Tuesday, December 11, 2012

White Christmas Odds Low Historically

Bing Crosby dreamed of a White Christmas...and every year you hear it, perhaps ten times or more in a day thanks to the utter lack of variety on B101 or WOGL during the holiday season (sorry guys, it's true though).  Given that our historical odds of a White Christmas aren't much north of ten percent, it's also some hallucinations to dream of ones that Philadelphia locals used to know.

Over the last 50 years, it has snowed at least a trace in Philadelphia six times on Christmas Day, with the largest snowfall in that time frame 4.1" (part of a three day snowfall event that produced 5.2").  Pretty close to the 10% curve -- it's not a frequent chance but one that can happen.  If we expand that window from the 25th to include the 24th and 26th  -- the number of snow events increases to ten (one every five years), with the most recent snowfall the Boxing Day Storm in 2010 which brought a foot to Philadelphia and over two feet at the Shore.  That storm wasn't the largest in the last 50 years over the 24th-26th timeframe -- it is Christmas Eve's snowstorm in 1966, bringing 12.7" to Philadelphia.

20 percent history in the last 50 years means 80 percent history of no snow in the three day window.

Do we have a shot at a White Christmas this year?  Well, depending on a storm system that moves through next week, perhaps...but odds favor that storm being rain at this point.  Computer modeling suggests snow (or a mix of snow, rain, and some ice) is possible for the Poconos and Northern Pennsylvania, with everyone else getting rain.  We'll keep an eye on Sunday just in case the trend shifts colder...as of now, odds don't favor that.

Modeling from the GFS (yesterday) suggests there will be plenty of snow cover across Canada and Northern New England, however, with potentially over a foot of snow (the 1 on the right equals 10" of snow cover on a 10:1 ratio) on the ground as you work into northern New Hampshire and even more in Northern Maine.  Canada looks pretty good too.

Around here?  For now, don't get your hopes up too much...history isn't completely on your side.