FIVE-WEEK SUMMARY

As many of you have probably guessed by now, we’re quickly approaching a more summer-like circulation pattern, with a few nuances and caveats. Upper 70s to mid-80s are possible roughly next Wed. May 28 into early June.

But, in part due to residual surface coupling effects from the very strong polar vortex disruption in early March, we could see at least a three-week period of somewhat frequent flip-flopping and instability.

The strongest signal for another warm spell of upper 70s to 80s is roughly June 7–12. And there’s some possibility that the last 10 days of June will not be super hot.

FIVE-WEEK FORECAST

Temperatures will increase this weekend as inland high pressure (+PNA) moves towards the coast and builds up.

(A +PNA refers to the Pacific-North America pattern in which high pressure tends to linger over the western states.)

Sunday, May 25 should reach the low 70s. Early Memorial Day morning there’s a 20–35% chance for a few sprinkles between 4–10 am.

Blustery winds are likely Tuesday, May 27 due to an increase in east Asian mountain torque back around May 18–19.

6–10 Day Temperature Outlook

Warmer weather will then return through the end of May into the first week of June. Highest temperatures will most likely happen around Wed. May 28–Sat. May 31.

More east Asian mountain torque (+MT) predicted for May 27–28 could help bring a low pressure system around June 4–6.

So, looking at the larger regional scenario (see Havmöller diagram), it’s likely that mild clashes will periodically continue in the next few weeks between troughing (blue lines) and high pressure (red lines), in part due to the periodic regression of a north Atlantic ridge from May 20 into early June, particularly as ridging increases over the western U.S.

There’s another possible warming trend around June 7–12. There are also signs for a strong blocking regime of high pressure building over Scandinavia, then maybe expanding towards Greenland around June 10, and this could perhaps help to maintain mostly warm weather in western Oregon into the third week of June.

And, although it’s a long ways off, there’s a slight chance the last 10 days of June will not be super hot, depending on the strength of any high pressure that develops in the vicinity of Greenland after June 10.

LATE SPRING INTO LATE SUMMER SEASONAL TRENDS

• Wildland Fire Potential – June

Fire potential is increasing in parts of the West. There are numerous wildfires burning in the red areas, even up in central Canada.

Seasonal Temperature Outlook

The NWS Climate Prediction Center (CPC), among others, predicts well above average temperatures through most of
Summer (June–August) across most of the western states.

CPC Seasonal Precipitation Outlook

The CPC also predicts very dry conditions (40% below normal, or less) for most of the Northwest, northern Rockies, and northern Plains.

Other seasonal prediction models are pointing towards continued hot and dry conditions in the Northwest through late Summer into early Autumn.

May 2025 ENSO Probabilities

But there’s GOOD NEWS for late Summer, Autumn, and possibly early Winter–

As of May 15, the El Niño Southern Oscillation Index (ENSO) has reached average neutral conditions. Neutral ENSO conditions are now favored through Summer (74% chance for June–August), with chances exceeding 50% through August–October 2025. And there’s even a mild possibility for weak La Niña conditions to re-emerge before the end of Autumn. (La Niñas tend to help keep global temperatures lower than El Niños do.)

We probably won’t know for sure until Summer comes along, but I’ll be keeping an eye on this.

(At the end of my April 20 forecast, I suggested a shift towards El Niño could occur in early 2026, but an opposite shift towards La Niña seems more plausible.)

– Praise