Update issued in December 2010
Updated 445 pm CST Fri Dec 17 2010
As advertised, a moderate to strong La Nina has formed with much colder than normal ocean water temperatures along the equatorial Pacific Ocean. We wanted to make a check to the "scorecard" and see what has actually happened for temperatures, precipitation, and snowfall so far. We also want to take a close look at what is expected for the winter and early spring. The original story is below these updated charts for reference.
The quick summary: The 2009-2010 winter was an El Nino episode and the 2010-2011 winter is a La Nina episode. So we went back through the records to find out what happens when El Ninos one year are followed immediately by La Ninas the next year. The first few charts below show what happened from June to November in those years and compares to normal and what has been observed so far this summer and fall in 2010. The 2nd series of charts show what could potentially happen during the winter and early spring if history is any indication. So far, the results are mixed...
The chart below shows the 30 year normal monthly temperature for Milwaukee, the average of the El Nino followed by La Nina years, and what has actually happened so far in 2010. The temperature has been above normal in 2010 most of the time, but especially in the summer. This is consistent with we have seen in the past.
This following chart is the same as the previous, except for precipitation. History indicated that dry early summers are followed by near normal late summer and fall precipitation. This is NOT what happened in 2010. A very wet early summer was followed by a dry late summer and fall.
The next chart is the same as the above charts, except for observed snowfall in 2010. No snow fell in October and November, which is consistent with the historical below normal snowfall when El Nino winters are followed the next year with La Nina winters.
The next charts show the same as the above charts, except they indicate the difference between normal Milwaukee monthly conditions from December through March and what has happened historically when El Nino winters are followed immediately the next winter by La Nina winters.
For Milwaukee temperatures, history shows that December and January readings are near normal with readings about 2-4 degrees below normal in February and March.
For precipitation, there is a tendency for above normal amounts with the exception of December.
For snowfall, there is a strong tendency for above normal snowfall, especially in March.
The following charts are the same as those above, except for Madison.
Madison temperatures were above normal for most of the summer and fall. This is consistent with El Nino to La Nina transitions. The summer of 2010 was especially warm relative to normal.
The precipitation pattern was similar to Milwaukee's, and completely different than what we have seen historically. The early summer was extremely wet in 2010 in Madison, while the remainder of the late summer and fall was dry. This is basically the opposite of what has happened in the past with the El Nino to La Nina transition .
It did not snow in Madison in October and November 2010, which is consistent with historically below average snowfall in the El Nino to La Nina transition years.
The following charts compare normal Madison monthly conditions with those that are historically observed in El Nino to La Nina transition years. Like Milwaukee, Madison temperatures start out around normal in December and January and then turn about 2-4 degrees colder in late winter.
For Madison precipitation, it tends to be wetter than normal in January and February.
For snowfall, Madison tends to be near normal for December snowfall and then above normal for the remainder of the winter when El Nino winters are followed by La Nina winters the next year.
Jeff Craven, Science and Operations Officer
National Weather Service, Milwaukee/Sullivan WI
Original Story released in August 2010.
Over the past few months, a transition from an El Nino event to a La Nina event has occurred. What does the La Nina mean for Milwaukee and Madison, especially this coming winter season?
A La Nina means that the eastern equatorial Pacific Ocean sea surface temperatures are 0.5 degrees Celsius or greater below normal. La Nina events affect weather patterns around the world. Typically, La Nina events tend to affect our weather the most during the winter months, when the jet stream is strongest over the country. La Nina winters in southern Wisconsin, especially during moderate to strong La Nina events, tend to be slightly colder than normal, and tend to be wetter and snowier than normal. The current La Nina is favored to become a moderate to strong event.
To find out the effects on Milwaukee and Madison, a local study was performed to compare average temperature, precipitation and snowfall values, from El Nino to La Nina transition periods, and the following winter seasons, with normal values. There were seven transition/following winter periods that were identified to have occurred in a similar timeframe to the one this year: 1964-65, 1970-71, 1973-74, 1988-89, 1995-96, 1998-99, and 2007-08. Specifically, trends for average temperature and snowfall were sought during the winter season.
The results are below:
1) Average Temperature - Milwaukee:
For Milwaukee, average temperatures tend to be slightly above normal for the summer months. Autumn and winter through January tend to be near normal. However, February and March tend to show slightly below normal average temperatures.
2) Average Temperature - Madison:
For Madison, the trends were similar to Milwaukee. Average temperatures tend to be slightly above normal for the summer months. Autumn and winter through January tend to be near normal. However, February and March tend to show slightly below normal average temperatures.
3) Precipitation - Milwaukee:
For Milwaukee, precipitation tends to be below normal in June and July, then tends to be near normal for August through October. Slightly below normal precipitation is found in November and December, with above normal precipitation in January. The February and March months are near normal, with April and May below normal.
4) Precipitation - Madison:
For Madison, June and July tend to be below normal as well. Precipitation then tends to become above normal in August, and remains slightly above normal through October. November and December tend to be slightly below normal, with January back above normal. Precipitation is near to slightly below normal in February and March, before above normal values in April, then below normal in May.
5) Snowfall - Milwaukee:
For Milwaukee, snowfall tends to be near normal for the October and November months. December tends to be slightly above normal, with January through March being well above normal for snowfall. April tends to be slightly below normal.
6) Snowfall - Madison:
For Madison, the trends are similar to Milwaukee. Near normal snowfall tends to occur in October, slightly below normal in November, and near normal in December. Like Milwaukee, the January through March period is above to well above normal for snowfall, with April below normal.
Thus, this coming winter season appears to show greater potential for above normal snowfall, especially in the January through March period. There also appears to be a trend for slightly below normal average temperatures in the February through March period. Confidence in this study will become greater if the moderate to strong La Nina event becomes more likely.
Stay tuned for further updates!
J. J. Wood
National Weather Service