Summer this year in Juneau has nearly tied the record set in 1970 for lowest average high temperature, forecasters said this week. And since May 1, 25.38 inches of precipitation have fallen at Juneau International Airport, setting a new record for summer rainfall so far.
But the El Niño climate pattern that forecasters say appears to be developing in the equatorial Pacific Ocean is expected to bring warmer weather to Juneau this winter. The effect it will have on precipitation is less clear.
“Generally, the Pacific Northwest and Alaska Panhandle tends to be a little bit dry in the falls of an El Niño,” said David Unger, a meteorologist at the National Weather Service’s Climate Prediction Center in Maryland. “Precipitation tends to be below normal along the Pacific coast, from the southeast of Alaska down to Oregon or so, in the Pacific Northwest.”
But Carl Dierking, science operations officer in the Juneau Forecast Office, said that observation does not necessarily hold true for Juneau.
“When we’re getting a lot of fronts or Pacific storms in the Gulf of Alaska, it tends to be wetter and warmer,” Dierking said. “In an El Niño winter, we tend to have more storms in the Gulf of Alaska.”
Perhaps explaining the apparent disparity, Dierking said El Niño’s effects on temperature are typically much more reliable than its effects on precipitation.
“The predictability is pretty small,” said Dierking. “It’s difficult to find a good relationship with precipitation. So it’s mainly temperature-based.”
Most of Alaska typically experiences warmer than normal temperatures during an El Niño event, according to Unger.
But a warmer winter, whether wetter or drier than normal, could mean any number of things for Juneau’s snowpack.
“Snowfall generally tends to be, for a lot of the country … related to individual storms, which we really can’t predict beyond a few weeks,” Unger said.
With warmer temperatures, Dierking added, “You can assume, I guess, that it’ll be less snow. And often there is. But certainly, we’ve had some El Niño winters that are snowy.”
One of those winters came in 2006, during the second-most recent El Niño event — on the heels of another gloomy summer.
“If you want a comparison to the last time we had a really bad … summer, ’06-’07 was a record-breaking snow year,” Juneau forecaster Aaron Jacobs observed. “Not to say we’re going to have another record-breaking snow year, but last time, we did have a kind of dismal winter that followed (summer).”
Part of the reason why snowfall is hard to predict this far out is that snow only forms and falls to ground under fairly specific weather conditions.
“The snow occurs right in that kind of ‘sweet spot,’” said Dierking. “You’re warm enough that you’re getting a lot of precipitation, but you’re cold enough that all of that precipitation is snow.”
Juneau’s average wintertime temperature hovers around or just below the freezing point, Jacobs noted. At those temperatures, precipitation often falls as snow, but above them, that snow can turn to rain.
“Even though we are saying above normal temperatures and precipitation, it all depends on how much above normal it is,” Jacobs concluded.
It is not a certainty that El Niño will be felt at all. Unger said the NWS currently predicts a 70 percent chance of El Niño impacting the Northern Hemisphere this fall and winter, and a 75 percent chance of it impacting Southeast Alaska particularly.
“The (climate) models that I speak of do not call for a very strong El Niño,” said Unger. “Very few are predicting a strong El Niño, so the effects on the weather are not quite as reliable as they’d be if El Niño was predicted to be stronger.”
So far, though, everything is proceeding as expected, Unger said.
“The models that were predicting the El Niños have turned out to be quite accurate so far,” said Unger. “El Niño conditions can be expected to emerge in the coming few months … if the observations continue to support the model predictions.”
The most recent El Niño developed in the early summer of 2009 and persisted into early 2010. Precipitation in Juneau that winter, especially snowfall, was below normal.
• Contact reporter Mark D. Miller at 523-2279 or at email@example.com.