Pete Parsons: El Niño will flip the switch in September

Presenter: Pete Parsons looks at the forecast for the rest of the summer and the transition into fall. A strengthening El Niño plus cold ocean temperatures off the Oregon coast should protect us from the worst of the heat.

Pete Parsons: Hey, I’m meteorologist Pete Parsons with the Oregon Department of Forestry. This is the seasonal climate forecast or a three-month forecast using analogs or years in the past that I line up with tropical Pacific Ocean temperatures that are similar to this year to try to predict what could happen this year. Let’s jump into it. 

Here’s the temperature forecast: near average temperatures, precipitation about average to a little on the dry side. But there can be some thundershowers mixed in there, much like we’ve had—go through periods of dry, really nice weather, and then you get a couple days or a day of thundershowers, but not enough to put the overall precipitation much above average. I would expect that general pattern to continue through August.

As I mentioned, off the top, I look at the tropical Pacific Ocean temperatures. And that’s what we want to look at down here across the equator. That’s where we look for the El Niño phenomenon. You’ve probably heard of that. When you get warmer surface temperatures down here, they’re high enough for long enough that we categorize that as an El Niño event.

And the reason that’s important is because the extra heat coming off of the tropical oceans down here impact the jet stream patterns globally. It doesn’t just affect our weather, it affects the weather all over the world in rather unique ways that we can utilize to predict what’s going to happen because the ocean temperatures change first, then the weather patterns.

There’s a little bit of a lag and it holds because these ocean temperatures are slower to change than the air, because water is slower to change in temperature than the air.

The Climate Prediction Center thinks that this El Niño is going to become very strong. And my analog years represent that as well, even though I don’t take into account their forecast when I pick analog years. 

I use an index for that. It’s the Oceanic Niño Index. When it’s above zero, that means the temperatures are warmer than normal. And when it’s below zero, they’re cooler than normal. 

We’ve been cooler than normal over the last couple of winters. We had La Niña winters, not real cold, but colder than normal. And now we’re going into a really strong. It looks like El Niño. I’ve been around long enough. I remember the El Niño of ‘82-‘83, a very strong one.

In fact, I was an atmospheric science student at Oregon State University at that time. And that’s when there was a big buzz about El Niño, when we really started to learn a lot about how much El Niños affect the worldwide weather patterns, and especially here in the Pacific Northwest, some of the influences it has, because that one really affected our weather, as this one is likely to do this winter.

The ‘97-‘98 El Niño, also a real strong one that had huge impacts on our weather. 

Another thing that I check is something called the Pacific Decadal Oscillation. It’s a North Pacific phase of the ocean. And right now we’re in the cold phase because the colder waters are right off our coast and off the coast of Alaska. That’s what we have right now. That’s why we haven’t been having big 100+ (degree) plus heat waves that have lasted for a long period of time.

And the analogs would say that we’re not going to get that. Maybe you get a day that’s one day that’s 100 or something. But it doesn’t last very long because then you get that southwest flow kicks back in. 

And this also increases the moisture feed coming out of the tropics. You can get moisture that gets sucked up in the flow, and also monsoonal moisture that can get sucked up in the flow and you can increase the thunderstorm threat.

So that’s the pattern that’s going to prevail here for July and in August: warm but not horrifically warm; increased chance of periods with thunderstorms, which isn’t the best for fire danger situation because we’ve got the drought situation over us. But perhaps those thunderstorms are going to have some moisture with them, and that does help.

September: Now we’re beginning to see the signature of El Niño show up. What is the El Niño signature? It’s a split-flow jet stream pattern right on the West Coast. So you have one branch of the jet strengthening and coming down here into California. That’s why El Niños tend to produce a lot of rainfall—or much above average—in California. And this one’s likely to be no exception.

Watch out for California to get some flooding rains coming in this winter. But we’ll start to see that split flow develop. They all do it to some degree. And in the upper atmosphere that means more troughing, more jet stream energy than normal coming into the West Coast in September.

So how is that going to play out for us? Well, it’s still a mild pattern. It’s not going to be probably horrifically hot, but there’s still going to be some warm spells, probably nice weather, generally speaking.

However, as that jet stream picks up, it’s also going to have enhanced moisture with it. From that, the subtropical branch or the southern branch of that jet stream and as that moisture gets spun up, we can get showers and thundershowers.

So the analogs tended to give us near or above normal precipitation with the showers, and especially a pick up in the shower activity and thundershower activity in the middle and latter portion of September. That’s what happened in these years.

As we go into October, a pretty strong westerly flow, some with more splitting than others, but a westerly flow into the coast—that is a wet pattern as we go into October. It’s not a real wet pattern in the middle of summer, but as we make the transition into October, it becomes a wet pattern.

Below normal temperatures in the upper atmosphere—that’s just going to make for a little cooler temperatures at the surface as well.

So here’s how things play out: In October, probably flipping to below normal temperatures. So we’re going to see a marked transition. Pleasant weather in the summer, maybe a little above normal on the temperatures with some showers and thundershowers at times.

And then a flip late September and October to cooler, wetter weather than normal. It always gets cooler and wetter in general as we go into September and October, of course, but more so than normal. So you’ll see that transition more pronounced and above normal on the rainfall in October.

It also—we’ve enhanced the chance of showers and thundershowers late in September and October. And if we still have the drought conditions, we might have to deal with some wildfire starts. Hopefully they come with enough rain to help us get to them quickly, get them out and we don’t have a big issue.

But as we go into late September and October is when it turns wetter than average. And even the analogs do show about average amount, a little bit of snow into the highest elevations of the mountains by the end of October.

Usually like Crater Lake, it’s about six or seven inches of snow by the end of October. And that’s about what the analog showed about average. But I’m jumping ahead of myself.

August through October: starting off warm, then turning cool. So near-average temperatures, precipitation starting off drier, then turning wetter, but above average precipitation for the period.

That transition could be tricky because we got a lot of drought going on. Much of the state in drought; expect all of the state to be in drought within about 30 days or so. So fire starts can be an issue if we get the lightning. Got to get to those quickly. Hopefully we’ll get rain with the thunderstorms. That’ll help as well if we get that.

Looks like we have some generally very pleasant weather coming up for much of the summer, so I hope you have a chance to get outside and enjoy it, and we’ll catch up with you next month. Take care.

Presenter: Pete Parsons shares the three-month forecast no sooner than the third Thursday of each month, and this one came out July 16. You can follow Pete by searching for ‘Seasonal Climate Forecast’ at oregon.gov.

Drought map courtesy National Drought Mitigation Center, July 2026.