Matt Makens

It's increasingly likely that La Niña is very near the end of this latest phase
as the likelihood of La Niña conditions this spring falls to 14% in the latest
CPC forecast released this week.

Here is that forecast, shown as the probability of ENSO phase going forward
through three-month seasons. 

For March-April-May we have an 82% probability of neutral conditions. That
forecast is based on multiple model and statistical projections for sea surface
temperatures (SSTs) within the ENSO 3.4 region.

In some respects we believe we have already seen evidence of the attempted
transition out of La Niña. The atmosphere, at times since November, has
fluctuated between La Niña and El Niño-like conditions. November was one of
weakest months since this phase began, yet, December then became one of the
strongest in this current phase. 

To address what's happened recently, there's the moisture situation out West
which probably isn't fair to say is part of an atmospheric change just yet. A
case study performed after this season may shed light on how much these ENSO
phase changes impacted the West's wet winter, and what the main driver of the
pattern has been. So far this 'winter', wetness has not behaved like La Niña but
the temperature pattern has been as expected.

Shown above is a precipitation analysis from observed precipitation during the
past 180 days, which shows a wet Southwest/West and very dry conditions over the
Plains.

Now we will look at the temperature anomalies for the past 90 days.

How do those images compare to history? Consider the following two images,
representing cold season temperature and precipitation anomalies between El Niño
and La Niña phases. 

There's no exact match to historical correlations, but a blend between phases as
it appears to me.

As we go forward, let's use this summer as an example of the difference between
El Niño and La Niña phases.

Those are notable differences - for Colorado's Front Range and Plains, the
difference is dry/warm and wet/cool between those phases. You can see how the
weather pattern changes dramatically across the country. This is during the
summer, when ENSO has a statistically lower correlation on conditions. 

I don't want to get too far out there, but assuming we have an El Niño in place
this summer, our thunderstorm chances increase with a better monsoon season and,
fingers crossed, a calm wildfire situation. Into next fall and winter, we'd
increase the chances for widespread snowpack in our mountains and wetness on the
plains, too.

I didn't mention much about a neutral phase, because during those times the
dominating driver is not longer 'behind the wheel' and we can see scattered
precipitation and temperature patterns across the states. It'll depend on how
much La Nina's influence lingers and if there's any El Niño influence
developing. That's a bit of a wait and see as it stands now.

Then there's this...the Pacific Decadal Oscillation (see more here: 
https://www.ncei.noaa.gov/access/monitoring/pdo/) . You've heard us talk about
the PDO before and how important it is in determining long range weather
patterns. Since January 2020, the PDO has largely been in a negative/cold phase.
The graphic below clearly shows this:

When the PDO is negative/cold, more frequent and potentially longer lasting La
Niña episodes occur. When the PDO is positive/warm, more frequent and
potentially longer lasting El Niño episodes occur. Want to know why we've
essentially been dealing with La Niña for 3 winters? The PDO is a pretty good
explanation... However, even in times when the PDO is in a decadal phase, there
will be "blips" that occur that are the opposite of the prevailing phase. I
believe we are on the cusp of seeing one of those "blips" occur. The last time
we dealt with one of those "blips" was from late 2013 through 2016. Look at what
the PDO did during that time:

The PDO was largely negative from the middle of 2010 until the middle of 2014.
We dealt with a period of La Niña and drought during that time too. However,
when it changed phase in 2014, a powerful El Niño developed. When this warm
"blip" in the PDO phase reverted back to the predominant cold phase, we started
having issues with La Niña again...ala the last 3 years. So, if history is any
indicator, as this La Niña episode ends, we should see another warm "blip". Here
is a look at what happened with the PDO and precipitation trends the last time
this happened:

You might ask yourself this... If the PDO was warming and we were out of La Niña
in early 2014, why was it so dry in the mid-section of the country and for parts
of the Southwest US? Well, if you look at the PDO graphic I posted above (from
late 2013 throgh 2014) you can see that the PDO didn't fully shift until later
in 2014. Historically speaking, here are the expected precipitation anomalies
when the PDO is negative, from January through May:

My message is this... Given that the PDO is more negative/cooler now than it was
during the late 2013/2014 transition, I would expect this transition to El Niño
and subsequent precipitation pattern changes to be a bit slower. Cautious
optimism is certainly the best way to go here, until La Niña is gone, and the
PDO shows signs of changing phase. But cautious optimism is certainly better
than where we were last year at this time.

January update: Death to La Niña?