Residents of the Mendenhall Valley have been looking up at fresh snow accumulating on the Mendenhall Towers for about a week now. And on Monday, some of them woke up to find frost on the ground in their own back yards.
Winter is coming — and according to National Weather Service meteorologist Rick Fritsch, it’s right about on schedule so far.
Since record-keeping began at Juneau International Airport in 1943, Fritsch said, the average day for the first frost in Juneau has been Sept. 29.
But an average of the entire record-keeping period is not the most precise tool for measuring expected conditions, according to Fritsch.
“Average looks at the entire data set,” said Fritsch. “The quote-unquote ‘normal’ is a climatology term that we use to gauge how much things are different from what should be expected at this time of the year, and a lot of that is predicated on just what is normal in this time of changing climate.”
The climate normal period from 1981 to 2010, according to Fritsch’s data, yields an average first-frost date of Oct. 3.
Those measurements were taken at the airport, where temperatures actually stayed above freezing Monday even as they dipped to 32 degrees Fahrenheit or below in other parts of the Valley, including at Riverbend Elementary School and the Juneau Forecast Office on the Back Loop, where Fritsch and his colleagues work.
“The only reports of frost that I have are back here in the Valley,” Fritsch said.
Freezing temperatures were also recorded at the top of the Mount Roberts Tramway and at Eaglecrest Ski Area early Monday morning, Fritsch added.
As of 4:20 p.m. Monday, the NWS was forecasting similar conditions for Monday night and Tuesday morning, with temperatures possibly slightly lower than they were the previous night. That could mean the airport sees its first freeze of the season.
“If a (Juneau Empire) reader said, ‘Well, I had frost on Sunday night,’ that same person should expect to see frost again tonight,” Fritsch said.
The NWS issues advisories and warnings only for freezes that occur outside the expected season, Fritsch noted.
“Our official growing season has ended, and because the growing season has ended, we don’t issue a frost advisory or a freeze warning,” said Fritsch.
Fritsch said the reason for the cold temperatures has to do with a mostly clear sky.
In most people’s minds, a clear sky is usually associated with warming from sunlight during the day, but at night, Fritsch said, it can cause heat to dissipate quite quickly — a phenomenon called radiational cooling.
“When there’s clouds, like is typically the case around here, the clouds act as an insulating shield,” said Fritsch. “Long-wave energy can’t get back out, so it reflects back down to the ground. One of the reasons our winters are relatively moderate compared to, like, the Yukon or British Columbia, (is) because we have the cloud cover (which) traps the heat in at night.”
In situations like this, and like last night, he said, there was very little cloud cover to trap in that heat so more of that long-wave radiation escaped back to space. That “just allows us to get colder during a clear night than we would during a cloudy night.”
Due to Juneau’s moist climate, when temperatures hit freezing at ground level, it typically causes frost, according to Fritsch.
With temperatures now dropping to freezing overnight in some parts of Juneau, Fritsch advised people to get prepared.
“If you do still have any flowers outside, and you don’t want them to die, put them in the garage, or put them inside the front door,” Fritsch said.
Tom Mattice, emergency programs manager for the City and Borough of Juneau, advised residents to “just start thinking about winter, obviously.”
“If you have outbuildings or things that need to be winterized, now’s the time,” Mattice added.
• Contact reporter Mark D. Miller at 523-2279 or at email@example.com.