National Weather Service United States Department of Commerce

Even though November and the first half of December across central and northeastern South Dakota and west central Minnesota began much above normal temperature-wise, December snows and the recent cold temperatures may have you wondering about how El Niño and how its "very strong" status was expected to bring mild conditions overall.

So let's start by taking a look at where we stand at this point by referencing the Midwestern Regional Climate Center's Accumulated Winter Season Severity Index (AWSII). This research has been targeted around providing an answer to the question "how severe was/is this winter?" It does so by taking into account max/min temperatures, snowfall, and snow depth and accumulates the information from the moment wintery conditions begin to when they stop... not just from one set date to another. Though it does not take into account wind (or blowing snow and wind chills as a result), it does an excellent job in providing a perspective on how the current winter ranks to winters since 1950-51. Five categories are assigned to 52 locations across the US, 5 in South Dakota, and they range from Mild/W1 to Extreme/W5. Record extreme/mild conditions are also indicated when appropriate. The charts below show Aberdeen and Pierre's winters up to January 2nd based on the AWSII.

 

You can see how these locations began as "Mild," but the area's recent snowstorms and cooler weather has brought the index to "Moderate" in Aberdeen and "Average" in Pierre. This makes sense since central South Dakota has been hit especially hard by snowstorms this past month. However, take a look at how the country ranks through January 2nd. Much of the Midwest and Eastern United States have experienced a "Mild" winter to this point. Also notice all the diamond icons - these are locations that are observing "Record Mild" winters to this point (again, since the 1950-51 season).

This information and more updates frequently and can be found at https://mrcc.isws.illinois.edu/research/awssi/indexAwssi.jsp#info

 

Now, compare the current winter to what the country typically experinces temperature-wise during December and January when strong El Niño events, like the current one, are present (from the National Centers for Environmental Information at https://www.ncdc.noaa.gov/monitoring-references/dyk/elnino-2015-2016). Notice how, on a whole, the country is behaving as we would expect. The exception? Right here at home!

 

 

Why? December's flow aloft was a little atypical for strong El Niños. Instead of the jet stream splitting across the West Coast and diverting system after system across the southern US leaving the Northern Plains high and dry, low pressure has been allowed to propagate further north. This has led to snowfall after snowfall. When you have white snow covering on the ground, the sun's already weakened rays (solstice on the 21st of December) reflect a great deal back into space. This means solar radiation can't heat the darker ground and thus the air which, if allowed to happen, would result in overall warmer temperatures.

See this youtube video for a better explanation on what El Niño is and how it influcences the weather across the United States: https://www.youtube.com/watch?v=_Tuou_QcgxI

 

Looking ahead to the remainder of the winter season, the status of our El Niño isn't likely to change. And by the way, by many measures, it's currently one of the top 2 strongest in recorded history (records go back to 1950). This means above normal temperatures are still certainly possible, and the climate prediction center has outlined the northern United States in heightened probabilities for above normal temperatures for their one and three month outlooks accordingly (http://www.cpc.ncep.noaa.gov/). However, it's also possible for existing snowcover to continue having an impact, and more importantly other climatic factors as well. For instance, the Arctic Oscillation is currently in its negative phase and is expected to remain that way for the next couple weeks. In other words, the polar vortex is set to weaken which will allow for the intrusion of arctic air across the Plains among other locations. We're already expecting a manifistation of arctic cold air across the area as a result this weekend and again for the middle of next week. May this serve as a reminder that all El Niño winters are different, and that a great number of factors can affect any given winter's outcome. For the current state of the 2015-16 El Niño, visit https://www.esrl.noaa.gov/psd/enso/enso.current.html