Peoria, Ill. (WMBD) – On Thursday the National Oceanic and Atmospheric Administration (NOAA) released their outlook for the 2020-2021 winter season which matches up with a typical La Niña winter forecast…warm and dry in the south and cold and wet in the north.
La Niña is a phase within the El Niño Southern Oscillation (ENSO) which monitors sea surface temperatures across the Pacific Ocean. When sea surface temperatures across the eastern Pacific are above average ENSO is in a positive phase, or El Niño. However, when those sea surface temperatures are below average ENSO is in a negative phase and is referred to as La Niña. When sea surface temperatures are neither above or below average the phase is neutral.
These sea surface temperatures impact the amount of convection that occurs near the Equator and the placement of subtropical and polar jet streams. Depending on the strength and phase of ENSO, forecasters can often get a good starting point in predicting how an upcoming season will play out.
La Niña conditions are ongoing and are expected to continue into this upcoming winter. This pattern often brings below average temperatures and above average precipitation to the northern U.S. and above average temperatures and below average precipitation to the southern U.S. While these conditions are most common, the strengths of these patterns will also have an impact.
While moderate La Niña conditions are favored this winter, some models suggest we could see a moderate to strong one. Stronger La Niña events tend to bring much of the U.S. warmer than normal conditions but are also highly variable which can leave much of the eastern U.S. with wild temperature swings.
Impact from ongoing drought
The latest Drought Monitor shows that nearly 65% of the U.S is experiencing drought conditions with nearly 30% of the U.S. in the midst of a severe drought or worse. In Illinois, 58% of the state was experiencing drought conditions, with moderate drought conditions in place across much of SE Illinois as of October 15th. Thankfully, there are minimal drought conditions in our local area.
Unfortunately, the strengthening La Niña means that drought conditions are likely to continue or worsen in the coming months, especially across the western U.S. As drought continues it can actually have an impact on how wet or dry an area will be in the future. With less moisture in the soil there will be less moisture in the air. This creates a feedback loop which makes dry areas drier and wet areas wetter.
What it means for Central Illinois
With La Niña conditions expected this winter the, the impacts to our area will generally come from the strength and position of the polar jet stream along with the corresponding phases of the Arctic Oscillation (AO) and the North Atlantic Oscillation (NAO), short term pattern shifts that last for one to two weeks.
While the official forecast from NOAA calls for a greater probability of above average precipitation, based on conditions from of the 15 weak to moderate La Niñas since 1955, only 20% of those produced above average precipitation and 23% of them produced above average snowfall. So, seeing above average precipitation and snowfall is not a common occurrence under these conditions.
In an email to WMBD Chris Miller, the Warning Coordinator Meteorologist from the National Weather Service office in Lincoln, stated “…we aren’t going to hit the Above Normal precipitation too hard (especially since many areas are in a moderate drought), and definitely will NOT say that we favor above normal snowfall.”
When it comes to the temperature outlook, the official forecast gives Central Illinois an Equal Chance (33.3% probability) of seeing above normal, below normal or near normal temperatures. Looking back at the 15 weak to moderate La Niñas since 1955, 37% of them had above normal temperatures. According to Chris Miller this type of outlook typically ends up with “highly variable temperatures for the season, which is similar to the “rollercoaster” pattern we are already in.”
Be sure to stay tuned for my Bold Winter Prediction in early November where I’ll give you my take on this upcoming winter and give you my best guess on where we’ll land with snowfall.