Chicago just experienced its hottest September in 64 years, but Chicagoans know how quickly the weather can change.
The National Oceanic and Atmospheric Administration’s official winter forecast won’t be released until Oct. 17, but there are already signs the winter could be colder and wetter. Of course, there are many caveats when it comes to long-term forecasts.
Currently, a “La Niña Watch” is in effect.
There is a 71% chance of a La Niña winter occurring between December and February, but the strength of La Niña is always in question.
Sea surface temperatures in the equatorial Pacific are cooling, but the question remains how much they will cool. The latest data suggests that we are more likely to see a weak La Niña rather than a strong one.
Typically, a La Niña winter places the jet stream in a position to bring overall colder and sometimes snowier conditions during the winter months in the Midwest. The correlation between La Niña and a colder winter in Chicago is greater during a strong La Niña season, but significantly less during a weak La Niña season.
Last year was marked by the most violent El Niño winter since 2015 and it ended up being the 5thth the warmest winter on record in Chicago.
It is still too early to say whether the winter will actually be very cold. Keep in mind that the last few winters have been very mild in Chicago, so even an “average” winter will be colder than previous years.
What are El Niño, La Niña and ENSO?
ENSO stands for El Niño Southern Oscillation, and the ENSO pattern alternates between El Niño, La Niña, and neutral ENSO. ENSO neutral conditions mean that no El Niño or La Niña phenomena are present.
In general, each El Niño or La Niña episode can last one or two years.
El Niño occurs when trade winds near the equator in the Pacific Ocean blow from the west. This carries warm waters to the surface of the Pacific Ocean.
A La Niña phenomenon occurs when winds blow from the east. An easterly wind coming off the coast of South America causes deep, cool ocean water to rise to the surface and drift westward.
These alternating patterns ultimately impact the position of the jet stream and can have overall impacts on weather conditions.
NOAA will release its monthly update on our emerging La Niña on October 10.
