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Next Storm Wet, Not Wintry
A look back at our performance on the last storm

Dan Stillman @ 8:45 AM

Quiet weather with temperatures typical of late February is what we'll get today and much of tomorrow, before a large storm system brings rain -- yes, we promise just rain -- to the area late tomorrow.


Forecast Confidence: High-Very HighAbout as normal as you can get. Under the influence of high pressure, we should see mostly sunny skies today with high temperatures right around average -- near 50. Tonight, increasing clouds with lows near 35 in town, and near 30 in the burbs.


Forecast Confidence: Medium-HighTurning cloudy, chance of rain late. It'll become mostly cloudy as the next storm system approaches, with a chance of showers during the late afternoon or early evening and highs in the upper 40s. The chance of showers increases as the evening goes on, with rain -- possibly heavy at times -- likely overnight. With thick cloud cover and a steady breeze out of the southeast, overnight temps should be largely unchanged from earlier in the day, nearly steady in the mid-to-upper 40s.


Forecast Confidence: MediumEventual clearing, mild afternoon. Thursday night's rains may linger into the morning hours on Friday, followed by breezy and clearing conditions with afternoon highs in the mid 50s. Overnight, a few clouds with lows in the mid-to-upper 30s in town, low 30s in the burbs.

The Weekend

Forecast Confidence: Medium-HighColder by Sunday. As of now, the weekend looks like this: Partly cloudy with a slight chance of a shower each day. Highs in the low 50s on Saturday, lows in the upper 20s to low 30s on Saturday night, and highs in the mid 40s on Sunday.

Post-mortem on Sunday Storm: A Poor Performance

Before the explanations and excuses, let's be straight about one thing: No one -- neither the National Weather Service, on-air meteorologists or -- did a very good job predicting Sunday's "surprise" snowstorm, which yielded a solid 3-6 inches across the metro area and a band of 6-8 inches in the far north and west suburbs, according to snowfall reports from visitors and those collected by the National Weather Service. did not include the potential of wintry weather in its forecast until late Wednesday night, about the same time other outlets caught on to the threat. At that time, we called for the possibility of "several hours (or more) of sleet 0R freezing rain (especially N&W of town) before precipitation changes to plain rain in the afternoon hours." On Friday, we continued to mention the potential for frozen precipitation on Sunday, but downplayed it saying, "Accumulations of snow and ice are not likely to be substantial, especially given the predicted 36°-40° afternoon highs."

By Saturday morning, we had posted an accumulation map suggesting about 1" or less of snow and ice for the metro area. Late Saturday night, after analyzing the latest model data, we sensed that freezing rain was becoming less likely and snow and sleet more likely. Despite differing opinions amongst the team, our forecast late Saturday night called for slightly higher accumulations in DC and points south and east (0.5-2" of snow/sleet) before a changeover to rain, and significantly higher accumulations immediately north and west (2-3").

Finally, by mid-morning on Sunday, as it became clear that snow was going to be the dominant precipitation type and that a change to rain would probably come later than expected, we increased our forecast to 2-4" for DC and south and east, and 4-8" north and west. Our reaction to the changing forecast came quicker than some outlets -- a 9:40am forecast from the National Weather Service mentioned only rain, freezing rain and sleet transitioning to all rain, even as heavy snow was already falling in the area, while a commenter noted that one relatively new weekend TV meteorologist didn't seem to have "a clue." But, admittedly, it's hard to call the prediction we made at that point a forecast as much as a "nowcast."

Could we have done better? Although still an underestimate, our forecast for increased accumulations as of late Saturday night wasn't all that bad, relatively speaking, and it was issued before any similar forecasts from the National Weather Service or local broadcast media. Problem is that most people wouldn't have seen our revised prediction until they woke up Sunday morning. For us to really deserve a pat on the back we would've had to post that forecast earlier on Saturday, by late afternoon or early evening.

Looking back, we might have been able to pick up sooner than we did on model trends showing colder and colder temperatures in the middle and upper levels of the atmosphere. Had we paid more attention to that, we might have recognized earlier the greater potential for snow versus ice, thus the potential for higher accumulations. Also, in retrospect, the surface winds from the east that were supposed to push surface temperatures above freezing were never predicted to be all that strong -- surface winds need a certain amount of "umph" to dislodge cold air. It's possible we could have better factored this into our earlier forecasts.

Given the uncertainty regarding precipitation type, and the large impact on the end result that a one-degree swing either at the surface or in the mid-to-upper levels of the atmosphere can have, we probably should've been less deterministic and more probabilistic in our forecasts during the days leading up to the event, though at some point you have to commit to a forecast and be held accountable for it. The team will take these lessons learned and, hopefully, use them to improve future forecasts.

Pictured: This little guy was created thanks to more snow than expected this past Sunday. Photo by Joe_Runs_26. Submitted to via Flickr.

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