The Big Thompson Storm Part I

© 2016 by Fernando Caracena

Introduction-The Deceptive weather situation

Forty years ago, was the year of the Bicentennial Celebration of the United States. In Colorado, itself the Centennial State, the summer camping season was in full swing on the hot, still day of July 31, 1976. Then, a sudden storm brought torrential rain to the rocky canyons of the Big Thompson River, creating a devastating, flash flood. Many people were in the mountains escaping the July heat. Over a foot of rain fell in some parts of the Big Thompson river drainage mainly in an hour's time. Accounts of eyewitness and of devastation of this epic storm are described a Denver Post article:

“Approximately 4,000 people were in the canyon... most[ly] from outside the area [and warned largely] by word of mouth. Telephone lines were ripped and mangled – power poles and bridges destroyed.

The monstrous flood took 143 lives and injured 150 people ... The flood caused $35 million in damage, destroying 418 homes and 52 businesses, 438 vehicles, bridges, roads, the highway and power and telephone lines.”

Weather Conditions Preceding the Storm

The morning of July 31 July, a weak cool front brought cool moist air into the Front Range area, which would offer hope of showers that afternoon to relieve the dry heat. The afternoon, however, was a bit disappointing in this regard. As the day progressed, the possibility of rain seemed to diminish to the north of Denver. By mid afternoon, the cool moist air had burned off. The dew point temperature had fallen into the low 50s oF[10-12 oC].

Suddenly, like a bolt out of the blue a big, slow-moving Thunderstorm appeared over the Big Thompson River drainage and rained with a vengeance.

The Afternoon Storms had Formed in the Typical Places

A few high based thunderstorms had formed in the drier air south of the Denver area and over the Palmer Divide, ahead of a second cold front. To the northeast of Denver, however, a cooler wet airmass behind that frontal surge advanced from the Nebraska border up the South Platte River Valley. The Big Thompson River is a tributary of the South Platte, as is the northern neighboring Cachet de la Poudre River. The dew point temperatures behind this second front were above 65 oF (18.3 oC). This is the air that fed the Big Thompson Storm, but here is was strongly capped below the frontal inversion.

Between four o'clock and and six PM, [1600-1800 MDT] a line of pre-frontal thunderstorms formed along an arc extending from the Denver area across central western Kansas and south-east-ward across Kansas. As showers advanced across the Denver area, the stage was set for the epic storm.

Moving into a New Home

The day of the storm is a very memorable one for me. I had accepted a new position at The Atmospheric and Chemistry Laboratory (NOAA) in Boulder, coming on board in April. Months later in July 31, 1976, we were moving into our new home that we had had built in an area north-east of Boulder called Gunbarrel.

I had promised my family that we would go our for supper after we had finished moving into the house. After a day of hauling and unpacking things, we were ready to pause, go out to eat, and come back for a restful night; but first, daddy would look over the garden.

I had been looking up at the sky the whole day of our big move. A member of Dr. Charlie Chappell's Mesoscale Research Group of the Atmospheric and Chemistry Laboratory (APCL), I had been taking part in discussions about the unusual weather events that were unfolding. Charlie had brought his long experience as a weather forecaster to his graduate weather studies. He was no ordinary academic. He had a keen feeling for how the atmosphere operated and how the business of weather forecasting should be conducted.

Charlie had had a map room set up for our group that became a magnet for people from other of NOAA's Boulder Research laboratories to come by, look at the charts, and discuss the weather. Students and interns kept current the paper charts hung on the wall. The source of the charts was a large weather fax machine.

So we knew that something big was about to happen in our area weather; but we did not know what it would be. And that evening, from my new backyard, I saw it, and was quite impressed.

I do not know how long I stood gaping at the storm. There was the big event that we anticipated. It looked like a big squall line to the northwest, but unlike a squall line, it did not move. Lightning was flashing from it continuously. I would have stared at that storm for hours, oblivious to hunger and the rest of the world; but, I was jarred back to my promise to the family by my little kids jerking my pants leg. “Daddy!! You promised!”

At the restaurant, we had a window seat that faced north. It had a view of the storm, which sat there flashing continuously during the entire time we were having our supper. Later in the evening and back home, I continued to watch the storm, until my wife dragged me off to bed. From our bedroom window, I could still see the big storm cloud flashing frequently, which by now had drifted a bit north of where it started. After that weekend, I returned eagerly to the office to discuss the big storm.

P.S., Between now and July 31, I intend to write a series of small posts that feature the Big Thompson Storm in Colorado. The idea is to get the information out about the kind of conditions that campers and others who live in the high country should be aware of, especially this time of year.

This discussion continues with The Big Thompson Storm:Weather Patterns I.

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