Brian Bledsoe

Time for a comprehensive look at what lies ahead over the next several months.
Here is what the sea surface temperature anomalies look like.

You can see some "blue" off the west coast of South America. These are sea
surface temperatures that are cooler than average. Certainly not La Niña status
yet, but still moving slowly in that direction.

Why are we only moving slowly toward a La Niña episode? For one, the atmosphere
and the ocean have to work in tandem to produce impacts. One piece of
atmospheric evidence, the SOI (Southern Oscillation Index), is far from La Niña.
Sustained positive numbers reflect La Niña, while sustained negative numbers
reflect El Niño. See below:

If we were moving rapidly toward La Niña, the SOI would be making a significant
move in the other direction. That doesn't mean it won't change, but certainly
not in a rapid fashion. 

Another element that has been lacking for a push toward La Niña is the lack of
strong easterly trade winds along the equatorial Pacific (ENSO region) Ocean.
The graphic below shows that is changing as well... 

Yellow/red coloring represents westerlies along the equator, and blue/green
represents easterlies. We are likely to see some pretty sustained and strong
easterly trade winds for the rest of the month. This allows for cooler water
beneath the surface to upwell, thus cooling the sea surface. This is something
that is necessary for La Niña to develop. Is there cool water beneath the
surface? Yes, the animation below clearly shows that: 

You are looking at west (north of Australia) to east (South America) across the
equatorial Pacific, and from the surface down to 450 meters. The blue represents
cooler than average water, while the orange represents warmer than average
water. You can clearly see that there is plenty of cool water to upwell off the
west coast of South America. If we can continue to see those easterly trades
persist, that water will rise to the surface and continue to push us toward La
Niña. Basically, we have some work to do before we get there...IF we are to get
there. 

So, what do the models suggest? Here is a look at the latest model spread when
it comes to determining what phase of ENSO (El Niño, La Niña, or neutral) we
will likely see.




Zero = neutral

Above the horizontal dashed line = El Niño

Below the horizontal dashed line = La Niña

The first graphic is what the US models are showing. The US models remain the
most aggressive when it comes to La Niña development. The second graphic is what
the international models are showing. The international models are the least
aggressive with La Niña development. This includes the EURO seasonal model,
which essentially doesn't show any La Niña taking place! However, there's one
thing that continues to stick out to me... The PDO (Pacific Decadal Oscillation)
has been strongly cold/negative for the past few years, is still strongly
cold/negative, and is likely to stay cold/negative in the coming months. You can
see in the graphic below: 

The significance of the cold/negative PDO is that it makes it easier for us to
slip into La Niña, even if it is just a weak event. And, yes, even weak La Niña
events have impacts. 

In short, we continue to progress toward at least a weak La Niña event, but
still some ingredients that need to fall in place for that to occur, and at
least some model guidance suggesting we may never get there – plenty to watch as
we head toward fall. 

Long-range precipitation outlook
Regarding precipitation anomalies for the next several months, here is what the
EURO and the NMME models have to say.

EURO Seasonal Model Precipitation Anomaly Forecast September - February


NMME Model Precipitation Anomaly Forecast September - March

While the precipitation anomaly signal is not as stout in the EURO model as it
is in the NMME model, the La Niña signal is still present. By that I mean, there
is a dry flavor for the southern tier and a wetter flavor for the northern tier,
with the NMME drier overall, and the EURO showing at least some better moisture
prospects, especially the higher in latitude you go. This makes sense given what
we discussed about the EURO being less bullish on La Niña prospects overall. 

As for temperatures... Both models really don't handle the temperature anomaly
forecast very well, which is why I don't show them to you. I will stay this... I
suspect that the Pacific Northwest, Northern Rockies, Northern Plains, and the
upper Midwest / Corn Belt will all likely end up cooler than average for the
upcoming fall and winter seasons. 

Farther south, I think it is a little early to tell. But, the tip toward a
drier/warmer fall and early winter is likely the play here. 

I think it is worth noting that if the La Nina signal ends up being stronger,
then the La Nina impact will likely end up being stronger too. This is something
that we hope continues to become more clear, as we finish summer and enter fall.

Stay cool everyone!

Weather5280 Insider: Extended outlook as we inch toward the cold season