La Niña Watch Issued for Upcoming Fall and Winter Seasons


Cooler than normal Sea Surface Temperatures (SST) have developed across the Central and eastern Pacific. Just two years after experiencing one of the strongest El Niños on record, it looks like the El Niño Southern Oscillation may be changing as cooler than normal sea surface temperatures develop across the eastern Pacific Ocean. This trend of cooler water temperatures has increased confidence that a La Niña climate pattern will develop across the U.S. this fall and continue through winter. Based on current projections, this La Niña may be the strongest La Niña since the 2011-2012 Winter season but not as strong as the one that took place from 2010 through early 2011. 

What is La Niña?

La Niña is a phase of the El Niño Southern Oscillation or ENSO, which is identified by monitoring sea surface temperatures across central and eastern portions of the Pacific Ocean. When water temperatures are below average, the ENSO is in a negative phase and is called a La Niña while warmer than normal conditions are called El Niño. When sea surface temperatures are near average, the El Niño Southern Oscillation is considered to be in a Neutral phase.

Why is this Important?

The El Niño Southern Oscillation can significantly impact the type of weather the United States may see during an upcoming season. These impacts tend to be more pronounced during the fall and winter seasons when the jet stream is stronger over the country. The location and strength of the jet stream will impact the placement of colder temperatures and snowfall across the U.S. 

How will a La Niña affect us?

During La Niña climate patterns, a blocking area of high pressure will develop over the Gulf of Alaska. This creates a strong ridge over the western U.S. and troughing across the eastern U.S. Typically, colder and snowier weather tends to affect northern parts of the country while warmer and drier weather takes shape over the southern U.S. 

Even though the state of the El Niño Southern Oscillation can offer somewhat of a glimpse of what may come in the upcoming fall and winter season, it's only a portion of the seasonal outlook forecasting pie. Variations in conditions across the north Atlantic (North Atlantic Oscillation) and the Arctic (Arctic Oscillation) can also influence weather across North America.

The Arctic Oscillation (AO) monitors the strength of high and low-pressure centers around the Arctic. When the AO is in a negative phase, it means that relatively high pressure is in place over the Arctic while lower pressure is further south over middle latitudes. This tends to send colder air southward across the continental U.S. When the AO is in a positive phase the opposite is true. Lower pressure across the Arctic will keep colder temperatures with in the Arctic Circle leaving warmer conditions further south. 

The North Atlantic Oscillation (NAO) is similar to the Arctic Oscillation but is based on the difference in pressure between the Polar Low and the Azores High over the Atlantic Ocean. A negative phase of the NAO tends to bring colder and snowier conditions to the eastern U.S. while a positive phase tends to bring warmer conditions to the eastern U.S.

So, while the presence of a La Niña can mean a colder and snowier Winter is in the cards for Central Illinois, it's really the interaction between the Arctic Oscillation, the North Atlantic Oscillation and La Niña that will bring those conditions to fruition. We'll continue to monitor conditions in the Pacific, Atlantic and Arctic and I will release my bold winter prediction in late October or early November. 


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