Tuesday, May 12, 2020

Spring Floods

Summer-like warmth has suddenly arrived in interior Alaska, and with plenty of snow still in place across the higher hills, creeks and rivers are in flood.  Fairbanks recorded 82°F on Sunday and 80°F yesterday; this is only the second time in the modern climate record (1930-present) that more than one day has reached 80°F before May 20.  The other time was the ridiculous early heat wave of 1995 (88°F on May 11).

Looking at the SNOTEL snowpack data up on Munson Ridge (east of Fairbanks at 3100' elevation), there's a long way to go to eliminate the very substantial snowpack: more than 12" of liquid equivalent water remains at this time.  The massive snowpack accumulation in late winter really was remarkable: as the chart below demonstrates, the snowpack went from being only modestly above normal in mid-March to nearly twice the normal amount just one month later.  It didn't reach the excess of 2018, which I reported on here, but it's an impressive snowpack nonetheless (click to enlarge).

I've commented before on the fact that above-normal snowpack has become the norm in recent years - where "normal" is defined as the 1981-2010 median.  The winter before last (2018-19) turned out to be the first below-normal snowpack in the Chena Basin since 2009-10, although it was barely below normal on April 1.

So with this winter now in the books, it's been 10 consecutive winters without a seriously deficient snowpack in this part of Alaska.  But of course the same is not true elsewhere; the Kenai peninsula had a dry winter this year, and my impression (although I haven't examined the data) is that the low-snow theme has been as dominant there as the excess snow near Fairbanks.  Here's the April 1 snowpack map from NRCS:

And here are the SNOTEL sites that contributed to the above basin-wide analysis:


  1. Hi, I am curious where the snowpack data comes from that represents the Southcentral Region that encompasses Anchorage and the Mat-Su. Due to multiple mountain ranges (Talkeetna & Chugach), it seems difficult to characterize. Naturally data from sea level at the Anchorage airport vs. a mountain gage would be very different.
    On a side note, it is interesting that the Mat-Su ski club continues to groom nordic trails at Hatcher Pass in mid-May.

  2. Hi Tracy, it's all SNOTEL data, so mostly at elevation. I've added a map to the post showing the sites - there are only a couple of valley-level locations.

    The basin analysis is aimed at river drainage basins, and the individual site reports do look consistent with the big gradient across the Matanuska valley (as of April 1). Of course it's mostly a very sparse network in a huge area.

  3. Further evidence that the Yukon and Kuskokwim River drainages (outside of the flooding in Napaimute) have dodged a serious bullet to this point in the spring. If April temperatures had stayed below normal until this recent interior warm spell, I think record flooding would have occurred at multiple locations.

    1. Yes, I'm a little surprised there haven't been bigger problems. Perhaps the abundant sunshine in the last week of April and first week of May also helped bring forward a lot of runoff before the heat arrived. It could have been very different.

    2. On the lower Kuskokwim and Yukon I believe a major factor was the 10 days or so of rain in mid-April. By the time the surge of water came down in late April, the ice was substantially weakened.