Tag: prescribed fire

We looked further into what led to the smoky conditions that resulted in numerous vehicle crashes on Interstate 40 west of Flagstaff, Arizona early Wednesday morning, October 19. The smoke on the highway from a prescribed fire was referred to as “pea soup”, and was clearly the cause of some of the accidents, but investigators are not yet ready to say the smoke caused the one fatality when a vehicle was sandwiched between two semi trucks.

During the very early morning hours of Wednesday, October 19, smoke settled into the areas around Interstate 40 between Parks and Williams. An electronic sign warned motorists about smoke, but the severely reduced visibility was not anticipated by the U.S. Forest Service. After the accidents started occurring the Interstate was closed for five hours.

The Kaibab National Forest ignited the Green Base Prescribed Fire on Tuesday, October 18, 2016 immediately north of Interstate 40. The smoke permit issued by the Arizona Department of Environmental Quality anticipated moderate “impacts on sensitive areas”:

Smoke impacts to the following communities of Flagstaff, Parks, Spring Valley, Pittman Valley, Sherwood Forest Estates and some smoke may impact Williams. I-40 may experience moderate smoke impacts in low-lying areas.

The Flagstaff office of the National Weather Service issued a Spot Weather Forecast for the Green Base Prescribed Fire at 5:28 a.m. MST on Tuesday October 18, 2016. It anticipated a “good” maximum ventilation rate for Tuesday, did not specify one for Tuesday night, and for Wednesday it was described as “fair”.

The ventilation rate provides an estimate of how high and how far smoke will disperse. A high ventilation rate suggests that smoke will spread out quickly and through a deep layer of the atmosphere, so that surface concentrations downwind will be lower than they would be with a lower ventilation rate.

The transport winds (from the ground to the mixing height) for Tuesday night were predicted to be “west 5 to 10 knots shifting to the north after midnight, then shifting to the northeast early in the morning.” The wind speed after the shift was not specified. Perhaps this was interpreted by fire managers to mean it would continue at 5 to 10 knots.

The prescribed fire was just north of the east-west Interstate, so a wind out of the northeast would likely push the smoke toward the highway. And if an area is prone to nighttime inversions, visibility can be compromised.

Fox News reported that Cory Mottice, a National Weather Service meteorologist in Flagstaff, said, “[Smoke] always gets trapped after dark,” he said. “It’s just a question of where the wind blows it.”

However that analysis was not described in detail in the spot weather forecast issued by Mr. Mottice’s office. As in many spot weather forecasts, much of the information appears to be generated by a computer, with little interpretation or discussion about how the information will affect the fire. Meteorologists are not expected to be Fire Behavior Analysts, but sometimes a little human-created discussion and interpretation can add value to a computer product.

Below are excerpts from an article at the Arizona Daily Sun:

… A Highway Patrol captain at the scene said smoke in the area reduced visibility down to about 20 feet, Department of Public Safety spokesman Bart Graves said. Sherwood Forest Estates Fire Chief Wayne Marx said even his crews had to stick their heads out the window to watch the yellow stripe on the road as they escorted commuters away from the interstate.

“You couldn’t see past the end of the hood,” Marx said.

[…]

One retired meteorologist who lives in the area believes more precautions should have been taken. Parks resident Byron Peterson, who retired from National Weather Service station in Bellemont, said the smoke was already bad on Old Route 66 Tuesday afternoon. Firefighters waved him on, he said, even though there were times when he could not see 10 feet in front of him.

“It was very frightening to say the least,” Peterson said.

He said strong southwest winds coming up over Bill Williams Mountain near Williams formed an eddy of swirling air that then dove down over the prescribed burn, keeping the smoke from dissipating.

“I tried to explain that to people at the Forest Service and it was just like talking to a wall,” he said.