Forecasting snow in D.C. area is hard and humbling. Over the years, we’ve had our share of misses. Understandably, in the days leading up to the event, many were skeptical that Snowzilla would live up to the hype.

But the predictions for the storm proved amazingly accurate and illustrate how far weather forecasting has come.

Let’s review a timeline of our forecasts:

  • Eight days before the storm (Jan. 14): A signal for storminess first emerged. Capital Weather Gang winter weather expert Wes Junker, in his two-week outlook, noted that “the next period for potential storminess would arrive around Friday, January 22.”
  • Seven days before the storm (Jan. 15): Models were waffling on the storm potential some, but there were still indications that trouble could brew. “Some storminess is possible with precipitation of unknown forms,” I wrote in a discussion. I also showed model simulations that showed a storm off the coast of North Carolina, in a classic position for snow in the Mid-Atlantic.
  • Five days before the storm (Jan. 17): The signal for a storm was becoming strong, with several models showing the risk. Junker wrote his first winter threat article of the season, saying:  “Forecast models have converged on the idea that a significant storm will impact the area between next Thursday night and Sunday. If they are right, it could bring us our first meaningful winter weather event of the season. There is some chance this could produce heavy amounts of snow on parts of the region.”
  • Four days before the storm (Jan. 18): We said that the storm had the potential to be most significant one since 2010 and that model forecasts were converging toward a severe storm. We offered our preliminary forecasts, conveying three possible scenarios, stating that the most likely would produce “a crippling snowstorm with accumulations of 1 to 2 feet across much of the area with near-blizzard conditions possible, especially northeast of the city.”
  • Three days before the storm (Jan. 19): We sounded the alarm, with a headline “Forecasts converging on severe, potentially historic Friday-Saturday snowstorm.” We issued our first snowfall prediction, calling for 12 to 20 inches in the I-95 corridor and 15 to 25 inches to the north and west and wrote: “The agreement among forecast models for a severe winter storm in this case is remarkable and a hallmark of some of our most memorable snow events.”
  • Two days before the storm (Jan. 20): The National Weather Service hoisted a Blizzard Watch. Like the Capital Weather Gang and meteorologists across the region, it started emphasizing the likelihood of severe impacts from the storm. We stressed that it would be a long event, that some areas could receive up to 30 inches of snow and that it would rival historic storms.
  • One day before the storm (Jan. 21): Our headline read, “Blizzard Warning: High winds, about two feet of snow forecast for D.C. area.” Capital Weather Gang was not alone in forecasting such heavy amounts, all the various media outlets and the National Weather Service also were right on top of the situation. That day, we also upped our forecast amounts. “This storm shares characteristics of many of D.C.’s greatest snow events, such as Snowmageddon in 2010 and the blizzard of 1996,” we wrote.

In short,  we, along with other forecasters, were able to accurately predict a long duration, blockbuster snowstorm four days in advance with relatively high confidence.

Not only did we get the big picture right, but also were fairly close to nailing down the specifics with an exception or two. Let’s review what we forecast and what happened:

Storm timing and duration: While it took a bit of time to come into focus, we correctly captured the onset and end time for the storm … starting early Friday afternoon and tapering off just before midnight Saturday.


Final forecast timeline for storm issued Friday morning.

Amounts: If we compare amounts that fell to what was forecast, the match is a pretty good. Forecast amounts in the 16- to 30-inch range verified for much of the Washington metro region, greatest to the northwest and lowest to the southeast. Yes, the storm overachieved in some of our areas west and northwest areas where “boom” totals of more than 30 inches occurred.

Also, the heavy snow ended up expanding farther north than we expected into northern Maryland and southern and eastern Pennsylvania. Only a sliver of that region represents our forecast area, but illustrates that forecasting along the periphery of a storm is challenging (more on that below).

Forecast amounts


Capital Weather Gang snowfall forecast for Jan. 22-24. Second call. Made 1:45 p.m. Thursday. Jan 21.

Actual amounts


Snow amounts during Snowzilla, Jan. 22-23. (Jordan Tessler)

The dry slot: We correctly noted that the lowest amounts would occur south and east of the city because of the potential for a dry slot. Indeed, a dry slot did greatly reduce the snow intensity on Saturday morning into the early afternoon from I-95 and to the southeast, even expanding a bit farther west than we thought. That made the difference between a top two and top five snowfall in the District.

Sleet to the south and east: As we mentioned as a possibility, a period of sleet also impacted areas south and southeast of the Beltway overnight Friday into early Saturday, reducing totals a bit.

Thundersnow: Another good specific prediction we made was that there was potential for thundersnow, particularly Saturday morning. Like clockwork, BOOM — the rumbles sounded off from the District and to the northwest in the 6 a.m. hour.

The brunt of the storm: While we were right that some of the worst conditions would develop late Friday night into  Saturday morning, especially in western areas, an argument could be made that the window from 3 to 6 p.m. on Saturday was the worst area-wide. That’s when Reagan National Airport experienced true blizzard, white-out conditions when winds gusted over 35 mph and heavy snow and blowing snow plastered the area. We missed this detail.

Impacts and outages: We said the storm would bring the region to a standstill, which was unquestionably true. However, we warned about the risk of power failures, which were mercifully isolated rather than widespread.  We were fortunate temperatures were a little colder than forecast (mostly in the low-to-mid 20s rather than 25 to 30) which meant the snow was light and powdery rather than heavy and wet.

Conclusions

The entire Capital Weather Gang team worked very hard for this event and provided detailed, round-the-clock analysis and coverage. I’m proud of what we achieved.

Also, the National Weather Service and other media forecasters deserve credit for their predictions and tireless efforts to communicate the vast storm threat.

This forecast success would not have been possible without the state-of-the-art forecast modeling available to us thanks to decades of investment into computing resources by the U.S. and international weather enterprise (public, private and academic sectors), as well as advances in our theoretical understanding of these storms.

As good as this snowfall forecast was in the Mid-Atlantic, it was not as successful in New York City, where forecasts were initially for a modest event of 6 to 12 inches, but totals ended up closer to 25 to 30 inches.

New York City was on the fringe of the storm where forecasting is always hardest, while D.C. was consistently in the bulls-eye making it a much easier forecast.

Continued investment in weather forecasting is critical for improving forecasts not just in the storm sweet spots, but also along the edges where minor shifts make the difference between a modest or non-event and a historic one.

The D.C. area frequently straddles the rain-snow line and sometimes lies on the razor-sharp edge of a storm, so the forecast for the next storm may not be so easy and successful as this one was, despite our best efforts.

We welcome your feedback on how we forecast and communicated this storm.

(Wes Junker contributed to this post.)

More on Snowzilla:

Why this incredible, ‘textbook’ snowstorm is so potent

Photos: Snowzilla’s gloriously deserted roads

Where Snowzilla fits into D.C.’s top 10 snowstorms

The nuts and bolts meteorology of an epic snowstorm in Washington, D.C.

How much snow fell from Snowzilla in the D.C. area, in detail

Snowzilla makes history from Northern Virginia to New York

Lower-than-expected D.C. snowfall total raises questions about its measurement

Mid-Atlantic coastline flooded by blizzard’s storm surge. ‘This is worse than Sandy.’

The 12 best meteorological images of the Blizzard of 2016