La Nina – Fact and Fiction

***Watch WISN 12 News for the latest weather information!***

Thank you for stopping by the Weather Watch 12 Blog!  Weather conditions this past weekend were amazing!  A high of 72 on Saturday, and 82 on Sunday in Milwaukee!  If highs hit 70 today(Monday), it would be the 6th day in a row with highs in the 70s or 80s!

Some locations in southern Wisconsin set record highs over the weekend, including Madison and LaCrosse.  Rockford, IL also set a record with a high of 90 on Saturday!  Considering that last October the high never hit 70 once, it may be years before we see a stretch of weather like this again in October.

Anytime our weather turns extreme there are a whole host of reasons or explanations why.  One of my ‘pet pevees’ is when someone simply says it is because of El Nino or La Nina.  It didn’t take long this weekend to find an article blaming our warm-up on La Nina!

I feel that if you are going to talk about the weather; some numbers, facts, or historical data should be used to support a claim.  In the example I’m going to show below I don’t feel this was the case, and I want to make sure the viewers of WISN know what is fact and fiction when it comes to El Nino and La Nina.

The article I’m going to reference was in the Wisconsin State Journal’s website on Sunday, and was titled “Another record-setting warm day possible, thanks to La Nina”

Let’s start by looking at the meat of the article which refers to La Nina below.

Madison has seen 73 days with temperatures above 80 degrees this year, not including today, which will likely make the number 84, Sears said. The average number of days with temperatures above 80 degrees in Madison is 54, according to the weather service.

“We have had an abnormally warm year,” Sears said.

We can thank La Niña — El Niño’s counterpart — for that, as it’s brought more southerly winds to the region, she said. Those winds brought warmth and moisture, which also accounts in part for how wet the summer was, she said.

NOAA’s Climate Prediction Center, which is part of the National Weather Service, declares the onset of an El Nino/La Nina episode when the 3-month average sea surface temperature departure exceeds 0.5C/-0.5C in the east-central equatorial Pacific.  July, August, and September had an average of -0.5C.  This means that La Nina just started in October, by definition.  Here are the sea surface water temps in the east-central Pacific in the past year.

Now that we have established that a La Nina has just formed, what are some of the impacts on our weather in the western Great Lakes.  Here are just a few of the highlights.


  • Strong La Niña (SST departure of -2°C or colder):
    • Temperature – tendency for warmer-than-average weather from January through March but not as strong or consistent as a strong El Nino.
    • Precipitation – tendency for drier-than-average weather July and August before becoming highly variable
  • Weak La Niña (SST departure between -1 and -2°C):
    • Temperature – tendency for warmer-than-average weather from October to December and again from February to May.
    • Precipitation – tendency for drier-than-average weather from November to January, and again from April to June.

Generally, La Niña impacts are not as clear-cut because there are fewer strong ones in recent years (1970-71, 1973-74, 1975-76, 1988-89).

  • Summers have a tendency to be warmer and drier.
  • Winters are typically warmer and wetter than average with more snow and winter storms
  • Springs tend to be cooler across most of the region
  • Tends to increase tornado activity in the High Plains and Midwest, while reducing it in the South.

Here is a chart from Illinois(couldn’t find Wisconsin, but it would be very close) showing precipitation associated with 3 past La Nina years.

Notice that this La Nina charts shows average to below average precipitation for July.  Milwaukee recorded its 2nd wettest month all-time in July, and Chicago finished the month over 5″ above average for rainfall!

The chart also shows a tremendous amount of variability in the late winter and spring.  This amount of variability makes it tough to pinpoint how the weather should trend.

Here are the facts in Milwaukee over the past few months to a year:

  • Average monthly temperatures have been above average from November 2009-September 2010
  • Rainfall: Wettest July on record, 2nd wettest month ever.  June rainfall well above average.

Taking all of the above information into account, saying that La Nina accounted for a warm and wet summer in southeast Wisconsin, and is responsible for our recent warm-up is FALSE based on when La Nina formed and comparing to past historical La Nina events.

Remember, when it comes to weather patterns and circulations I believe that many combine to help produce our overall weather pattern.  The weather pattern that will produce our weather over the next 9 months or so is just forming!  Check back for more updates this month, and look for the winter forecast in the blog later in November! 

What are your thoughts?

Jeremy Nelson


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