Weather Checker: "False Alarmageddon"?
Weather Checker is an outsider's analysis of CWG's forecast accuracy. See previous Weather Checker posts.
After the historic snow last season, it seemed too good to be true (for snow lovers, at least) that yet another significant snow event was going to hit the DC area yesterday. And ultimately it was, defying the predictions of most forecasters, including the Capital Weather Gang.
First, there is no disputing that CWG expended an amazing amount of energy in covering the storm that ultimately lacked energy inside the Beltway. Their less than accurate prediction certainly couldn't be attributed to lack of effort. Technical guidance from Wes Junker, the CWG's new winter weather expert, and real-time updates from Jason Samenow late into the night on the two days prior to the storm while watching new model runs come out, showed the seriousness with which CWG performs its mission, and added significant color to the story.
Likewise, the continual chatter in the comment boxes by all of the members of CWG supplementing Jason and Wes' commentary was very helpful. If the content produced by CWG on this particular storm is anywhere close to representative of what future storms will bring, then Washingtonians will be treated to top-of-the line analysis that simply cannot be found in other cities. Of course, additional analysis from CWG leads to an enhanced risk that something said will be amiss, and may lead to over-thinking the result of a storm by twisting with each model run, which seems to have happened here.
The genesis of "False Alarmageddon" on the long-range models was picked up by CWG at least one week before the storm. The Snow Lover's Crystal Ball on Dec. 19 contained the following quote from Wes Junker regarding the models: "This is a rare storm where we can have 50% probability five days in advance of at least 1 inch of snow falling. Models are in unusual agreement."
Forecasts on Dec. 20 and Dec. 21 pegged the likelihood of an accumulating snow (1" or more) on Saturday into Sunday at 55%, with a 20% chance of a significant snow (4" or more). The probabilities did not appreciably change on Dec. 22 (50-60% of an accumulating snow, and a 20-30% chance of a significant snow, except that the timing of the storm was pushed back to a Sunday to Monday event. On Dec. 23, CWG's measured probability of snow actually fell considerably, to 30-40% for an accumulating snow, and 10-15% for a significant snow. The SLCB on Dec. 24 tweaked these percentages to 30% and 15% respectively. So less than 48 hours prior to the start of the storm, the analysis seemed to be "dead on," suggesting that a miss to the east was very likely.
If the forecast had stayed the same from that point on, I think that the CWG would have been universally commended for its call. The problem was that several of the models looked considerably more promising from a snow perspective on each run, and Jason gradually pushed the odds of accumulating snow to 60-65% and the odds of significant snow to 35-40% by 1:20 a.m. on Saturday. Still not an indefensible call considering the result - a miss on the accumulating snow prediction, as by and large, places inside the Beltway did not receive an inch of snow, but not necessarily a miss on the significant snow prediction (less than 50% likelihood means the event still is a no go). My thought was that if a snow map were issued at that time, a 1-3" prediction would have been supportable.
By 10:30 am on Saturday, however, the odds of accumulating snow were raised to "better than 70%" and the odds of significant snow crossed the 50% threshold. At 1:15 pm on Saturday, a snow accumulation chart was formulated, and basically suggested that anything from a dusting to a snowstorm was possible, with 3-6" the most likely result.
The original snow map, which was released at 3:55 p.m. on Saturday, again suggested that the entire DC metro area would receive 3-6" of snow, with lesser amounts north and west, and greater amounts north and east toward the other major I-95 cities.
With respect to the timeline, CWG expected that there was a fair (40%) chance of light snow on Saturday night, and that light to moderate snow would last practically all day Sunday. When the forecast timeline was originally released with details on the intensity of the storm, I was somewhat concerned that it is normally very difficult to get 3-6" of accumulating snow during the daytime in this area with marginal temperatures (even in December, when the sun angle is at its lowest) without a fair amount of moderate or heavy banding, especially when starting without any snow cover.
On Saturday evening, seeing that the snow was not going to start until Sunday morning at the earliest and that some of the models were backing off amounts, CWG noted that the snowfall amounts were likely to be on the low end of the 3-6" range.
However, the accumulations for the storm were not lowered until after 9 a.m. on Sunday, when it was clear that the storm was only going to brush the D.C. metro area, at best. The revised accumulation map put the D.C. metro area in the 1-3" range, with higher amounts a few miles east of DC and Baltimore, and practically nothing west of Warrenton and Charlottesville. While many of the snowfall amounts well away from D.C. verified (Eastern Shore, Delmarva Peninsula), the snowfall in the immediate D.C. metro area did not reach the 1" threshold.
So overall, not a good result, even though CWG walked back the optimistic 3-6" prediction by Sunday morning. On the positive side, it should be noted that CWG never raised its confidence level on the storm above "low-medium," and plenty of cautionary language was used whenever the potential for large accumulations was raised. In the CWG frequently asked questions section following the issuance of its team forecast, the significant possibility of lower and higher snowfall totals than forecast was discussed early and often. And it might provide cold comfort that none of the media outlets called this one correctly for D.C. But this storm really calls into question the reliability of the models, and one question that CWG might want to consider in their internal post-mortem on the storm is how much weight CWG should put on these models in the future.
About the author: 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.
| December 27, 2010; 11:00 AM ET
Categories: Capital Weather Gang, Latest, Weather Checker, Winter Storms
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