. These cool colors represent lower than average pressures, while the warm colors farther north across northeastern Canada and Greenland indicate above average pressures. The significance of these pressure anomalies is twofold; they affect the strength of upper level jet stream, and they create dynamical temperature anomalies. So in the case of this past December and January the lower pressure across the eastern United States acted to produce colder temperatures across the region and it decreased the strength of the upper level jet, which produced weaker and less frequent storm systems. Therefore, the presence of the negative NAO acted to overwhelm some of the typical influences associated with La Nina during the December through January timeframe. Overall, this helped lead to abnormally cold and relatively dry conditions across much of the eastern third of the nation, including the Southeast, where warmer than average conditions would otherwise be expected during La Nina.
Things changed by February, however, as the NAO rapidly shifted to its positive phase. This allowed that typical flow pattern associated with La Nina to take a firm grip of across much of North America. This is around the time that we experienced the blizzard of 2011. Ultimately, the pattern shift lead to a stronger jet across the mid section of the country, which helped to produce above average precipitation and near average temperatures across northern Illinois in February. The figure below plots the same mid and upper level pressure anomalies, except it is during February 2011. It is apparent that above average pressures in the mid and upper levels of the atmosphere dominated the North Pacific near Alaska and also across the southeastern United States. Meanwhile, lower pressure was observed across much of Canada and the western United States. This is a classic La Nina pattern and supports a strong upper level jet right across the mid section of the nation and near average to slightly warmer than average temperatures across northern Illinois and northwest Indiana, as well as much greater temperature variability. As mentioned, this February ended up very close to normal temperature wise, but featured very cold temperatures at the beginning of the month, including a record low of -20 at Rockford on February 10, followed by a pronounced two week warm spell before seasonably cold air returned for the end of the month.
Overall, this past winter shows just how difficult long range forecasting can be across the United States. There are numerous large scale climate oscillations that occur naturally, and they can either act constructively or destructively with one another. If this was not complex enough, these oscillations have different timescales of variability ranging from weeks to over a decade. Unfortunately, much of this natural climate variability, including phenomena such as the NAO, is not predictable more than a few weeks in advance making it a typical long range forecast buster.
Article Author: K Birk - Forecaster National Weather Service Forecast Office Chicago IL