EARLY AUTUMN SUMMARY
A counterpart south of Asia to the El Niño-Southern Oscillation (ENSO), in the Pacific, is called the Indian Ocean dipole (IOD). That oscillation can have an outsized effect on our weather in North America and beyond, at times.
In tandem with easterly Pacific trade winds producing La Niña conditions in the coming weeks, warm, moist air is on the rise around the Southeast Asian islands (aka the “Maritime Continent”), and the MJO tropical convection oscillation is also strongly predicted to kind of get stuck there for awhile.
This situation should generally lead to a lack of precipitation in the northeast Pacific, although I wouldn’t rule out systems up north curving back towards the northern Pacific Northwest later in September.
Heading into the second week of September, global winds are predicted to reach more normal levels. BUT, recent velocity potential charts suggests mostly dry conditions will continue in Oregon beyond September 5.
However, general atmospheric instability and surface troughing seem likely to give September kind of a wide range of possible weather, just not nearly enough rain quite yet.
SEPTEMBER FORECAST
As many people know by now, there’s another significant heatwave coming this week, which I’ve been predicting for about 3 weeks. Having a persistently strong signal for over 3 weeks is almost a dead giveaway that this will be unusual for this time of year.
The 106.3° recorded in Lytton, British Columbia last Wednesday was a harbinger of this next event which is likely to hit B.C. and Washington state the hardest.
Folks in Eugene-Springfield should prepare for temperatures to reach somewhere between 93–98°, just in case. The more urban metro areas are predicted to suffer from major heat risk.
(This unusually late, fairly strong heatwave is almost certainly being enhanced by the major marine heatwave stretching across the northern Pacific, along with a strong blocking regime farther east.)
Temperatures should begin to fall by next Friday or Saturday, Sept. 4–6.
Some instability and possible thunderstorms are likely to develop near the end of the first week of September, setting us up for periods of showers or maybe rain heading into the second week, with possibly some downstream effects of a weak polar vortex disruption.
More warm weather could arrive into the third week of September, followed by low pressure and more instability as we enter a transition towards more Autumn-like weather.
AUTUMN and EARLY WINTER SEASONAL OUTLOOK
The September 2025 C3S multi-system precipitation forecast suggests generally dry conditions for much of Oregon, with the probable exception of the northwest corner and maybe parts of the northern Oregon Cascades.
But the upcoming La Niña is still likely to bring us significant amounts of precipitation heading into the heart of Autumn.
The overall region of above normal wildland fire potential is still predicted to contract in September, but the more southern near-coastal areas will likely be MORE at risk as soils and vegetation continue to dry out.
And again, as I mentioned in May, the expected development of weak La Niña conditions heading towards Autumn bodes well in terms of promoting lower global temperatures.
Multiple sea surface temperature forecasts suggest this La Niña will be very weak and short-lived, however, with neutral ENSO conditions redeveloping by late winter or March/April 2026.
Other indicators strongly suggest temperatures will generally be colder than usual this winter, at least through January, with an increased chance for one or more major sudden stratospheric warnings. But a more neutral ENSO could help to improve conditions heading into late winter.
Heading into next summer and autumn, the window of possibility for El Niño conditions to develop begins to open up, but it’s really anybody’s guess when the next El Niño occurs and how strong it gets.