Anyone in Missoula could tell you that Sunday was a clear night, but that wasn't good enough for a couple of airlines flying here from Seattle.

An after-hours equipment malfunction at Missoula International Airport delayed the last two flights, a rare occurrence that had local officials working on a backup plan Monday so it won’t happen again.

Though the weather was fine and visibility at Missoula International Airport was 10 miles or more, Delta and Alaska airlines officials in Seattle weren’t getting that message from the Automated Surface Observation System, or ASOS.

“We’re a little bit mystified,” airport director Cris Jensen said. “In my 10 years here I’ve never seen this before.”

Missoula’s ASOS is owned and maintained by the National Weather Service. It’s what airlines rely on after the Missoula control tower closes at 10 p.m. to determine if it’s safe to land here.

Jensen said a check Monday showed hourly readings missing at 9:53 p.m. and 10:53 p.m. By 11:53 p.m. the system was working again.

Travelers on Alaska and Delta expected to land in Missoula at 11:50 p.m. and 12:17 a.m., respectively. The Alaska flight was delayed 30 minutes and Delta landed 61 minutes late.

“For some unknown reason the visibility sensor on the ASOS was sporadically inoperative last night, though all other sensors continued to function normally,” Jensen said.

Weather Service electronic technicians worked on the ASOS equipment remotely Sunday night and then physically Monday morning, meteorologist David Noble said. 

Jensen said it appeared to be functioning normally again. But he and deputy director Brian Ellestad planned to meet with tower chief Keith Eberhard to discuss a plan to keep the tower open until all flights have landed in Missoula for the night. A staffed tower is the primary way airlines get their weather information between 6 a.m. and 10 p.m.

“Literally, it has been so stable in the past it never dawned on us this was going to be an issue,” Jensen said. “The Weather Service response to anything like this is very quick. They have trained technicians here and even when we do have a problem, they’re able to jump on it so quickly. We just never thought to have a backup plan.”

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