Brian Bledsoe

It is time to revisit our seasonal weather outlook. First, let's check out what
the sea surface temperature anomalies look like.


1) The sea surface temperature anomalies along the equator in the Pacific Ocean
continue to cool. This is especially true near the South American coast and
westward... This is the developing La Niña. The cooler those sea surface
temperature anomalies get, the stronger the La Niña episode. As of right now, we
have just started to enter weak La Niña status. However, this setup has to
persist for longer than just a few weeks or a month to be classified as La Niña. 


2) The northern Pacific Ocean continues to be warmer than average. However, the
warmer than average water off the west coast of North America will likely start
to cool off with the seasonal change. The warmer than average water near and
east of Japan, will likely still stay quite a bit warmer than average. This
signature is a big part of why the PDO is still so negative/cold...-2.88 August
reading. I don't see the PDO changing over the next several months.The warmth in
the northern Pacific Ocean is important, because a ridge of high pressure will
usually set up where that warmer than average water resides. 


3) The warmth in the northern Pacific Ocean is important, because a ridge of
high pressure will usually set up where that warmer than average water resides.
The position of that north Pacific ridge is critical in determining how the
fall/winter northern branch of the jet stream pattern will set up. It is also
important to note that during a La Niña episode there is RARELY ever a
sub-tropical (southern branch) of the jet stream. That feature is much more
prevalent during an El Niño episode, ala last year.


4) Most of the northern Atlantic Ocean remains much warmer than average, with
the exception of the water temperatures around Greenland. This colder than
average feature has caused some to think that the Atlantic might be starting its
cooling phase. However, this is likely just a seasonal thing and we will likely
continue to see a positive/warm AMO (Atlantic Multi-Decadal Oscillation)
continue... And remember, a -PDO and a +AMO usually increases drought frequency
across the western Plains, southern Plains, and throughout much of the Desert
Southwest. The graphic below which I have referenced many times, shows this very
clearly. We are firmly in the lower right side of the graphic...-PDO +AMO.

 Alright, with all of that current analysis out of the way, 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 be in...

0 = 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 EURO model is showing. While the US models are still showing a stronger La
Niña than the EURO or any other international models, the EURO model is coming
in with a relatively stronger event than previous runs. Also, the PDO (Pacific
Decadal Oscillation) is still quite negative/cold, with the latest reading at
-2.88. With the background state of the PDO being so negative/cold and the
equatorial Pacific (ENSO region) region continuing to cool, even a weak La Niña
will produce an impact. As you'll see below, the September run of the models
will be similar to what the August run showed. There are a few differences, but
the most noticeable difference is that the EURO is showing a more traditional La
Niña impact. You'll see that below...


EURO Seasonal Model Precipitation Anomaly Forecast October - March


NMME Model Precipitation Anomaly Forecast October - April

Both models show a pretty classic La Niña look to their precipitation outlook.
Generally speaking, wetter for northern areas with drier conditions farther
south. I've been saying this for quite a while, I think that the Pacific NW,
northern Rockies, northern Plains, eastward into the Midwest could see a MUCH
different winter than what was witnessed last winter. By that I mean, quite a
bit more cold and snow... Areas farther south will likely end up on the windy,
warmer, and drier side of the storm track. This is especially true for the
western/southern Plains... Parts of the Southeast US could also end up drier
than average, as the main storm track stays too far north.


I will caution you that when you look at the precipitation anomalies, just
remember that you are looking at ANOMALIES. Just because you reside in the
"brown area" doesn't necessarily mean that it isn't going to rain or snow. It
simply means that the likelihood of being drier than average is enhanced. Could
you see a big storm? Sure... These models do not account for individual storms.
They account for the perceived pattern over a period of months. 

Q and A

1) How strong will this La Niña episode be? Always tough to say, but all
evidence at this time suggests that this will be a weak to possibly briefly
moderate event. I'm not saying a strong event can't happen, but the odds of a
strong event are pretty low.

2) How long will this La Niña last? As of right now, it appears this event will
peak in December or possibly early January (pretty typical) before weakening and
or disappearing as we finish winter and move into spring. I am always reluctant
to show a La Niña the door given the negative/cold PDO, but that is what I am
thinking right now.

3) What happens after La Niña exits? Very tough to say, especially given the
negative/cold PDO. However, some models are suggesting we move into an El Niño
for spring. I am certainly not ready to call for that yet. However, could we see
a flip back to El Niño after La Niña? I suppose it is possible, but I would
certainly like to see the PDO moderate before I even entertain that idea.
Nonetheless, it is something that bears watching for the spring and
summer...but, nothing more.

4) Analogs? Yes, analogs are becoming more clear for at least the next several
months while La Niña is present. I am working on that now, and will release more
about them in the coming days.

Still some questions to figure out in the coming weeks, but the overall players
are becoming more and more clear. It is important to note that El Niño or La
Niña is not the only player in the forecast. Determining the prevailing phase of
the Madden-Julian Oscillation (MJO) in the months to come is very important. We
will obviously be keeping you updated on that in the coming weeks.

Weather5280 Insider: Seasonal forecast update, September 2024