Weather continues to be in the headlines in Alaska, with the town of Yakutat declaring a "local disaster emergency" on Tuesday because of excessive snow loads that have caused significant damage. There's been similar trouble in Juneau, with at least two commercial building collapsing earlier this week, and the governor issued a disaster declaration yesterday.
The photos in the second news link above appear to show a snow depth of at least several feet in Yakutat, but I'm not aware of any actual measurements of the current snow pack or how much snow has fallen so far this winter. Regrettably, Yakutat's airport climate data hasn't included snowfall since the winter of 2017-2018, and there are no other observing platforms (e.g. Snotel) in the area. The daily climate data only include liquid-equivalent precipitation, and actually this has been far below normal for the time of year - see below. Yakutat is an extremely wet place in winter, but typically it's mostly rain.
But let's not allow the lack of snow measurements to get in the way of a bit of science. We do still have hourly ASOS reports from Yakutat, and these include weather type, such as "snow", "rain", "fog", etc, and also hourly liquid-equivalent precipitation. So I went back through the history to 1973 and calculated the fraction of the November 10 - January 10 precipitation that fell in hours when snow was reported as the weather type. Of course many hours have both snow and rain reported, so I excluded those hours. The November 10 - January 10 window corresponds roughly to the period when accumulating snow seems to have occurred in Yakutat so far this winter, judging from temperatures.
I then applied the "snow fraction" to the total precipitation from the daily climate data, resulting in an estimate for snow liquid equivalent each year. Comparing these estimates to the actual snow data that ended in 2017, we see a pretty good relationship (R=0.89) between measured snowfall and estimated snow liquid equivalent.
Remarkably, this year's snow liquid equivalent (estimated) is 10.4", which is the third highest since 1973 despite total precipitation being the lowest since 1990 for this two-month window! Using the simple linear regression, we can estimate about 100-120" in total snowfall.
While this is a lot of snow, it's unlikely to have been a record for total snowfall in the two-month window, let alone a full winter; Yakutat has seen well over 300" of snow in a winter before (most recently 2011-12). However, the key issue is of course how much melts in between fresh accumulations. Looking at thaw degree days (accumulation of daily mean temperatures above freezing) since November 10, the past two months have been so cold that there has been very little melting in Yakutat; total TDDs were the third lowest on record for the date window.
The following chart summarizes the situation: a lot of snow has fallen (judging from hourly data), and very little melting has occurred. The combination appears to be unprecedented in recent decades, as the only other years with a lot of snow also had much more meltout.
While this simple analysis provides quite a pleasing explanation for the recent difficulties in Yakutat, a couple of puzzling aspects remain. If we go back to earlier decades in the climate history, there are a number of years when similar or greater amounts of snow fell in a two-month window with just as little melting; for example, December 1971 - January 1972 saw 138" of snow (and over 18" of liquid equivalent) with very little meltout. January-February 1988 produced 118" of snow with cold conditions.
We also have the fact that the reported snow depth was as high as 96" in March 2012, and it's difficult to imagine that today's snow load is greater than it was at the end of that epic winter. But perhaps it is, with the very heavy snow and rain earlier this week pushing it over the edge. Without ground-truth data, we'll never know for sure.
Good sleuthing, Richard,
ReplyDeleteHere in Haines we've been cold and snowy too, and yes, cold can mean much more snow even with below normal precipitation, the case here, YAK, the region. But look at the photo in this tweet https://twitter.com/ResusCGMedia/status/1480896209531260935. Then look at the photos in my recent blog post on this. Your estimate of 100-120 inches in Yakutat is valid based on the precip data from the ASOS, but compare that to the 10.98 inches we got Nov 11 through Jan 8, virtually all snow and less melting than YAK (6 TDD!). First, since when has Haines been wetter than Yakutat. Second the photo in the tweet shows maybe 5-6 feet of snow on that roof, compared to 3-4 feet around here. If the density is similar to ours (conservative assumption) I'd say there is closer to 22 inches of water in that snow. So I'd like to propose that the ASOS is likely under-reporting precipitation. BTW, from Jan 9 to today I've logged 14.47 official inches precip in the 8 inch rain gauge while PAHN (ASOS) 1 mile away has reported 6.34", plus whatever fell on the 8 missing days.
I meant from *Nov 11* to today for the 14.47 inches.
ReplyDeleteHi Jim,
ReplyDeleteThanks very much for the comments. I've made a follow-up post, including a comparison with the Yakutat CRN data, which I forgot about in my earlier analysis. Indeed the CRN shows much more precip, although possibly still not enough.
Richard