The State of Play: Parched Western Lands
It’s not a good time for water resources in the south-western USA. From New Mexico to Nevada, most places are suffering an extreme to exceptional drought as of 18th March. This is despite rainfall close to the long-term average; it’s typically a very dry time of year anyway (averaging 0.5 to 1.5 mm per day), so it’s not made much difference.
Meanwhile, Nevada, California and much of the Midwest have been experiencing similar rainfall rates – but here, it’s below the norm. Many areas are now in moderate to extreme drought.
As we head through spring and into summer, rising temperatures make it easier for drought conditions to sustain. It’s going to take a good deal of rain, ideally with below normal temperatures, to overcome that. How likely is this during the next few months?
Analogue Years Paint a Picture of Persistence
The MetSwift Analogues can give us a good idea. They consist of a selection of ‘most representative years’ from 1950-2020. Whether a year qualifies depends on the behaviour of numerous ‘key climate drivers’ (teleconnections) and how they compare with what’s been observed in the runup to the forecast period. The more similar that behaviour, the more likely that year is to be selected.
By comparing the typical conditions between those years and a ‘climate baseline’ such as the 1981-2010 average, we can see what ‘signals’ the MetSwift analogues are giving us for the forecast period. Below, this has been done for each month April-June 2021, considering both temperature and rainfall.
Map of North America showing the weather tendency suggested by the MetSwift analogues for April 2021.
The weather tendency suggested by the MetSwift analogues for May 2021.
The weather tendency suggested by the MetSwift analogues for June 2021.
Please click on an image to view it in full resolution.
April: Troubling for the Southwest but Hopeful in the Midwest
Unfortunately, we’re starting out with a bad look for the drought-stricken states of New Mexico, Arizona, Colorado, and Utah. The analogues suggest it will be either warmer than average, drier than usual, or both.
For the Midwest, it’s a different story: Rainfall near normal and temperatures near or below. Suppressed temperatures may allow typical rainfall amounts to do some good work on the land, especially in North Dakota’s extreme drought region.
May: Alarmingly Dry Southwest and Hints of Midwest Heat
This month has an abnormally dry signal for a large part of the extreme to exceptional drought region south of the Kansas to Utah line. This should be interpreted as a raised probability of such conditions, not a prediction that it will be that way, but still – it’s an unpleasant sight.
In the Midwest, there’s some suggestion of above average temperatures for the month. That means a raised chance of heatwaves and associated strong drying of the land. On the other hand, rainfall is signalled to be near normal, which suggests some appreciable pauses or reversals of drought development.
June: Concerning Risk of a Hot and Dry Start to Summer
From Oregon to South Dakota in the north and Oklahoma to California in the south, the analogues suggest a particularly hot start to summer 2021. There are 14 analogue years, yet their mean temperature anomaly (not shown) is widely 1-2°C above average in this region. Even when accounting for the climate trend, this is an impressive – and worrying – signal.
On top of this, below normal rainfall is also signalled for most places west of the South Dakota to Oklahoma line.
Such an outcome could not only sustain moderate to exceptional drought conditions but cause them to expand in coverage.
Overall, the outlook has far more negatives than positives – which begs the question, why are such conditions being suggested?
What’s Behind These Concerning Signals?
Put simply, it’s a combination of ocean temperature patterns and climate change.
In recent weeks and months, below average temperatures have dominated the eastern half of the equatorial Pacific waters. This is the fingerprint of what’s known as a La Niña event. It’s the cold phase of a slow-shifting oscillation of tropical Pacific temperatures known as the El Niño Southern Oscillation (ENSO).
Warm and cold phases (El Niño and La Niña events) typically last at least 6 months, sometimes more than a year. The current La Niña got going last autumn and is expected to continue until at least early this summer.
In tandem with this event, sea surface temperatures in the eastern North Pacific Ocean have also been predominantly below normal. Meanwhile, they’ve been above normal to the north and northwest of Hawaii. This setup matches the negative phase of another oscillation: The Pacific Decadal Oscillation (PDO). It changes even more slowly than ENSO, tending to favour a certain setup for over a decade at a time (hence the name).
Meanwhile, warmer than average conditions have been widespread in the North Atlantic.
Below, I have shown the weather tendency for April-June associated with La Niña alone (left) and with a combination of that and the other two ocean states (middle).
Map of North America showing the weather tendency during years with a La Niña event.
Weather tendency for years with a La Niña event + Negative PDO + warm North Atlantic.
Weather tendency for all years 2000-2020.
As you can see, La Niña is a significant player: It’s made a notable contribution to the warm, dry, or both, tendencies that the analogues suggest for the drought-stricken areas of the western USA.
When this joins forces with a negative PDO and warm North Atlantic, the theme of a warm and dry west amplifies.
Then there’s the climate trend to consider (above-right). The 21st century has so far been generally warmer, also drier in some spots, than the 1981-2010 average. There’s been a shift in favour of drought conditions.
The MetSwift Analogues account for climate change, so this trend has influenced what they suggest for the coming few months. Which is to say, heightened drought risk in parts of the western USA is not so unusual nowadays as it was in the 20th Century.
This makes it more important than ever to keep track of how large-scale phenomena are likely to steer weather patterns in the weeks and months ahead.
For the sake of those suffering from drought, I’m hoping this proves to be one of those occasions where the most likely outcome doesn’t occur!
James Peacock MSc
Head Meteorologist at MetSwift