Despite a cool finish to the month, overall August was a very hot and dry month across the Northwest. With only a few exceptions, temperatures were above average and precipitation was below average as a ridge of high pressure dominated the Western U.S.
Summer was back-loaded this year. July was relatively cool compared to average, but the heat arrived in August.
Drought conditions remain prevalent across much of Oregon, as well as across portions of Central Washington, Eastern/Central Wyoming, and the Wood River Valley of Idaho.
Warm and Dry Pattern Returns from September 2nd-6th
After an early cool snap brought freezing temperatures to high elevation valleys in the Northern Rockies and light snow to the higher elevations, a shift back to above-average warmth will occur over the next 5 days as high pressure strengthens over the Western U.S. again.
The most impressive warmth will be along the Pacific Coast, particularly in Oregon which will be near the center of high pressure. Winds will shift to offshore across Western Oregon, and as a result, temperatures will soar into the 90s in Portland, Eugene, and Bend over Labor Day weekend.
Here are the projected temperature anomalies for September 2-6 from the European Model.
Strong Cold Front and Possible Early Season Snow Near the Continental Divide – September 7th-8th
A highly amplified pattern will continue to develop after Labor Day as the ridge of high pressure further strengthens and shifts west off of the Pacific Coast.
Around this time, a deep trough will develop over the High Plains and a strong Canadian cold front is projected to dive southward along and east of the Continental Divide through Montana and Wyoming.
Colder air will also spill over west of the Divide into Idaho and even into Eastern Washington and Oregon as well.
Keep in mind, this is still a week away, but here are the projected temperature anomalies from the European Model for the afternoon of September 8th (next Tuesday).
Based on current projections, this would be an impressively cold airmass for this early in September, and the potential will exist for widespread frosts and freezes east of the Continental Divide in Montana and Wyoming, as well as across the mountain valleys west of the Divide.
Believe it or not, there is at least an outside chance for early season upslope snow behind this cold front outside of just the higher mountain peaks.
Areas along the eastern slopes of the Continental Divide that are favored in upslope regimes will have the best chance of picking up some snow during this time.
However, snow also couldn’t be ruled out over the adjacent plains in higher elevation cities above 5k such as Casper or Cheyenne either.
It is still much too early to have any confidence in this scenario, but ensemble models are hinting at the possibility given the extent of the cold air expected to arrive along with easterly upslope winds behind the front.
There is no need to panic just yet if you live in these areas outside of the mountains — this is still a low probability/high impact scenario at the moment until we see more consistency in the model trends moving forward.
However, it would be wise to keep the possibility in the back of your mind and stay tuned moving forward as accumulating snow (IF it were to happen) this early in the year could be damaging to plants and crops.
For hikers, climbers and hunters, be prepared for the possibility of a significant early season snowfall in the mountain ranges along the Continental Divide, such as the Beartooths, Absarokas, and Wind Rivers — this is a higher probability occurrence.
On the plus side, beneficial moisture will be possible along and east of the Divide all the way to the Dakota borders. We could all use some good rainfall right now.
West of the Cascades, stubborn high pressure will persist and the late summer warm and dry pattern will continue.
September 9th-16th Outlook
From September 9th-12th, a northwest flow pattern should generally result in warmer and drier conditions west of the Continental Divide and cooler than average temperatures and somewhat unsettled conditions along and east of the Continental Divide as additional cold fronts slip in from the north.
From the 13th-16th, quiet conditions with warmer and drier conditions across all areas is the most plausible scenario in my opinion, though the GFS Model shows a trough could sneak into the picture from the Pacific Northwest and bring more unsettled weather.
There is a degree of uncertainty during this time, but the GFS is an outlier at this point so I’m leaning toward the warmer/drier solution until new evidence suggests otherwise.
Week 3-4 Outlook and Overall September Outlook
By mid-September and beyond, long range models are projecting a gradual weakening of the strong blocking ridge of high pressure across the west. This eventually may open the door to the occasional trough from the Pacific, which could bring shots of cooler and wetter weather.
However, I don’t see any significant pattern changes on the horizon in the longer term, and I think that overall, areas from the Washington and Oregon Coasts east toward the Continental Divide are in for a warmer and drier than average September.
The warmest signal will be along and west of the Cascades, while temperature anomalies shouldn’t be quite as high across the Interior Rockies due to occasional cold fronts.
East of the Continental Divide in Montana and Wyoming (and Alberta if looking north of the border), trends are pointing toward warmer temperatures compared to earlier in the month. Depending on the extent of the cold air next week, I suspect we’ll see temperatures closer to average if not below average for the month of September as a whole.
Looking Ahead to October
Although it’s not a sure thing yet, we’re continuing to trend toward a La Nina pattern for the winter season as cooler than average sea surface temperature anomalies continue to emerge across the Eastern Equatorial Pacific.
At the same time, a warm “blob” of above-average temperatures is currently in place across the Gulf of Alaska, which is traditionally not consistent with La Nina patterns, though has occurred more often in recent years.
Finally, the Quasi-Biennial Oscillation is near neutral and trending toward positive, all of which give us a starting point of analogs to work with as we head into the cool season.
As for October, we should eventually start to transition into a more active pattern, though it’s too early to say whether or not this occurs earlier in the month or holds off until later in the month.
My early outlook for October calls for:
1) Above average precipitation and near average temperatures along and west of the Cascades in Washington and Oregon.
2) Near average precipitation and above average temperatures across the Inland Northwest and Interior Rockies west of the Continental Divide, including Eastern Washington/Oregon, Idaho, and Western Montana/Wyoming.
3) Below average precipitation and above average temperatures east of the Continental Divide in Montana and Wyoming.