We tracked the storm dubbed “Snurlough” for close to a week, and it was clear all along it would likely produce meaningful snow accumulation. But the storm stood out for being a surprisingly strong closer, forcing forecasters like us to constantly play catch-up. And, ultimately, we never quite did.
In the 36 hours leading up to the first flakes, we increased our most likely accumulation forecast three times, from 2 to 4 inches to 3 to 6 inches to, ultimately, 4 to 8 inches (slightly more south and less north). And even that wasn’t enough in some locations.
The storm’s snowy grand finale pushed totals in the immediate metro region to 8 to 12 inches, hitting our “boom scenario.”
If you look at the pattern of observed snowfall versus our final-call prediction issued before the first flakes began Saturday afternoon, our forecast was actually pretty good in broad strokes. What stands out is that the jackpot zone of heaviest snowfall ended up just slightly north of where we predicted, maximizing right over the metro region and our northern suburbs — rather than in the zone from Fredericksburg to Southern Maryland.
The consensus of computer models was that this jackpot zone of heaviest snowfall would occur south of D.C., and we believed it. The trouble with snow prediction, and precipitation forecasting in general, is that it is extremely difficult to pinpoint where the heaviest will occur until it is actually happening. This is the whole basis for including boom and bust scenarios in our snow predictions.
Before the final phase of the storm, which pushed snowfall totals into the boom zone in the metro area, our forecast of 4 to 8 inches seemed right on track. About 4 to 6 inches had fallen through Sunday morning, and we predicted the final impulse would produce another 1 to 2, maybe 3 inches Sunday afternoon and evening. During the early afternoon pause in the snow Sunday, we accurately warned readers that conditions would go downhill again.
But this final impulse was more vigorous and longer-lasting than we anticipated. Instead of an additional 1 to 3 inches, it produced another 2 to 6 inches — the heaviest amounts focused from the District into its immediate northern suburbs, where storm totals reached 10 to 12 inches. This final impulse, whose effects were maximized right over the metro area, was the difference-maker between an okay forecast and a good forecast.
More generally, although it took us until late Friday and Saturday to get a reasonably good handle on the storm’s evolution, I think we did a decent job of conveying its overall timing and impacts.
We made the right call in upgrading the storm from a Category 2 “disruptive” event (Friday) to a Category 3 “significant” storm on our winter storm impact scale Saturday. Running our algorithm for assigning these categories, it verified as a high-end Category 3 storm (not far off from a Category 4 “major” storm). Although a number of areas registered double-digit totals, the fact that snow fell gently over a long duration allowed road crews to keep up with it, setting aside the closing burst Sunday evening.
Overall, we took a slow and steady approach in forecasting this storm, doing our best to consider and synthesize model forecasts that were jumping all around. For example, one model, the NAM, was predicting 0.3 inches Thursday and 17 inches on Saturday for Washington.
Perhaps we should have been more aggressive in our predictions, especially when you consider our snowfall totals were a bit low for the Nov. 15 storm as well. This past year was the wettest in recorded history, and El Niño events, like the one currently developing, tend to produce storms with a lot of precipitation.
These are lessons to carry forward for future storm prediction this winter, but not to get carried away with.