Recapping the 2011-12 Snow Potential Index


Snow Potential Index plotted against daily snowfall by date during meteorological winter. Note that maximum daily snowfall in D.C. was only 0.6 inches. (Justin Grieser)

We introduced the SPI in late 2010 to assess any upcoming potential for accumulating snow in the D.C. area on a 0-10 scale. When we brought it back for the start of meteorological winter on December 1, snow enthusiasts had three months of winter to look forward to. In hindsight, it’s clear that the snow potential didn’t really amount to much this year.

Today we’ll take a look at how the SPI fared during the winter of 2011-2012…

Overall Averages and Winter Recap

Almost mirroring the paltry 2.0” of snow recorded at National Airport (DCA) this winter, the SPI averaged a low 1.7 from beginning to the end of the season (Dec. 1-Mar. 9). For meteorological winter (Dec-Feb), the average was also about 1.7. This is more than a full point lower than the average of 2.8 from the previous winter.

Broken down by month, the SPI averaged 1.1 in December, 1.5 in January and ticked up to a more optimistic 2.3 in February.

While the SPI trended higher in the final weeks of winter, the “bulk” of the snow (1.7”) actually fell in January. The first accumulating snow of the season brought an unexpected 0.6” on Jan. 9. The Snow Potential Index remained at 1 for several days before the event, and was then bumped to 2 the night before. I’m guessing we probably could have gone higher on that one…

The more notable snow event was a combined 1.1” of snow on January 20-21. For three days leading up to this wintry mix the SPI remained at 2, and on the day of, it climbed to 4. In hindsight, the SPI could have gone higher than 4 on Jan. 20, but the initial forecast did call for a mix of snow, sleet, and freezing rain. In D.C. such wintry mix events often underperform in the snow department.

During February, some slightly better odds of accumulating snow brought the digit up to 6 for the first time all winter (Feb. 8 and 17). Yet as the above graph shows, a mere 0.3” was all that fell in D.C. the entire month. The only other SPI of 6 was assigned in anticipation of a possible Presidents’ Day snow event, which ended up missing D.C. entirely. After this “no show snow,” the SPI tumbled quickly, and averaged only 0.9 for the remaining three weeks of the season.


Number of days each SPI number was assigned during the last two winters. (Justin Grieser)

The chart above compares the number of times each SPI number was assigned during the past two winters. Unlike 2010-11 when the SPI once reached 10, winter 2011-12 never saw a Snow Potential Index higher than 6 on a 0-10 scale. In fact, 82 percent of all days this winter were assigned an SPI of 2 or lower. At least that shows our forecasters were honest and never hyped anything up too much!

In a nutshell…

Here’s a sampling of some witty SPI commentary from our forecasters, summing up the tough year it was for D.C. area snow lovers:

Plenty of chilly conditions, and a little bit of moisture in our forecast... just not at the same time.

- Brian, 12/11/11 (SPI: 0)

Dreaming is likely all we will be doing as far as a white Christmas is concerned or anytime in the next week for that matter.

- David, 12/22/11 (SPI: 1)

Just not enough cold air around when we get weather systems.

- Matt, 12/27/11 (SPI: 0)

Another cold front Tue night/Wed a.m. could include a very few flakes. If only we could add up all these 1s...

- Ian, 1/14/12 (SPI: 1)

About the only hope for flakes is an errant flurry on Sunday and that is being a real optimist.

- David, 1/26/12 (SPI: 1)

Hmm… a flake -- ok, maybe two -- on Sunday? (Insert continued disappointment ->here<-).

- Camden, 1/27/12 (SPI: 1)

Monday & Wednesday a few flakes may potentially mix with raindrops. Accumulation unlikely as it appears now - condolences, snow lovers.

- Jason, 2/5/12 (SPI: 2)

Models hang on to possible weekend snow threat. Rain or the storm missing us are also in play.

- Dan, 2/15/12 (SPI: 4)

I’m going to take the plunge and drop to zero. Just don’t see any opportunity for accumulating snow next 7 days.

- Ian, 2/25/12 (SPI: 0)

And last but not least…

I know there have been plenty of snows in March but this year it feels like time to wrap this up.

- David, 3/8/12 (SPI: 0)

Caveat

Although we have a historically warm pattern right now, snow is not unheard of in Washington, D.C. in late March and early April. Should snow potential re-emerge, we would need to dust off the SPI. But more likely than not, it’s in hibernation until November or December 2012.

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