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Posted at 02:32 PM ET, 11/30/2011

Return of the Snow Potential Index

With the start of meteorological winter on December 1, comes the resumption of Capital Weather Gang’s Snow Potential Index. As a reminder, the 0-10 index describes the potential for measurable snow (at least 0.1”) over the next seven days.

Here’s more from our overview of the index last year:

A 10 would indicate a virtual certainty of accumulating snow in the next seven days and a 0 would indicate essentially no chance of accumulating snow. If there’s potential for four or more inches of snow, we’ll add a “+” to the SPI number and “++” for the potential for more than eight inches.

We’ll also include the trend from the previous day, so if the potential increases, we’ll include an up arrow, and if the potential decreases, a down arrow. The text “no change” [update: we actually just as a dash “-” symbol instead of text] conveys the same SPI value from the previous day.

Here’s an example (note: this is from a day in March last year, it is NOT current) of what it looks like...

Snow Potential Index: 1 (↑) - Any flakes Thurs. night shouldn't accumulate. Slight snow chance early next week, wouldn't put much stock in it yet.
 
The SPI is a daily assessment of the potential for accumulating snow for the next week on a 0-10 scale. Get the 'SPI' on Twitter
 


A word cloud created at wordle.net, using SPI text from December 1, 2010 through February 28, 2011. (Ian Livingston)

At the tail end of last winter, concerns were raised by readers that the SPI did not clearly enough define the region it applied to. That’s constructive feedback, so this year, we’ll define the region as encompassing the general D.C. metropolitan area. For the purposes of “ground-truthing” the SPI, we’d consider at least 0.1” falling at any of the three local airports (DCA, IAD and BWI) as the markers for accumulating snow in the “area”.

Admittedly, that criteria doesn’t capture snow that might fall in southern Maryland or in Frederick and western Loudoun counties, but it’s the best we can do objectively.


Snow Potential Index plotted against daily snowfall by date during meteorological winter. (Ian Livingston)
What were typical SPI values last winter? Ian Livingston provided a comprehensive summary last March 15; you can see the daily time series above. Here’s an excerpt from his post summarizing monthly SPI averages:

The average SPI from when it began on November 30 (SPI 3) through when it ended on March 15 (SPI 0) was 2.8. In December, the SPI average was a 3, in January a 4.6, February fell to a 2, and by March the SPI average was a pathetic 0.3.

Interestingly, we never used a “+” or “++” to communicate potential for 4” or 8”-type events last winter as the snow potential only once climbed into that range. That was on January 26 (the day of the Commutegaddon storm) but the SPI that day inadvertently left out the “+” even though we were forecasting 3-8”.

Looking ahead to this season, we should’ve dusted off the SPI for the October 29 event (0.6” measured at Dulles), but we didn’t have our SPI ducks in a row yet!

I’m sure everyone now wants to know if it will snow on December 5 (like it has in 5 of the last 9 years). Stay tuned to tomorrow’s SPI!

By  |  02:32 PM ET, 11/30/2011

Categories:  Capital Weather Gang, Latest, Winter Storms

 
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