A snowy surprise, 10 years ago today
"Major Snowstorm Ambushes Washington"
Now that I have your attention, this is not a speculative (wishful-thinking) headline for a storm that might come along later this winter. Rather, it is the front-page, above-the-fold, large bold-print headline of the Washington Post 10 years ago when reporting on the "surprise" snowstorm of Jan. 24-25, 2000. Related stories ran under the headlines "Blindsided and Snowed Under" and "Snow Job."
This major East Coast storm blanketed heavy snow from North Carolina northward to New England and eastern New York State (see recap here). As much as 20 inches were recorded in sections of North Carolina, and up to 12-15 inches in the D.C. area between the early morning and evening of Tuesday Jan. 25. The storm's notoriety arises from the fact that heavy snow was unpredicted for D.C. and vicinity until about six hours before beginning to accumulate.
All the operational forecast models routinely available at the time gave little, if any, clue to the imminence of this major weather event until late afternoon on the 24th. Even then, notwithstanding the fact that radar and satellite imagery appeared to indicate the area was in for at least a close call, since all models insisted on keeping the main area of precipitation to the east, the late-afternoon (4 p.m.) National Weather Service forecast for Tuesday called only for "Cloudy and Cold. Chance of snow...accumulation of an inch possible."
Likewise, the local early-evening broadcast meteorologists unanimously and categorically dismissed the possibility of a big snowstorm for Washington.
It was not until 10 p.m. that an official "Winter Storm Warning" was issued with the forecast now calling for 4-8 inches. Unfortunately, many folks had already gone to bed blissfully content that the next morning would dawn with a light accumulation of snow at worst and possibly nothing at all. Suffice it to say, when these same people arose the following morning to unexpected blizzard conditions, consternation was abundant. The snow lovers, on the other hand, some of whom surely stayed up all night monitoring the situation, welcomed the excitement of a raging snowstorm -- surprise or not!
The dismay and embarrassment on the part of the NWS and local TV meteorologists was especially haunting given the announcement one week before by NWS Headquarters that the introduction of a new supercomputer "puts us closer to reaching our goal of becoming America's no-surprise weather service." Washington Post columnist Tony Kornheiser wrote at the time, "Models? Next time, read pig entrails." Syndicated cartoonist Pat Oliphant depicted an NWS person digging out from a snow drift saying "with our new equipment, supercomputer models and enhanced programming, we will now be able to be wrong much quicker."
What went wrong in the forecast process in this case is well described in a paper (section 2b) by highly respected research and teaching meteorologist, Lance Bosart. Critical mistakes included forecasters taking the models at face value (with virtual disregard for radar and satellite) and relying too much on "deterministic thinking." That is, to forecasters then it was essentially an absolute yes/no for a snowstorm in the D.C. area, and the models said "no" until very late in the game.
As I've written previously, virtually every forecast has some level of uncertainty. Recognizing this spurred development of the National Centers for Environmental Prediction Short Range Ensemble Forecast (SREF) system, which explicitly estimates levels of uncertainty (confidence). In retrospective experiments, SREF gave at least a 40% chance of significant snow for the Washington area 24 hours in advance of the storm. Had this information been available in real time. it would have provided at least a probabilitic "heads up" of a possible snowstorm rather than a deterministic "no."
For more on dealing with uncertainties in weather forecasts, see this PowerPoint presentation.
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