We have been in an El Niño-Southern Oscillation-Neutral phase since the beginning of June. This was the expected pattern to set up for the duration of the summer and with a month and a half left to go of meteorological summer, things are lining up as expected. Long story short, this summer was supposed to be hotter than normal with less than normal precipitation across the state.
A La Niña Watch has been issued because when conditions are favorable for the development of El Niño or La Niña conditions within the next six months. We have been in an ENSO-Neutral phase recently which is defined by the surface water temperatures in the equatorial Pacific ocean running near normal compared to long-term average.
Since June 1, the waters near the equatorial Pacific have been running near normal. The sub-surface of the ocean has also been running near normal and the atmosphere above the ocean in the equatorial Pacific has also been running near normal further securing the fact that we are in an ENSO-Neutral phase. The folks that handle forecasting ENSO are calling for a 66% chance of La Niña conditions starting sometime this Fall.
Before jumping into what happens when we switch ENSO phases, let’s look at this summer to date. Since June 1, we have had a few heat waves and a couple bouts of monsoon moisture. The monsoon moisture we’ve seen is something that we were hoping to see occur this year since it’s been several years since there has been a notable monsoon season. In terms of precipitation, some of the expected areas have benefitted from this monsoon moisture.
The central and south mountains of Colorado, excluding the western San Juans, have seen some beneficial moisture over the last 40 days. The Sangre de Cristo and Wet Mountains have seen the most benefit to this surge of moisture as they are running a surplus of 1 to 2 inches of rain. We also had some beneficial rains sneak into the western areas of the state near Grand Junction. The far northeast Plains are running a deficit of 1 to 3 inches of rain but notably, a good majority of the state has seen below normal precipitation.
In terms of heat, there has been plenty of that to go around since the beginning of June.
There is no area in Colorado that is running cooler than normal for this period, but a few areas, mainly in southeast Colorado, are running near normal. For the remainder of the state, temperatures have been warmer than expected. East of the Continental Divide, temperatures have been running about 1 to 4 degrees above normal. On the western side of the divide, where the heat has been most intense the last few weeks, temperatures are running two to seven-degrees above normal! That is a large departure from normal value for any time of the year.
A big story this summer season already has been the heat and drought across all of the west so to pay it the attention it deserves, here is what the Western US looks like when look at the same variables as above.
Some areas are facing temperatures that are more than 10 to 15 degrees above normal and thanks to the exceptional heat the was seen all over the country, June 2021 was the hottest June on record for the US.
Most areas, with the exception of the Northern Rockies, are running a slight deficit in terms of precipitation and only a few areas are running a surplus. That surplus is most notable in New Mexico where monsoon moisture has been very beneficial as of late and one spot in east-central Utah that got hammered by a couple days of summer storms.
Overall, the trend that we are seeing in Colorado so far this summer is not alarming. This is what was expected with ENSO-Neutral conditions. Over the next month to month and a half, we can expect more of the same. A few bouts of rainy times with above normal temperatures keeping headlines.
Once we begin to switch into a La Niña pattern this fall, the ENSO phase will begin to impact us differently. La Niña brings more notable impacts during the wintertime than the fall but we’ll need to watch the hurricane season since it can be influenced by La Niña conditions during September and October. Winter 2020-2021 was a La Niña winter and we all saw how that ended with a lot of the Front Range getting hammered with snow. But that is not always how it plays out. The two previous winters where La Niña conditions were present — the 2016-17 winter and the 2017-18 winter — were some the the least snowy winters on record for Denver.
What is typical with a La Niña winter is that the north-central mountains get the most winter action. The Front Range is normally windier than normal with the southern portion of the state missing out on some big storms thanks to the Jet Stream being held further north due to those La Niña conditions.
The Climate Prediction Center is forecasting the fall months of September, October and November to feature slightly hotter-than-normal temperatures with slightly below normal precipitation in Colorado. The CPC is calling for normal precipitation to fall during the winter months of December, January and February with marginally warmer temperatures when compared to normal across the state. This is a long ways out to forecast and most long-range forecasts include data from events such as the phase of ENSO we are in or expected to be in so we’ll need to check back in on this.
For now, we are still, and will continue to be in, ENSO-Neutral conditions through August and that means we could continue to see slightly drier and slightly warmer conditions last through then.
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