DES MOINES, Iowa — The September monthly analysis of the El Nino/Southern Oscillation is in from the Climate Prediction Center and the NWS and the odds of La Nina continuing for a rare third consecutive winter are increasing.
La Nina and El Nino conditions are defined by sea surface temperatures in the equatorial Eastern Pacific region, with La Nina representing cooler temperature anomalies and El Nino warmer ones. Throughout August, sea temperatures remained cool with little change, and the forecast is now for a 91% that La Nina will continue through fall and a 65% chance that it will continue through Winter, up from 60% last month.
What Does It Mean?
La Nina years are typically noteworthy for persistent high pressure over the Pacific Ocean that can tend to drive the jet stream north, closer to the Alaskan Coast. This can then lead to colder conditions across the northern tier of the United States, and potentially wetter, snowier winters as the storms follow along jet stream.
With the jet coming in farther north, the southwestern and southern US can end up drier without great support for storm systems, potentially leading to or exacerbating drought conditions.
How About Iowa?
El Nino/La Nina years tend to be most pronounced in their effects closer to the coasts, and with Iowa obviously far away from those, the relationship here can be a bit muddled.
In fact, for the past two seasons, both La Nina years, Des Moines received 56.0″ of snow for 2020-21 and only 36.0″ of snow for 2021-22. The average total snow for Des Moines in a year is 36.5″.
Even trickier, snowfall may possibly be impacted by the strength of La Nina. Overall for La Nina years, their isn’t a strong signal either way for snowfall in Iowa. However, in strong La Nina years, Iowa’s snowfall average skews lower, while appearing to be on the higher side in weak La Nina years.
This may be because the jet stream will be more consistently pushed to the north, with storms coming into Iowa from the NW and more generally cut off from gulf moisture. With a weaker La Nina and Pacific high, it may be easier for the jet stream to have a more southern angle of attack and push storms up from the south.
Breaking down La Nina years that far also ends up resulting in small numbers of years to compare, so the trends can’t necessarily be counted on to be statistically significant.
Remember, Des Moines had a 14″ snow storm last year…than came from the typically drier northwest! And of the last two La Nina winters, the stronger one produced more snow in Iowa, which isn’t necessarily expected! One or two storms can be all it takes sometimes to make or break a snow season.
Another trend sometimes associated with La Nina is an active Atlantic Hurricane season, which has not be the case yet this year.
The Bottom Line
Overall, La Nina is still forecasted to weaken into the spring, with odds dropping below 50% of La Nina continuing for the Feb-Apr time frame. While it may not provide the best odds for predicting Iowa’s fate this winter, skiers should likely be happy in the Northwestern United States this year.