MAY–AUGUST SUMMARY
(mostly somewhat technical)

I apologize for not posting a forecast last month. I was busy after June 16 preparing for a big conference and also had some ongoing personal struggles.

One glance at this first chart should tell you there’s a lot going on in the atmosphere, and there’s going to be a significant shift in about 10 days.

Through May & early June, global atmospheric winds had swung negative, which set us up for more stagnant weather patterns through much of June.

This pattern generally continued into late June, exemplified by the lack of a strong heat wave– just as I suggested in my late May forecast. (I had meant to say that we could see a decent heat wave at the end of June, but I wasn’t very certain.)

Near the end of June into early July, global winds began to rebound, inducing a weakly positive east Asian mountain torque (EAMT+), thus inducing a stronger Pacific Jet Stream.

More recently, Pacific wind speeds were (and STILL are) also getting strengthened by two planetary waves that generally move east across the tropics– namely the Madden-Julian Oscillation (MJO) and a convectively-coupled Kelvin wave (CCKW).

By the end of July, MJO and CCKW-induced global mountain torque will likely produce more regular Rossby wave flow. And this could help to prevent wildfire smoke from moving back towards Eugene until sometime after August 5–6, or so.

But, heading into the second week of August, global winds are very likely to drop back to more normal levels. Velocity potential charts also suggest persistently dry conditions will continue through the last three weeks of August.

FOUR-WEEK FORECAST

Monday through Friday, July 28 to August 1 will bring a mild heatwave, along with a 15–35% chance of thunderstorms, mainly for the far eastern portions of Lane
and Linn counties roughly from Monday afternoon, July 28 through Thursday evening, July 31.

Then, from August 3–7, a trough is predicted to drop down from the Gulf of Alaska along the British Columbia coast, bringing another welcome cooling trend and another mild chance for mountain thunderstorms. Parts of east Lane and Linn counties could see occasional precipitation around then.

Temperatures are then predicted to warm up again roughly August 5–6. This could be the beginning of two heat waves, and there may not be much relief in between. The first heatwave looks a bit stronger, with some potential for highs above 92°.

Temperatures could even remain generally above average until near the end of August, which has been predicted to be warmer and drier than average since April/May 2025.

LATE SUMMER AND EARLY AUTUMN SEASONAL TRENDS
(all these charts were issued July 1st)

The August 2025 C3S multi-system precipitation forecast, for example, suggests generally dry conditions will persist, particularly away from the more northern coastal areas and heading east into the northern Plains.

For August temperatures I want to highlight the C3S 850 millibar temperature probability forecast. This basically shows a classic La Niña-like pattern of persistent heat centered over the northern Great Basin and northern Rockies.

More importantly for Lane County, this chart is also heavily influenced by the major marine heatwave in the northern Pacific. Nobody knows for sure how that marine heatwave will evolve in the next three months, but I would expect it to begin shifting a little bit north in the next several weeks and extend a little farther east as we approach genuine La Niña conditions.

(See the September temperature forecast, below.)

Closely aligned with the August precipitation forecast, wildland fire potential should stay significantly above normal for most of northern California, all of Oregon and Washington, and most of Idaho and Montana.

While the persistent August heat dome is predicted to contract and move a little farther east, please NOTE that surface temperatures in the northeast Pacific are predicted to surge heading into September.

I would guess that atmospheric instability will probably increase over western Oregon, that summer-like temperatures could persist into a big chunk of September, and that mostly DRY conditions could persist for most of the Pacific Northwest, particularly away from the more northern coastal areas.

But, while the overall region of above normal wildland fire potential is predicted to contract slightly in September, the more southern coastal areas may actually become MORE at risk as soils and vegetation continue to dry out before Autumn rains arrive.

But again, as I mentioned in May, the expected weak La Niña conditions heading towards Autumn bodes well for promoting lower global temperatures. What this upcoming La Niña brings us in the heart of Autumn, however, could involve more rain than we usually get.