This past Saturday's snow forecast: Subpar
Weather Checker is an outsider's analysis of CWG's forecast accuracy. See previous Weather Checker posts.
By Jamie Yesnowitz
A few notes on CWG's forecasting performance for the surprisingly potent storm this past weekend:
Good pre-storm coverage. Coverage of a potential threat in the days leading up to the storm was as usual, extensive, and covered the run-to-run fluctuations of the forecast models. Jason Samenow mentioned in his Monday forecast last week the threat of a storm occurring between late Thursday and early Saturday, and consistent updates followed as the event approached. The first Snow Lover's Crystal Ball was posted early Monday morning as well, with a probability chart, which leads to the next point . . .
The probability tables are extremely helpful. They provide adequate warning for the outlier event. It is interesting to note that in the accounting profession, practitioners are required to measure for uncertainty in income tax positions by using probability tables remarkably similar to those used by CWG.
Keep reading for more analysis of CWG's coverage of the past weekend's snowstorm...
It's a good topic for a future column, but suffice to say, the most likely outcome is not necessarily the outcome that is used for accounting purposes -- rather, the outcome that occurs at the 50% probability line is what matters (I characterize this as the "50% rule"). And the issue of how to read a probability map was raised by CWG and readers alike in the comments section in the days prior to the storm. A good example was the probability chart in last Tuesday's SLCB, which read as follows:
Here is the current (VERY EARLY) probabilities on accumulations: 40% chance: Less than 1"; 20% chance: 1-4"; 20% chance: 4-8"; 20% chance: 8"+
If one looked at the probability chart using the most likely option listed, a prediction of less than 1 inch would have been made. Using the 50% rule, on the other hand, would yield a 1-4 inch prediction. While both these predictions ultimately failed to verify, the early call that there was a 40% chance of a 4+ inch snowfall four days before the event was noteworthy. Clearly, there was optimism early on that some accumulating snow would make it to the D.C. area.
Issuing the snow map is in many respects "the end game." Probability tables are great, especially when gauging a potential outlier threat, but when the snow map comes out, there's a level of finality inherent in the map which doesn't come through in the probability charts. The very first map issued by CWG on Friday showed D.C. in the "dusting to 1 inch" category (backtracking from the 1-4 inch prediction above, if one considers the 50% rule to be in effect), with a 1-4 inch band extending from just south of D.C. to about Fredericksburg, deeper snows south of Fredericksburg and flurries north of DC. Obviously, this map missed by a wide margin, with only Richmond's 8+ inch forecast verifying. Later maps -- late Friday morning and Friday afternoon at least had the magnitude of the snow moving in the right direction, though the substantial accumulations north of D.C. never really got incorporated into the forecast until CWG entered the "nowcasting" phase. Other local forecast outlets didn't do much better through late Friday night.
A whole lot of nowcasting. The forecast was well off until the flakes were flying throughout the metro area. By late morning Saturday, CWG had updated its snow prediction to 4-8 inches, which did mostly verify throughout the region (though some spots to the south attained higher amounts). The good news is that CWG then stuck with its 4-8 inch call through the afternoon and evening, rather than continuing to tweak the forecast with each snowflake, thereby avoiding the "double-miss" phenomenon that occurred in the early March 2009 snowstorm.
The bad news is that, for the most part, CWG's overall forecast missed on two levels, underplaying the snow depth in the immediate metro area until the day of the storm, and underestimating the additional push of snow to the northern suburbs.
Post-mortems in the comments section are getting better and more extensive. It's getting increasingly difficult to add much to a the conversation about what CWG could have done to improve a particular forecast, and that's a good thing -- CWG and their readers are engaging in discussions on this topic even before a forecast may be going awry, and continue the discussion long after the event is past.
Incorporating this winter's storms into future snow predictions? This year, the D.C. area has had two very unusual snowstorms, the Dec. 19 megastorm that was in part enhanced by unusually warm waters off the Atlantic, and last Saturday's "cold southern storm," with temperatures hovering in the teens and a southern track that, contrary to most early predictions, moved much farther north than initially expected (accumulating snows all the way up to southern New Jersey). Hopefully, the experiences gleaned from these two events will be incorporated into future forecasts in an effort to improve the long- and short-range predictions that often fall short of the mark during snow season.
Jamie Yesnowitz has been interested in the weather since he rooted for school-closing snowstorms while growing up in Brooklyn and East Rockaway, N.Y. After graduating from Dartmouth College with a bachelor's degree in economics and government, his focus on the accuracy of weather predictions took hold when he moved to Coral Gables, Fla., to attend the University of Miami School of Law. Class was scheduled to begin on August 24, 1992. Hurricane Andrew had other ideas, however, shutting down the school for weeks. But what stuck in Jamie's mind was the final unpredicted swerve of the eye that saved those living in Miami and points north, and completely devastated areas about 20 miles south of Miami.
Undeterred by the hurricane, Jamie ultimately served as editor-in-chief of his law school newspaper, and earned both a juris doctorate and master's degree in taxation. Following law school, Jamie practiced corporate and securities law in New York before shifting to the state and local tax consulting world. Jamie moved from New York to the Washington area in 2003, and he is presently a state and local tax senior manager at a major accounting firm. Jamie lives in Potomac with his wife, Sandra, and their two daughters, Sarah and Carly.
Capital Weather Gang
| February 3, 2010; 10:45 AM ET
Categories: Weather Checker
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