JACKSON, Wyo. – Following three consecutive years of La Niña, sea surface temperatures in the Eastern Equatorial Pacific Ocean have quickly reversed course this spring. As of early June, El Niño conditions are officially present.
We are already seeing impacts on weather conditions in Wyoming and the Western U.S. with this emerging pattern early this summer. El Niño’s influence tends to be even more pronounced with regard to winter weather conditions, which will be something to keep an eye on later this year.
What is El Niño?
El Niño represents the warm phase of the El Niño Southern Oscillation Cycle (ENSO). This occurs when sea surface temperatures in the Eastern Equatorial Pacific Ocean off the coast of South America are warmer than average. This is the reverse of a La Niña, in which sea surface temperatures in this region are colder than average.
From late 2020 through early 2023, La Niña conditions were present for most of this period, which spanned the course of three winter seasons – an unusual occurrence.
The pattern quickly flipped this spring, and as of early June, there is a noticeable warm tongue of above-average sea surface temperatures near the equator in the Pacific Ocean off the coast of South America. This is a classic El Niño signature.
As we head into the fall and winter months, El Niño is a near certainty to continue based on long-range forecasts and is also likely to strengthen. El Niño strength is determined by the departure from average sea surface temperatures, with warmer sea surface temperatures relative to average indicating stronger El Niño’s (and vice versa for La Niña).
How Does El Niño affect Jackson Hole and Western Wyoming Weather Patterns?
First, it should be noted that seasonal forecasts have their limitations and every El Niño (or La Niña phase) is unique. However, looking at past data, there are signals which indicate which type of weather patterns are more or less likely at various times of year during El Niño cycles.
In some years, El Niño does not emerge until the fall months. However, this year El Niño has already arrived as of early summer.
During years in which El Niño emerges and/or strengthens during the summer months, low pressure troughs are common across the Southwest/Southern California with southwest winds often transporting moisture into Wyoming. We are also more likely to experience wetter than average conditions and cooler than average temperatures across Northwest Wyoming during these El Niño summers.
So far, the pattern we have experienced in late May and early June has fit the “typical” El Niño look with frequent showers and thunderstorms. While July and August are our two driest months on average, during El Niño summers, rainfall is more likely to be above-average than below-average during these two months.
Temperatures have actually been a little warmer than average in May and early June, largely because of mild overnight temperatures. However, if recent history is any indication, we are more likely to have below-normal temperatures during the two hottest months of the year in July and August, or at least cooler compared to the past two summers.
There are some weaker signals that indicate higher odds of warmer and drier conditions emerging as the summer season winds down in September.
As we head later into the fall, temperatures are more likely to be on the colder side of average during October and November. However, there are no obvious precipitation signals during these months.
Skiers and riders will not want to hear this, but El Niño winters are more likely to feature below-average snowfall and warmer-than-average temperatures in the Tetons. This is not always the case, however, as our last El Niño winter (2018-2019) featured heavy snowfall and colder-than-average temperatures.
Also, the warmer signal for temperatures shows up primarily during late winter. Temperatures in December-January only show a slight warmer-than-average signal (and in some years, mid-winter temps have been very cold during El Niño’s), while temperatures in February show a strong above-average warmth signal.
Once again, every El Nino phase is different and subject to other atmospheric patterns that can’t be predicted far in advance, so take these historical comparisons with a grain of salt. But at the very least, these historical comparisons give us an idea of temperature and precipitation patterns that are more or less likely.
Current Weather Outlook – Active Pattern Continues Into Next Week
We have been stuck in a showery pattern for several weeks now with almost daily rounds of showers and thunderstorms. We will start to see some subtle changes to the large scale pattern over the next week, but we still have plenty of active and wet weather to get through first.
First, a trough of low pressure is moving across the Northern Rockies on Wednesday, which is helping to break down the stagnant pattern we’ve seen for a while now. A cold front is moving across Teton County on Wednesday afternoon with drier air working its way in behind the front.
On Thursday, we will see more sunshine compared to recent days along with just a slight chance of isolated afternoon thunderstorms. Temperatures will also be cooler than average with highs in the mid 60s.
On Friday, a weak disturbance arriving from the southwest will result in a better chance of showers and thunderstorms, during both the morning and afternoon hours. Moisture will be more limited compared to recent weeks and any rainfall will be light. However, dangerous cloud-to-ground-lightning is still a threat with any thunderstorm.
Heading into the weekend, Saturday looks quite nice with highs in the upper 60s along with a low/medium chance of isolated afternoon thunderstorms.
The pattern will begin to change again early next week as a deep trough of low pressure originating in the Gulf of Alaska deepens across the Western U.S. The result will be a good chance of showers and thunderstorms on Sunday and Monday with locally heavy downpours possible.
Eventually, a shot of colder air is expected to arrive, likely by Tuesday, resulting in chilly temperatures and additional rain chances. Also, there is a chance we could see accumulating snow across the higher elevations of the Tetons and surrounding ranges, possibly down to around pass level.
This cooler/wetter pattern is projected to linger through about the middle of next week. Once we get into the second half of next week and beyond, this system is expected to exit to the east and it’s possible we could head into a drier and sunnier pattern.
Alan Smith, Meteorologist
