SEELEY LAKE — John Thompson has a pretty good way of measuring how dry a summer it was in western Montana.
“I made the comment in Ovando the other night that I think I’ve spent 42 or 43 nights in a tent this summer. I’ve had dew on my tent fly once,” Thompson said on Labor Day, his final full day as incident commander on the Rice Ridge fire.
At the end of the driest summer since 1929, fire camps across the region will wake up Thursday to rain or the promise of it.
“This is definitely going to be our last Red Flag Warning,” said Trent Smith of the National Weather Service in Missoula on Wednesday, as winds kicked up activity on some of the 22 fires and fire complexes still burning in the state.
“We've needed it. We've wanted it. It's finally here!” proclaimed the U.S. National Weather Service of Missoula on Facebook. “The end of fire season and summer will occur Thursday through Friday for western Montana and north central Idaho.”
Up to 3 inches of precipitation will fall in selected spots Thursday through Saturday morning. There’s a winter storm watch out for the mountains above 6,000 feet, and some of the higher peaks will see 4-10 inches of snow.
Temperatures that reached the low 80s on Wednesday will be 30-50 degrees colder on Friday, when “we might struggle to get out of the 40s,” Smith said.
It’ll be wetter in southwest Montana, including Missoula County and the Bitterroot Valley, than it will be to the north and west, but everyone can expect a least a sprinkling.
If this trough of low pressure doesn’t do it, another one with even more punch is expected at midweek next week.
The massive sigh of relief from firefighters, evacuees, sheriff departments, and just about everyone else comes with a question.
Rain, what took you so long?
From the first day of summer through all of July and August and most of the first two weeks of September, the National Weather Service at the Missoula airport registered just .23 of an inch of precipitation — and almost all of that on one day in August.
It tied the record low rainfall for the same period set in 1929.
The high atmospheric pressure that parked so long over Montana and the Dakotas was the result of a shut-down of the normal cycling, said state climatologist Kelsey Jencso.
Jencso, an associate professor of hydrology at the University of Montana, said the region is mired in ENSO-neutral, that “dead time” between the effects of when El Nino and La Nina are felt. El Nino is when the ocean surface temperatures off the Pacific coast get abnormally warm, and La Nina, when they’re cooler than usual. ENSO stands for El Nino Southern Oscillation.
The moribund high pressure we experienced all summer is due to “atmospheric blocking,” Jencso said.
In a normal summer, a cold front or low-pressure system from the west will come along periodically to push the hot, dry high-pressure system toward the Atlantic.
This year, “the normal cycling of high- and low-pressure systems came to a halt because two large weather patterns that occur over the Atlantic and over Asia have become abnormally stationary," said Jencso. "This has shifted the jet stream above Montana and into the Canadian Rockies.”
Eastern Montana suffered a double whammy. Smith said all the winter flow that gave western Montana more than 10 inches of precipitation in the first six months didn’t reach far east of the mountains. Many parts of the region were in extreme drought conditions before the ridge of high pressure stalled out over the whole state. Now many areas are in “exceptional drought,” the worst of five drought levels.
The bad news, Jencso said, is that the summer conditions of 2017 are similar to what are predicted to be the norm by 2040. Those climate projections are in the Montana Climate Assessment due to be released on Monday.
The good news is the systems wetting down the west this week and next should rumble right on over to the eastern plains, said Smith.
What’s not guaranteed is a wet fall or winter. Jencso pointed to the latest Climate Prediction Center discussion that says there’s a better chance than not that ENSO-neutral will remain over North America until February.
It becomes more difficult to predict the weather during ENSO-neutral conditions, especially in the summer.
“There are other large-scale weather patterns ... that can interact with the ENSO and lead to different conditions,” Jencso said, citing Arctic, Pacific Decadal and Atlantic oscillations.
“It can give you a headache. Meteorologists and climate forecasters look at all of these variables to determine the likelihood of precipitation or temperature trends using a bunch of different models.”