Tuesday, February 13, 2007
Major Winter Storm to Affect Region
Significant ice event likely north and west suburbs
Team Forecast @ 12:25 PM
The DC Metro area is still facing a significant wintry event through Wednesday afternoon. A major ice event is probable north and west of town, and still possible elsewhere.
A low pressure system over the Tennessee Valley will transfer its energy to the southeastern coast today, and a powerful coastal storm will result. The biggest threat from this system for our area is ice by way of sleet and freezing rain this afternoon into Wednesday morning.
Although the track of the storm is fairly clear at this point, small shifts can result in meaningful changes in our area. The amount of low level cold air that makes it into our area and how easily it gets scoured out is not known with certainty. These factors will determine whether you receive frozen precipitation or just plain rain during the height of the storm -- most likely to be tonight.
When/where will the heaviest frozen precipitation fall? Most likely Tuesday night in the north and west suburbs. Significant and even dangerous ice accumulations are possible there, with impacts lasting into the morning hours. The District and points south and east could also see overnight icing problems, but toward dawn, temperatures approaching or slightly surpassing freezing may help make conditions in these areas slushy or wet rather than icy. Considering the uncertainty about precipitation type and amount, and keeping in mind that both of these could vary quite a bit across the metro area, we've kept the Storm Impact at three flakes for now.
Will the snow stick? Yes -- the ground is relatively cold so it should not take that long for snow to begin sticking on the grass and untreated, lightly traveled surfaces.
When will travel be most difficult? From this evening's rush hour through Wednesday morning. Freezing rain could make for dangerous conditions after dark tonight, especially north and west.
Could there be power outages due to ice? Yes. Especially north and west of DC where temperatures will be colder and the heaviest precipitation could fall as ice. This is the biggest concern from this storm as downed trees and powerlines are likely in some areas due to ice and wind.
What about precipitation type? This is the most difficult aspect of this storm to forecast. Some precipitation will fall initially as snow, but amounts will be light. By the time the heaviest precipitation moves in, mid layers of the atmosphere will have warmed enough so that the precipitation will have changed to sleet or freezing rain (late afternoon today and overnight). There's a good chance that precipitation may even change to plain rain in DC and points south and east which would reduce the storm's impact early Wednesday morning in those areas. There is a possibility that the precipitation could turn back to snow early Wednesday, particularly to the west, given that the coastal low will be bombing out and cold air rushing in. Stay tuned for more on this as the day progresses.
When will conditions improve? Most likely midday Wednesday, but lingering snow showers, cold surface temps, and very windy conditions could cause travel and power woes throughout the day on Wednesday.
Could it fizzle out? Yes -- especially from the perspective of those who want snow. Models have been trending warmer and warmer in the layer of the atmosphere where it's required to be cold for it to snow. So snow amounts could be quite modest. While the threat of ice is real, if the storm track takes a more northerly track, an east or southeast wind could warm temperatures enough for plain old rain, even in some of the close-in northern and western suburbs. We also need to watch out for a dry slot that could form when the initial storm transfers its energy to the coast. If we get "dry slotted," the amount of precipitation we get could be significantly reduced. But the latest information suggests that the chances of a complete fizzle are unlikely.
Could the storm bring more snow or ice than expected? Perhaps. The key for the storm to outperform expectations from the perspective of snow lovers would be if it takes a more southerly track. There isn't a lot of hope for this, however. Also, if the coastal low rapidly intensifies and shifts the winds to the northerly direction more rapidly than forecast (this scenario isn't shown by the models, however), this could help produce more snow. The storm could be even more of an ice threat if the low-level cold air holds strong and doesn't retreat as much to the west. This is a possibility.
What are other forecasters saying? As of last night, most forecasters were mentioning light snow accumulations followed by the threat of icing, especially north and west, and a changeover to rain in the District and points south and east. Bob Ryan downplayed the storm as a 3 on a scale from 1-10. Doug Hill was more serious -- and warned of up to 0.5" of ice in the metro area. Topper Shutt's perspective fell somewhere in the middle -- not as dismissive as Bob nor as serious as Doug.
What's the bottom line? Big-time snow amounts are no longer likely. In fact, just a couple inches of snow seem like the best estimate at this point. But the impact of the storm is likely to be significant due to icing issues, particularly north and west of DC.
