Post Mortem: Successful Forecast With Its Flaws
During the course of the storm and in its wake, there has been much debate about how well our forecast panned out. Our independent "Weather Checker" (i.e. our weather ombudsman), Jamie Yesnowitz, will weigh in with his own thoughts on this tomorrow. But I'll kick things off with a self-assessment reflecting on what I think went well and went wrong.
The short version of the assessment is that west of I-95 we overestimated the impact of the first half of the storm and slightly underestimated the impact of the second half of the storm (southeast of I-95 forecast we were close to being accurate throughout). But when all was said and done, our initial forecast and impact projections were spot on (pretty much everywhere) -- notwithstanding a modestly flawed mid-storm adjustment for areas west of I-95.
Keep reading for details about our successes and mistakes in forecasting the storm and vote for what grade you'd give the Capital Weather Gang's forecast...
SUCCESS: IDENTIFIED THREAT IN ADVANCE
This storm snuck up on a lot of people who considered winter mostly over, especially in light of the fact March had arrived and we had received a meager 2 inches of snow to date. However, CWG's Matt Rogers first mentioned the threat of wintry weather a week ago today. We specifically noted the possibility of a coastal storm on Thursday, but cautioned there was a "decent" chance it would go out to sea. By Friday, the threat of a storm started to seem legitimate, so we issued a Snow Lover's Crystal Ball that spoke of the potential of two weekend waves of wintry weather -- though we indicated the second (but ultimately the more important) was a low probability.
Early Saturday morning (5 a.m., but post was updated at 11 a.m.) we started sounding the alarm bells noting a "[m]oderate risk of significant snow accumulation." Late Saturday morning, in a comprehensive assessment of storm possibilities , we went on the record with a preliminary accumulation estimate of 3-6" with more east -- which was pretty close to on target. As far as I know, no other local forecasting outlet in the area identified the threat as early as we did nor provided as early and accurate of an accumulation outlook. Although I give props to AccuWeather's Joe Bastardi who suggested a 6-12" storm was possible for D.C. north on Friday afternoon.
MISTAKE: WRONG TIMING AND IMPACT OF FIRST HALF OF STORM NEAR AND WEST OF I-95
In our storm timeline, we indicated snow would begin between 1 and 4 p.m. and begin accumulating by early evening. In reality, snow didn't arrive until between 4 and 5 p.m. east and south, and until between 5 and 6 p.m. in the metro area. Also, the snow was mixed with sleet and rain in spots, which we didn't specifically call for. Most importantly, little meaningful accumulation occurred until after midnight along and west of I-95 even as areas 10 miles or more east of I-95 received several inches like we called it.
What happened was that the initial surge of moisture got eaten away by dry air coming in from the north. So even though radar showed precipitation surging through central Virginia Sunday afternoon, it hit a wall north and west Fredericksburg, Va. Then, even as the precipitation moved in, it was so light that some of the snowflakes forming in the cold air aloft melted near the surface (where temperatures were above freezing) causing a mix of rain/sleet and little accumulation. Had it been precipitating harder (like it was to the east), the air would've cooled via evaporation, we wouldn't have had rain, and the initial snow would've started sticking earlier. None of the models we use as a tool in forecasting predicted the dry air intrusion until it was practically happening. In the future, this is something we'll have to analyze more closely for an incoming storm of this sort.
MISTAKE: INCORRECTLY REDUCED SNOW AMOUNTS WEST OF I-95 MID-STORM
Around 9:15 p.m., when the snow had temporarily stopped from the District and points west and not accumulated more than a sloppy half inch or so, we decided to lower accumulations for those areas. Because we had expected at least a couple inches of snow in these spots by then and the radar to the west and northwest was underwhelming, it seemed like the obvious decision. We didn't throw in the towel entirely though, noting the redevelopment of snow to the southwest as well as the likelihood of more snow late at night associated with an "energetic upper low." But we did drop accumulation estimates to 2-5" (from 4-8") in the immediate metro area and 1-3" north and west of the Beltway (from either 2-4" or 4-8"). That downward adjustment of 2-3 inches was unnecessary.
As it turned out, the "energetic upper low" (the same feature had produced thundersnow in Atlanta) dropped a couple more inches than even we expected. That made up for the lack of snow along and west of I-95 early on.
The lesson here is unclear. If we had stuck to our guns, our forecast would've been more successful. However, if for some reason the second phase of the storm had underperformed, the forecast would have been a horrendous bust.
SUCCESS: OVERALL STORM ASSESSMENT OF STORM ACCUMULATION AND IMPACTS
If you look back at our post from 11 a.m. Sunday -- which basically served as an "executive summary" of what to expect -- it was on target save the second bullet ("snow to start between 1 and 4 p.m. this afternoon, and may quickly become heavy"). And the storm impact scales (StormCast, TravelCast, SchoolCast, and FedCast) provided in the preceding post verified nicely.
Overall, I'd give our performance a B (on an A-F scale): we were onto the storm early, got the big picture right but missed on some important details in the storm's evolution. We'll see if the Weather Checker agrees tomorrow...
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