Friday, October 21, 2016

Snow Cover - Will It Last

A fair portion of the interior now has at least a little snow on the ground after yesterday's modest snowfall.  Half an inch of accumulation was reported at Fairbanks airport, but the snow depth was rounded up to 1".  It's little more than a dusting of snow, so what are the odds that it will last and prove to be the start of the winter snow pack?

An analysis from a couple of years ago (see here) indicates that we're now late enough in October that there is less than a 20% chance that a one-inch snow cover will melt out to a trace or less.  This suggests that this year is likely to be one of the 10% or so in which the first measurable snowfall doesn't melt out.  On the other hand, the forecast for next week looks warm, with major storminess in the Bering Sea importing warm air over interior Alaska.  About half of all years reach 40°F or warmer after October 25 in Fairbanks, so if it doesn't snow any more - which looks likely in the short-term - then I'd say there's a fair chance of seeing bare ground again.

The chart below shows the number of days with zero or trace snow cover occurring after the first measurable snowfall, versus the date of the first snowfall on the horizontal axis.  Obviously the earlier the first snow occurs, the more subsequent snow-free days there tend to be before winter sets in.  But it's interesting to see that very late first snows do tend to melt out - which makes sense if the weather pattern is unusually warm in those years.


The outlier on the lower left is from the infamous early winter onset of 1992 - the only time the winter snow pack was established in September, and mid-September at that.

7 comments:

  1. The moisture content was only 0.03" so I'm guessing a sublimation loss is possible. Unless there is cloud cover next week the day time highs should be marginally cool enough to support snow cover. (Due to our famous inversion). However with a still brief but mildly decent sun angle anything is possible. Shady areas will remain snow covered though. Just my guess.

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    1. Mike - great point and a good guess, I think. The forecast looks very sunny by day, with low humidity; but the airport snow measurement location is shady.

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    2. If you look at the Nenana ice classic webcam the southeast facing hillside (facing the camera) was white this morning but is now brown this afternoon. I saw a high of 22 degrees there today.

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    3. Well spotted, Mike - thanks. I've noticed before that the hillside there often loses its snow cover quickly even during winter.

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  2. The bare ground had cooled some prior to the recent snow under clear skies. Frost was visible in bare spots exposed between vegetation and open to our low angle insolation.

    I suppose if the storms predicted for next week materialize and advect inland we may see some more of my unraked leaves.

    Gary

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    1. Gary - yes, Thursday morning was the coldest with no snow on the ground since October 1991. Very unusual.

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  3. Minus 5F here Sunday am at sunrise when the cold air started stirring. Dark or bare ground spots are growing in size and any dark surfaces with a southern exposure are melting surface snow mid-day in the mid-teens temps, even at a low 13* max Sun angle.

    Gary

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