The National Weather Service issued its annual river breakup forecast this week. The forecast calls for a relatively mild breakup arriving about on schedule across Alaska. But what factors determine the timing and severity how it plays out? We put that question to climatologist Brian Brettschneider.
Brettschneider said snow pack in Feb. and March generally has almost no bearing on river breakup. Instead, it’s closely correlated to temperatures in April and May.
“So we could have a warm winter up through then or a cold winter, all the way through March, but it really doesn’t have much of an influence on what breakup is going to be like and when it’s going to be,” Brettschneider said. “You really need to focus on April and May temperatures and to a lesser degree what the snow conditions are like going into the month of April.”
So, you could have the coldest winter on record, but a warmer-than-normal April will mean breakup happens fairly quickly. But you could also have a warm winter and a cold April, which would lead to a later breakup.
Breakup is all about temperature.
“When we have sunny conditions and warm temperatures it thaws the ice and it also tends to melt whatever snow pack we have,” Brettschneider said. “So if we do have a thick snow pack, those warm temperatures can melt that snow quickly and send a rush of water into the streams…The temperature also drives the the snow melt, which can push the ice out of the way.
Brettschneider hasn’t used his understanding of spring breakup to play the Nanana Ice Classic, but he does have some forecasting tips:
“Historically, the best predictor of when river breakup is going to be, specifically with the Tanana river in Nenana, is to look at the calendar and look at last year to see when breakup is going to be. And that does better than any other kind of forecast technique,” Brettschneider said. “Having said that, last year was a record early breakup; this year will not be a record early breakup, so picking last year’s date would be a losing strategy this year.”