Johnnie St. Vrain: Longs Peak might have greater effect on the mind than on Longmont's weather

Johnnie St. Vrain, Daily Times-Call, Longmont, Colo.
·3 min read

Jul. 16—Dear Readers: This column appeared originally on July 20, 2012. I have updated the precipitation averages.

Dear Johnnie: I've lived in Longmont for 35 years and have wondered about our weather pattern here. This time of year it's not unusual to watch Denver TV and hear the lamentations of the weather crew, complaining about yet another heavy rain day that has snarled traffic, flooded underpasses and generally made a nuisance of itself. At the same time, we can look out our window and see ... sun!

The old-timers I've talked to mention a "Longs Peak effect," which involves that wonderful mountain parting the clouds west of here so they go either north or south of us.

I realize anecdotal experience isn't the same thing as scientific proof. Is there anything to this or am I just feeling paranoid? — Gentleman Rancher

Dear Gentleman Rancher: Back in 2007, the Times-Call published an article about winter weather in Longmont, in which state climatologist Nolan Doesken said that it's possible that Longs Peak might "split" weather systems that come from the west, but that other factors were more likely to play a role in Longmont's weather, especially during colder times of the year.

One of those factors is Rabbit Mountain, which Doesken said catches moisture from storms coming from the northwest.

But the more important player in Longmont's precipitation totals is elevation, he said. The reason, he explained, has to do with the ability of air to hold moisture.

"Anything that causes air to rise will increase precipitation. Anything that causes air to fall decreases precipitation," Doesken told the Times-Call at the time. "Air moving toward Longmont, with exception from the east, is moving down."

That works for summer thunderstorms, too, Times-Call weather consultant David Larison said.

"Thunderstorms often develop in convergence zones, a complex interaction of moisture, low level winds and terrain. Longmont is in a valley at roughly 5,000 feet above sea level, while the elevation rises by several hundred feet as you go south from town. (Lafayette's elevation is 5236 feet.) The landscape is also slightly higher immediately north of Longmont. All things considered, heavy thunderstorm formation may well favor the higher terrain south of Longmont into the Denver area," Larison responded when I asked him about the "Longs Peak effect" this week.

"I tend to discount the Longs Peak theory relating to Longmont thunderstorms. For one reason, mountaintop winds in the summertime are much lighter than the winter season and a little more variable in direction. From my experience, it would be hard to determine any prevailing steering pattern for Longmont due to Longs Peak alone."

I did a little research of my own, comparing precipitation totals for Longmont, Denver and Colorado Springs, which I chose because it has a massive mountain (Pikes Peak) just to its west. Here's what I found for July and August precipitation averages, from usclimatedata.com:

Longmont (4,979 feet) — July, 1.1 inches; August, 1.38 inches;

Denver (5,280 feet) — July, 2.17 inches; August, 1.69 inches;

Colorado Springs (6,035 feet) — July, 2.84 inches; August, 3.34 inches.

So, Gentleman Rancher, you may draw your own conclusions. But it's nice to think of Longs Peak playing a special role in Longmont's climate.