Defying National Weather Service forecasts for minor flooding Friday morning, Mendenhall Lake and Mendenhall River swelled to water level peaks well below their flood stages shortly after midnight as subglacial drainage brought high water to parts of the Mendenhall Valley.
Forecasters issued a Flood Advisory Thursday and predicted waters would rise high enough to cover parts of View Drive and Mendenhall Lake Spur Road and flood a few houses’ yards and garages by 4 a.m. Friday. The peak was much lower, and much earlier than forecast.
Forecaster Rick Fritsch said the deviation from forecast indicates there was less water in the Mendenhall Glacier’s Suicide Basin than there was last July, when a similar phenomenon caused minor to moderate flooding in the area.
“This is consistent with the idea that Suicide Basin had not filled up this year as much as last year,” Fritsch said.
The reported water levels tracked the forecast quite closely until about midnight, when the rise neared its zenith — 8.36 feet at Mendenhall Lake and 9.98 feet at Mendenhall River. Shortly thereafter, water levels began to fall rapidly toward normal.
On Thursday, the NWS forecast a peak of 9.54 feet at Mendenhall Lake and 12.08 feet at Mendenhall River. The forecast suggested water levels would hit those peaks by 10 a.m. Friday.
“It’s never nice when your forecast busts, as we say, but it’s always nice to know that people’s property wasn’t put in jeopardy,” Fritsch said.
Although waters never reached their flood stage, meaning the Flood Watch that the NWS replaced with a Flood Advisory at noon Thursday would have sufficed, Fritsch said the decision to issue a Flood Advisory was made based on the data that forecasters had at the time.
“When we looked at what was happening relative to the forecast, and there was such a good match between the two, we, at the time, yesterday considered that we had a high confidence event that we were actually going to see some flooding,” said Fritsch. “So in retrospect, I have to say that, you know, if I was sitting in a position where I had to make that choice with the data that we had to work with yesterday, I still probably would have gone out with the Flood Advisory when we did.”
Tom Mattice, emergency programs manager for the City and Borough of Juneau, said it was his job to prepare for the worst-case scenario, even though experts had reported indications the amount of water in Suicide Basin could have been less than it was last July. He defended the work the NWS did in forecasting the event.
“You’re dealing with an event that’s only occurred once ever,” Mattice said. “I don’t think they were sensationalizing or being alarmist. They were using the information from a one-time event to create a model.”
While water levels on the lake remained elevated when the Mendenhall Glacier Visitor’s Center opened Friday morning, interpretive specialist Nikki Hinds said they had receded enough that the U.S. Forest Service had been able to reopen all hiking trails.
“It was much lower by even 7 (o’clock) this morning,” Hinds said of the lake.
• Contact reporter Mark D. Miller at 523-2279 or at firstname.lastname@example.org.