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Posted at 10:40 AM ET, 03/17/2011

Recapping the 2010-11 Snow Potential Index


A word cloud created at wordle.net, using SPI text from December 1 through February 28.
Capital Weather Gang introduced the Snow Potential Index (SPI) on November 30, one day before the start of meteorological winter (December-February). After searching for flakes well into March – and arguably longer than we should have – we’ve decided (as announced Tuesday) to send SPI into hiding until later this year.

We brought you (and our buds at Twitter and Facebook) SPI to give a quick look at accumulation risks over the week following its issuance in D.C. and surrounds. SPI faced a multitude of tests, even if no storms produced results like the previous winter. A look back tells the story of SPI’s mostly timid, but sometimes tumultuous, first winter on CWG.

Keep reading for more on the SPI over the course of the winter....

National Airport (DCA) has recorded just shy of 10” of snow this season – 9.9” to be exact. If we get no more (which is not a crazy idea given the forecast and historical precedence), that’s only 46.2” below last season! Or equal to part of the morning during Snowmageddon. Anyway…


Snow Potential Index plotted against daily snowfall by date during meteorological winter.

While a first glance at the graph above, which tracks SPI against daily snowfall during “SPI season” might make a snow lover further long for last season, two things stand out right away. There was not a lot of big snow, and there was not a lot of really high hope for accumulation of any sort.

Within the period, there were the highs: 10 - A glimpse back to last season (almost). Late-day snow thump should prove impactful. Weekend flakes? (1/26). There were also lows: 0 - This pattern is horrible for snow. (2/14).

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. Keep in mind, the SPI is a scale of 11 (0 to 10).

During meteorological winter, when all the accumulating snow fell, there were 61 days of an SPI of 4 or less. A whopping 0.6” snow occurred on these days.


Number of days of each SPI ranking during meteorological winter.

Of the five days of an SPI of 8 or greater, including the one and only SPI 10, 8.3” snow or 84% of the seasonal total occurred. Each day with an SPI of 8 or above featured accumulating snow. Of course, Commutageddon alone accounted for 5” of that total. There were no 9s this year. We’ll try for one next time!

Five days (i.e., 12/12 before 12/16) prior to each daily snowfall greater than .4” (6 occurrences total), the SPI averaged a 4 which is a little higher than the average SPI overall while it ran. On the day before accumulation in these events, the SPI average rose to 5.5. A coin flip for snow 24 hours out? It is the D.C. area, and the trends love to go wrong for snow lovers.

The biggest SPI “controversy” followed an atypically warm and dry end of meteorological winter that included 4 days above 70 in D.C. The ensuing storm had, perhaps, slightly more wintry aspects than forecast. Some accused CWG of starting spring training a little too early.

The Saturday, March 5, SPI of 0 read: Now accepting bribes from snow lovers to raise the SPI. The Sunday SPI 0 followed up with: “Nada, naught, zip, zilch, goose egg, bupkis, need I go on?. As the author of the former, I offer up one “oops,” though maybe a half-hearted one.

After a rare (for the winter) soaking of rain amounting to about 1-2” across most of the area on March 6, colder air quickly filtered in and precipitation lingered as upper-level energy rotated through. By evening, snow was falling across the western suburbs and before all was done, snow fell into the city and east.

Still, CWG’s implied call for no accumulation snow held (or did it?). Despite the fact that little snow accumulated until into elevation well northwest of the city, and none of note within the Beltway, comments like the following have provided cause to reflect:

The day of… “I love you guys but saying the SPI is for accumulating snow only is a cop-out. The SPI is designed for snow lovers & we would have really wanted to know about the potential of our last conversational snow of the season!” -caphillse

The day after... “12:45 AM: As we slip slide backwards down the giant hill that is the final 2 miles of the 70 mile trek from National back to our home in Frederick County I mutter slowly to myself, “Snow Potential Index: 0 (→) - Nada, naught, zip, zilch, goose egg, bupkis.” 1:40 AM: As we waken a sister in law in Hagerstown begging a room for the night, I hum softly, “even the good get it wrong” (to tune, of course, of only the good die young). 10:00 AM: Finally make it up hill to Frederick home. Measure 3.5” snow, take snow pics for Weather Gang, read philosophical treatises posted regarding true meaning of SPI, resolve to drive Subaru in future if there is even the slightest hint of snow. 11:00 AM: Remind myself CWG is the best, decide to send this little post anyway, and to tell you all you have many, many devoted fans who live on the periphery of what you are defining as the Washington area, are within what is officially called the Washington-Baltimore metro area, and who often need a heads up as much as, and pay a higher price when one is not forthcoming, than do your closer in fans.” -dustygroundhog

When the SPI returns in the fall (it could come back before then if there is a snow risk, but that seems unlikely), there will undoubtedly be at least a few adjustments as well as some further clarifications. As viewers of the SPI here at CWG: If you could offer up ideas for modifications, what would they be? Please feel free to share in the comment section of this post.

By  |  10:40 AM ET, 03/17/2011

Categories:  Local Climate, Capital Weather Gang

 
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