Petersburg Pilot -

 
 

Warm summer forecasted to extend through fall and winter

 


This summer’s warmer than average temperatures could carry over into the winter if current climate trends continue.

Two main factors affect winter weather in Southeast Alaska—the Pacific Decadal Oscillation, or PDO, and the El Nino/La Nina Oscillations or ENSO. Each factor has two phases—positive and negative and both affect sea surface water temperatures.

The positive PDO phase happens when increased storm activity in the north Pacific blows warmer water into the Gulf of Alaska and raises the average sea surface temperatures. The winds also bring air laced with moisture contributing to rain and snow.

The negative phase occurs when cold dry air flows from Canada. Winds blow over coastal mountain peaks, through valleys and passes and flows out through inner channels like Frederick Sound and Sumner Strait. The cold dry winds blow surface water out to sea and colder water underneath replaces it.

Rick Fritsch, Climatologist for the National Weather Service in Juneau, said the positive and negative phases occur over long periods of time.

“This is something that does not flip flop back and forth in any time scales less than about a decade,” Fritsch said.

And a negative PDS has been cooling sea surface temps since 2006. Of course, anyone who goes outside knows weather is not one for being put in a box.

“It just so happened, just to spice up our lives, the Pacific decadal index jumped around August and became positive so right now we’re in a situation where the ocean is warmer than normal,” Fritsch said.

Fritsch anticipates a rainy October and a warmer than average winter with increased precipitation. But as winter progresses Fritsch, along with weather officials at the Climate Prediction Center in Washington, D.C. who monitor the PDO, anticipate the brief positive phase to flip back to negative. By February colder than average temperatures should jitter Southeast bones.

The second main factor, El Nino, which also deals with sea surface temperatures that occur along a sliver of water along the equator and extends west from South America isn’t up to much this year and, Fritsch said, shouldn’t impact Southeast.

There are a number of minor factors, called tele-connections, which all contribute to bursts of rain, snow, heat and cold.

“The bottom line is their variability are on much shorter time scales, as short as a week to as long as a month,” Fritsch said.

According to 42 years of weather service data, the average first freeze day in Petersburg occurs around October 4 or 5. The average date for the first measurable amount of snow is November 11.

 

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