 Note: 1-3" stripe will likely include rain, especially from D.C. toward points south and east, in addition to ice. | 
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 Travelcast: Schoolcast (Wed.):  | |
Summary
A low pressure system over the Tennessee Valley will transfer its energy to the southeastern coast today, and a powerful coastal storm will result. The biggest threat from this system for our area is ice by way of sleet and freezing rain this afternoon into Wednesday morning.
Although the track of the storm is fairly clear at this point, small shifts can result in meaningful changes in our area. The amount of low level cold air that makes it into our area and how easily it gets scoured out is not known with certainty. These factors will determine whether you receive frozen precipitation or just plain rain during the height of the storm -- most likely to be tonight.
Frequently Asked Questions
When/where will the heaviest frozen precipitation fall? Most likely Tuesday night in the north and west suburbs. Significant and even dangerous ice accumulations are possible there, with impacts lasting into the morning hours. The District and points south and east could also see overnight icing problems, but toward dawn, temperatures approaching or slightly surpassing freezing may help make conditions in these areas slushy or wet rather than icy. Considering the uncertainty about precipitation type and amount, and keeping in mind that both of these could vary quite a bit across the metro area, we've kept the Storm Impact at three flakes for now.
Will the snow stick? Yes -- the ground is relatively cold so it should not take that long for snow to begin sticking on the grass and untreated, lightly traveled surfaces.When will travel be most difficult? From this evening's rush hour through Wednesday morning. Freezing rain could make for dangerous conditions after dark tonight, especially north and west.
Could there be power outages due to ice? Yes. Especially north and west of DC where temperatures will be colder and the heaviest precipitation could fall as ice. This is the biggest concern from this storm as downed trees and powerlines are likely in some areas due to ice and wind.
What about precipitation type? This is the most difficult aspect of this storm to forecast. Some precipitation will fall initially as snow, but amounts will be light. By the time the heaviest precipitation moves in, mid layers of the atmosphere will have warmed enough so that the precipitation will have changed to sleet or freezing rain (late afternoon today and overnight). There's a good chance that precipitation may even change to plain rain in DC and points south and east which would reduce the storm's impact early Wednesday morning in those areas. There is a possibility that the precipitation could turn back to snow early Wednesday, particularly to the west, given that the coastal low will be bombing out and cold air rushing in. Stay tuned for more on this as the day progresses.
When will conditions improve? Most likely midday Wednesday, but lingering snow showers, cold surface temps, and very windy conditions could cause travel and power woes throughout the day on Wednesday.
Could it fizzle out? Yes -- especially from the perspective of those who want snow. Models have been trending warmer and warmer in the layer of the atmosphere where it's required to be cold for it to snow. So snow amounts could be quite modest. While the threat of ice is real, if the storm track takes a more northerly track, an east or southeast wind could warm temperatures enough for plain old rain, even in some of the close-in northern and western suburbs. We also need to watch out for a dry slot that could form when the initial storm transfers its energy to the coast. If we get "dry slotted," the amount of precipitation we get could be significantly reduced. But the latest information suggests that the chances of a complete fizzle are unlikely.
Could the storm bring more snow or ice than expected? Perhaps. The key for the storm to outperform expectations from the perspective of snow lovers would be if it takes a more southerly track. There isn't a lot of hope for this, however. Also, if the coastal low rapidly intensifies and shifts the winds to the northerly direction more rapidly than forecast (this scenario isn't shown by the models, however), this could help produce more snow. The storm could be even more of an ice threat if the low-level cold air holds strong and doesn't retreat as much to the west. This is a possibility.
What are other forecasters saying? As of last night, most forecasters were mentioning light snow accumulations followed by the threat of icing, especially north and west, and a changeover to rain in the District and points south and east. Bob Ryan downplayed the storm as a 3 on a scale from 1-10. Doug Hill was more serious -- and warned of up to 0.5" of ice in the metro area. Topper Shutt's perspective fell somewhere in the middle -- not as dismissive as Bob nor as serious as Doug.
What's the bottom line? Big-time snow amounts are no longer likely. In fact, just a couple inches of snow seem like the best estimate at this point. But the impact of the storm is likely to be significant due to icing issues, particularly north and west of DC.
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