Tuesday, May 3, 2022

Nenana Tripod Out

Breakup occurred yesterday evening on the Tanana River at Nenana.  This is a couple of days later than the normal of recent decades - no doubt because of the chilly April (5°F below average in Fairbanks) - but it's several days earlier than was typical before about 1990.  Here's Rick Thoman's graphic:

Here's the view at Nenana this morning.

There's little doubt that breakup was brought forward several days by the large volume of water from melting of the deep snowpack upstream from Nenana.  For example, the Little Chena Ridge SNOTEL site (2000' elevation) has lost about half of its deep snowpack in the last 10 days.

Based on April 15 data from 5 SNOTEL sites in the Chena River basin, this year's snowpack water content is the greatest in the past 4 decades.  Looking at Nenana breakup dates since 1982, there is a modest inverse relationship between snowpack water and the amount of warmth needed to cause breakup at Nenana:

Breakup in 2018 and 2020 seemed to be good examples of breakup occurring after relatively little warmth (low thawing degree days) when the snowpack was very deep.  1991 and 1993 don't fit the pattern, but this is almost certainly because April was much warmer in those years - particularly in 1993, which had one of the warmest April's on record, leading to a very early breakup (April 23).  More heat input is required to shift the ice at an early date, because the sun has less time to work on the ice; whereas late breakup tends to occur with lower TDDs because of the long window for increasingly intense solar heat input.  It's a fun multivariate prediction problem.

As for 2002, breakup occurred with both low snowpack and minimal heat input, and that's because it was the wettest April on record, with well over 2" of (mostly) rain in the last week of the month; so the swollen rivers did the job without needing a big snowpack meltout.

1 comment:

  1. Now there's flood warnings for Eagle and Manley Alaska...plus more on the way no doubt. And, the WX forecast for Fairbanks includes rain and SNOW starting tomorrow 5/8. Nice! Even the early mosquitoes are wearing boots and coats.