Here’s the very same model run just six hours earlier (at 7 a.m.):

This run of the same model shows a giant snow hole – i.e. no accumulation over the exact same area during that same 5 a.m. to 3 p.m. time window Tuesday.

These two model runs represent the extreme ends of a large range of possible outcomes Tuesday. Neither is particularly likely, but neither can be entirely ruled out.

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The snow hole scenario portrays a case in which the heaviest snow bands develop over southern Pennsylvania, and D.C. gets mostly light, non-accumulating snow with temperatures above freezing.

Conversely, the 10-inch pasting reflects a narrow, but heavy band of snow parking itself over the District. The heavy snow helps to cool the air and, boom!, a nuisance event has become one for the ages.

We have seen such low probability but high impact scenarios unfold in the past. March 9, 1999 and Veteran’s Day 1987 come to mind for those with a keen recollection of D.C. weather history.

The forecast problem for this event all along has been exactly where will a heavy band of snow, if it materializes at all, occur? Models have tended to favor the locations in northern Maryland to southeast Pennsylvania, but we’ seen a few simulations shift it south towards D.C.

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We are not changing our forecast based on one run of the NAM model which has problems with consistency, but rather relying on the overall suite of model solutions as well as our knowledge of this particular weather pattern. We continue to favor relatively modest snowfall amounts for the immediate D.C. area, on the order of a coating to two inches. But we wouldn’t be shocked if we’re wrong.

Forecasting along the edge of an area of snow is one of the most difficult problems in weather prediction today, because small shifts are incredibly consequential.