Now that I'm back from my lengthy trip, the first order of business seems to be to look at how breakup is playing out. Today's NWS breakup map (below) shows that breakup is mostly complete in the southern and eastern interior, and the Yukon River is currently in the process of breakup, with the ice having gone out at Tanana on Friday and at Fort Yukon on Saturday. This is all considerably earlier than normal, of course.
As many readers know, the Nenana tripod moved out on April 23, which is only 1 day earlier than last year and does not match the April 20 record. However, breakup was the earliest on record by a significant margin on the Kuskokwim River at Bethel and on the Yukon River at Dawson in Yukon, Canada. The charts below show the annual breakup dates for the common period of record at these locations. It's interesting to see that the variance is a lot larger at Bethel than at Dawson, and the long-term trend towards earlier breakup is somewhat less at Bethel. Of the 3 locations, the trend is strongest at Nenana.
A scatterplot comparison of the breakup dates reveals that Bethel's breakup is rarely earlier than Nenana's (as occurred this year), although it has been as much as 11 days earlier (in 1927).
Breakup timing at Nenana is more highly correlated with Dawson than with Bethel, as might be expected based on closer proximity and a more similar (interior) environment.
Lastly, here's the final result for the chart of thawing degree days that I showed before. Breakup at Nenana didn't occur until TDDs reached 216, which is twice as high as last year. It's tempting to speculate that the resilience of the ice this year may have been related to the amazing lack of snow during most of the winter: a lack of insulation may have allowed the ice to become stronger than would be expected based on the mild temperatures. However, the peak ice thickness of 40 inches at Nenana was only slightly greater than last year, so I'm not sure if this idea holds water...