After a brief period of warm, springy weather, many Yukon-Kuskokwim Delta communities plunged back into negative temperatures the second week of April.
“Even though it might not feel like spring is coming, it's coming,” said Johnse Ostman, a hydrologist with the Alaska-Pacific River Forecast Center and the National Weather Service in Anchorage.
And with spring, and warming weather, comes river breakup.
“Breakup is complicated and changes every year, absolutely,” Ostman said. “And [when predicting breakup] we take into several different, measurable data points.”
One factor is snowpack – how it compares to previous years, and the “snow water equivalent,” or how wet the snow is.
Ostman said he and his colleagues primarily get that data from National Resources Conservation Service (NRCS) snow telemetry (SNOTEL) sites and monthly reports.
Another factor is ice thickness.
“We're relying upon community observers, whether they be from a program called Fresh Eyes on Ice, which is through University of Alaska - Fairbanks or maybe Bethel Search and Rescue and the ice road folks, [or] observers we have on the ground in different villages to just tell us [...] what is the ice thickness is compared to what it was in past years.”
Ice conditions are also important to consider, Ostman said. Community partners on snowmachines can report on ice conditions throughout the winter, and that information together with satellite imagery in the visible and microwave spectrums helps put together a more complex picture of the river ice.
It’s still early in the forecasting season, but Ostman said air temperature also plays an important role as breakup approaches.
“We're looking at: what do the climate models tell us about what we should expect our temperatures to be? So it's more of like a trend analysis at this point,” Ostman said. “Are we cooler than historically average for this time of year? Are we trending warmer? And then maybe how is that distributed across the state? So that we can predict where we're going to start melting that snowpack and rotting that ice.”
When talking about averages, Ostman said that generally compares against the “climate-normal” period, or between 1991 and 2020.
A broad look at Y-K Delta breakup
Around a month out from many villages’ median breakup dates, it’s hard to say exactly how breakup may look this year on the Yukon-Kuskokwim Delta.
“In the most broad terms, we are not as cold as we were, say, last year,” Ostman said. “We have less snow in most places than we had last year. And we think that breakup will not be as late as it was last year. And probably overall, we think [...] the impacts from breakup will be less severe.”
Last year, severe ice-jam flooding impacted communities on the Yukon, Kuskokwim, and Tanana rivers, prompting a disaster declaration from Gov. Mike Dunleavy on May 13, 2023.
Conditions vary widely this year. On the Yukon River, the Porcupine Basin area has above-average snowpack. Tanana and Galena have average to below average. And moving west of Galena, snowpack generally increases.
“That western coast, the Lower Kuskokwim has a higher than average snowpack by maybe 20 to 30%,” Ostman said.
Although it’s an incredibly educated guess, fed by all these complicated data streams, Ostman said predicting breakup is also something of art.
“It’s subjective. We have to make sure that we use what we know and what we've learned from the past to help predict what's going on,” Ostman said. “But really, what we're looking at is we're looking at a forecast temperature model going out about seven days, and then super smart people who come up with climate models that project out even further. So they're looking at other models to say, ‘What do we think is going to happen over that next week?’”
As potential breakup dates get closer, Ostman said the air temperature data becomes more and more important.
“If you can accumulate the number of days greater than freezing, we can take that in and with these temperature forecasts, continue to sort of close the window on a particular date, and then use historical breakup dates to better inform us how wide that window is to start with,” Ostman said. “And so we just keep shifting that around.”
Taking all the factors into account, Ostman said that communities west of Galena can expect breakup two to five days later than their median. East of Galena, it will likely be more like one or two days later than the median.
Thermal or dynamic?
There are two main types of breakup: thermal and dynamic. Thermal breakup is where temperatures gradually warm and the ice sort of mushes out and floats away. Dynamic breakup is the kind that can cause ice jams and flooding, where temperatures stay cold and then suddenly warm dramatically, unevenly melting river ice.
“Not only are we forecasting sort of a window of time for breakup date-wise, on that temporal scale, but we're also looking at what the basin flood potential is for most villages on the Yukon, most villages on the Kuskokwim, and other places across the state — but those two rivers in particular,” Ostman said. “Historically, we've assigned a range of flood severity, or flood potential at these sites.”
The Alaska-Pacific River Forecast Center's breakup outlook from April 12 indicates that many areas of the state are leaning more toward the dynamic type of breakup – the kind that can cause ice jams and flooding.
Midwinter surveys found some areas of jumbled ice on both the Yukon and Kuskokwim rivers which can increase flood potential.
“We don't have a way to quantitatively, say: ‘X, Y, and Z gives you a flood potential of 1, 2, 3 or 4,'” Ostman said. “There's nothing built that way. So it's using a lot of historical knowledge, past floods, and what we know about climate, snowpack, ice thickness, how high the river was when it froze up.”
“And,” he added: “observations from people on the ground. The community partners that we have, and community members that we reach out to each year, are super valuable for what we are doing.”
Ostman said he and his colleagues wouldn’t be able to make breakup predictions like they do without information from community partners.
“I got an email from someone in Bethel this morning who told us about a couple of areas that we really need to pay attention to as we look at break up,” Ostman said on April 9. “And these are places that it was like, ‘Okay, we'll look at those in the satellite imagery, and some of the remote sensing tools. We’ll take what you've told us and apply that to this qualitative and subjective breakup flood potential estimate that we're going to put out.’ Because we are putting it out for each of the communities on the Kusko, and he's identified a couple of those where we should be maybe paying attention.”
There are a lot of factors that go into modeling breakup and a lot of uncertainty. Alaska is huge.
“If I tell you it's going to be sunny two weeks from now, you're probably not going to believe me,” Ostman said. “But if I tell you it's going to be sunny two days from now, you might decide to plan a picnic, right? And then would be less likely to call me a liar if I was wrong. So that's kind of where we are – we're building up to that. And so each of these products will build on the other one. And we'll have more confidence because the model has more skill at that point.”
As breakup inches closer, so will the tools to help communities prepare for whenever and however the river ice crumbles.
The Alaska-Pacific River Forecast Center’s first Kuskokwim community breakup call is scheduled for 10 a.m. on Monday, April 22. To participate, call +1 (866) 203-1705, and use participant code 1145901.
