Snow and Cold Notes

Following up briefly on yesterday's post, 2.0" was the official snow measurement from Denali Park HQ yesterday.  This is actually fairly remarkable: in a century of climate observations, the earliest date with 2" of snow in one or two days was more than two weeks later: August 26-27, 1984.  The latest in the spring with such an event was June 18.  The only measurable snow recorded in July was 1.0" on July 17, 2003.

Yesterday is also the the only time with more than an inch of snow reported on the ground at the daily measurement time in either July or August: the previous earliest in the season was September 2, 1980.

The cold air mass also produced some notable freezes this morning, including 21°F at Chicken, 26°F at Eagle airport, and mid to upper 20s at many RAWS locations.  The 21°F in Chicken is the coldest on record for this early in the season, although there's only 25 years of data.  Previously the coldest observed in Chicken between June 15 and August 15 was 24°F, and perhaps surprisingly a sub-20° reading hasn't been observed prior to August 29 (2012).

On another note, it's also quite remarkable to see that persistent cold over the western Chukchi Sea has allowed sea ice to linger this summer along the Arctic coast of far eastern Russia, and in recent days some ice has made its way down through the Bering Strait.  Here's the latest NWS ice analysis:

The NSIDC commented recently that it seems unlikely that the Northern Sea Route will open up this year, so that will presumably help avoid a fiasco like last year with unprepared ships getting stuck at freeze-up.

Changing Ice

A couple of items of follow-up today.  First, all semblance of similarity to the early cold of 1992 has long since been lost this autumn in Fairbanks, with above-normal temperatures persisting now for more than two weeks, and a near-record high of 48°F yesterday.  It was the highest temperature for more than a month, which in itself is quite remarkable at this time of year, when the seasonal normal is dropping so quickly.  There's been no meaningful drop in this year's daily mean temperatures since late September.

The warmth has pushed back freeze-up of rivers; the Tanana is mostly ice-free at Nenana, as is the Yukon River across the border at Dawson, and the Salcha shows plenty of open water (photo below from Thursday, courtesy of Twitter user NateoftheNorth).

Second topic: following up on my post last week on a very different facet of the cryosphere - Arctic sea ice - I proceeded to examine both the opening and closing dates of the Northeast Passage in the past two decades, and the chart below is the result of this effort.  This is based on the NSIDC sea ice index images, which show the estimated daily location of ice concentration above 15%.

The note for 2013 highlights the fact that there was well over a month of open passage to the north of Severnaya Zemlya, but the near-shore route adjacent to the mainland (Taymyr Peninsula) was closed for nearly the entire season.  The more northerly route is pretty far north (north of 80°N) and I'm unsure if this would be considered an open Northeast Passage, but I allowed it for the chart.

Besides the striking trend towards a more navigable route, it's interesting to see the difference between this year and last.  This is largely the result of contrasting weather patterns, with 2020 having exceptionally warm conditions across the Arctic Ocean, but summer 2021 being relatively (but not extremely) cool and cloudy.  Obviously the window for open navigation is sensitive to the details of where the ice is located, with Severnaya Zemlya being - as this year - the most common bottleneck; Arctic-wide sea ice was not that much higher this summer than in recent years.

ERA5 Winter Precipitation Type

In light of recent posts on winter rain in Bethel and elsewhere, I thought it would be worthwhile to see what the new ERA5 reanalysis data has to say about winter precipitation type in Alaska.  I've mentioned the ERA5 data on previous occasions, for example here:

https://ak-wx.blogspot.com/2019/05/era5-data-for-alaska-including-download.html

The ERA5 dataset includes a precipitation type indicator, which makes it a simple task to assess the frequency of each category across the 0.25-degree latitude/longitude grid.  The six categories are: Snow, Wet Snow, Rain/Snow Mix, Rain, Freezing Rain, and Ice Pellets; the determination is based on the modeled surface temperature and the temperature profile aloft.  Here is some more information on the model algorithm:

https://confluence.ecmwf.int/display/FUG/Types+of+Precipitation

Based on the hourly ERA5 data for 40 winters since 1979-1980, I simply added up the number of occurrences of each category of precipitation type for the months of November through March.  Below are maps for the four simple categories, i.e. snow, rain, and in-between.  Note that a precipitation type is assigned whenever the model has a non-zero precipitation rate, no matter how small; so for example the model indicates snow falling more than 90% of the time over the Arctic waters, but rates would be extremely low much of the time.  Click to enlarge the maps.

Here are the two other categories, freezing rain and ice pellets, with a different scale to reveal areas of low frequency.  For reference, a frequency of 1% corresponds to 36 hours per winter.

And here are the rain, rain/snow mix, and "wet snow" maps on the same "zoomed in" color scale:

It's interesting to see relatively high frequencies extending around the north side of the Alaska Range to the middle Tanana River valley (e.g. Nenana and Fairbanks); the spatial distribution looks realistic to me, although the absolute values of the frequencies might be biased high or low (more on that in another post).

Finally, here's the sum of frequencies for everything other than dry snow.  I'd be interested to hear readers' perceptions of these results - feel free to leave a comment below.