Saturday, September 26, 2015

Heavy September Snow - Will It Survive?

Fairbanks received an extraordinary 6.7" of snow yesterday, which puts this year firmly in second place for September snowfall through the 25th of the month.  Of course the epic September of 1992 still holds the record, with an amazing 24.4" of snow by the 20th.  If no more snow falls this month, the month will end up in 5th place for total September snowfall (1930-present).

This year's early snowfall is consistent with a modest long-term trend towards earlier autumn snow in Fairbanks.  From 1930-1970, the median date of first measurable snowfall was October 5, but since 1971 the median date has been September 29.  Furthermore, from 1930 to 1990, only 7 years out of 61 saw at least 1" of snow by September 27, but this has occurred 7 more times in the 25 years from 1991 to present.  The chart below shows annual snowfall totals for September 1-27.

At least part of the trend appears to be attributable to increasing precipitation and "storminess" in September.  From 1930 to 1990, the median September precipitation was 0.80", but this increased to 1.19" in 1991-2014.  In the earlier period, only 59 September days (3.2%) saw 0.25" of precipitation or more, compared to 41 days (5.5%) since 1991.  Given that the atmosphere is often cold enough for snow in Fairbanks by late September, an increased frequency of substantial precipitation events may be enough to account for the increased snowfall.

The maps below show MSLP and 500mb height anomaly maps for the two periods, according to reanalysis data.  Note the more unsettled pattern in the recent period.


500mb height

An interesting question now is, will the snow cover melt out in Fairbanks before winter arrives?  From one perspective, it seems very likely: measurable snow cover (1" or more) in September has ALWAYS melted out to a trace or less, except in 1992 (i.e. 13 of 14 times).  This includes 1972, when 8" on the 30th melted out by October 17.  We are, after all, a full 3 weeks ahead of the average date of establishment of the permanent winter snow cover (October 17) and the normal high temperature is still 48 °F.

On the other hand, when September snow cover occurs, it is usually only 1-2", but yesterday brought a snow depth of 5" - the second earliest on record.  The first occurrence of 5" snow depth has stuck around all winter in 82 of 85 years (including 1992), with the only exceptions being 1934, 1936, and 1972.  The chart below shows the frequency of melt-out after the initial occurrences of various snow depths.   So from this perspective the odds look better; but of course most years don't see 5" on the ground until much later (median date October 30).

All in all, I suspect there is at least a two-thirds chance that the snow cover melts out in Fairbanks itself, even if more snow occurs on Tuesday and Wednesday, as seems possible.  However, with 9" of snow reported just a few miles north of Fairbanks, it seems quite likely that elevated and/or sheltered locations not far from Fairbanks may just have initiated their winter snowpack.

Here's a post from last year on a similar topic.  Last year's October 5 snowpack of 4" did manage to survive, so 2014 actually tied for 3rd earliest arrival date of the permanent winter snowpack.

[Update Sep 28: That was quick.  The latest snow depth observation from Fairbanks airport shows the snow cover has melted back to only a trace in just 2 days.  That's what steady rain and temperatures in the upper 30s will do.  But more snow appears to be on the way.]


  1. For Fairbanks and most of the rest of Interior Alaska:

    Amazing event if it happens as forecast. Rain again now to be followed by another snow event. It might be worth a look at the dynamics behind this September's closing weather.


  2. If it had remained cool enough to snow instead of rain, how many inches of snow would we probably have and where would that put us in the history books?

    1. Eric, the 6.7" of snow came out of 0.50" of liquid equivalent, so that's a ratio of 13.4. Total precip since Sep 25 is now 1.04", so at the same ratio you would have 14" of snow by now. September 1992 snowfall was 24.4".

    2. Update: the latest storm produced 11.9" of snow out of 0.89" liquid equivalent (through midnight), so the ratio is again 13.4 (interesting coincidence). The week's precip is at 2.23", so that would be 30" of snow if it were cold enough. But it wasn't close to being so: temps aloft were as high as +7.6C on Monday morning, when it was raining.