December 5 snow threat decreasing
The threat that the Washington area will receive its first touch of snow on Sunday keeps waxing and waning from one computer run to another. This potential snow producer is nothing like the monster snowstorms we experienced last year. Instead, the area will be flirting with a clipper type system, one that comes from the west-northwest and then passes to our south as opposed to last year's big storms that came up from the Gulf States with copious amounts of moisture. And this clipper may track too far south to give us any snow at all.
Clippers tend to be much drier than storms from the south so they rarely produce a major snowstorm and usually produce no more than a dusting to a couple inches. Occasionally, an unusually vigorous one will produce more than 4 inches but that is the exception, not the rule.
Even when a storm is only a day or two away, the snowfall associated with these storms can be tough to forecast. The remainder of the article will attempt to explain what makes snowfall forecasts with a clipper so difficult to forecast and will also outline uncertainty of the forecasts that are portrayed by the models. Finally, I'll make a guess at what I think the chances of getting measurable snow are from the system.
Because of their northerly origins and lack of moisture, clippers tend to have a rather narrow precipitation shield. Therefore, relatively small differences in the track of the storm can shift a stripe from one area into another. A forecaster calling for 1 to 3 inches of snow in Washington is liable to have his 1 to 3 inches accumulate over Richmond or Philadelphia leaving Washington high and dry. The track of the low pressure system needs to be almost perfect to get accumulating snow.
If the low tracks right over Washington or just to its north, the stripe of snow will usually be north of Washington. A track to far south may give Richmond rather than Washington. Of course in Washington, there is also always the question whether surface temperatures will support accumulating snow.
The 4-panel image above illustrates how seemingly small differences in the evolution in the strength and intensity of a clipper can lead to drastically different snow forecast for an area. The left hand panels are from a NWS model run yesterday afternoon, the right hand panel is from the same model run six hours later verifying at the same time (early Sunday morning). Note the differences between the two 500 mb projections (top panels).
One each, the location of troughs (the dips in the lines), are depicted by dashed blue lines. The two troughs are located farther apart on the earlier run allowing more of a ridge (the red line) to develop between them. Only a weak ridge develops on the right hand panel forecasts. The much flatter, less amplified flow favors less of a storm. The left hand panel 500 mb pattern with the stronger, deeper trough and more amplified ridge favors a deeper low with more precipitation than the weaker less amplified pattern. The bottom panels reflect how the differences in the mid level pattern and track impacts the low development and how far north the heavier precipitation (darker green area) can get.
The earlier run suggested the Washington area would get accumulating snow with it still snowing at 84 hours. The later run and most of the other operational models throughout the day were on a different page suggesting that any accumulating snow would stay south of the Washington area. Subsequent runs of the operational models last night supported the weaker more suppressed storm track suggesting that the probability of getting accumulating snow Saturday night into Sunday in the Washington area is low. The best chances for accumulating snow appear to be over the higher elevations of southern West Virginia and southwestern Virginia.
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