Wednesday, December 12, 2012

Talking You Off The Snowless Ledge...Sorta

So, we played Debby Downer this morning and posted about how it's been over 300 days since our last measurable snowfall in Philadelphia.  Don't despair...it's not the end of the world.  No sense pulling the Bermuda shorts out of the dresser and dawning them, thinking winter is over before we hit the shortest day of the calendar year.

Take a deep breath and relax.  We still have plenty of time for snow.  Plenty.

We may not get an uber dumping of snow along the lines of the 2009-2010 winter or the 2010-2011 winter but outside of the winter of 1972-1973, we've had at least some snow accumulate in the city annually.

Click on the above graphic to see all the data if we don't get measurable snow by December 31st...good, bad, and ugly.

Assuming we make it through December without any accumulating snowfall, it would be the 21st winter where we've made it through December without snow.  Some of the years are recent...but there have been many in the past without measurable snow, including some colder winters.  The average of the data set is 14.1" -- not a robust total by any stretch but still mighty respectable since our average snowfall is just above 20".  Given last winter was a tire fire for snow lovers, getting 14" between January and March isn't a terrible haul and is an improvement on last winter.

The range is pretty extreme -- from zero (1972-1973) to 33.1" (1913-1914), with six years of the twenty featuring at least 20" of snow.  Odds aren't terrible of getting to average but not great.

Let's expand this out and say that December gets less than an inch of snow but at least gets some accumulation before month's end.  If our first coating snowfall occurs in December before month's end -- the average doesn't improve markedly.  It's only 14.9" over a 16 year spread with a spread of 0.8" (1997-1998) to 30.4" (2004-2005).  Let's do the math -- 36 years have either their first sub inch coating in December or no coatings at all -- that's over a quarter of the time since 1884 where we're getting through December in such a scenario.  Cuing Tom Jones...it's not unusual.

Historically, about a third of our winters don't get an inch of snow between October, November, and December.  We've gone four winters in a row in such a scenario...and five in six at one point.

Snow Begets Snow Usually

It's not doomsday, nor time to pull out the sackcloth and starting mourning, fasting, or doing snow dances.  However, a healthy December snowfall tends to correlate well with snowfall the rest of the winter.  If we look at years where at least 5" of snow has fallen by year's end, your winter is pretty solid overall.

There have been 49 winters where we've picked up five inches by December 31st.  The average snowfall for January-April in those years is twenty inches and in only seven of those 49 years did we fail to pick up ten inches of snow after January 1st.  Included in those years are 2009 and 2010 (thank you big snows in late December both years).  There is truth to the thought process that you want it to start snowing early and in good quantity in order to get it to snow often through winter - the numbers generally support this.  However, even if we're skating through December in a relative snowless dearth, there are exceptions to the rule.

The winter of 1977-1978 (third snowiest on record in Philadelphia) -- 0.4" of snow in November, 54.5" of snow after January 1st.  Also, the winter of 1906-1907 -- 0.3" of snow between November and December, 38.3" of snow after January 1st.

Even if we end up with 14 or 15" of snow hereafter, it's not the worst of winters.  More importantly, given our tendency to be in a very blocky regime along the East Coast so far this cold season...the odds favor at least one decent storm at some point, which will likely bring snow.