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Posted at 10:07 AM ET, 02/09/2012

Without a trace: why no snow in D.C. yesterday?


Snow forecast (left) versus reality (right, see bigger) for Wednesday February 8.
Let’s begin by dispensing with the idea that yesterday’s forecast was a total bust. We correctly forecast the timing of the precipitation and where the most snow would fall. And when you compare actual snowfall amounts to the amounts we forecast, you’ll see our outlook was not far off.

While the right panel of the figure above is difficult to read (because I had to shrink it down to fit), you can see that the light blue bubbles indicating where 1” or more of snow fell pretty closely correspond with the 0.5-2” zone on our forecast map on the left. Also, the gray bubbles indicating where a trace to less than 1” fell closely coincides with the forecast zone for that amount on the north and west side.

But it’s completely correct that few flakes were found inside the beltway and to the south and east to around Fredericksburg where we called for at least a trace of snow (note: Reagan National Airport did actually record a trace according to the National Weather Service). And while we were clear we didn’t expect significant problems in this area, our call for at least “conversational flakes” was a bad one.

What went wrong?

The basic issue is that surface temperatures were too warm, rising into the low 40s in the area that didn’t see snow. That temperatures got that warm didn’t surprise us, but we expected temperatures to fall into the mid-to-upper 30s when precipitation commenced. They got that cold, but that proved not to be cold enough.


Forecast sounding shows temperatures in the atmosphere (indicated by the white line) were below freezing (indicated by the red line) all the way down to the very lowest level.
You may be thinking: well, of course, the mid-to-upper 30s aren’t cold enough for snow. But it can snow at 40 degrees or even a little higher if the precipitation is falling fast enough that the flakes don’t melt through the warm layer close to the ground. The problem yesterday was that the precipitation rates were too light to overcome the warm layer

Although the atmosphere down very close to the surface was cold enough for snow. the precipitation intensity was so weak that snow flakes melted at the last possible moment before reaching the ground.

It turns out the amount of precipitation forecast by the models was about half of what actually fell. The models generally predicted 0.2” of liquid equivalent precipitation, and 0.1” fell. The lower amounts manifested themselves in weaker precipitation rates.

We cautioned such a scenario could occur in our analysis posted Tuesday:

What could go wrong?

Weaker low scenario, tracking north

This scenario would reduce precipitation amounts, and raise temperatures, resulting in little more than conversational snowflakes, or even some rain in the immediate metro area and south and east. The colder north and west suburbs would get a coating to 1” in this scenario.

This weaker scenario became the reality. Perhaps we could be rightly criticized for not shifting our forecast to this weaker scenario sooner and for continuing to emphasize the potential for a snowy commute in our forecasts. But this was such a close call, that we didn’t see strong enough evidence to materially alter our forecast.

How would you grade our forecast? Vote below...

By  |  10:07 AM ET, 02/09/2012

Categories:  Latest, Recaps, Winter Storms

 
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